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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 with both channels being driven. Measurements made on left channel through the balanced inputs unless otherwise noted.

- Output power at 1% THD+N: 224.8W @ 8 ohms, 372.2W @ 4 ohms
- Output power at 10% THD+N (est., see text): 250W @ 8 ohms, 400W @ 4 ohms

- This amplifier does not invert polarity.
- AC-line current draw at idle: 1.52A, 0.75PF, 137W
- AC-line current draw at standby: 0.4A, 0.77PF, 37W
- Gain: output voltage divided by input voltage for balanced inputs: 18.61X, 25.4dB
- Input sensitivity for 1W output into 8 ohms, balanced inputs: 152.0mV
- Output impedance @ 50Hz: 0.15 ohm
- Input impedance @ 1kHz: 1M ohm
- Output noise, 8-ohm load, balanced inputs, termination 600 ohms, Lch/Rch
- Wideband: 0.369mV/0.363mV, -77.7 dBW/-77.8dBW
- A weighted: 0.078mV/0.076mV, -91.2dBW/-91.4dBW

The VX-R, one of Ayre Acoustics’ newest models, is a medium-power solid-state stereo power amplifier. Though of relatively small size, this dual-mono design, milled from a solid block of aluminum, is one heavy beast of an amp: It weighs almost 80 pounds.

Chart 1 shows the VX-R’s frequency response with varying loads. The high-frequency response is quite wide, with an approximate 3dB down point in excess of 200kHz.

The frequency response varies a small amount with load; therefore, the VX-R’s output impedance is reasonably low. The effect of the NHT dummy load is hard to see at the resolution at which this chart is usually shown; it amounts to a frequency-response deviation of about ±0.2dB.

Chart 2A illustrates how the VX-R’s 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 protection fuses in the “front end” of the VX-R blew repeatedly when I attempted to drive it to 10% THD+N, so the values listed in the Additional Data are estimates. The amount of distortion is reasonable for what is claimed to be a no-overall-feedback design.

Chart 3 plots the THD+N as a function of frequency at several different power levels. The amount of increase in distortion at high frequencies is admirably low up to 150W. Attempts to get data at the 200W level blew the front-end fuses.

The VX-R’s plot of damping factor vs. frequency (Chart 4) is unusually flat. I have seen only a few amps with this characteristic. This usually goes along with flat distortion amount with changing frequency, as is the case with this design.

A spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal is plotted in Chart 5. The magnitudes of the AC-line harmonics are very low, and the signal harmonics are predominantly the second and third; all higher harmonics are an order of magnitude lower.

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

Cyan line = NHT dummy load

(Line up at 10W to determine lines)

Top line = 4-ohm SMPTE IM distortion

Second line = 8-ohm SMPTE IM distortion

Third line = 4-ohm THD+N

Bottom line = 8-ohm THD+N

(8-ohm loading)

Red line = 1W

Magenta line = 10W

Blue line = 70W

Cyan line = 150W

Damping factor = output impedance divided into 8

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

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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.

Notes: Measurements were made at 120V AC line voltage with both channels being driven. Measurements were made on the left channel through the balanced input unless otherwise noted. The Audio Precision AUX-0025 measurement filter was used unless otherwise noted.

- Power output at 1% THD+N: 187.5W @ 8 ohms, 348.8W @ 4 ohms
- Power output at 10% THD+N: 272.7W @ 8 ohms, 448.8W @ 4 ohms

- This amplifier does not invert polarity.
- AC-line current draw at idle: 0.14A, 0.63PF, 10.0W
- AC-line current draw in operation: 0.59A, 0.67PF, 48.8W
- Gain: output voltage divided by input voltage, 8-ohm load
- Unbalanced inputs: 39.5X, 31.9dB
- Balanced inputs: 39.5X, 31.9dB

- Input sensitivity for 1W output into 8 ohms
- Unbalanced inputs: 71.6mV
- Balanced inputs: 71.6mV

- Output impedance @ 50Hz: 0.018 ohm
- Input impedance @ 1kHz
- Unbalanced inputs: 135k ohms
- Balanced inputs: 135k ohms

- Output noise, 8-ohm load, balanced inputs terminated with 600 ohms and unbalanced inputs terminated with 1k ohms, without AUX-0025 filter, Lch/Rch

- Wideband: 471mV/477mV, -15.6dBW/-15.5dBW

- Output noise, 8-ohm load, balanced inputs terminated with 600 ohms and unbalanced inputs terminated with 1k ohms, with AUX-0025 filter, Lch/Rch

- Wideband: 0.87mV/0.91mV, -70.2dBW/-69.9dBW
- A weighted: 0.29mV/0.34mV, -79.8dBW/-78.4dBW

The Rogue Audio Medusa is a high-power hybrid stereo power amplifier with a vacuum-tubed front end coupled to a pair of Hypex switching-amplifier modules.

