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Unless otherwise noted, measurements were taken at the balanced input and at 120V AC line voltage, with the Audio Precision AUX-0025 measurement filter.

- Power output at 1% THD+N: 358.4W @ 8 ohms, 658.4W @ 4 ohms
- Power output at 10% THD+N: 452.7W @ 8 ohms, 852.4W @ 4 ohms

- Input/output polarity: noninverting
- AC line current draw at idle: 16.0W, 0.32A, 0.44PF
- Gain: output voltage divided by input voltage, 8-ohm load
- Unbalanced input: 22.5X, 27.0dB
- Balanced input: 22.3X, 27.0dB

- Input sensitivity for 1W output into 8 ohms
- Unbalanced input: 125.7mV
- Balanced input: 126.9mV

- Output impedance @ 50Hz: 0.03 ohm
- Input impedance @ 1kHz
- Unbalanced input: 92.5k ohms
- Balanced input: 92.5k ohms

- Output noise, 8-ohm load, unbalanced inputs terminated with 1k ohms

- Wideband: 0.471mV, -75.6dBW
- A weighted: 0.022mV, -102.2dBW

- Output noise, 8-ohm load, balanced inputs terminated with 600 ohms

- Wideband: 0.476mV, -75.5dBW
- A weighted: 0.0206mV, -102.8dBW

- Output noise, 8-ohm load, unbalanced inputs terminated with 1k ohms, without the AP AUX-0025 filter

- 462kHz: 423.0mV, -16.5dBW

- Output noise, 8-ohm load, balanced inputs terminated with 600 ohms, without the AP AUX-0025 filter

- 462kHz: 423.0mV, -16.5dBW

The Bel Canto Design REF600M mono power amp is a switching design using the latest Hypex Ncore technology.

Chart 1 shows the REF600M’s frequency response with varying loads. One of the interesting aspects of the Ncore technology, in contrast to many other switching-amplifier designs, its amazing independence from load in the high frequencies. Here, though, there is a little anomaly between 50 and 60kHz. The output impedance is low enough that there was negligible variation with the NHT dummy speaker load.

Chart 2 illustrates how the REF600M’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE intermodulation test signals and amplifier output for loads of 8 and 4 ohms. The amount of distortion visible in this plot is quite low.

The Bel Canto’s THD+N as a function of frequency at different power levels is plotted in Chart 3. The rise in distortion with increasing frequency is quite pronounced, and the low-frequency region also shows more distortion at higher power levels.

The REF600M’s damping factor vs. frequency (Chart 4) is somewhat lower than Bel Canto’s specified >1000. My measurements of some other amplifiers have resulted in values higher than 1000, so I know the measurement technique is valid. However, of considerable interest is that the damping factor’s “bandwidth” is quite a bit wider than with most amplifiers.

Chart 5 plots a spectrum of the Bel Canto’s harmonic distortion and noise residue of when fed a 10W, 1kHz test signal. The AC line harmonics are extremely low and relatively simple. The signal harmonics are dominated by the third and second harmonic, with higher harmonics of decreasing magnitude.

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

Yellow line = 500W

Stereo mode

Damping factor = output impedance divided into 8

Stereo mode

1kHz signal at 10W into a 4-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.

Measurements were taken at 120V AC line voltage with both channels driven, and on both channels using balanced inputs. Unless otherwise noted, all results cited are for the left channel. The integrated amplifier reference volume setting was 500mV input (5W/8-ohm power output).

- Output power at 1% THD+N: 231.1W @ 8 ohms, 468.3 @ 4 ohms
- Output power at 10% THD+N: 281.3W @ 8 ohms, 564.7W @ 4 ohms

- This amplifier does not invert polarity.
- AC line current draw at idle: 1.02A, 0.63PF, 78.0W
- Gain: output voltage divided by input voltage
- Unbalanced inputs: 44.5X, 33.0dB
- Balanced inputs: 44.5X, 33.0dB

- Input sensitivity for 1W output into 8 ohms
- Unbalanced inputs: 43.5mV
- Balanced inputs: 43.5mV

- Output impedance @ 50Hz: 0.015 ohm
- Input impedance @ 1kHz
- Unbalanced inputs: 6.7k ohms
- Balanced inputs: 9.7k ohms

- Output noise, reference conditions, balanced inputs, termination 600 ohms, Lch/Rch
- Wideband: 0.561mV/0.580mV, -74.1dBW/-73.8dBW
- A weighted: 0.0745mV/0.0862mV, -91.6dBW/-90.3 dBW

