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All amplifier measurements are performed independently by BHK Labs. All measurement data and graphical information displayed below are the property of the SoundStage! Network and Schneider Publishing Inc. Reproduction in any format is not permitted.

Note: Measurements were made at 120V AC line voltage and through the balanced input unless otherwise noted.

- Output power at 1% THD+N: 355.2W @ 8 ohms, 520.7W @ 4 ohms
- Output power at 10% THD+N: 447.2W @ 8 ohms, 663.1W @ 4 ohms

- This amplifier does not invert polarity.
- AC-line current draw at idle: 1.39A, 0.64PF, 105W
- Gain: output voltage divided by input voltage, 8-ohm load

- Unbalanced inputs: 31.4X, 29.9dB
- Balanced inputs: 30.4X, 29.7dB

- Input sensitivity for 1W output into 8 ohms
- Unbalanced inputs: 91.1mV
- Balanced inputs: 93.0mV

- Output impedance @ 50Hz: 0.026 ohm
- Input impedance @ 1kHz
- Unbalanced inputs: 10.8k ohms
- Balanced inputs: 50.5k ohms

- Output noise, 8-ohm load, unbalanced inputs, termination 1k ohm
- Wideband: 0.175mV, -84.2dBW
- A weighted: 0.042mV, -96.6dBW

- Output noise, 8-ohm load, balanced inputs, termination 600 ohms
- Wideband: 0.991mV, -69.1dBW
- A weighted: 0.188mV, -83.6dBW

The Jones Audio PA-M300 Series 2 is a high-powered, solid-state, monoblock power amplifier.

Chart 1 shows the frequency response of the PA-M300 with varying loads. The response is quite wideband, with a -3dB point of over 200kHz. Although the NHT dummy load shows no appreciable variation in the audioband, it does have some visible effect above 50kHz.

Chart 2 illustrates how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output for 8- and 4-ohm loads. The amount of distortion and how it rises with output level is typical of most solid-state power amplifiers, except that the IM distortion is not materially higher than the harmonic distortion. Also of note: The low-power THD+N with the unbalanced input (not shown) is quite a bit lower due to that input’s lower noise.

Chart 3 plots THD+N as a function of frequency at several different power levels. The amount of increase in distortion at high frequencies is very pronounced as the power level rises.

Damping factor vs. frequency, shown in Chart 4, is of a value and nature typical of many solid-state amplifiers: high up to about 1kHz, then rolling off with increasing frequency.

Chart 5 plots the spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal. The magnitude of the AC-line harmonics is relatively complex, with mostly odd harmonics of 60Hz extending way up into the midrange. Signal harmonics are low, with the second, third, and fifth harmonics being visible in the spectrum.

Magenta line = open circuit

Red line = 8-ohm load

Blue line = 4-ohm load

Cyan line = NHT dummy load

(Line up at 100W to determine lines)

Top line = 4-ohm SMPTE IM distortion

Second line = 4-ohm THD+N

Third line = 8-ohm SMPTE IM distortion

Bottom line = 8-ohm THD+N

(8-ohm loading)

Red line = 1W

Magenta line = 10W

Blue line = 150W

Cyan line = 300W

Damping factor = output impedance divided into 8

1kHz signal at 10W into an 8-ohm load

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- Parent Category: Products
- Category: Amplifier Measurements

All amplifier measurements are performed independently by BHK Labs. All measurement data and graphical information displayed below are the property of the SoundStage! Network and Schneider Publishing Inc. Reproduction in any format is not permitted.

Note: Measurements were made at 120V AC line voltage with both channels being driven. Measurements made on right channel digitally fed via the AES/EBU input at a 24/96 sample rate. Unless otherwise noted, the Audio Precision Aux 0025 external low-pass filter was used to keep high-frequency spuria from contaminating the Audio Precision SYS 2722 measuring instrument.

See "Additional data" section (these are manufacturer-supplied specs)

- Output power: 190W @ 8 ohms, 240W @ 6 ohms

- This amplifier does not invert polarity through the digital or analog inputs.
- AC-line current draw at idle:
- Gain set +20 & 0, AES/EBU input, 96k Fs: 59W, 0.96PF, 0.5A
- Gain set -20 & below, AES/EBU input, 96k Fs: 30W, 0.94PF, 0.27A

- Gain: output voltage divided by input voltage, analog line input, gain set to 0dB: 13.0X, 22.3dB
- Input sensitivity:
- For 1W output into 8 ohms, analog line input, gain set to +30dB (max): 6.9mV
- For 1W output into 8 ohms, digital input, gain set to +30dB (max): -52.2 dBFS

- Output impedance @ 50Hz: 0.0024 ohm
- Input impedance @ 1kHz: 14.2k ohms
- Output noise, digital input, digital input level at 0, 8-ohm load, Lch/Rch:
- Gain at -30dB, wideband: 123/130uV, -87.2/-86.7dBW
- Gain at -30dB, A weighted: 40/40uV, -97.0/-97.0dBW
- Gain at 0dB, wideband: 137/143uV, -86.3/-85.9dBW
- Gain at 0dB, A weighted: 42/39uV, -96.6/-97.2dBW
- Gain at +30dB, wideband: 174/180uV, -94.2/-83.9dBW
- Gain at +30dB, A weighted: 74/71 uV, -91.6/-92.0dBW

- Output noise, analog input, input termination 1k ohm, 8-ohm load, Lch/Rch:
- Gain at -30dB, wideband: 124/118uV, -87.2/-87.6dBW
- Gain at -30dB, A weighted: 40/45uV, -97.0/-96.0dBW
- Gain at 0dB, wideband: 296/298uV, -79.6/-79.6dBW
- Gain at 0dB, A weighted: 154/160uV, -85.3/-85.0dBW
- Gain at +30dB, wideband (note: values in mV): 8.4/8.6mV, -50.5/-50.3dBW
- Gain at +30dB, A weighted (note: values in mV): 4.6/5.0mV, -55.8/-55.0dBW

The Devialet D-Premier is unique -- a volume-controlled power DAC that accepts both digital and analog inputs. Its uniqueness is in how it generates its output, being a combination of a low-powered, class-A analog output stage and a digital-switching section. The class-A stage controls the output voltage, and the switching section adds the necessary current to supply the output power. Also of note is the power-factor-corrected power supply, which measures close to unity. This is a good thing, as it makes the incoming AC line current sinusoidal rather than the usual 120Hz, 2-3ms rectifier-charging pulses of conventional capacitor input power supplies. The result is less crap on one’s AC power line, and less messing up of the sound of the other connected gear.

This D-Premier is extremely well protected against various things that might damage it or the load. As a consequence, it was difficult, if not impossible, to produce curves of power output vs. distortion that went into clipping below loads of 8 ohms, as is usual with other, more conventional amps that have been measured. Therefore, the usual measured output powers at 1% and 10% distortion are not shown in the additional data. A Devialet publication, "Advanced Practical Information," indicates that the D-Premier’s short-term RMS power output is doubled each time the load is halved, to a maximum total power of 600Wpc.

Another observation was that the D-Premier’s distortion and noise floor was pretty much the same at the 192kHz sample rate, so we used a 96kHz sample rate for most of the measurements. The output noise, measured without the Aux-0025 filter, varied from 5 to 12mV over the gain range of ±30 for the digital and analog inputs.

Chart 1 shows the frequency response of the D-Premier with varying loads. The Devialet’s output impedance is so low that no difference can be seen at the scale we usually use in testing analog amplifiers. Also of great significance is that the D-Premier lacks an output low-pass filter, as is necessary in almost all other switching amplifiers; as a consequence, the high-frequency response is not load dependent -- an interesting plus among many of this design.

Chart 1A is a plot of the D-Premier’s frequency response as a function of the incoming sample rate, at 44.1, 96, and 192kHz. (Note: This plot is exactly what one sees for regular D/A converters used to decode signals from the digital outputs of CD transports and other digital sources to produce analog outputs.) The frequency response for analog inputs is similar to that shown in Chart 1, as the sampling rate for the analog input is also 96kHz. Not shown is the low-frequency response, which was flat to below 10Hz at all sample rates. The pulse and squarewave response shape, with its symmetrical ringing, is indicative of FIR filters being used. This plot is done without the Aux-0025 low-pass filter, to allow the full bandwidth of the D-Premier to be measured.