Chart 1 shows the Medusa’s frequency response with varying loads. The amp’s high-frequency response is rolled off before the cutoff frequency of the Audio Precision AUX-0025 measuring filter used for measuring switching amplifiers. As a result, the high-frequency response remained virtually unchanged, regardless of whether or not it was sent through the filter. The -3dB point is about 26kHz. Further, the -3dB down point at 26kHz is independent of load -- a feature of the Hypex modules.

As can be seen, the Medusa’s output impedance is so low that the variations with the NHT dummy speaker load don’t show up on the response plot.

Chart 2 illustrates how the Medusa’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load for loads of 8 and 4 ohms. The amount of distortion, and how it rises with output level, are similar to those of tube power amplifiers, and are no doubt caused by the Medusa’s tubed front end.

THD+N as a function of frequency at several different power levels is plotted in Chart 3. The degree of increase in distortion at high frequencies is admirably low.

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 120Hz being the dominant one. Not surprisingly, the dominant signal harmonic is the second, coming from the tube circuitry. All higher harmonics quickly disappear into the noise 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

(8-ohm loading)

Red line = 1W

Magenta line = 10W

Blue line = 70W

Cyan line = 200W

Damping factor = output impedance divided into 8

1kHz signal at 10W into an 8-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: 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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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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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 Aron Garrecht on *SoundStage! Ultra* on August 15, 2021

**General Information**

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

The SPL Director Mk2 was conditioned for 30 minutes at 0dBFS (4.3Vrms out) into 200k ohms before any measurements were taken.

The Director Mk2 offers a multitude of digital and analog inputs, including one set of balanced outputs (XLR), a tape loop (single-ended RCA inputs and outputs), and a fixed single-ended line-level output (RCA). Comparisons were made between S/PDIF optical (TosLink), S/PDIF coaxial (RCA), and AES/EBU (XLR) digital inputs; total harmonic distortion plus noise (THD+N) was the same for all of them. For the measurements below, unless otherwise specified, the coaxial digital input (0dBFS) and the balanced analog input (2 or 4.3Vrms) were used, with the volume control set to maximum (-0.1dB). With the volume at maximum, a 0dBFS digital input yields 4.3Vrms at the output.

The Director Mk2 volume control appears to be a traditional potentiometer offering a range of attenuation from about -90dB to -0.1dB.

Whereas most preamplifiers offer at least 6dB of gain, one interesting design aspect of the Director Mk2 is that it offers no gain. In fact, in the table where we have our primary measurements, the gain for each channel is a little less than 0dB. As a result, potential users should ensure compatibility with whatever power amplifier and/or source component(s) the Director Mk2 will be partnered with.

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

Volume position | Channel deviation |

min | 0.9dB |

25% | 0.246dB |

50% | 0.200dB |

75% | 0.137dB |

max | 0.119dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by SPL for the Director Mk2 compared directly against our own. The published specifications are sourced from SPL’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 a measurement input bandwidth of 10Hz to 90kHz, 200k ohms load, and the worst-case measured result between the left and right analog balanced input.

Parameter | Manufacturer | SoundStage! Lab |

Maximum input and output voltage | 32.5dBu (33Vrms) | >26.7Vrms |

Input impedance (RCA) | 47k ohms | 89.1k ohms |

Input impedance (XLR) | 20k ohms | 21.7k ohms |

Output impedance | 75 ohms | 74.3 ohms |

Frequency range (-3dB) | 4Hz - 300kHz | 1Hz(-3dB), 200kHz(-1dB) |

Crosstalk (1kHz, ref 0.775Vrms) | -108dB | -111dB |

THD (1kHz, ref 0.775Vrms) | 0.000992% | <0.00009% |

Noise (A-weighted, ref 0.775Vrms) | -102.5dB | <-100dB |

Dynamic range (ref maximum output voltage) | 135dB | *132dB |

*The maximum input voltage available with the Audio Precision APx555 is 26.66Vrms. Since the SPL has no gain, roughly the same voltage is available at the output. At 26.66Vrms, the SNR is 130.2dB. The 132dB figure was calculated based on an assumed maximum output voltage of 33Vrms.