- Output noise, reference conditions, unbalanced inputs, termination 1k ohm, Lch/Rch
- Wideband: 0.546mV/0.569mV, -74.3dB/-73.9dBW
- A weighted: 0.0706mV/0.0803mV, -92.1dBW/-90.4dBW

- Output noise, 8-ohm load, volume at maximum, balanced inputs, termination 600 ohms, Lch/Rch
- Wideband: 0.652mV/0.707mV, -72.4dBW/-72.0dBW
- A weighted: 0.155mV/0.141mV, -87.8dBW/-86.1dBW

- Output noise, 8-ohm load, volume at maximum, unbalanced inputs, termination 1k ohm, Lch/Rch
- Wideband: 0.410mV/0.457mV, -76.8dBW/-75.8dBW
- A weighted: 0.0618mV/0.0784mV, -93.2dBW/-91.1dBW

- Output noise, 8-ohm load, volume at minimum, balanced inputs, termination 600 ohms, Lch/Rch
- Wideband: 0.403dBW/0.422mV, -76.9dBW/-76.5dBW
- A weighted: 0.0533mW/0.0655mV, -94.5dBW/-92.7dBW

- Output noise, 8-ohm load, volume at minimum, unbalanced inputs, termination 1k ohm, Lch/Rch
- Wideband: 0.405mV/0.422mV, -76.9dBW/-75.5dBW
- A weighted: 0.0533mV/0.0645mV, -94.5dBW/-92.8dBW

The H360 DAC-integrated amplifier builds on the success of Hegel’s H300, adding network-playing capability and AirPlay. The power outputs of the two models are similar.

Chart 1 shows the frequency response of the H360 with varying impedance loads. The output impedance is low enough that there was negligible variation with the NHT dummy speaker load.

Chart 2 illustrates how the H360’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE intermodulation test signals and amplifier output for 8- and 4-ohm loads.

Chart 3 plots the THD+N as a function of frequency at several different power levels. As the power level is increased, the increase in distortion with frequency is quite pronounced.

The H360’s damping factor vs. frequency is shown in Chart 4. Like the H300, the H360 shows a typical decrease as the frequency increases, but with a surprising decrease at low frequencies. Perhaps Hegel has discovered something by having this characteristic -- that it possibly improves the sound?

A spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal is plotted in Chart 5. The AC-line harmonics are low but relatively complex. The signal harmonics are dominated by the third harmonic, with second and higher harmonics of decreasing magnitude.

Some key measurements were taken of the H360’s digital section. The Coax 1 input was fed a full-scale, 0dBFS digital signal and the main amplifier outputs were set as close to 5W/8 ohms as possible with the volume control. At a sample rate of 192kHz, the frequency response was the same as at a sample rate of 96kHz. I have seen this behavior in a few other DACs. This means that files at sample rates of 176.4 and 192kHz won’t be played back with the extended high-frequency response such files can contain. Chart 6 is a plot of this response.

A revealing test that I always do on a DAC is to measure the THD+N of a 1kHz signal in a 20Hz-22kHz bandwidth at full-scale digital level with decreasing input signal level, down to where the distortion disappears into the noise floor. Doing this revealed that the H360’s noise floor was about -84dBFS, which is equivalent to about 0.42mV, -76.6dBW of output noise dominated by AC-line harmonics. This is somewhat more than with the analog inputs.

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm 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 = 30W

Cyan line = 70W

Green line = 200W

Damping factor = output impedance divided into 8

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

Red line = 44.1kHz

Magenta line = 96 and 192kHz

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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: The Cyrus Stereo 200 power amplifier was measured at 120V AC line voltage, both channels driven, using its balanced inputs. Measurements were taken for both channels, but unless noted otherwise, the results reported below are for the left channel only. Because the Stereo 200 is a switching amplifier, measurements were made with the Audio Precision AUX-0025’s low-pass filter, except as noted.