Chart 2 illustrates how the D-Premier’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load for 8- and 4-ohm loads. Amount of distortion is noise dominated up to perhaps 10-20W and then rises as distortion, per se, at higher power up to the power outputs shown on the chart. The amount of THD+N with the analog inputs was roughly twice as much. Looking at the D-Premier as a high-voltage-output D/A converter, I plotted the THD+N amplitude not as a percentage of reading, but as dB down from full scale as a function of decreasing input level below 0dBFS. I and others commonly do this to reveal any glitches in distortion level at various input levels, and also to easily illustrate the noise floor of the device when its input levels get way below where distortion, per se, occurs. This is shown in Chart 2A. A major aspect of this curve is that the noise floor is at about -115dBFS -- one of the lowest I have measured for a D/A converter over the years. However, the analog inputs were not so quiet, with a noise floor closer to -105dBFS.

Chart 3 shows the D-Premier’s THD+N as a function of frequency for 4-ohm loading at several different power levels. The apparent increase in distortion at high frequencies is reasonably low. Again, the Devialet’s protection circuitry prevented the taking of any measurements near the maximum amount of power the amp can deliver with music signals.

The damping factor vs. frequency, shown in Chart 4, is very high, and remains high to a far higher frequency than is typical of analog power amplifiers.

Chart 5 plots a spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal. The magnitude of the AC-line harmonics is relatively low, except for a prominent output at 120Hz. Signal harmonics are low in amplitude, the second, third, and fifth harmonics being the most significant.

I listened to this amplifier in my system quite a bit, and found it to be most revealing, clear, and musical. I wish I owned it!

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

(Note that the curves are so close together, it is not possible to see the different colors.)

**Additional: Chart 1A**

Red: 44.1kHz

Magenta: 96kHz

Blue: 192kHz

(Line up at 10W to determine lines)

Top line = 8-ohm SMPTE IM distortion

Second line = 4-ohm SMPTE IM distortion

Third line = 4-ohm THD+N

Bottom line = 8-ohm THD+N

**Additional: Chart 2A**

THD+N vs. decreasing input level in dB down from full scale; 0 dBFS = 35.8V output

(4-ohm loading)

Red line = 2W

Magenta line = 20W

Blue line = 60W

Cyan line = 150W

Damping factor = output impedance divided into 8

1kHz signal at 10W into a 4-ohm load

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- Category: Amplifier Measurements

All amplifier measurements are performed independently by BHK Labs. All measurement data and graphical information displayed below are the property of the SoundStage! Network and Schneider Publishing Inc. Reproduction in any format is not permitted.

Note: Measurements were made at 120V AC line voltage and through the balanced input unless otherwise noted.

- Output power at 1% THD+N: 428.2W @ 8 ohms, 618.2W @ 4 ohms
- Output power at 10% THD+N: 515.3W @ 8 ohms, 739.6W @ 4 ohms

- This amplifier does not invert polarity.
- AC-line current draw at idle: 0.31A, 0.58PF, 22W
- Gain: output voltage divided by input voltage for unbalanced and balanced inputs: 64.7X, 36.2dB
- Input sensitivity for 1W output into 8 ohms, unbalanced and balanced inputs: 43.7mV
- Output impedance @ 50Hz: 0.0052 ohm
- Input impedance @ 1kHz
- Unbalanced inputs: 23.6k ohms
- Balanced inputs: 45.3k ohms

- Output noise, 8-ohm load, unbalanced inputs, termination 1k ohm
- Wideband: 0.300mV, -79.5dBW
- A weighted: 0.095mV, -89.5dBW

- Output noise, 8-ohm load, balanced inputs, termination 600 ohms
- Wideband: 0.236mV, -80.0dBW
- A weighted: 0.085mV, -90.4dBW

The Simaudio Moon 400M is a high-powered, solid-state power amplifier, and the least expensive of three monoblock models in Simaudio’s line. Utilizing a full-bridge design, it has two output devices in each of the four corners of the bridge.

Chart 1 shows the frequency response of the 400M with varying loads. The high-frequency response is moderately wide, with a 3dB down point of about 120kHz. Because the 400M’s frequency response is quite invariant with load, the amplifier’s output impedance is quite low. As a consequence, the response with the NHT dummy-speaker load is not shown in the chart.

Chart 2 illustrates how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output for loads of 8 and 4 ohms. What is of interest in these results is that the distortion is relatively constant with power level over a very wide range. In its list of specifications, the 400M’s manual states that the amp’s level of IM distortion is “unmeasurable.” As Chart 2 shows, that is not the case.

THD+N as a function of frequency at several different power levels is plotted in Chart 3. The small increase in high-frequency distortion is one of the 400M’s admirable attributes. At higher powers, the amp’s protection circuitry activated and shut it down before the power sweep could be completed at low frequencies.

The plot of damping factor vs. frequency, shown in Chart 4, is of a value and nature typical of many solid-state amplifiers: high -- in this case, very high -- up to about 1kHz, and then rolling off with frequency.

Chart 5 plots the spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal with 8-ohm loading. The area of the AC-line harmonics is relatively free of discrete harmonics, but is up somewhat in level, with what would be a relatively higher noise level in this frequency range. The signal harmonics are dominated by the second, third, and fifth harmonics, with higher-order harmonics being lower but numerous in the spectral plot.

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

(Line up at 200W to determine lines)

Top line = 8-ohm SMPTE IM distortion

Second line = 4-ohm SMPTE IM distortion

Third line = 8-ohm THD+N

Bottom line = 4-ohm THD+N

(4-ohm loading)

Red line = 1W

Blue line = 10W

Cyan line = 300W

Green line = 500W

Damping factor = output impedance divided into 8

1kHz signal at 10W into a 4-ohm load

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Note: Measurements were made at 120V AC line voltage with both channels being driven. Measurements made on left channel through the balanced inputs unless otherwise noted.

- Output power at 1% THD+N: 216W @ 8 ohms, 381W @ 4 ohms
- Output power at 10% THD+N: 276W @ 8 ohms, 472W @ 4 ohms

- This amplifier does not invert polarity.
- AC-line current draw at idle: 1.3A, 0.61PF, 97W
- Gain: output voltage divided by input voltage for unbalanced and balanced inputs: 40.5X, 32.5dB
- Input sensitivity for 1W output into 8 ohms, unbalanced and balanced inputs: 69.8mV
- Output impedance @ 50Hz: 0.018 ohm
- Input impedance @ 1kHz
- Unbalanced inputs: 45.5k ohms
- Balanced inputs: 9.4k ohms

- Output noise, 8-ohm load, unbalanced inputs, termination 1k ohm, Lch/Rch
- Wideband: 0.32mV/0.32mV, -78.9dBW/-78.9dBW
- A weighted: 0.067mV/0.041mV, -92.5dBW/-96.8dBW

- Output noise, 8-ohm load, balanced inputs, termination 600 ohms, Lch/Rch
- Wideband: 0.58mV/0.59mV, -73.7 dBW/-73.6dBW
- A weighted: 0.11mV/0.10mV, -88.2dBW/-89.0dBW

The H20, a medium-powered solid-state stereo power amplifier, is the smallest of three models in the Hegel line.

Chart 1 shows the frequency response of the H20 with varying loads. The high-frequency response is wide, with an approximate 3dB-down point beyond 200kHz. The frequency response is quite invariant with load over the audioband, and so the response with the NHT dummy-speaker load is not shown in this chart. Of note, this design includes a low-frequency rolloff.

Chart 2 illustrates how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output for 8- and 4-ohm loads. The amount of distortion is dominated by noise up to perhaps 10W, then rises as distortion per se at higher power, up to clipping.

Chart 3 plots THD+N as a function of frequency for 4-ohm loading and at several different power levels. The apparent increase in distortion at high frequencies is admirably low.

The H20’s damping factor vs. frequency (Chart 4) is typical of that of many solid-state amplifiers: high up to about 1kHz, then rolling off with increasing frequency. At low frequencies, however, the effect of what causes the low-frequency rolloff also affects the output impedance, and causes the damping factor to decrease below 100Hz.

Chart 5 shows the spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal. The AC-line harmonics are relatively low in level but complex in nature. Signal harmonics are about equally second and third; the fourth and fifth harmonics are somewhat lower in level.

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

(Line up at 100W to determine lines)

Top line = 8-ohm SMPTE IM distortion

Second line = 4-ohm SMPTE IM distortion

Third line = 8-ohm THD+N

Bottom line = 4-ohm THD+N

(4-ohm loading)

Red line = 1W

Magenta line = 10W

Blue line = 70W

Cyan line = 150W

Green line = 300W

Damping factor = output impedance divided into 8

1kHz signal at 10W into a 4-ohm load

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Link: reviewed by Phil Gold on *SoundStage! Hi-Fi* on January 15, 2024

**General information**

All measurements taken using an Audio Precision APx555 B Series analyzer.