Our primary measurements revealed the following using the coaxial input, the balanced analog input and the balanced line-level outputs (unless specified, assume a 1kHz sine wave at 0dBFS or 4.3Vrms, 200k ohms loading, 10Hz to 90kHz bandwidth):

Parameter | Left channel | Right Channel |

Crosstalk, one channel driven (10kHz, analog) | -92.9dB | -111.9dB |

Crosstalk, one channel driven (10kHz, 16/44.1) | -97.8dB | -111.5dB |

Crosstalk, one channel driven (10kHz, 24/96) | -97.9dB | -111.1dB |

DC offset | <0.4mV | <0.3mV |

Dynamic range (A-weighted, 16/44.1) | 95.8dB | 96.2dB |

Dynamic range (unweighted, 16/44.1) | 93.0dB | 93.4dB |

Dynamic range (A-weighted, 24/96) | 110.5dB | 111.3dB |

Dynamic range (unweighted, 24/96) | 102.0dB | 104.4dB |

IMD ratio (18kHz and 19kHz stimulus tones, analog) | <-115dB | <-117dB |

IMD ratio (18kHz and 19kHz stimulus tones, 16/44.1) | <-96dB | <-96dB |

IMD ratio (18kHz and 19kHz stimulus tones, 24/96) | <-96dB | <-97dB |

Input impedance | 21.7k ohms | 21.4k ohms |

Maximum gain | -0.115dB | -0.234dB |

Maximum output voltage | >26.7Vrms | >26.7Vrms |

Output impedance | 74.3 ohms | 74.2 ohms |

Noise level (A-weighted, analog) | <8uVrms | <8uVrms |

Noise level (unweighted, analog) | <18uVrms | <17uVrms |

Noise level (A-weighted, 16/44.1) | <71uVrms | <70uVrms |

Noise level (unweighted, 16/44.1) | <106uVrms | <98uVrms |

Noise level (A-weighted, 24/96) | <17uVrms | <16uVrms |

Noise level (unweighted, 24/96) | <42uVrms | <30uVrms |

Signal-to-noise ratio (A-weighted, analog) | 115.1dB | 115.0dB |

Signal-to-noise ratio (unweighted, analog) | 108.6dB | 108.7dB |

THD ratio (unweighted, analog) | <0.00004% | <0.00004% |

THD ratio (unweighted, 16/44.1) | <0.001% | <0.001% |

THD ratio (unweighted, 24/96) | <0.00095% | <0.00095% |

THD+N ratio (A-weighted, analog) | <0.00018% | <0.00018% |

THD+N ratio (unweighted, analog) | <0.0004% | <0.0004% |

THD+N ratio (A-weighted, 16/44.1) | <0.002% | <0.002% |

THD+N ratio (unweighted, 16/44.1) | <0.0027% | <0.0025% |

THD+N ratio (A-weighted, 24/96) | <0.0011% | <0.0011% |

THD+N ratio (unweighted, 24/96) | <0.0013% | <0.0012% |

**Frequency response (analog)**

In our measured frequency-response plot above, the Director Mk2 is perfectly flat within the audioband (20Hz to 20kHz), and only about -0.25dB at 100kHz. SPL’s claim of a frequency range (-3dB) of 4Hz to 300kHz can be corroborated at 4Hz (we measured -0.1dB at 5Hz), but due to the limitations of the Audio Precision’s maximum 200kHz upper limit for a frequency sweep, the 300kHz figure can only be inferred. Since we measured -0.5dB (left) and -0.7dB (right) at 200kHz, it’s fairly safe to assume that the Director Mk2 makes or comes close to making the company’s -3dB 300kHz spec. The Director Mk2 can definitely be considered a high-bandwidth audio device. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue, purple, or orange trace) is performing identically to the right channel (red, green, or pink trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

**Frequency response vs. input type (16/44.1, 24/96, 24/192, analog)**

The chart above shows the Director Mk2’s frequency response as a function of sample rate. The blue/red traces are for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally orange/pink represents 24/192 from 5Hz to 96kHz. In addition, for comparison, the analog frequency response is shown in green (up to 80kHz). The behavior at low frequencies is the same for all plots—near perfectly flat down to 5Hz. There is an oddity at high frequencies, however, where the right channel showed a softer attenuation around the corner frequency at all sample rates compared to the left channel. All three sample rate data for the right channel were at -0.5dB at 20kHz, while the left channel at all sample rates was at -0.1dB at 20kHz. The behavior of the left channel at high frequencies for all three digital sample rates is as expected, offering sharp filtering around 22, 48, and 96kHz (half the respective sample rate). The -3dB point for each sample rate (left channel) is roughly 21, 46, and 90kHz, respectively. It is also obvious from the plots above that the 44.1kHz sampled input signal (left channel) offers the most “brick-wall”-type behavior, while the attenuation of the 96kHz and 192kHz sampled input signals approaching the corner frequencies (48kHz and 96kHz) is gentler.