- Power output at 1% THD+N: 174.3W @ 8 ohms, 262.4W @ 4 ohms
- Power output at 10% THD+N: 220.0W @ 8 ohms, 345.0W @ 4 ohms

- Input/output polarity: noninverting
- AC-line current draw at idle: 26.3W, 0.30A, 0.74PF
- Gain: output voltage divided by input voltage, 8-ohm load (Lch/Rch): 26.4X/26.2X, 28.4dB/28.4dB
- Input sensitivity for 1W output into 8 ohms (Lch/Rch): 107mV/107mV
- Output impedance @ 50Hz: 0.028 ohm
- Input impedance @ 1kHz (Lch/Rch): 62.8k ohms/60.1k ohms
- Output noise, 8-ohm load, balanced inputs, termination 600 ohms (Lch/Rch)

- Wideband without AUX-0025: 1.86V/1.92mV, -3.64dBW/-3.36dBW
- Wideband with AUX-0025: 3.94mV/4.0mV, -57.1dBW/-57.0dBW
- A weighted: 0.114mV/0.124mV, -87.9dBW/-87.1dBW

The Stereo 200’s switching-amplifier output circuit is powered by a conventional nonswitching power supply. It has a unique circuit for adjusting the output filter to the optimal conditions for the speaker load used.

Chart 1 shows the frequency response of the Stereo 200 with varying loads. This was done by going through the output-filter startup program with 8-ohm resistive loads. As can be seen, the curve for the 8-ohm load is the flattest, with more rolloff for a 4-ohm load, and some pretty bad peaking with an open-circuit load. The NHT dummy speaker load is nicely contained within about +/-0.8dB throughout the audioband.

Chart 2 illustrates how the Stereo 200’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE intermodulation (IM) test signals and amplifier output for loads of 8 and 4 ohms. The THD+N curves have some strange kinks on the way up to clipping. The IM curves show significant increases in the amount of distortion in the 5-30W range; these are worse for the 4-ohm load.

THD+N as a function of frequency at several different power levels is plotted in Chart 3. The Cyrus Stereo 200 had some trouble producing high-frequency power. I had to modify the Audio Precision’s settling routines from the normal exponential settling to no settling to get the measurements I did get. The amount of HF rise is pretty substantial; I felt I had to stop at a 30W power level to avoid damaging the review sample.

Chart 4 plots the Stereo 200’s damping factor vs. frequency. Although it seems typical of many amplifiers -- high at low frequencies, then declining over the audioband -- it begins to decrease at 50Hz, which is quite a bit lower than the norm, and declines to a very low value at 20kHz.

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 about as low as I have seen in any amplifier, showing no really identifiable line frequency harmonics. The signal harmonics are also low, and consist mainly of a declining series of odd harmonics.

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

Cyan line = NHT dummy-speaker load

(Line up at 50W 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 = 10W

Cyan line = 30W

Stereo mode

Damping factor = output impedance divided into 8

Stereo mode

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 of the NAD C 275BEE power amplifier were taken at 120V AC line voltage, both channels driven. Both channels were measured, using the fixed-level inputs. Unless otherwise indicated, the data reported below are for the right channel.

- Power output at 1% THD+N: 192.0W @ 8 ohms, 201.0W @ 4 ohms
- Power output at 10% THD+N: 203.8W @ 8 ohms, 243.8W @ 4 ohms

- Power output at 1% THD+N: 354.1W @ 8 ohms
- Power output at 10% THD+N: 468.7W @ 8 ohms

- Input/output polarity: noninverting
- AC-line current draw at idle: 65.0W, 0.78A, 0.71PF
- Gain (stereo, Lch/Rch): output voltage divided by input voltage, 8-ohm load
- Fixed input: 28.6X/28.5X, 29.1dB/29.1dB
- Variable input, full clockwise: 50.7X/50.0X, 34.1dB/34.0dB

- Gain (mono), output voltage divided by input voltage, 8-ohm load
- Fixed input: 57.2X, 35.2dB
- Variable input, full clockwise: 101.1X, 40.1dB

- Input sensitivity for 1W output into 8 ohms (stereo, Lch/Rch)
- Fixed input: 98.9mV/99.2mV
- Variable input, full clockwise: 55.8mV/56.6mV

- Input sensitivity for 1W output into 8 ohms (mono)
- Fixed input: 59.4mV
- Variable input, full clockwise: 28.0mV

- Output impedance @ 50Hz
- Stereo mode: 0.042 ohm
- Mono mode: 0.084 ohm

- Input impedance @ 1kHz (stereo)
- Fixed input: 10.3k ohms
- Variable input, full clockwise: 205k ohms

- Input impedance @ 1kHz (mono)
- Fixed input: 10.3k ohms
- Variable input, full clockwise: 163k ohms

