Hegel Music Systems H600 Integrated Amplifier-DAC Measurements

Link: reviewed by Philip Beaudette on SoundStage! Hi-Fi on August 1, 2023

General Information

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

The Hegel H600 was conditioned for 1 hour at 1/8th full rated power (~37W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The H600 offers two unbalanced (RCA) and two balanced (XLR) line-level analog inputs, seven digital inputs (one BNC, one RCA, three optical, one ethernet, and on USB), two pairs of line-level outputs (fixed and variable over RCA), and two pairs of speaker-level outputs (left and right). For the purposes of these measurements, the following inputs were evaluated: RCA digital coaxial, and the analog line-level balanced inputs (a 1kHz FFT using the unbalanced inputs is also provided).

Most measurements were made with a 2Vrms line-level analog input or a 0dBFS digital input. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 300W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 300W output.

Based on the accuracy and non-repeatable results at various volume levels of the left/right channel matching (see table below), the H600 volume control is likely digitally controlled but operating in the analog domain. The volume control offers a total range from -62.3dB to +32.8dB between the line-level balanced analog inputs and the speaker outputs. Volume step sizes varied, from 5 to 2dB for the first nine steps, then 1dB from levels 10 to 55, then 0.5dB steps from 56 to 100.

The analyzer’s input bandwidth filter was set to 10Hz–22.4kHz for all measurements, except for frequency response (DC to 1 MHz), FFTs (10Hz to 90kHz), and THD vs. frequency (10Hz to 90kHz). The latter to capture the second and third harmonics of the 20kHz output signal. Since the H600 is a conventional class-AB amp, there was no issue with excessive noise above 20kHz.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Hegel for the H600 compared directly against our own. The published specifications are sourced from Hegel’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was extended from DC to 1MHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

* Hegel measures damping factor directly at the output stage whereas we measure at the output terminals.

Our primary measurements revealed the following using the line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the H600 was able to sustain 520W into 4 ohms (~2% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (52W) for 5 seconds, for 5 continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the H600 was hot enough to the touch to cause pain after a few seconds.

Frequency response (8-ohm loading, line-level input)

In our measured frequency-response chart above, the blue and red plots show the speaker-level outputs (relative to 1kHz, at 10W into 8 ohms). The H600’s speaker outputs are near flat within the audioband (-0.15dB at 20Hz, 0dB at 20kHz), and show a very extended bandwidth (0dB at 80kHz). The H600 appears to be AC-coupled, due the attenuation in response below 20Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response (8-ohm loading, line-level input)

Above is the phase response plot from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The H600 does not invert polarity. Here we find essentially no phase shift at 20kHz, and 10 degrees at 20Hz, owing to the H600’s extended bandwidth and AC-coupled design.

Frequency response vs. input type (8-ohm loading, left channel only)

The chart above shows the H600’s frequency response (relative to 1kHz) as a function of input type measured across the speaker outputs at 10W into 8 ohms. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 10Hz to 22kHz using the coaxial digital input, and the purple trace is for a 24/96 dithered digital input signal from 10Hz to 48kHz, while the pink trace is for a 24/192 dithered digital input signal from 10Hz to 96kHz. The green trace is the same analog input frequency response seen above. All three digital responses show a rise in output above 20kHz, with the 16/44.1 data peaking at +0.15dB at 19.2kHz, the 24/96 data peaking at +0.72dB at 43.3kHz, and the 24/192 data peaking at +1.8dB at 78.3kHz. All three digital responses exhibit brick-wall-type filtering, with -3dB points at 21.2kHz (16/44.1), 46.8kHz (24/96), and 93.8kHz (24/192).

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

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the fixed line-level outputs of the H600 for a 2.4Vrms (at 0dBFS) output signal. The digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. Both data were essentially perfect as of -100dBFS down to 0dBFS. At -120dBFS, the 16/44.1 data were about +2dB (left/right) above reference, while the 24/96 data were just below +1dB above reference.

Impulse response (24/44.1 data)

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the fixed line-level outputs of the H600. We can see that the H600 DAC utilizes a reconstruction filter with no pre-ringing.

J-Test (coaxial)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outs of the H600. J-Test was developed by Julian Dunn in the 1990s. It is a test signal—specifically a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc.). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits some low-level peaks below 1kHz, between -120 and -140dBrA. This is a relatively strong J-test result, indicating that the H600 DAC should be adequate at rejecting jitter.

J-Test (optical)

The chart above shows the results of the J-Test test for the optical digital input measured at the fixed line-level pre-outs of the H600. The optical input produced essentially the same result as with the coax input.

J-Test with 100ns of injected jitter (coaxial)

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved strong. Although sidebands were visible at the 100ns jitter level, they did not exceed -135dBrA in amplitude. The optical input jitter result was very similar to the coaxial input result.

J-Test with 300ns of injected jitter (coaxial)

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sinewave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity again proved to be solid but not perfect, with visible sidebands at a very low -125dBrA at the 300ns jitter level. The H600 DAC did lose sync with the signal when jitter was increased beyond approximately 400ns. The optical input jitter result was very similar to the coaxial input result.