**Phase response (analog)**

Above is the phase response plot from 20Hz to 20kHz. The Director Mk2 does not invert polarity, and the plot shows less than -10 degrees of phase shift at 20kHz.

**Phase response vs. sample rate (16/44.1, 24/96, 24/192)**

Above are the phase response plots from 20Hz to 20kHz for the coaxial input, measured at the balanced output. The blue/red traces are for a dithered 16/44.1 input at -20dBFS, the purple/green for 24/96, and the orange/pink for 24/192. Here again we see the differences between the left and right channels. Since the left channel exhibits sharper attenuation than the right for all sample rates, predictably, there is more phase shift at 15-20kHz than the right channel. At 15kHz, the phase shift is at around +144/+128 (left/right) degrees for the 16/44.1 input data, +45/+30 (left/right) degrees for the 24/96 input data, and +24/+8 (left/right) for the 24/192 input data.

**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 line-level output. 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 is a straight flat line at 0dB (*i.e.*, the amplitude of the digital input perfectly matches the amplitude of the measured analog output). Both input data types exhibited exemplary linearity. The 16/44.1 and 24/96 data showed a worst-case deviation of only +2dB around -120dBFS. At -100dBFS, both input data yielded essentially perfect results down to 0dBFS. The sweep was also performed down to -140dBFS (not shown) where both input data showed significant deviations below -120dBFS.

**Impulse response (16/44.1 and 24/96 data)**

The chart above shows the impulse responses for a 16/44.1 dithered input stimulus at -20dBFS (blue), and a 24/96 dithered input stimulus at -20dBFS (purple), with both measured at the balanced line-level output. The implemented filter appears to be designed for minimized pre-impulse ringing.

**J-Test (coaxial input)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output. The J-Test was developed by Julian Dunn in the 1990s. It is a test signal, specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bit). 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 sine wave (the main peak). In addition, an undithered 250Hz square wave 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 (fundamental at 250Hz) down to -170dBrA for the odd harmonics. The test file can also be used in conjunction with artificially injected sine-wave jitter by the Audio Precision, to also show how well the DAC rejects jitter.

The coaxial input shows obvious peaks in the audioband from -90dBrA to just below -130dBrA. This is an indication that the Director Mk2’s DAC may be susceptible to jitter through the coaxial input.

**J-Test (optical input)**

The optical input shows close to the same but slightly worse J-Test FFT result compared to the coaxial input. The peaks adjacent to the primary signal reach almost -85dBrA.

**J-Test (coaxial input, 2kHz sine-wave jitter at 10ns)**

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output, with an additional 10ns of 2kHz sine-wave jitter injected by the APx555. The results are very clear, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/-2kHz jitter signal) manifest at near -70dBrA. This is a clear indication that the DAC in the Director Mk2 has poor jitter immunity. For this test, the optical input yielded defectively the same results.

**J-Test (coaxial input, 2kHz sine-wave jitter at 100ns)**

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output, with an additional 100ns of 2kHz sine-wave jitter injected by the APx555. The poor jitter-immunity results are further corroborated, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/-2kHz jitter signal) manifest at near -50dBrA. For this test, the optical input yielded similar results.

**Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)**

The chart above shows a fast Fourier transform (FFT) of the Director Mk2’s balanced-line level output with white noise at -4dBFS (blue/red) and a 19.1 kHz sine-wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green). The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of the brick-wall type reconstruction filter. There are minor imaged aliasing artifacts in the audioband between -100 and -110dBrA. The primary aliasing signal at 25kHz is just below -100dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone range from -90 to -100dBrA.