- Output noise with fixed input (stereo mode), 8-ohm load, termination 1k ohm, Lch/Rch
- Wideband: 0.45mV/0.33mV, -76.0dBW/-78.9dBW
- A weighted: 0.0257mV/0.0264mV, -100.8dBW/-100.6dBW

- Output noise with variable input set full clockwise (stereo mode), 8-ohm load, termination 1k ohm, Lch/Rch
- Wideband: 0.38mV/0.37mV, -77.4dBW/-77.7dBW
- A weighted: 0.0391mV/0.0433mV, -97.2dBW/-96.3dBW

- Output noise with fixed input (mono mode), 8-ohm load, termination 1k ohm
- Wideband: 0.70mV, -72.1dBW
- A weighted: 0.0540mV, -94.4dBW

- Output noise with variable input set full clockwise (mono mode), 8-ohm load, termination 1k ohm
- Wideband: 0.64mV, -72.9dBW
- A weighted: 0.0821mV, -90.7dBW

The C 275BEE appears to be a linear design with load-sensing circuitry that causes it to output about the same amount of power into 4 or 8 ohms; usually, a solid-state power amp produces quite a bit more power into 4 ohms.

Chart 1 shows the frequency response of the C 275BEE with varying loads. In mono mode (not shown), the high-frequency rolloff was about twice that shown in Chart 1. In both cases, the output impedance was low enough that there was negligible variation with the NHT dummy speaker load.

The distortion measured in the C 275BEE’s left channel was quite a bit better than in the right. Chart 2 illustrates how the NAD’s total harmonic distortion plus noise (THD+N) vs. power varied for 1kHz and SMPTE intermodulation test signals, and with amplifier output for 8- and 4-ohm loads. Chart 2A shows the results for 8-ohm loading in mono mode. NAD does not recommend 4-ohm loading for the C 275BEE in mono mode.

Chart 3 plots the C 275BEE’s THD+N as a function of frequency at different power levels. The rise in distortion with frequency in the right channel is quite pronounced. Chart 3A shows the same measurement taken for the left channel.

The NAD’s plot of damping factor vs. frequency, shown in Chart 4, is typical of most solid-state power amplifiers: high at low frequencies, then declining throughout the audioband. In mono mode, the damping factor (not shown) was about half that indicated in Chart 4 -- a normal situation, as the two output channels are in series with the load.

Chart 5 shows a spectrum of the C 275BEE’s harmonic distortion and noise residue in a 10W, 1kHz test signal. AC-line harmonics are low but relatively complex. Signal harmonics are dominated by the third harmonic, with the second and higher harmonics of decreasing magnitude.

Stereo mode

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

Chart 2A

Stereo mode

(Line up at 50W 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

Chart 2B

Mono mode

(Line up at 100W to determine lines)

Top line = 8-ohm SMPTE IM distortion

Second line = 8-ohm THD+N

Chart 3A

Stereo mode

(8-ohm loading, Rch)

Red line = 1W

Magenta line = 10W

Blue line = 30W

Cyan line = 70W

Green line = 150W

Chart 3B

Stereo mode

(8-ohm loading, Lch)

Red line = 1W

Magenta line = 10W

Blue line = 70W

Cyan line = 150W

Stereo mode

Damping factor = output impedance divided into 8

Stereo mode

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

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Notes: Measurements of the Benchmark AHB2 were taken at the balanced inputs of both channels at 120V AC line voltage, both channels driven. Unless otherwise noted, the data reported below are for the left channel.

- Power output at 1% THD+N: 110.0W @ 8 ohms, 201.0W @ 4 ohms
- Power output at 10% THD+N: 130.0W @ 8 ohms, 255.9W @ 4 ohms

- Power output at 1% THD+N: 403.6W @ 8 ohms
- Power output at 10% THD+N: 450.0W @ 8 ohms

- Input/output polarity: noninverting
- AC line-current draw at idle: 20.0W, 0.37A, 0.46PF
- Gain (stereo): output voltage divided by input voltage, 8-ohm load
- 2V sensitivity: 14.2X, 23.1dB
- 4V sensitivity: 7.5X, 17.5dB
- 9.8V sensitivity: 2.92X, 9.4dB

- Gain (mono), output voltage divided by input voltage, 8-ohm load
- 2V sensitivity: 25.2X, 28.0dB
- 4V sensitivity: 14.2X, 23.1dB
- 9.8V sensitivity: 5.8X, 15.3dB

- Input sensitivity for 1W output into 8 ohms (stereo)
- 2V sensitivity: 199.1mV
- 4V sensitivity: 377.0mV
- 9.8V sensitivity: 968.5mV