The PRE was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The PRE offers two sets of line-level unbalanced (RCA) inputs, one set of line-level balanced (XLR) inputs, one set each of unbalanced (RCA) and balanced (XLR) outputs (both always on). The PRE offers a maximum of 6dB of gain from input to output for the same input type. That is to say, if the volume is set to unity gain, an input of 2Vrms will yield 2Vrms at the output for the unbalanced input/output scenario, and the balanced input/output scenario. For the unbalanced in/balanced out scenario, 12dB gain is available. For the balanced in/unbalanced out scenario, 0dB of gain is available.

Based on the accuracy and non-repeatable nature of the channel deviation (table below), the volume control is in the analog domain, but digitally controlled. It offers between 2 and 3dB step increments for the first 12 volume steps. From steps 12 to 22, 1dB steps were measured. Beyond level 22 up to 100, the volume control offers 0.5 dB steps. Overall gain was measured at -68.7dB for volume step one, up to +6dB at the maximum position (100). Volume channel tracking proved exquisite, ranging from 0.000dB to 0.008dB.

There is a difference in terms of THD and noise between unbalanced and balanced signals in the PRE (see both the main table and FFTs below). The balanced outputs have about 6dB more uncorrelated thermal noise, whereas using the balanced inputs yields about 10dB less THD compared to the unbalanced inputs. Unfortunately, the lower distortion is only apparent in the FFTs, because they allow averages over multiple data runs, which averages out and lowers the noise floor, making the very low distortion peaks visible. During normal real-time THD measurements, the analyzer is set to measure for 2-3 seconds (maximum) and cannot assign a THD value below the measured noise floor. This explains why in the primary table below, THD appears lower for the unbalanced input/output compared to the balanced input/output. The true THD ratio figure for the balanced configuration, based on the balanced input/output FFT, is an astounding 0.00002% (about -135dB), compared to the 0.00007% (about -123dB) or so for the unbalanced input.

Unless otherwise stated, balanced input and output was evaluated, with an input and output of 2Vrms into a 200k ohm-load, with the analyzer’s input bandwidth filter set to 10Hz to 22.4kHz (exceptions include FFTs and THD vs frequency sweeps where the bandwidth is extended to 90kHz, and frequency and squarewave response where the bandwidth is extended from DC to 1MHz).

**Volume-control accuracy (measured at preamp outputs): left-right channel tracking**

Volume position | Channel deviation |

1 | 0.003dB |

10 | 0.000dB |

20 | 0.008dB |

30 | 0.001dB |

40 | 0.001dB |

50 | 0.003dB |

60 | 0.005dB |

70 | 0.005dB |

80 | 0.004dB |

90 | 0.002dB |

100 | 0.001dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Meitner for the PRE compared directly against our own. The published specifications are sourced from Meitner’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), assume, unless otherwise stated, a 1kHz sinewave, 2Vrms input and output into 200k ohms load, 10Hz to 22.4kHz bandwidth, and the worst-case measured result between the left and right channels.

Parameter | Manufacturer | SoundStage! Lab |

SNR (4Vrms output, 20Hz-20kHz BW) | >116dB | *108.1dB |

Gain control range | 74dB | 74.6dB |

THD (1kHz) | 0.004% | <0.0001% |

Frequency range | 0Hz-200kHz | 0Hz-200kHz (0/-0.14dB) |

System gain | 6dB | 6dB |

Maximum input level | 6.2Vrms | 13.5Vrms |

Input impedance (XLR) | 20k ohms | 47.9k ohms |

Input impedance (RCA) | 10k ohms | 11.6k ohms |

Output impedance (XLR) | 150 ohms | 149.4 ohms |

Output impedance (RCA) | 75 ohms | 150.7 ohms |

*SNR measured with unbalanced in/out = 115.3dB

*SNR calculated with residual noise (volume at 0) and unbalanced in/out = 118.6dB

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sinewave, 2Vrms input and output into 200k ohms load, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, once channel driven (10kHz) | -122.2dB | -89.1dB |

DC offset | <-1.7mV | <0.6mV |

Gain (default) | 6dB | 6dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-113dB | <-113dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <-100dB | <-100dB |

Input impedance (balanced) | 47.9k ohms | 47.9k ohms |

Input impedance (unbalanced) | 11.5k ohms | 11.6k ohms |

Maximum output voltage (at clipping 1% THD+N) | 20.2Vrms | 20.2Vrms |

Maximum output voltage (at clipping 1% THD+N into 600 ohms) | 16Vrms | 16Vmrs |

Noise level (with signal, A-weighted) | <12uVrms | <12uVrms |

Noise level (with signal, 20Hz to 20kHz) | <15uVrms | <15uVrms |

Noise level (no signal, volume min, A-weighted) | <7.9uVrms | <7.9uVrms |

Noise level (no signal, volume min, 20Hz to 20kHz) | <10uVrms | <10uVrms |

Output impedance (balanced) | 149.4 ohms | 149.9 ohms |

Output impedance (unbalanced) | 150.7 ohms | 150.7 ohms |

Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in) | 104.2dB | 104.1dB |

Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in) | 102.1dB | 102.3dB |

Signal-to-noise ratio (2Vrms out, A-weighted, max volume) | 100.7dB | 100.7dB |

THD (unweighted, balanced) | <0.0001% | <0.0001% |

THD (unweighted, unbalanced) | <0.00009% | <0.00009% |

THD+N (A-weighted) | <0.0006% | <0.0006% |

THD+N (unweighted) | <0.00082% | <0.00082% |

**Frequency response**

In our measured frequency-response plot above, the PRE is perfectly flat within the audioband (0dB at 20Hz and 20kHz). The PRE appears to be DC-coupled, as it yielded 0dB of deviation at 5Hz. The PRE can certainly be considered an extended-bandwidth audio device, as it is only 0.14dB down at 200kHz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

**Phase response**

Above is the phase-response plot from 20Hz to 20kHz. The PRE does not invert polarity, and exhibited zero phase shift within the audioband.

**THD ratio (unweighted) vs. frequency**

The plot above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sinewave input stimulus. The blue and red plots are for the left and right channels into 200k ohms, while purple/green (L/R) are into 600 ohms. THD values were flat across most of the audioband at 0.0001% into 600 ohms and 200k ohms, with a small rise to 0.0002% at 20kHz. This shows that the PRE’s outputs are robust and would yield identical THD performance feeding an amplifier with either a high or low input impedance.

**THD ratio (unweighted) vs. output voltage**

The plot above shows THD ratios measured at the output of the PRE as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD values measured around 0.03%, dipping down to around 0.00006% at 6-8Vrms, followed by a rise to 0.0003% at around 18Vrms. The 1% THD point is reached at 20.2Vrms. It’s also important to mention that anything above 2-4Vrms is not typically required to drive most power amps to full power.

**THD+N ratio (unweighted) vs. output voltage**

The plot above shows THD+N ratios measured at the output of the PRE as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD+N values measured around 0.2%, dipping down to around 0.0003% at 10-18Vrms.

**FFT spectrum – 1kHz (balanced in, balanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load, for the balanced inputs and outputs. We see that the signal’s second harmonic, at 2kHz, is extremely low at around -135dBrA, or 0.00002%, and subsequent signal harmonics are not visible above the -145dBrA noise floor. Below 1kHz, we can see very small peaks at 60, 120, 148, 180, and 300Hz. These peaks are all below the -130dBrA, or 0.00003%, level. This is a very clean FFT.

**FFT spectrum – 1kHz (unbalanced in, balanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and balanced outputs. The main difference here compared to the FFT above is the higher second signal harmonic, at -125dBRa, or 0.00006%, versus the -135dBrA 2kHz peak seen when the balanced inputs are used. Noise peaks left of the signal peak are at similar levels as the FFT above.

**FFT spectrum – 1kHz (unbalanced in, unbalanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and outputs. The same distortion profile with the higher 2kHz peaks can be seen here as with the FFT above. The common denominator is the use of the unbalanced inputs. The overall noise floor is at its lowest here, at -150dBrA.

**FFT spectrum – 1kHz (balanced in, unbalanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the balanced inputs and unbalanced outputs. The same distortion profile with the lower 2kHz peaks can be seen here as with the first FFT above. The common denominator is the use of the balanced inputs.