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

The chart above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sine-wave input stimulus of 2Vrms. The blue and red plots are for left and right channels into 200k ohms, while purple/green (left and right) are into 600 ohms. THD values are extremely low: about 0.00005-0.0002% into 200k ohms from 20Hz to 3kHz, climbing to 0.0005% at 20kHz. The 600-ohm data yielded higher THD values, especially at frequencies above 2kHz, where THD values were measured as low as 0.00007% (100Hz) and as high as 0.005% (20kHz). The Director Mk2’s analog THD values are extremely low, and in most cases, the signal harmonic peaks that the Audio Precision is “looking” for to calculate THD are buried amongst noise peaks, which may cause errors in the measurements, exhibited as peaks in the data above. For example, there is a sample point just above 1kHz in the plots above, where the Audio Precision would look for signal harmonics just above 2kHz and 3kHz. Unfortunately, the Director Mk2 has a noise peak at 3.02kHz, which causes a false and unnaturally high THD rating at 1kHz. See FFT charts below for a full explanation.

**THD ratio (unweighted) vs. frequency vs. load (24/96)**

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 a 24/96 dithered 1kHz signal at the coaxial input. The 200k and 600 ohms data are very close from 20Hz to 6kHz, hovering around 0.001%. At 20kHz, THD increased into 600 ohms vs 200k ohms, where we see 0.005% vs 0.002%.

**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 as a function of frequency for a 16/44.1 (blue/red) and a 24/96 (purple/green) dithered 1kHz signal at the coaxial input. Both data input types performed almost identically. We see THD values around 0.001% from 20Hz to 10kHz, then a climb to 0.002% at 20kHz.

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

The chart above shows THD ratios measured at the output as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 1mVrms level, THD values measured around 0.06%, dipping down to nearly 0.00002% at 3-5Vrms. It’s important to highlight just how low the Director Mk2’s THD values are, as they are flirting with the inherent THD performance of the Audio Precision of 0.000015% at these voltage levels. Also important to note here is that it was not possible to sweep the input voltage high enough to see the 1% THD point. This is because the Director Mk2 can handle up to 33Vrms (input or output), while, the AP can only output 26.7Vrms. Also, the Director Mk2 has a maximum gain of -0.1dB, thereby limiting the output to around 26Vrms.

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

The chart above shows THD ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). The 24/96 outperformed the 16/44.1 data, with a THD range from 0.3% to 0.0002%, while the 16/44.1 ranged from 2% down to 0.0005%.

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

The chart above shows THD+N ratios measured at the output as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 1mVrms level, THD+N values measured around 2%, dipping down to around 0.0002% at 20Vrms.

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

The chart above shows THD+N ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). The 24/96 outperformed the 16/44.1 data, with a THD+N range from 5% down to 0.001% (right channel), while the 16/44.1 ranged from 20% down to 0.003% at 4Vrms. For the 24/96 data, the right channel outperformed the left by about 1-2dB.

**FFT spectrum – 1kHz (analog at 2Vrms)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output into a 200k-ohm load. Below 1kHz, we see peaks due to power-supply noise at 60Hz (-135dBrA, or 0.00002%), 120Hz (-135dBrA), 180Hz (-125dBrA, or 0.00006%), and beyond. Above 1kHz, at first glance, it appears that there’s a peak at 3kHz (third signal harmonic) at -115dBrA. However, when zoomed in . . .

. . . we find that this is actually a noise peak at 3.02kHz, and that the signal harmonic is at a vanishingly low -149.5dBrA, or 0.000003%. All signal-harmonic peaks are extremely low for the Director Mk2, and buried below and between a multitude of noise peaks.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1. We see clear signal harmonics at -110dBrA, or 0.0003% (2kHz), and -100dBrA, or 0.001% (3kHz).

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96. We see signal harmonics at -110dBrA, or 0.0003% (2kHz), and -100dBrA, or 0.001% (3kHz), as well as lower-level signal harmonics at 4/5/6kHz at around -130dBrA, or 0.00003%, and below. Power-supply noise peaks are just visible to the right of the main signal peak, at 60Hz (-140dBrA, or 0.00001%) and 180Hz (-140/130dBrA, or 0.00001/0.00003%, for the left and right channels).

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1 at -90dBFS. The primary signal peak is at the correct amplitude and there are no visible signal harmonics. The peak that appears to be at 3kHz is actually just above 3kHz and is a noise artifact.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96 at -90dBFS. The primary signal peak is at the correct amplitude. The peak that appears to be at 3kHz is actually just above 3kHz and is a noise artifact. Power-supply noise peaks are clearly visible to the right of the main signal peak, at 60Hz (-140dBrA, or 0.00001%) and 180Hz (-140/130dBrA, or 0.00001/0.00003%, for the left and right channels).