- Input sensitivity for 1W output into 8 ohms (mono)
- 2V sensitivity: 112.2mV
- 4V sensitivity: 199.4mV
- 9.8V sensitivity: 487.6mV

- Output impedance @ 50Hz
- Stereo mode: 0.027 ohm
- Mono mode: 0.054 ohm

- Input impedance @ 1kHz, stereo and mono: 47.0k ohms
- Output noise 2V sensitivity (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch
- Wideband: 0.120mV/0.099mV, -87.5dBW/-89.1dBW
- A weighted: 0.0114mV/0.0112mV, -107.9dBW/-108.0dBW

- Output noise 4V sensitivity (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch
- Wideband: 0.104mV/0.0905mV, -88.7dBW/-89.9dBW
- A weighted: 0.0084mV/0.0085mV, -110.5dBW/-110.4dBW

- Output noise 9.8V sensitivity (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch
- Wideband: 0.098mV/0.088mV, -89.2dBW/-90.1dBW
- A weighted: 0.0072mV/0.0076mV, -111.9dBW/-111.4dBW

The AHB2 is Benchmark’s attempt to produce a very-low-distortion, low-noise power amplifier using THX’s AAA Technology, which linearizes the amp’s class-AB output stage without using large amounts of overall negative feedback. I must say that they’ve succeeded; I measured less distortion and noise in the AHB2 than in any other of the many power amps I’ve measured over the years.

Because the AHB2 can be switched between stereo and bridged-mono modes, some measurements were taken with the amp in both modes; the charts labeled “B” indicate measurements in bridged mode. The AHB2 has input sensitivities of 2V, 4V, and 9.8V. Most of the testing was done at the 2V sensitivity; the distortion results were pretty much the same at the 9.8V sensitivity.

Chart 1 shows the AHB2’s frequency response into different loads. The slightly greater high-frequency rolloff in bridged-mono mode (not shown) is due to the series-connected nature of that mode, which caused the HF deviation with load to show up more than in stereo mode. Because the AHB2’s output regulation is so good, its measured performance into the IHF dummy load showed no significant variation within the audioband.

Chart 2A illustrates how the AHB2’s total harmonic distortion plus noise (THD+N) vs. power varied with 1kHz and SMPTE intermodulation test signals and amplifier output load into loads of 8 and 4 ohms. Chart 2B shows the mono results into 8 ohms; in bridged mono, the AHB2 is not rated for use into a load of 4 ohms.

The AHB2’s THD+N as a function of frequency at different power levels is plotted in Chart 3. High-frequency THD+N is admirably low, and in stereo *and* mono modes, the AHB2’s level of distortion throughout most of the power range is *amazingly* low.

The plot of the AHB2’s damping factor vs. frequency (Chart 4) is of a value and nature typical of many solid-state amplifiers: high up to 1-2kHz, then rolling off with increasing frequency.

Spectra of the THD+N residue of a 10W, 1kHz test signal are plotted in Charts 5A and 5B. The magnitudes of AC line harmonics are relatively low, and the signal harmonics -- consisting of the third and fifth harmonics in stereo mode -- are extremely low in amplitude. Chart 5B shows this to be similar in mono mode, but with the fifth harmonic being higher, and the seventh and higher harmonics visible but at extremely low levels.

Stereo mode

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

Chart 2A

Stereo mode

(Line up at 50W 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

Chart 2B

Mono mode

(Line up at 100W to determine lines)

Top line = 8-ohm SMPTE IM distortion

Second line = 8-ohm THD+N

Stereo mode

(4-ohm loading)

Red line = 2W

Magenta line = 20W

Blue line = 60W

Cyan line = 120W

Green line = 180W

Stereo mode

Damping factor = output impedance divided into 8

Chart 5A

Stereo mode

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

Chart 5B

Mono mode

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

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Measurements were made at 120V AC line voltage with all three channels driven. Unless otherwise noted, all measurements were taken at the balanced inputs. The data reported below are for channel one unless otherwise noted.