**FFT spectrum – 50Hz**

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output into a 200k-ohm load. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. Signal-related peaks can be seen at the second (100Hz) and third (150Hz) harmonics, at an extremely low -140dBrA, or 0.00001%. Noise-related peaks are all below -135dBrA, or 0.00002%.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output into a 200k-ohm load. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -125dBrA, or 0.0006%, while the third-order modulation products, at 17kHz and 20kHz, are at roughly the same level. This, like the 1kHz FFTs, is an extremely clean result.

**Intermodulation distortion FFT (line-level input, APx 32 tone)**

Shown above is the FFT of the output of the PRE into 200k ohms with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms into 200k ohms. The intermodulation products—*i.e.*, the “grass” between the test tones—are distortion products from the amplifier. Distortion products are at a vanishingly low -140dBrA, or 0.00001%. Thus, even with a complex input signal, the PRE does not add any audible coloration to the input signal.

**Square-wave response (10kHz)**

Above is the 10kHz squarewave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the PRE’s slew-rate performance. Rather, it should be seen as a qualitative representation of its relatively high bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (*e.g.*, 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The PRE’s reproduction of the 10kHz squarewave is extremely clean with sharp corners.

*Diego Estan*

Electronics Measurement Specialist

- Details
- Parent Category: Products
- Category: Preamplifier Measurements

Link: reviewed by Doug Schneider on *SoundStage! Hi-Fi* on December 1, 2023

**General Information**

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Simaudio Moon North Collection 791 was conditioned for 30 minutes with 2Vrms in/out into 200k ohms before any measurements were taken.

The 791 offers a multitude of inputs, both digital and analog (balanced and unbalanced), and line-level analog balanced outputs over XLR and unbalanced over RCA. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated: digital coaxial S/PDIF (RCA), analog balanced (XLR), as well as phono (RCA), configured both using the default settings for moving-magnet (MM) and moving-coil (MC) cartridges. Comparisons were made between unbalanced (RCA) and balanced (XLR) line inputs and outputs, and no appreciable differences were seen in terms of gain and THD+N (FFTs for different configurations can be seen in this report).

Most measurements were made with a 2Vrms line-level and 0dBFS digital input with the volume set to achieve 2Vrms at the output. For the phono input, a 5mVrms MM level and 0.5mVrms MC level were used to achieve 1Vrms at the output. The signal-to-noise ratio (SNR) measurements were made with the same input-signal values and, for comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 2Vrms output.

The 791 also offers a range of gain settings (40 in total) by using the Offset feature in the onscreen menu system. The menu allows for a setting between -10dB to +10dB, in 0.5dB steps, individually assignable to each input. Note that this changes the gain for the input, it does not offset the volume to level. Also of note, despite the -10dB to +10dB in the menu, actual gain varies from roughly -6dB to +14dB. The default setting in the menu is +6dB (+10dB of actual gain), which is, unless otherwise stated, what was used for these measurements.

Based on the accuracy and random results of the left/right volume channel matching (see table below), the 791 volume control is likely digitally controlled but operating in the analog domain. The 791 offers 140 volume steps from -69dB to 9.8dB for the line-level inputs. The first 20 steps (0 to 20dB) are in 1dB increments, and then the 20dB to 80dB volume positions can be changed in 0.5dB increments.

**Volume-control accuracy (measured at preamp outputs): left-right channel tracking**

Volume position | Channel deviation |

1 | 0.019dB |

10 | 0.014dB |

20 | 0.014dB |

30 | 0.000dB |

40 | 0.022dB |

50 | 0.000dB |

60 | 0.014dB |

70 | 0.016dB |

80 | 0.005dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Simaudio for the 791 compared directly against our own. The published specifications are sourced from Simaudio’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), assume, unless otherwise stated, assume a 1kHz sinewave at 2Vrms or 0dBFS at the input, 2Vrms at the output into 200k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter | Manufacturer | SoundStage! Lab |

Input impedance (line level, RCA) | 22k ohms | 25.8k ohms |

Maximum gain (line level) | 10dB | 9.8dB (default), 13.7dB (max) |

Phono gain | 40/54/60/66dB | 40.3/54/60/66.4dB |

Phono input resistance | 10/100/470/1k/47k ohms | 11.7/99.8/466/0.97k/46k |

Output impedance (RCA) | 50 ohms | 50.8 ohms |

Crosstalk (1kHz) | -125dB | -141dB |

Frequency response (line-level) | 2Hz-200kHz (0, -3dB) | 2Hz-200kHz (0, -3dB) |

SNR (line-level, A-weighted, 2Vrms out) | 120dB | 119.8dB |

Dynamic range (digital input, 24/96, fixed output) | 125dB | 124/125dB (L/R) |

THD+N (at 1kHz, 10Hz to 22.4kHz bandwidth) | 0.0004% | 0.00025% |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | 0.0003% | 0.00015% |

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 2Vrms output, 200kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -145dB | -137dB |

DC offset | <0.1mV | <0.1mV |

Gain (RCA in/out, default) | 9.7dB | 9.7dB |

Gain (XLR in/out, default) | 9.8dB | 9.8dB |

Gain (RCA in/out, maximum) | 13.6dB | 13.6dB |

Gain (XLR in/out, maximum) | 13.7dB | 13.7dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-117dB | <-117dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <-115dB | <-115dB |

Input impedance (line input, RCA) | 25.8k ohms | 25.7k ohms |

Input impedance (line input, XLR) | 53.2k ohms | 53.2k ohms |

Maximum output voltage (at clipping 1% THD+N) | 20Vrms | 20Vrms |

Maximum output voltage (at clipping 1% THD+N into 600 ohms) | 16Vrms | 16Vrms |

Noise level (with signal, A-weighted)* | 2.4uVrms | 2.4uVrms |

Noise level (with signal, 20Hz to 20kHz)* | 3.0uVrms | 3.0uVrms |

Noise level (no signal, A-weighted, volume min)* | 1.23uVrms | 1.23uVrms |

Noise level (no signal, 20Hz to 20kHz, volume min)* | 1.58uVrms | 1.58uVrms |

Output impedance (RCA) | 50.7 ohms | 50.8 ohms |

Output impedance (XLR) | 96.6 ohms | 96.7 ohms |

Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in)* | 119.8dB | 119.9dB |

Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in)* | 117.8dB | 117.9dB |

Signal-to-noise ratio (2Vrms out, A-weighted, max volume)* | 116.7dB | 116.7dB |

Dynamic range (2Vrms out, A-weighted, digital 24/96)* | 119.2dB | 119.8dB |

Dynamic range (2Vrms out, A-weighted, digital 16/44.1)* | 96.0dB | 96.0dB |

THD ratio (unweighted) | <0.00019% | <0.00019% |

THD ratio (unweighted, digital 24/96) | <0.00021% | <0.00019% |

THD ratio (unweighted, digital 16/44.1) | <0.0004% | <0.0004% |

THD+N ratio (A-weighted) | <0.00025% | <0.00025% |

THD+N ratio (A-weighted, digital 24/96) | <0.00027% | <0.00025% |

THD+N ratio (A-weighted, digital 16/44.1) | <0.0016% | <0.0016% |

THD+N ratio (unweighted) | <0.00025% | <0.00025% |

*due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz sinewave at 5mVrms, 1Vrms output, 200kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -101dB | -103dB |

DC offset | <0.2mV | <0.2mV |

Gain (default phono preamplifier) | 40.3dB | 40.3dB |

IMD ratio (18kHz and 19 kHz stimulus tones) | <-101dB | <-101dB |

IMD ratio (3kHz and 4kHz stimulus tones) | <-101dB | <-101dB |

Input impedance | 45.5k ohms | 46.0k ohms |

Input sensitivity (1Vrms out, max volume) | 3.15mVrms | 3.15mVrms |

Noise level (with signal, A-weighted) | <19uVrms | <19uVrms |

Noise level (with signal, 20Hz to 20kHz) | <40uVrms | <40uVrms |

Overload margin (relative 5mVrms input, 1kHz) | 22.2dB | 22.2dB |

Signal-to-noise ratio (1Vrms out, A-weighted) | 93.3dB | 93.5dB |

Signal-to-noise ratio (1Vrms out, 20Hz to 20kHz) | 88.0dB | 87.7dB |

THD (unweighted) | <0.0004% | <0.0004% |

THD+N (A-weighted) | <0.0019% | <0.0019% |

THD+N (unweighted) | <0.005% | <0.005% |

Our primary measurements revealed the following using the phono-level input, MC configuration (unless specified, assume a 1kHz sinewave at 0.5mVrms, 1Vrms output, 200kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -94dB | -93dB |