**FFT spectrum – 50Hz (analog at 2Vrms)**

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. Here we can clearly see how vanishingly low the signal harmonics are, where we see the second harmonic (100Hz) at -145/-150dbRA, or 0.000006/0.000003% (left/right), and the third harmonic (150Hz) at -140dBrA, or 0.00001%. The worst-case power-supply-noise peaks are at 180Hz (third harmonic) and 300Hz (fifth harmonic), both around -130dBrA, or 0.00003%.

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

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 -125dBrA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz, are at worst at -120dBrA, or 0.0001%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 16/44.1. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 4.3Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is near -115dBRA, or 0.0002%, and the third-order modulation products, at 17kHz and 20kHz, are slightly higher, at or above -110dBrA, or 0.0003%.

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 24/96. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 4Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -120dBRA, or 0.0001%, while the third-order modulation products, at 17kHz and 20kHz, are higher, at just above and below -110dBrA, or 0.0003%.

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

Above is the 10kHz square-wave 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 Director Mk2’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very high bandwidth. An ideal square wave 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 Director Mk2’s reproduction of the 10kHz square wave is squeaky clean, with very sharp edges devoid of undershoot and overshoot, confirming its high bandwidth.

*Diego Estan*

Electronics Measurement Specialist

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

Link: reviewed by Doug Schneider on *SoundStage! Hi-Fi* on July 15, 2021

**General information**

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

The Accuphase C-2850 was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The C-2850 (as tested) is an analog line-level preamp offering several balanced (XLR) and unbalanced (RCA) inputs and outputs, and a headphone output (¼″ TRS). The volume control is implemented using a proprietary process Accuphase calls “Accuphase Analog Vari-gain Amplifier (AAVA).” This system works by converting the incoming analog signal from a voltage to a current in 16 weighted steps. Each step is digitally controlled and switched in or out of the circuit depending on the encoded position of the volume knob. The current from each step switched into the circuit is summed and converted back to a voltage. The 16 circuit steps are analogous to on/off bits, and therefore, the volume system allows for 65536 (2^{16}) discrete positions. Accuphase has configured the volume control to provide 251 steps ranging from -95dB to 0dB. Between -95 and -85dB, step sizes are 5dB; between -80 and -74dB, 3dB; -74 to -60dB, 2dB; -60 to -50dB, 1 dB; -50 to -30dB, 0.5dB; -30 to -8, 0.2dB; and finally between -8 to 0dB, 0.1dB. Considering both the exquisite channel tracking (see table below) and the variable, ultra-fine adjustments, this may be the finest digitally controlled analog volume control available in a consumer product.

The C-2850 also offers three gain settings, both for line-level (12, 18, and 24dB) and for the headphone output (Low, Mid, and High). The preamp gain setting affects the headphone gain, where Low is -10dB relative the preamp setting, Mid is 0dB, and High is +10dB. This means there are nine possible gain settings for the headphone amp: 2, 8, 12, 14, 18, 22, 24, 28, and 34dB. Unless otherwise stated, all measurement data below were taken with the 12dB gain setting for the preamp, and the Mid gain setting for the headphone amp.

When using the unbalanced and balanced inputs and outputs, the C-2850 provides the same gain regardless of combination. That is to say, with the volume set to unity gain, if I fed 2Vrms into the unbalanced input, I measured 2Vrms at the unbalanced and balanced outputs. If I fed 2Vrms into the balanced input, I measured 2Vrms at the unbalanced and balanced outputs. It’s also important to highlight that Accuphase assigns pins 2/3 on their XLR connectors as inverting/noninverting, which is the opposite to what we typically find in North-American or European products. For example, if I fed an unbalanced input and measured phase at the balanced output, it was 180 degrees out-of-phase. To compensate for this, Accuphase provides a polarity-inverting switch on the front panel, which was tested and flips the polarity as advertised.

I found small differences in THD and noise between the RCA and XLR inputs and outputs for the same output voltage. The RCA outputs exhibited about 11dB (unweighted) more noise than the XLR outputs, while the RCA inputs (when measured at the XLR outputs) measured slightly worse in terms of THD compared to the XLR inputs (0.0005% vs 0.0003% at 1kHz). Unless otherwise stated, all measurement data below are with the balanced inputs and outputs, at 2Vrms with volume set to unity gain (-12dB). Signal-to-noise ratios (SNR) were measured with the volume at maximum position.