- Output power at 1% THD+N: 232.8W @ 8 ohms, 352.0W @ 4 ohms
- Output power at 10% THD+N: 285.0W @ 8 ohms, 422.9W @ 4 ohms

- This amplifier does not invert polarity.
- AC-line current draw at idle: 0.95A, 0.68PF, 76.0W (see Note 1)
- Gain: output voltage divided by input voltage, 8-ohm loads (see Note 2)
- Balanced inputs
- Normal gain: 13.16X, 22.4dB
- Low gain: 6.22X, 15.9dB
- High gain: 27.73X, 28.9dB

- Unbalanced inputs
- Normal gain: 29.42X, 29.4dB
- Low gain: 13.85X, 22.8dB
- High gain: 61.84X, 35.8dB

- Balanced inputs
- Input sensitivity for 1W output into 8 ohms (see Note 3)
- Balanced inputs
- Normal gain: 216mV
- Low gain: 455mV
- High gain: 102mV

- Unbalanced inputs
- Normal gain: 96.1mV
- Low gain: 204mV
- High gain: 45.7mV

- Balanced inputs
- Output impedance @ 50Hz: 0.0135 ohm
- Input impedance @ 1kHz
- Balanced inputs: 105.6k ohms
- Unbalanced inputs: 42.8k ohms

- Output noise, 8-ohm load, balanced inputs, termination 600 ohms (see Note 4)
- Nominal gain
- Wideband: 0.233mV, -82.1dBW
- A weighted: 0.076mV, -91.4dBW

- Low gain
- Wideband: 0.173mV, -84.3dBW
- A weighted: 0.085mV, -90.4dBW

- High gain
- Wideband: 0.436mV, -76.2dBW
- A weighted: 0.130mV, -86.8dBW

- Nominal gain
- Output noise, 8-ohm load, unbalanced inputs, termination 1k ohm (see Note 4)
- Nominal gain
- Wideband: 0.275mV, -80.2dBW
- A weighted: 0.089mV, -90.0dBW

- Low gain
- Wideband: 0.257mV, -80.8dBW
- A weighted: 0.087mV, -90.2dBW

- High gain
- Wideband: 0.538mV, -71.4dBW
- A weighted: 0.154mV, -85.3dBW

- Nominal gain

Note 1: As a matter of interest, the AC power draw on turn-on is up to about 210W, which slowly comes down as the unit warms up.

Note 2: Gain values were averaged for the three channels.

Note 3: Input sensitivity values were averaged for the three channels.

Note 4: Noise values for the three channels averaged for the three gain settings.

The high-powered Halo A 31 is the only three-channel power amplifier in Parasound’s extensive line of Halo products.

Chart 1 shows the frequency response of the Halo A 31 with varying loads. Unusual is the uniformity of the high-frequency rolloff with changing load. The Halo’s output impedance is so low that its response with the NHT dummy speaker load would not show up.

Chart 2 illustrates how the A 31’s 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.

The THD+N as a function of frequency at several different power levels is plotted in Chart 3. High-frequency rise with frequency is moderate, and the amount of distortion is quite low through most of the power range. Interesting is that there is a dip in distortion at around 500Hz at the higher powers.

I wonder if that dip in distortion is related in some way to the peak in the curve of damping factor vs. frequency, shown in Chart 4. Most unusual is that this curve, too, peaks at about 500Hz, then decreases and shelves off as the frequency decreases. This almost suggests that some open-loop frequency shaping was done to tune the sound -- speculation on my part.

Chart 5A plots the spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal for the Halo A 31’s balanced inputs; Chart 5B plots the same for the unbalanced inputs. The unbalanced input is quite a bit worse, with the AC line harmonics extending up into the signal harmonics. The measurements shown are of channel three, which had more transformer-induced noise than channels one and two, on the other side of the amp.

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 THD+N

Second line = 4-ohm THD+N

Third line = 8-ohm SMPTE IM distortion

Bottom line = 4-ohm SMPTE IM distortion

(8-ohm loading)

Red line = 1W

Magenta line = 10W

Blue line = 70W

Cyan line = 150W

Green line = 200W

Damping factor = output impedance divided into 8

Chart 5A - balanced inputs

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

Chart 5B - unbalanced inputs

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

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Notes: Measurements of the unbalanced (UB) and balanced (B) inputs of both channels were made at the 120V AC line voltage, both channels driven. Since the Luxman M-900u is switchable from stereo to mono mode, measurements were made in both modes. (The suffix “B” indicates a mono-mode measurement.) Unless otherwise noted, the data reported below are for the balanced inputs and the left channel.