DC offset | <0.6mV | <0.6mV |

Gain (default phono preamplifier) | 60dB | 60dB |

IMD ratio (18kHz and 19 kHz stimulus tones) | <-85dB | <-85dB |

IMD ratio (3kHz and 4kHz stimulus tones) | <-80dB | <-80dB |

Input impedance | 99.5 ohms | 99.8 ohms |

Input sensitivity (1vrms out, max volume) | 0.39mVrms | 0.39mVrms |

Noise level (with signal, A-weighted) | <250uVrms | <250uVrms |

Noise level (with signal, 20Hz to 20kHz) | <520uVrms | <520uVrms |

Overload margin (relative 0.5mVrms input, 1kHz) | 24.1dB | 24.1dB |

Signal-to-noise ratio (2Vrms out, A-weighted) | 71.1dB | 71.2dB |

Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz) | 65.9dB | 66.1dB |

THD (unweighted) | <0.004% | <0.004% |

THD+N (A-weighted) | <0.025% | <0.025% |

THD+N (unweighted) | <0.06% | <0.06% |

**Frequency response (line-level input)**

In our measured frequency response (relative to 1kHz) plot above, the 791 is near perfectly flat within the audioband (0dB at 20Hz, -0.05dB at 20kHz). At the extremes the 791 is 0dB at 5Hz, and -0.8dB at 100kHz, and -3dB just past 200kHz. These data corroborate Simaudio’s claim of 2Hz to 100kHz (0/-3dB). The 791 appears to be DC-coupled, as there is no attenuation at low frequencies, even at 5Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

**Phase response (line-level input)**

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input. The 791 does not invert polarity and exhibits, at worst, less than -10 degrees (at 20kHz) of phase shift within the audioband.

**Frequency response vs. input type (left channel only)**

The chart above shows the 791’s frequency response (relative to 1kHz) as a function of input type. The green trace is the same (but limited to 80kHz) analog input data from the previous graph. The blue trace is for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz (using the coaxial input). The behavior at low frequencies is the same for all the digital sample rates, as well as the analog input—flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering filtering around 22, 48, and 96kHz (half the respective sample rate). The 44.1kHz sampled input signal exhibits typical “brick-wall”-type behavior, with a -3dB point at 21kHz. The -3dB point for the 96kHz sampled data is at 46kHz, and 91kHz for the 192kHz sampled data.

**Frequency response (MM input)**

The chart above shows the frequency response (relative to 1 kHz) for the phono input (MM configuration) and shows extremely small maximum deviations within the audioband of about +0.05 (100-200Hz) and -0.1dB (20kHz). What is shown is the deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision (*i.e.*, zero deviation would yield a flat line at 0dB). This is an example of exceptionally accurate RIAA tracking.

**Frequency response (MC input)**

The chart above shows the frequency response for the phono input (MC configuration). We see essentially the same result as with the MM configuration.

**Phase response (MM and MC phono inputs)**

Above is the phase response plot from 20Hz to 20kHz for the phono input (MM and MC configurations behaved identically). For the phono input, since the RIAA equalization curve must be implemented, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case of about +40 degrees at 20Hz and +20 degrees at 1kHz.

**Digital linearity (16/44.1 and 24/96 data)**

The chart above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced outputs of the 791. For this test, the digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. At -120dBFS, the 16/44.1 data overshot by only 1-2dB, while the 24/96 remained perfect. To verify how well the 24/96 data would perform down to -140dBFS, we extended the sweep in the chart below.

Here we can see that the 24/96 data only overshot the mark by +2/+1dB (left/right) at -140dBFS. These tests show exceptional digital-linearity results for 16/44.1 and 24/96 data.

**Impulse response (24/48 data)**

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the balanced outputs of the 791. We can see that the 791 utilizes a reconstruction filter that favors no pre-ringing.

**J-Test (coaxial input)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 791. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (*i.e.*, 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least-significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (*e.g.*, 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below. The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial S/PDIF input of the 791 shows a near-perfect J-Test result, with only two very small peaks on either side of the 12kHz fundamental at a vanishingly low -155dBrA.

**J-Test (optical input)**

The chart above shows the results of the J-Test test for the optical digital input measured at the balanced outputs of the 791. The results here are similar but not quite as pristine as the coaxial input above. Here, the peaks adjacent to the 12kHz fundamental reach -145dBrA.

**J-Test (coaxial, 2kHz sinewave jitter at 10ns)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 791, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as the J-Test result without additional jitter. The same was true for the optical input.

**J-Test (coaxial, 2kHz sinewave jitter at 100ns)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 791, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. This time, the tell-tale peaks at 10kHz and 12kHz can be seen; however, they are very small in amplitude, just below -120dBrA.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input)**

The chart above shows a fast Fourier transform (FFT) of the 791’s balanced outputs with white noise at -4dBFS (blue/red) and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows that the 791 uses a brick-wall-type reconstruction filter. There are no aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -95dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are even lower.

**THD ratio (unweighted) vs. frequency vs. load (analog)**

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for the analog balanced inputs. The 200k and 600 ohms data are identical throughout the audioband, which is in indication that the 791’s outputs are robust and can handle loads below 1k ohms with no difficultly. THD ratios are very low, from 0.0003% to 0.0002%.

**THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)**

The chart above shows THD ratios at the balanced line-level output into 200k ohms for a 16/44.1 (blue/red) dithered 1kHz signal at the coaxial input and a 24/96 (purple/green) signal, as a function of frequency. The 16/44.1 THD ratios were slightly higher than the 24/96, at 0.0005% to 0.0002%. The 24/96 data ranged from 0.0003% down to 0.00015%.

**THD ratio (unweighted) vs. frequency (phono input, MM and MC)**

The graph above shows THD ratio as a function of frequency plot for the phono input. The MM configuration is shown in blue/red (left/right channels), and MC in purple/green (left/right channels). The input sweep is EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.003% (20Hz) down to just above and below 0.0002% (1kHz to 2kHz), then up to 0.0005% at 20kHz. The MC THD values were higher, although these are limited by the higher noise floor (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). THD ratios ranged from 0.03% (20Hz) down to 0.002% (2kHz to 20kHz).

**THD ratio (unweighted) vs. output (analog)**

The chart above shows THD ratios measured at the balanced outputs of the 791 as a function of output voltage for the balanced line-level input. THD values start at 0.05% at 1mVrms, down to a low of 0.00005% at 3Vrms, then a steep rise past 10Vrms to the 1% THD mark at 20Vrms.

**THD+N ratio (unweighted) vs. output (analog)**

The chart above shows THD+N ratios measured at the balanced outputs of the 791 as a function of output voltage for the balanced line level-input, with the volume control at maximum. THD+N values start at 0.4% at 1mVrms, down to a low of 0.00015% at 5Vrms, then a steep rise past 10Vrms to the 1% THD mark at 20Vrms.

**THD ratio (unweighted) vs. output (16/44.1 and 24/96)**

The chart above shows THD ratios measured at the balanced outputs of the 791 as a function of output voltage for the digital coaxial S/PIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD values start at 3%, and predictably, reach their low at the maximum output voltage of about 6.5Vrms, at 0.0002%. For the 24/96 data, the right channel outperformed the left by 5-10dB, and THD ratios ranged from 0.2% down to 0.0001% at the maximum output voltage.

**THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)**

The chart above shows THD+N ratios measured at the balanced outputs of the 791 as a function of output voltage for the digital coaxial S/DPIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD+N values start at 20% and reach their low at the maximum output voltage of about 6.5Vrms, at 0.002%. For the 24/96 data, THD ratios ranged from 1% down to 0.0002% at the maximum output voltage.

**Intermodulation distortion vs. generator level (SMPTE, 60Hz:4kHz, 4:1, 16/44.1, 24/96)**

The chart above shows intermodulation distortion (IMD) ratios measured at balanced output for 16/44.1 (blue/red) and 24/96 input data (purple/green) from -60dBFS to 0dBFS. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 16/44.1 data yields IMD ratios from 2% down to 0.002% at 0dBFS. The 24/96 data yields IMD ratios from 0.15% down to 0.0004% near 0dBFS.