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

Volume position | Channel deviation |

-95.0dB | 0.006dB |

-52.0dB | 0.002dB |

-28.0dB | 0.001dB |

-18.0dB | 0.002dB |

-12.0dB | 0.001dB |

-6.0dB | 0.000dB |

-3.0dB | 0.000dB |

0.0dB | 0.000dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Accuphase for the C-2850 compared directly against our own. The published specifications are sourced from Accuphase’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 measurement input bandwidth of 10Hz to 90kHz, and the worst case measured result between the left and right channel.

Parameter | Manufacturer | SoundStage! Lab |

Input impedance | 40k ohms | 31.7k ohms |

Output impedance | 50 ohms | 96 ohms* |

Maximum output level (1% THD+N, 200k ohms) | 7Vrms | 8.8Vrms |

Maximum output level (1% THD+N, 600 ohms) | 7Vrms | 7.6Vrms |

Gain | 12/18/24dB | 11.9/18/24dB |

Frequency response (20Hz-20kHz) | +0, -0.2dB | -0.35, -0.06dB |

Frequency response (5Hz-200kHz) | +0, -3dB | -3.5, -3dB |

Sensitivity (ref 2Vrms output, 18dB gain) | 252mVrms | 252mVrms |

THD (1kHz, 2Vrms, 200k ohms) | 0.005% | <0.00033% |

SNR (2Vrms output, A-weighted, 18dB gain) | 111dB | 111dB |

* The discrepancy in balanced output impedance may be due to Accuphase specifying this value for the inverting and noninverting pins separately. Our measurement considers both inputs on the balanced connector together. Treated separately, our measurement would be halved, or 48k ohms.

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sinewave, 2Vrms output into 200k ohms load, 10Hz to 90kHz bandwidth, 12dB gain setting):

Parameter | Left channel | Right channel |

Crosstalk, once channel driven (10kHz) | -109.8dB | -108.2dB |

DC offset | 0.03mV | 0.25mV |

Gain (switchable) | 11.9/18/24dB | 11.9/18/24dB |

IMD ratio (18kHz and 19kHz stimulus tones) | <-104dB | <-104dB |

Input impedance | 31.7k ohms | 31.7k ohms |

Maximum output voltage (at clipping 1% THD+N) | 8.89Vrms | 8.84Vrms |

Maximum output voltage (at clipping 1% THD+N into 600 ohms) | 7.67Vrms | 7.62Vrms |

Noise level (A-weighted) | <2.4uVrms | <2.4uVrms |

Noise level (unweighted) | <6uVrms | <6uVrms |

Output impedance | 96.0 ohms | 95.6 ohms |

Signal-to-noise ratio (A-weighted, 12dB gain) | 115.7dB | 115.8dB |

Signal-to-noise ratio (unweighted, 12dB gain) | 107.7dB | 107.8dB |

Signal-to-noise ratio (A-weighted, 18dB gain) | 110.8dB | 110.9dB |

Signal-to-noise ratio (unweighted, 18dB gain) | 102.7dB | 102.7dB |

Signal-to-noise ratio (A-weighted, 24dB gain) | 105.1dB | 105.3dB |

Signal-to-noise ratio (unweighted, 24dB gain) | 96.9dB | 96.9dB |

THD (unweighted) | <0.00033% | <0.00033% |

THD+N (A-weighted) | <0.0004% | <0.0004% |

THD+N (unweighted) | <0.00045% | <0.00045% |

Our primary measurements revealed the following using the balanced analog input and the headphone output (unless specified, assume a 1kHz sinewave at 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth, 12dB and Mid gain setting):

Parameter | Left channel | Right channel |

Maximum output power into 600 ohms (1% THD+N, unweighted) | 116mW | 115mW |

Maximum output power into 300 ohms (1% THD+N, unweighted) | 229mW | 227mW |

Maximum output power into 32 ohms (1% THD+N, unweighted) | 1650mW | 1627mW |

Gain (Low/Mid/High) | 2.4/12.4/22.2dB | 2.4/12.4/22.2dB |

Output impedance | 1.3 ohms | 1.4 ohms |

Noise level (A-weighted) | <5uVrms | <5uVrms |

Noise level (unweighted) | <18uVrms | <20uVrms |

Signal-to-noise (A-weighted, ref. max output voltage, Low gain) | 118.5dB | 117.1dB |

Signal-to-noise (unweighted, ref. max output voltage, Low gain) | 105.7dB | 104.3dB |

Signal-to-noise (A-weighted, ref. max output voltage, Mid gain) | 123.7dB | 123.1dB |

Signal-to-noise (unweighted, ref. max output voltage, Mid gain) | 113.2dB | 112.1dB |