- Power output at 1% THD+N: 212.5W @ 8 ohms, 368.2W @ 4 ohms
- Power output at 10% THD+N: 259.2W @ 8 ohms, 445.4W @ 4 ohms

- Power output at 1% THD+N: 749.0W @ 8 ohms
- Power output at 10% THD+N: 904.3W @ 8 ohms

- Input/output polarity
- Unbalanced inputs: noninverting
- Balanced inputs (switch set for Normal, pin-3 hot): inverting

AC-line current draw
- Operating: 276.0W, 3.02A, 0.74PF
- Standby: 1.4W, 0.03A, 0.38PF
- Gain: output voltage divided by input voltage, 8-ohm load
- Stereo mode B inputs: 27.4X, 28.8dB
- Stereo mode UB inputs: 28.2X, 29.0dB
- Mono mode B input: 55.6X, 34.9dB
- Mono mode UB input: 55.0X, 34.8dB

- Input sensitivity for 1W output into 8 ohms
- Stereo mode B inputs: 103.2mV
- Stereo mode UB inputs: 100.0mV
- Mono mode B input: 50.8mV
- Mono mode UB input: 51.4mV

- Output impedance @ 50Hz
- Stereo mode: 0.012 ohm
- Mono mode: 0.024 ohm

- Input impedance @ 1kHz

- Stereo mode B inputs: 34.0k ohms
- Stereo mode UB inputs: 52.0k ohms
- Mode mode B input: 34.0k ohms
- Mode mode UB input: 52.0k ohms

- Output noise (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch

- Wideband: 0.532mV/0.546mV, -74.5dBW/-74.3dBW
- A weighted: 0.164mV/0.166mV, -84.7dBW/-84.6dBW

- Output noise (stereo mode), 8-ohm load, unbalanced inputs terminated with 1k ohms, Lch/Rch

- Wideband: 0.167mV/0.154mV, -84.6dBW/-85.3dBW
- A weighted: 0.043mV/0.040mV, -96.4dBW/-97.0dBW

- Output noise (mono mode), 8-ohm load, balanced inputs terminated with 600 ohms

- Wideband: 0.750mV, -71.3dBW
- A weighted: 0.231mV, -81.7dBW

- Output noise (mono mode), 8-ohm load, unbalanced inputs terminated with 1k ohms
- Wideband: 0.656mV, -72.7dBW
- A weighted: 0.200mV, -83.0dBW

Charts 1A and 1B show the frequency response of the Luxman M-900u with varying loads. Otherwise essentially identical to the stereo mode in the audioband, the mono response shows a bit more dependence on load above that range. The M-900u’s output regulation is so good that the IHF dummy load showed no significant variation in the audioband.

Chart 2A illustrates how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE intermodulation (IM) test signals and amplifier output load for loads of 8 and 4 ohms. Chart 2B is for mono mode and a load of 8 ohms, as the M-900u is not specified for a mono load of 4 ohms.

THD+N as a function of frequency at several different power levels is plotted in Charts 3A and 3B. High-frequency rise with frequency is moderate, and the distortion is quite low over most of the power range in both stereo and mono modes.

The plot of damping factor vs. frequency (Chart 4A) is of a value and nature typical of many solid-state amplifiers: high up to about 1-2kHz, then rolling off with increasing frequency. The damping factor in mono mode (4B) is about half that in stereo mode, with the same curve shape. This is normal; the channels’ separate output impedances are in series with the load in mono or bridged mode.

The M-900u’s spectrum of harmonic distortion and noise residue of a 10W, 1kHz test signal is shown in Charts 5A and 5B. The AC line harmonics are relatively low in magnitude and simple in nature. The signal harmonics in stereo mode consist of a single second harmonic of low amplitude. In mono mode, the single second harmonic seems to have been canceled and does not show at all!

Chart 1A

Stereo mode

Blue line = open circuit

Red line = 8-ohm load

Magenta line = 4-ohm load

Chart 1B

Mono mono

Blue line = open circuit

Red line = 8-ohm load

Magenta line = 4-ohm load

Chart 2A

Stereo mode

(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

Chart 2B

Mono mode

(Line up at 100W to determine lines)

Top line = 8-ohm SMPTE IM distortion

Second line = 8-ohm THD+N

Chart 3A

Stereo mode

(8-ohm loading)

Red line = 1W

Magenta line = 10W

Blue line = 30W

Cyan line = 120W

Green line = 150W

Chart 3B

Mono mode

(8-ohm loading)

Red line = 1W

Magenta line = 20W

Blue line = 60W

Cyan line = 120W

Green line = 600W

Chart 4A

Stereo mode

Damping factor = output impedance divided into 8

Chart 4B

Mono mode

Damping factor = output impedance divided into 8

Chart 5A

Stereo mode

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

Chart 5B

Mono mode

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

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All amplifier measurements are performed independently by Warkwyn Associates. 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 (stereo mode). Measurements made on left channel through the balanced input for stereo and mono modes unless otherwise noted.