**FFT spectrum – 1kHz (XLR line-level input)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the balanced line-level input. We see that the signal’s second harmonic, at 2kHz, is at -130dBrA, or 0.00003%, and around -115dBrA, or 0.0002%, at the third (3kHz) harmonic. The subsequent signal harmonics are at a vanishingly low -140dBrA, or 0.00001%, and below. Below 1kHz, we see two small peaks from power-supply noise artifacts at 60Hz and 120Hz. These are extraordinarily low at around -150dBrA, or 0.000003%, and it must be highlighted that they are actually from the Audio Precision’s sinewave generator, and not inherent to the 791. We can say this with confidence for two reasons: these same peaks can be seen at roughly the same amplitudes when the Audio Precision’s sine-wave generator is connected directly to the inputs of its analyzer (loopback), and, these peaks are non-existent in the digital 24/96 FFT below, where the Audio Precision’s DAC generator is connected to the 791, not its sine-wave generator. It should also be stressed how extraordinarily low the 791’s noise floor is—quite possibly the quietest preamp we’ve ever measured. The 791 does not seem to have any correlated power-supply (60Hz and harmonics) related noise (what we would describe as “hum”). The residual A-weighted noise from the 791 (volume control set to minimum with no signal) due to non-correlated thermal noise (what we would describe as “hiss”) was measured at 1.2 uVrms, compared to the analyzer’s self-noise of 0.66uVrms. Even with the volume set to the reference position (70.5dB) for our measurements and with a 2Vrms output signal present (notched out by the analyzer), A-weighted noise was measured at 2.4uVrms. Given all of the digital circuitry inside the 791, this is an impressive feat accomplished by Simaudio.

**FFT spectrum – 1kHz (RCA line-level input)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the unbalanced line-level input. We see effectively the same results as with the balanced input FFT above, except for a slightly lower 2kHz signal harmonic peak for the left channel (-140dBrA instead of -130dBrA).

**FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 16/44.1. We see similar results in terms of the second (2kHz) and third (3kHz) signal harmonics compared to the FFTs above. The noise floor is much higher due to the 16-bit depth limitation.

**FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 24/96. We see essentially the same signal harmonic profile within the audioband as with the balanced analog FFT above. There are zero noise-related peaks to be seen above the -160dBrA noise floor.

**FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)**

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal- or noise-related harmonic peaks above the -140dBrA noise floor.

**FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)**

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and signal-related harmonic peaks at 2kHz (left, -135dBrA, or 0.00002%) and 4kHz (-140dBrA, or 0.00001%). Other signal-related harmonics can be see but at the extremely low -150dBrA, or 0.000003%, level.

**FFT spectrum – 1kHz (MM phono input)**

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs across for the phono input, configured for MM (default 40dB of gain in phono stage). The dominant signal-related harmonic can be seen at 2kHz, at -115dBrA, or 0.0002%. Power-supply-related noise peaks can be seen at the -100dBrA, or 0.001%, level at 60Hz and 180Hz.

**FFT spectrum – 1kHz (MC phono input)**

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs across for the phono input, configured for MC (default 60dB of gain in phono stage). There are no visible signal-related harmonic peaks above the -110 to -120dBrA noise floor. Power-supply-related noise peaks can be seen at the -80dBrA, or 0.01%, level at 60Hz and 180Hz.

**FFT spectrum – 50Hz (line-level input)**

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the balanced line-level input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peaks are that of the signal’s second harmonic (100Hz) at -120dBrA, or 0.0001%, and the third signal harmonic (150Hz) nearing -110dBrA, or 0.0003%. Power-supply-related peaks can be seen at 60Hz (-140dBrA or 0.00001%) and 120Hz (-150dBrA or 0.000003%), but as discussed above, these are inherent to the Audio Precision sinewave generator.

**FFT spectrum – 50Hz (MM phono input)**

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the phono input configured for MM. The most predominant (non-signal) peaks are that of the signal’s second harmonic (100Hz) at -110dBrA, or 0.0003%, and the primary (60Hz) and third (180Hz) power-supply-noise harmonics at -100dBrA, or 0.001%.

**FFT spectrum – 50Hz (MC phono input)**

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the phono input configured for MC. The most predominant (non-signal) peaks are that of the signal’s second harmonic (100Hz) at -90dBrA, or 0.003%, and the primary (60Hz) and third (180Hz) power-supply noise-harmonics at -80dBrA, or 0.01%.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the balanced line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -125dBRA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz, are at -120dBrA, or 0.0001%. This is a very clean IMD result.

**Intermodulation distortion FFT (line-level input, APx 32 tone)**

Shown above is the FFT of the balanced outputs of the 791 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms. The intermodulation products—*i.e.*, the “grass” between the test tones—are distortion products from the amplifier and are around the extremely low -150dBrA, or 0.000003%, level.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)**

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 16/44.1. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -130dBrA, or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz, are at around the same level. We also see the main aliased peaks at 25.1kHz and 26.1kHz at around -100dBrA.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 24/96. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -130dBrA, or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz, are at around the same level.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MM phono input)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the phono input configured for MM. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -110dBrA, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz are at around -130dBrA, or 0.00003%.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MC phono input)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the phono input configured for MC. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -100dBrA, or 0.001%, but only visible for the left channel, while the third-order modulation products, at 17kHz and 20kHz, are at around -105dBrA, or 0.0006%, but only visible at 20kHz.

**Square-wave response (10kHz)**

Above is the 10kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the 791’s slew-rate performance. Rather, it should be seen as a qualitative representation of its extended bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (*e.g.*, 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The 791 reproduction of the 10kHz squarewave is very clean, with only extremely mild softening in the edges.

*Diego Estan*

Electronics Measurement Specialist

- Details
- Parent Category: Products
- Category: Preamplifier Measurements

Link: reviewed by Jason Thorpe on *SoundStage! Ultra* on November 15, 2023

**General information**

All measurements were taken using an Audio Precision APx555 B Series analyzer.

The Angela-Gilbert Yeung C312 was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The C312 under test offers three sets of line-level unbalanced (RCA) inputs, two sets of line-level balanced (XLR) inputs, one set of unbalanced outputs, two set of balanced outputs, and a set of fixed line-level unbalanced outputs. There was no difference in terms of gain between unbalanced and balanced inputs, while there was a 6dB increase in terms of gain for the balanced outputs compared to the unbalanced outputs. There was effectively no difference in terms of THD and noise between balanced and unbalanced inputs and outputs; however, 1kHz FFTs are included in this report with all four i/o combinations for comparison purposes. The volume control does not have a numerical display. Based on the accuracy and non-repeatable nature of the channel deviation (table below), the volume control is a potentiometer operating in the analog domain.

The C312 is a very unusual preamp, as it offers three different adjustments on the front panels via three dials. These are labeled Warm, Tube S, and SS. Unless otherwise stated, measurements were made with the volume set to unity gain, using the XLR inputs and outputs, with a 2Vrms input, and the three control dials set to the same positions as were used by the reviewer Jason Thorpe (for the most part): Warm and Tube S at the 10 o’clock position (about 1/3 of full deflection), and SS at the 9 o’clock position (about ¼ of full deflection). The short description as to what these dials do is to control the gain of various stages in the preamp. If the dials are set to minimum, there is no usable output from the preamp with the volume at maximum, while the total gain measured from the preamp with all dials at maximum is an astonishing 52dB (in order to avoid clipping, a very small input signal of 10mVrms was applied). At the end of this report, an attempt was made to characterize the measured difference (if any) to the output signal that the dials have when adjusted.

**Volume-control accuracy (measured at preamp outputs): left-right channel tracking**

Volume position | Channel deviation |

min | 2.2dB |

7.5 o'clock | 0.184dB |

9 o'clock | 0.067dB |

10.5 o'clock | 0.071dB |

12 o'clock | 0.023dB |

1.5 o'clock | 0.106dB |

3 o'clock | 0.388dB |

4.5 o'clock | 0.357dB |

max | 0.292dB |

**Primary measurements**

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sine wave, 2Vrms input and output into 200k-ohm load, 10Hz to 90kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -46.5dB | -45.8dB |

DC offset | <-3.8mV | <14.7mV |

Gain (default) | 7.2dB | 6.9dB |

Gain (all controls to maximum) | 52.6dB | 52.6dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-100dB | <-100dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <-96dB | <-96dB |

Input impedance (balanced) | 59.2k ohms | 57.0k ohms |

Input impedance (unbalanced) | 52.7k ohms | 52.8k ohms |

Maximum output voltage (at clipping 1% THD+N) | 13.5Vrms | 13.5Vrms |

Maximum output voltage (at clipping 1% THD+N into 600 ohms) | 12.9Vrms | 12.9Vrms |

Noise level (with signal, A-weighted) | <65uVrms | <69uVrms |

Noise level (with signal, unweighted) | <47uVrms | <50uVrms |

Noise level (no signal, volume min, A-weighted) | <14uVrms | <14uVrms |

Noise level (no signal, volume min, 20Hz to 20kHz) | <17uVrms | <17uVrms |

Output impedance (balanced) | 4.2 ohms | 4.1 ohms |

Output impedance (unbalanced) | 2.3 ohms | 2.35 ohms |

Signal-to-noise ratio (A-weighted) | 92.6dB | 92.6dB |

Signal-to-noise ratio (20Hz to 20kHz) | 90.4dB | 90.1dB |

Signal-to-noise ratio (max volume, 2Vrms out, A-weighted) | 87.5dB | 87.4dB |

THD (unweighted, balanced) | <0.0012% | <0.0012% |

THD (unweighted, unbalanced) | <0.0012% | <0.0012% |

THD+N (A-weighted) | <0.0027% | <0.0027% |

THD+N (unweighted) | <0.0034% | <0.0035% |

**Frequency response**

In our measured frequency response (relative to 1kHz) plot above, the C312 is essentially flat within the audioband (0dB at 20Hz, less than -0.1dB at 20kHz). The C312 appears to be AC-coupled, as it yielded about -0.2dB of deviation at 5Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace) and so they perfectly overlap, indicating that the two channels are ideally matched.