Signal-to-noise (A-weighted, ref. max output voltage, High gain) | 115.9dB | 115.8dB |

Signal-to-noise (unweighted, ref. max output voltage, High gain) | 107.6dB | 107.2dB |

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

THD+N ratio (A-weighted) | <0.0005% | <0.0005% |

THD+N ratio (unweighted) | <0.0009% | <0.001% |

**Frequency response**

In our measured frequency response plot above, the C-2850 is near perfectly flat within the audioband (20Hz to 20kHz). The blue/red traces are without the 10Hz filter engaged, the purple/green traces with the 10Hz filter. These data do not quite corroborate Accuphase’s claim of 3Hz to 200kHz +0/-3dB (measured down to 5Hz). While at the upper end of the frequency spectrum, the -3dB point was measured at 200kHz, at low frequencies, Accuphase’s claim would imply that the C-2850 is DC coupled, whereas our measurements indicate AC coupling. Nevertheless, at the extremes of the audioband, we measured only -0.35dB at 20Hz (-1dB with filter on) and -0.04dB at 20kHz. The C-2850 can be considered a high-bandwidth audio device. 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.

**Frequency response (Compensator dial 1, 2, and 3 positions)**

Above are four frequency response plots for the balanced line-level input, with the Compensator control set to Off (blue/red), 1 (purple/light green), 2 (pink/cyan), and 3 (brown/dark green). We see what appears to be conventional bass-control EQ with various degrees of gain. At position 1, just under +3dB at 20Hz, position 2 yields about +5.5dB at 20Hz, and position 3 about +8.3dB.

**Phase response**

Above is the phase response plot from 20Hz to 20kHz, with the Phase control disabled (blue/red) and enabled (purple/green). The C-2850 does not invert polarity, while setting the Phase control to Invert does exactly that—it provides -180 degrees of shift. Since these data were collected using the balanced input and output, there is no phase inversion. However, since Accuphase assigns pins 2/3 on their XLR connectors as inverting/noninverting, the opposite to what we typically find in North American or European products, feeding the signal into an unbalanced input and measuring on the balanced output would yield the exact opposite of what is shown above.

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

The chart above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a 2Vrms sine-wave 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 are very low, near 0.0001% around 50-60Hz 20Hz, and around 0.0003-0.0004% through most of the audioband. The worst-case THD values are at 20Hz (0.001%) and 20kHz (0.001% into 600 ohms and 0.0007% into 200k ohms). Overall, the 600 and 200k-ohms load THD data are nearly identical.

**THD ratio (unweighted) vs. output voltage at 1kHz**

The plot above shows THD ratios measured at the output of the C-2850 ^{ }as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 10mVrms level, THD values measured around 0.003%, dipping down to around 0.00009% at 0.4Vrms. The “knee” occurs at around 7Vrms, hitting the 1% THD just past 8Vrms.

**THD+N ratio (unweighted) vs. output voltage at 1kHz**

The plot above shows THD+N ratios measured at the output the C-2850 as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD+N values measured around 0.05%, dipping down to around 0.0005% from 1.5 to 5Vrms.

**FFT spectrum – 1kHz**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus at 2Vrms, measured at the output into a 200k-ohm load. We see that the signal’s second harmonic, at 2kHz, is at -110dBrA or 0.0003%, while the third harmonic, at 3 kHz, is at -125dBrA or 0.00005%. Below 1kHz, we see some noise artifacts, with the 60Hz peak due to power supply noise visible at -145/-130dBrA (left/right), or 0.000006/0.00002%, and the 120Hz (second harmonic) peak just below -130dBrA.

**FFT spectrum – 50Hz**

Shown above is the FFT for a 50Hz input sinewave stimulus at 2Vrms 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. Here we find the second harmonic of the signal (100Hz) and the third harmonic of the signal (150Hz) at -120/-125dBrA respectively, or 0.0001/0.00006%. The worst-case power supply peak is at 120Hz measuring just below -130dBrA, or 0.00003%.

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

Shown above is an FFT of the intermodulation (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 and just above -120dBrA, or 0.0001%.

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

Above is the 10kHz square-wave 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 C-2850’s slew-rate performance. Rather, it should be seen as a qualitative representation of its high bandwidth. An ideal square wave 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 C-2850’s reproduction of the 10kHz square wave is squeaky clean, with very sharp edges devoid of undershoot and overshoot.

*Diego Estan*

Electronics Measurement Specialist

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