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

The plot above shows a fast Fourier transform (FFT) of the H600’s line-level pre-outs with white noise at -4 dBFS (blue/red), and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The medium roll-off above 20kHz in the white-noise spectrum shows the implementation of a typical brickwall-type reconstruction filter. There are no clear aliased image peaks in the audioband above the -135dBrA noise floor. Some very low frequency peaks can be seen in the FFT, however; these are at 60/120Hz due to power-supply noise. The main 25kHz alias peak is highly suppressed at -110dBrA. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are below -100dBrA.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we can see only minor deviations of about 0.04dB from 4 ohms to no load through most of the audioband, reaching as high as 0.08dB at 20kHz. This is an indication of a very high damping factor, or low output impedance. The variations in RMS level when a real speaker was used are smaller, deviating by at worst 0.04dB through most of the audioband.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 240W. The power was varied using the volume control. At 1W and 10W, THD ratios ranged from 0.006% at 20Hz, down to 0.003% from 100Hz to 4kHz, then up to 0.01% at 20kHz. The 240W THD values were slightly higher and ranged from 0.004% at 20Hz through to about 1kHz, then up to 0.04% at 20kHz.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the H600 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm and 4-ohm data track closely and are relatively constant below the “knees,” at 0.002 to 0.005%, with the 8-ohm data outperforming the 4-ohm THD data by only 4-5dB. The “knee” for the 8-ohm data is just past 250W, then up to the 1% THD mark at 313W. With a 4-ohm load, the “knee” occurs at about 400W, and the 1% THD value was reached at 519W.

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the H600 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Overall, THD+N values for both loads were similar, ranging from about 0.02-0.03%, down to just above 0.003% (8-ohm). The 4-ohm THD+N ratios were a few dB worse than the 8-ohm ratios through most of the sweep.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the H600 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yielded 40W at the output into 8 ohms (and roughly 80W into 4 ohms, and 160W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find a roughly 3-4dB increase between the 8- and 4-ohm data, then another 5-7dB increase from 4 to 2 ohms. We find the 8-ohm trace ranging from 0.003% from 60Hz up to 4kHz, then up to 0.007% at 20kHz. These data show that the H600 is not only stable into 2-ohm loads, but will perform well in terms of THD, comparable to an 8- and 4-ohm load.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the H600 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Overall, THD ratios were mostly within 5-10dB of the 8-ohm THD data, sometimes above, sometimes below. The worst-case result was at 20Hz with the two-way speaker, where THD ratios reached 0.03%. By and large, THD ratios ranged from 0.01 to 0.001%. This shows once again, that the H600 will yield consistent and stable THD results into different loads.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the H600 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The results for the dummy load were fairly consistent, hovering between 0.005 and 0.01% throughout the sweep. We find that the two-way speaker yielded IMD ratios 2-10dB lower than the dummy load, whereas the 3-way speaker yielded IMD ratios roughly 5dB lower than the dummy load at lower frequences, and 5dB higher at higher frequencies.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the H600 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar, at around the 0.01% level.

FFT spectrum – 1kHz (balanced line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the balanced analog line-level input. We see that the signal’s second (2kHz) and third harmonic (3kHz) dominate at near -90dBrA, or 0.003%. The other signal harmonics are around or below -110dBrA, or 0.0003%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60/180/300Hz peaks dominating at near -110dBrA or 0.0003%.

FFT spectrum – 1kHz (unbalanced line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the unbalanced analog line-level input. While the signal’s second (2kHz) and third harmonics (3kHz) are nearly identical to the balanced input FFT above, this FFT shows more higher odd-order signal harmonics, all the way out to 100kHz at -120dBrA, or 0.0001%, and below. The power-supply-related noise peaks are very similar to the balanced input FFT above.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. The signal and noise-related harmonics are very similar to the balanced analog input FFT above, but with a slightly elevated noise floor due to the limitations of the 16-bit word depth.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. The signal and noise-related harmonics are very similar to the balanced analog input FFT above.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and non-existent signal harmonics. Power-supply-related noise peaks can be seen at the fundamental (60Hz) and higher harmonics, at -110dBrA, or 0.0003%, and below.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and non-existent signal harmonics. Power-supply-related noise peaks can be seen at the fundamental (60Hz) and higher harmonics, at -110dBrA, or 0.0003%, and below.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the analog line-level input. 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. We see that the signal’s third harmonic (150Hz) dominates at around -90dBrA, or 0.003%. The second signal harmonic (100Hz), and subsequent signal harmonic peaks, are at or below -100dBrA, or 0.001%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 200Hz peak dominating at -110dBrA (left), or 0.0003%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBrA, or 0.001%. The third-order modulation products, at 17kHz and 20kHz, are higher at almost -90dBrA, or 0.003%.

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBrA, or 0.001%. The third-order modulation products, at 17kHz and 20kHz, are higher at almost -90dBrA, or 0.003%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBrA, or 0.001%. The third-order modulation products, at 17kHz and 20kHz, are higher at almost -90dBrA, or 0.003%.

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

Shown above is the FFT of the speaker-level outputs of the H600 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the "grass" between the test tones—are distortion products from the amplifier and reach between the -120 and -110dBrA, or 0.0001-0.0003%, level.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the H600’s slew-rate performance. Rather, it should be seen as a qualitative representation of the H600’s extended bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. Here we can see a very clean squarewave reproduction, with sharp corners, and no overshoot.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. Both channels track closely, with a higher damping factor (as high as 550) between 20Hz and 2kHz. Above 2kHz, we see a decline in the damping factor, as low as 220 at 20kHz.

Diego Estan
Electronics Measurement Specialist