- Power output at 1% THD+N: 333.3W @ 8 ohms, 499.4W @ 4 ohms
- Power output at 10% THD+N: 365.2W @ 8 ohms, 576.0W @ 4 ohms

- Power output at 1% THD+N: 1200W @ 8 ohms, 1953W @ 4 ohms
- Power output at 10% THD+N: 1453W @ 8 ohms, 2303W @ 4 ohms

- This amplifier does not invert polarity.
- AC-line current draw at idle: 1.29A, 0.62PF, 98W
- Gain at 1W at into 8 ohms at 1kHz, unbalanced and balanced inputs
- Stereo and mono modes: 39.9X, 32.0dB

- Input sensitivity for 1W into 8 ohms, unbalanced and balanced inputs
- Stereo and mono modes: 71.1mV

- Output impedance @ 50Hz
- Stereo mode: 0.0093 ohm
- Mono mode: 0.0182 ohm

- Input impedance @ 1kHz

- Stereo mode, unbalanced input: 8.58k ohms
- Stereo model, balanced input: 26.6k ohms
- Mono mode, unbalanced input: 5.33k ohms
- Mono mode, balanced input: 15.7k ohms

- Output noise (stereo mode), 8-ohm load, unbalanced inputs terminated with 1k ohms, Lch/Rch

- Wideband: 0.133mV/0.142mV, -86.5dBW/-86.0dBW
- A weighted: 0.101mV/0.099mV, -88.9dBW/-89.1dBW

- Output noise (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch

- Wideband: 0.129mV/0.138mV, -86.8dBW/-86.2dBW
- A weighted: 0.095mV/0.093mV, -89.5dBW/-89.6dBW

- Output noise (mono mode), 8-ohm load, unbalanced inputs terminated with 1k ohms

- Wideband: 0.164mV, -84.7dBW
- A weighted: 0.121mV, -87.4dBW

- Output noise (mono mode), 8-ohm load, balanced inputs terminated with 600 ohms

- Wideband: 0.160mV, -84.9dBW
- A weighted: 0.119mV, -87.5dBW

All charts: A = stereo mode, B = mono mode.

Charts 1A and 1B show the frequency response of the Hegel H30 with different resistive loads. With loads of 4 and 8 ohms, there are small deviations in output response at 20kHz in both stereo and mono modes, but the dummy-speaker load shows very little deviation at 20kHz, which reflects real-world load conditions.

Charts 2A and 2B illustrate how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals. The results show very low and uniform distortion levels in both modes until clipping is reached.

Charts 3A and 3B show THD+N as a function of frequency at several different power levels into 4 ohms. The high output power achieved in stereo and mono modes with low THD+N is admirable. (Note that the decrease in distortion above 10kHz in these charts is a result of a 22kHz cutoff filter, which helps to improve the accuracy of this test below 10kHz.)

Charts 4A and 4B show the H30’s damping factor vs. frequency. The result in mono mode is about half that in stereo mode, but has a similar shape.

Charts 5A and 5B show the spectrum of harmonic distortion and noise residue for a 10W, 1kHz test signal.

Chart 1A

Stereo mode

Red line = open circuit

Cyan line = 8-ohm load

Blue line = 4-ohm load

Magenta line = dummy-speaker load

Chart 1B

Mono mode

Red line = open circuit

Cyan line = 8-ohm load

Blue line = 4-ohm load

Magenta line = dummy-speaker load

Chart 2A

Stereo mode

(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

Chart 2B

Mono mode

(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

Chart 3A

Stereo mode, 22kHz cutoff filter

(4-ohm loading)

Cyan line = 2W

Green line = 20W

Blue line = 100W

Red line = 200W

Magenta line = 400W

Chart 3B

Mono mode, 22kHz cutoff filter

(4-ohm loading)

Cyan line = 2W

Green line = 20W

Blue line = 600W

Red line = 1400W

Chart 4A

Stereo mode

Damping factor = output impedance divided into 8

Chart 4B

Mono mode

Damping factor = output impedance divided into 8

Chart 5A

Stereo mode

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

Chart 5B

Mono mode

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

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