**Phase response**

Above is the phase response plot from 20Hz to 20kHz. The C312 does not invert polarity, and it yielded a worst-case -20 degrees of phase shift at 20kHz.

**THD ratio (unweighted) vs. frequency**

The plot above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sinewave input stimulus. The blue and red plots are for left and right into 200k ohms, while purple/green (L/R) are into 600 ohms. THD values range from 0.0003-0.0005% from 20Hz to 200Hz, then up to 0.02% at 20kHz into 200k ohms. Into a 600-ohm load, THD ratios were nearly identical, but 2-3dB higher through most of the frequency sweep.

**THD ratio (unweighted) vs. output voltage**

The plot above shows THD ratios measured at the output of the C312 as a function of output voltage into 200k ohms with a 1kHz input sinewave, with the volume set to maximum. At the 10mVrms level, THD values measured around 0.1%, dipping down to around 0.0004% at 5-6Vrms, followed by a rise to 0.0007% at the “knee,” at around 12Vrms. The 1% THD point is reached at 13.5Vrms. It’s also important to mention that anything above 2-4Vrms is not typically required to drive most power amps to full power.

**THD+N ratio (unweighted) vs. output voltage**

The plot above shows THD+N ratios measured at the output of the C312 as a function of output voltage into 200k ohms with a 1kHz input sinewave, with the volume set to maximum. At the 10mVrms level, THD+N values measured around 1%, dipping down to around 0.0015% at 12Vrms.

**FFT spectrum – 1kHz (balanced in, balanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load, for the balanced inputs and outputs. We see that the signal’s second harmonic, at 2kHz, is at around -100dBrA, or 0.001%, while the third harmonic, at 3kHz, is much lower at -125dBrA, or 0.00006%. Higher order harmonics are non-existent above the -130dBrA noise floor. Below 1kHz, we can see power-supply-related noise peaks at the fundamental (60Hz) and second harmonic (120Hz) at -110dBrA, or 0.0003%, and higher harmonics at -115dBrA, or 0.0002%, and below.

**FFT spectrum – 1kHz (unbalanced in, balanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and balanced outputs. The FFT is essentially identical to the balanced-in/balanced-out FFT above.

**FFT spectrum – 1kHz (unbalanced in, unbalanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and outputs. Again, the FFT is essentially identical to the balanced-in/balanced-out FFT above.

**FFT spectrum – 1kHz (balanced in, unbalanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the balanced inputs and unbalanced outputs. Yet again, the FFT is essentially identical to the balanced in/balanced out FFT above.

**FFT spectrum – 50Hz**

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output into a 200k-ohm load. The X axis is zoomed in from 40Hz to 1kHz so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant non-signal peaks are from the power-supply-related noise peaks at 60/120Hz at -110dBrA, or 0.0003%. The second (100Hz) and third (150Hz) signal harmonics are very low at -125dBrA, or 0.00006%.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output into a 200k-ohm load. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -110dBrA, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz, are at -120/-115dBrA, or 0.0001/0.0002%. This is a clean IMD result.

**Intermodulation distortion FFT (line-level input, APx 32 tone, 24/96)**

Shown above is the FFT of the speaker-level output of the C312 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—*i.e.*, the “grass” between the test tones—are distortion products from the preamplifier and below the -120dBrA, or 0.0001%, level. This is another clean IMD result. The peaks that reach the -110dBrA level at lower frequencies are not IMD products but power-supply-related noise peaks.

**Square-wave response (10kHz)**

Above is the 10kHz squarewave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the C312’s slew-rate performance. Rather, it should be seen as a qualitative representation of its relatively high bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (*e.g.*, 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The C312’s reproduction of the 10kHz squarewave is clean, with only mild softening in the corners.

**What do the Warm, Tube S, and SS control dials do?**

Each dial controls the gain in different stages of the preamp. The Warm dial provides the most significant changes in gain: from -42dB to +27.8dB (with the other two dials held at the 9 o’clock position). Both the Tube S and SS dials varied the gain from about -10dB to +15dB (in each case with the other two dials held at the 9 o’clock position). The effects of changing each dial, while maintaining the other two dials at the 9 o’clock position, were explored. When varying the dial positions, we found no appreciable changes in terms of: frequency response, phase, crosstalk, and output impedance. Because each dial affects gain, we predictably found changes in terms of noise and distortion (and IMD). In terms of the dials yielding differences in noise and distortion from one dial to the other, we found the effects of varying Tube S and SS to be essentially identical, while Warm yielded more distortion, with more high frequency harmonics.

We first explored changing the dials while maintaining low distortion, and below are the 1kHz FFTs with each dial at the 3 o’clock position, while maintaining the other two dials at the 9 o’clock position. In each case, an input voltage of 1Vrms was maintained and an output of 2Vrms (using the volume control). We found that with the Warm dial set to 3 o’clock, there was more distortion with a clear peak at the third harmonic (3kHz) at -110dBrA, or 0.0003%, that was not there when Tube S and SS were set to the same position. Having the Warm dial set to the 3 o’clock position did yield less noise compared to when Tube S and SS were set to the same position; however, this may be due to having the overall volume set to a lower position. There was absolutely no difference in the 1kHz FFTs between Tube S and SS set to the 3 o’clock position.

**FFT spectrum—1kHz (Warm at 3 o’clock)**

**FFT spectrum—1kHz (Tube S at 3 o’clock)**

**FFT spectrum—1kHz (SS at 3 o’clock)**

We then explored changing the dials to achieve high distortion (~5% THD). This was done with a baseline of maintaining all dials at the 12 o’clock position with a 2Vrms input, and 2Vrms output, then adjusting one dial at a time to achieve 5% THD, all the while adjusting the overall volume to maintain 2Vrms at the output. We also included a scope capture to display the shape of the 1kHz waveform. In addition, we show an FFT and scope capture for the scenario where Warm is set to maximum. We found that the Warm dial yields “harder” clipping, which can be seen in the distorted peaks of the sinewaves compared to when Tube S and SS were adjusted to yield 5% THD. Once again, no differences were seen between Tube S and SS in the 5% THD scenario.

**FFT spectrum—1kHz (all dials at 12 o’clock—the baseline)**

**FFT spectrum—1kHz (Warm causing 5% THD)**

**Scope—1kHz (Warm causing 5% THD)**

**FFT spectrum—1kHz (TUBE S causing 5% THD)**

**Scope—1kHz (TUBE S causing 5% THD)**

**FFT spectrum—1kHz (SS causing 5% THD)**

**Scope—1kHz (SS causing 5% THD)**

**FFT spectrum—1kHz (Warm at maximum)**

**Scope—1kHz (Warm at maximum)**

Because adjusting Tube S and SS seemed to yield identical results, we explored this further by maintaining Warm at the 10 o’clock position and alternating between Tube S at maximum with SS at minimum, and vice versa, while maintaining the input at 1Vrms and the output at 2Vrms. Below you will find FFTs for a 1kHz sinewave, IMD (CCIF, 18+19kHz, 1:1) and 32-tone, as well as frequency response. We also explored (not shown) IMD (SMPTE, 60Hz+7kHz, 4:1), crosstalk, phase, and output impedance. With the exception of a very small difference in frequency response, there were absolutely no differences between Tube S and SS adjustments. With the Tube S set to maximum, there was a small dip at very low frequencies (-0.2dB at 5Hz), whereas with SS set to maximum, we measured 0dB at 5Hz. This would be inaudible.

