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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 and 8-ohm outputs unless otherwise noted.

- Output power at 1% THD+N: 26.0W @ 8 ohms, 15.0W @ 4 ohms
- Output power at 10% THD+N: 40.0W @ 8 ohms, 32.0W @ 4 ohms

- This amplifier does not invert polarity.
- AC-line current draw at idle: 1.42A, 0.89PF, 151W
- Gain: output voltage divided by input voltage, volume at maximum, Lch/Rch: 18.61X, 25.4dB:
- Balanced inputs: 39.75X, 32.0dB / 40.31X, 32.1dB
- Unbalanced inputs: 50.32X, 34.0dB / 52.18X, 34.4dB

- Input sensitivity for 1W output into 8 ohms, volume at maximum, Lch/Rch:
- Balanced inputs: 0.071mV / 0.70mV
- Unbalanced inputs: 0.56mV / 0.54mV

- Output impedance @ 50Hz: 2.15 ohms
- Input impedance @ 1kHz: 50k ohms
- Output noise, 8-ohm load, balanced inputs, termination 600 ohms, Lch/Rch
- Volume control at reference position

Wideband: 0.89 mV, -70.0dBW / 4.88mV, -55.3dBW

A weighted: 0.21 mV, -82.6dBW / 0.70 mV, -72.1dBW - Volume control full clockwise

Wideband: 1.11mV, -68.2dBW / 5.08mV, -54.9dBW

A weighted: 0.28mV, -80.1dBW / 0.71mV, -72.0dBW - Volume control full counterclockwise

Wideband: 0.89mV, -70.0dBW / 4.89 mV, -55.2 dBW

A weighted: 0.18mV, -83.9dBW / 0.68 mV, -72.4 dBW

- Volume control at reference position
- Output noise, 8-ohm load, unbalanced inputs, termination 1k ohm, Lch/Rch
- Volume control at reference position

Wideband: 1.12mV, -68.1dBW / 5.09mV, -54.9dBW

A weighted: 0.20mV, -83.0dBW / 0.74mV, -71.6dBW - Volume control full clockwise

Wideband: 1.59mV, -65.0dBW / 4.97mV, -55.1dBW

A weighted: 0.25mV, -81.1dBW / 0.73mV, -71.8dBW - Volume control full counterclockwise

Wideband: 0.95mV, -69.5dBW / 5.32mV, -54.5dBW

A weighted: 0.19 mV, -83.5 dBW / 0.72 mV, -71.9 dBW

- Volume control at reference position

The JE Audio VS70.1 is a medium-power tubed power amplifier with a front-panel volume control. Its overall gain is 6-7dB over the "nominal" power gain of 26dB. However, the extra gain would be useful for low-output CDs, such as Reference Recordings’ HRx discs.

Chart 1 shows the VS70.1’s frequency response with varying loads. This was made at the usual 1W/8-ohm level, with the volume control turned fully clockwise. The frequency-response variation with the NHT dummy load is of the order of ±1.5dB. The high-frequency rolloff is nicely controlled in that its shape is quite independent of load.

With the reference condition set up for measuring integrated amplifiers -- *i.e.*, volume control set for 5W output into 8 ohms for 500mV input -- another response was run, and is shown in Chart 1A. Here we see that the HF response is a little more extended, and the very low-frequency level is starting to roll off because of output-transformer limitations. Channel tracking as a function of volume-control position down from this reference condition was generally within ±1dB down to 50dB of attenuation, below which hum, noise, and capacitive coupling altered the response and the channel separation.

Chart 2 illustrates how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. It is apparent that the power is maximized for the 8-ohm load, whereas the power for a 4-ohm load is lower at higher distortion. Further, the amounts of intermodulation distortion are rather excessive at the higher power levels.

Chart 3 plots THD+N as a function of frequency for 8-ohm loading at several different power levels. The rise in distortion at higher frequencies is reasonably low. Because of output-transformer characteristics, low-frequency distortion gets progressively worse as the bass frequency rises and the power level increases.

The VS70.1’s damping factor vs. frequency (Chart 4) is fairly low, but not unusually so for a tubed power amplifier. What is good is that it is quite flat with frequency.

A spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal into 8 ohms is plotted in Chart 5. The magnitudes of the AC-line harmonics are quite high and complex, as are the signal harmonics. Intermodulation components of line harmonics with signal harmonics are numerous, and can be seen at the baseline of the signal harmonics.

(Volume control fully clockwise)

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

Cyan line = NHT dummy load

Red line = left channel

Blue line = right channel

(Line up at 20W 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 = 5W

Blue line = 15W

Cyan line = 30W

Green line = 35W

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.

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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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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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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