**FFT spectrum—1kHz (Tube S maximum, SS minimum)**

**FFT spectrum—1kHz (Tube S minimum, SS maximum)**

**FFT spectrum—IMD (Tube S maximum, SS minimum)**

**FFT spectrum—IMD (Tube S minimum, SS maximum)**

**FFT spectrum—32-tone (Tube S maximum, SS minimum)**

**FFT spectrum—32-tone (Tube S minimum, SS maximum)**

**Frequency response (Tube S maximum, SS minimum)**

**Frequency response (Tube S minimum, SS maximum)**

*Diego Estan*

Electronics Measurement Specialist

- Details
- Parent Category: Products
- Category: Preamplifier Measurements

Link: reviewed by Jason Thorpe on *SoundStage! Hi-Fi* on January 1, 2023

**General information**

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Hegel Music Systems P30A was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The P30A offers three sets of line-level unbalanced (RCA) inputs, two sets of line-level balanced (XLR) inputs, two sets of unbalanced outputs, and one set of balanced outputs. There’s no difference in terms of gain between unbalanced and balanced inputs/outputs. That is to say, if the volume is set to unity gain, an input of 2Vrms will yield 2Vrms at the output, regardless of the input and output type configuration (*i.e.*, all of these configurations yield the same results in terms of gain: RCA in/XLR out, XLR in/RCA out, RCA in/RCA out, XLR in/XLR out). The volume control does not have a numerical display. Based on the accuracy and non-repeatable nature of the channel deviation (table below), the volume control is digitally controlled but passes the signal in the analog domain. It offers between 8dB and 2dB increments for the first eight volume steps. Beyond the eighth step to just below the 12 o’clock position, 1dB steps were measured. Beyond the 12 o’clock position, the volume control offers 0.5 dB steps. Overall gain was measured at -85dB for volume step one, up to +5.3dB at the maximum position.

As Hegel claims, there is a difference in terms of THD between unbalanced and balanced signals in the P30A (see both the main table and FFTs below). We found that the difference lies in whether the unbalanced or balanced inputs (not outputs) are used—the unbalanced inputs yielded a little over twice as much THD at 1kHz. Unless otherwise stated, measurements were made with the volume set to unity gain, using the XLR inputs and outputs, with a 2Vrms input.

**Volume-control accuracy (measured at preamp outputs): left-right channel tracking**

Volume position | Channel deviation |

min | 0.009dB |

9 o'clock | 0.069dB |

12 o'clock | 0.059dB |

3 o'clock | 0.078dB |

max | 0.073dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Hegel for the P30A compared directly against our own. The published specifications are sourced from Hegel’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth is set at its maximum (DC to 1MHz), assume, unless otherwise stated, a 1kHz sine wave, 2Vrms input and output into 200k-ohm load, 10Hz to 90kHz bandwidth, and the worst-case measured result between the left and right channels.

Parameter | Manufacturer | SoundStage! Lab |

SNR (14Vrms output, volume at unity gain, A-weighted) | >130dB | 122.4dB |

Crosstalk | <-100dB | -107dB |

THD | 0.005% | 0.002% |

IMD ratio (19kHz and 20kHz stimulus tones, 2Vrms) | <0.01% | <0.008% |

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sine wave, 2Vrms input and output into 200k-ohm load, 10Hz to 90kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -87.3dB | -96.8dB |

DC offset | <0.6mV | <0.6mV |

Gain (default) | 5.33dB | 5.26dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <0.0075% | <0.0072% |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <0.0073% | <0.0073% |

Input impedance (balanced) | 15.2k ohms | 15.5k ohms |

Input impedance (unbalanced) | 20.1k ohms | 20.5k ohms |

Maximum output voltage (at clipping 1% THD+N) | 14Vrms | 14Vrms |

Maximum output voltage (at clipping 1% THD+N into 600 ohms) | 10Vrms | 10Vrms |

Noise level (A-weighted) | <10uVrms | <10uVrms |

Noise level (unweighted) | <22uVrms | <22uVrms |

Output impedance (balanced) | 1763 ohms | 1764 ohms |

Output impedance (unbalanced) | 23 ohms | 23 ohms |

Signal-to-noise ratio (A-weighted) | 105.6dB | 105.7dB |

Signal-to-noise ratio (unweighted) | 99.6dB | 99.6dB |

Signal-to-noise ratio (max volume, 2Vrms out, A-weighted) | 105.6dB | 105.7dB |

THD (unweighted, balanced) | <0.0020% | <0.0020% |

THD (unweighted, unbalanced) | <0.0048% | <0.0048% |

THD+N (A-weighted) | <0.0024% | <0.0023% |

THD+N (unweighted) | <0.0023% | <0.0022% |

**Frequency response**

In our measured frequency-response plot above, the P30A is essentially perfectly flat within the audioband (0dB at 20Hz, less than -0.1dB at 20kHz). The P30A appears to be DC-coupled, as it yielded 0dB of deviation at 5Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

**Phase response**

Above is the phase response plot from 20Hz to 20kHz. The P30A does not invert polarity, and yielded a worst-case 25 degrees or so of phase shift at 20kHz.

**THD ratio (unweighted) vs. frequency**

The plot above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sine-wave input stimulus. The blue and red plots are for left and right channels into 200k ohms, while purple/green (L/R) are into 600 ohms. THD values were essentially flat across the audioband at 0.002% into 600 ohms. Into a 200k-ohm load, THD ratios were at 0.002% from 20kHz to 1kHz, then a rise to 0.01% at 20kHz.

**THD ratio (unweighted) vs. output voltage**

The plot above shows THD ratios measured at the output of the P30A as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 10mVrms level, THD values measured around 0.01%, dipping down to around 0.0005% at 0.6-0.7Vrms, followed by a rise to 0.05% at 10Vrms. The 1% THD point is reached at 14Vrms. It’s also important to mention that anything above 2-4Vrms is not typically required to drive most power amps to full power.

**THD+N ratio (unweighted) vs. output voltage**

The plot above shows THD+N ratios measured at the output of the P30A as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 10mVrms level, THD+N values measured around 0.2%, dipping down to around 0.002% at 1.5Vrms.

**FFT spectrum – 1kHz (balanced in, balanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output into a 200k-ohm load, for the balanced inputs and outputs. We see that the signal’s second harmonic, at 2kHz, is at around -115dBrA, or 0.0002%, while the third harmonic, at 3kHz, is higher at -95dBrA, or 0.002%. Higher-order odd harmonics can be seen to beyond 20kHz, at -135dBRa, or 0.0002%, and below. Below 1kHz, we can see only a very small peak at 120Hz, the second harmonic of the power-supply fundamental, at -125dBra, or 0.00006%, just above the noise floor.

**FFT spectrum – 1kHz (unbalanced in, balanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and balanced outputs. The main difference here compared to the FFT above is the much higher second signal harmonic, at -90dBRa, or 0.003%, versus the -115dBrA 2kHz peak seen when the balanced inputs are used.

**FFT spectrum – 1kHz (unbalanced in, unbalanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and outputs. The same distortion profile with the higher 2kHz peaks can be seen here as with the FFT above. The common denominator is the use of the unbalanced inputs.

**FFT spectrum – 1kHz (balanced in, unbalanced out)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output into a 200k-ohm load for the balanced inputs and unbalanced outputs. The same distortion profile with the lower 2kHz peaks can be seen here as with the first FFT above. The common denominator is the use of the balanced inputs.

**FFT spectrum – 50Hz**

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output into a 200k-ohm load. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant non-signal peak is from the signal’s third harmonic (3kHz) at -95dBrA, or 0.002%. The second signal harmonic (100Hz) is at -115dBrA, or 0.0002%. Peaks from the power-supply fundamental (60Hz) and the second (120Hz), fourth (240Hz), and fifth (300Hz) harmonics can be seen at very low levels (-130dBrA, or 0.00003%, and below) just above the noise floor.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output into a 200k-ohm load. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -125/-135dBrA (left/right), or 0.00006/0.00002%, while the third-order modulation products, at 17kHz and 20kHz are at -95dBrA, or 0.002%.

**Square-wave response (10kHz)**

Above is the 10kHz squarewave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the P30A’s slew-rate performance. Rather, it should be seen as a qualitative representation of the P30A’s relatively high bandwidth. An ideal squarewave can be represented as the sum of a sine wave and an infinite series of its odd-order harmonics (*e.g.*, 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The P30A’s reproduction of the 10kHz squarewave is clean, with only mild softening in the corners.

*Diego Estan*

Electronics Measurement Specialist

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