Hegel Music Systems H400 Streaming Integrated Amplifier Measurements

Link: reviewed by Philip Beaudette on SoundStage! Hi-Fi on August 15, 2024

General Information

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

The Hegel Music Systems H400 was conditioned for one hour at 1/8th full rated power (~30W 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 H400 offers three sets of line-level analog inputs (two unbalanced over RCA, one balanced over XLR), six digital inputs (three S/PDIF optical, two S/PDIF BNC, one S/PDIF RCA, one USB), left/right unbalanced line-level outputs (fixed and variable over RCA), and one set of speaker level outputs. For the purposes of these measurements, the following inputs were evaluated: digital coaxial (RCA), balanced analog (XLR) line-level. There were no appreciable differences in terms of gain, THD, and noise between the unbalanced and balanced analog inputs.

Most measurements were made with a 2Vrms line-level analog input and 0dBFS digital input. The signal-to-noise ratio (SNR) measurements were made with the default input signal values but with the volume set to achieve the rated output power of 250W into 8 ohms. For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum.

Based on the accuracy and randomness of the left/right volume channel matching (see table below), the H400 volume control is digitally controlled but operating in the analog domain. The H400 overall volume range is from -59dB to +28dB (line-level input, speaker output) using 100 volume steps. It offers 2 to 3dB increments from position 0 to 10, 1dB increments from positions 10 to 56, and 0.5dB from 57 to 100.

Our typical input bandwidth filter setting of 10Hz to 22.4kHz was used for all measurements except FFTs and THD versus frequency, where a bandwidth of 10Hz to 90kHz was used. Frequency-response measurements utilize a DC to 1MHz input bandwidth.

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 H400 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 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.

* measured at speaker 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 H400 was able to sustain 440W into 4 ohms (~2% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (44W) for 5 seconds, for 5 continuous minutes without inducing a fault protection circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the H400 was warm to the touch.

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

In our frequency-response plots above (relative to 1kHz), measured across the speaker outputs at 10W into 8 ohms, the H400 is near flat within the audioband (20Hz to 20kHz, -0.25/-0.1dB). The -3dB point is near 180kHz, validating Hegel’s frequency-response claim. The H400 appears to be AC coupled, yielding just under -2dB at 5Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

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

Above are the phase-response plots from 20Hz to 20kHz for the line level input, measured across the speaker outputs at 10W into 8 ohms. The H400 yielded just under +20 degrees of phase shift at 20Hz, and -20 degrees at 20kHz.

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

The chart above shows the H400’s frequency response (relative to 1kHz) as a function of input type measured across the speaker outputs at 10W into 8 ohms. The two green traces are the same analog input data from the speaker-level frequency-response graph above (but limited to 80kHz). The blue and red traces are for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, the purple and green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and the pink and orange traces are for a 24/192 dithered digital input signal. At low frequencies, the digital signals yielded the same response as the analog input (-2.2dB at 5Hz). All three digital input data yielded brick-wall-type responses. The -3dB points are: 21.1kHz for the 16/44.1 data, 46.2kHz for the 24/96 data, and 92.4kHz for the 24/192 data. Also of note, the digital input data response show a rise in frequency response above 20kHz while the analog data did not.

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 H400, where 0dBFS yielded 2.6Vrms. The digital input was swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output was 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. The 24/96 data were at -1dB just under -110dBFS, while the 16/44.1 data were +2dB at -120dBFS. We also extended the sweep down to -140dBFS, where . . .

. . . we see that both input data grossly overresponded. Above +10dB at -140dBFS for the 16/44.1 data, and at +10dBFS for the 24/96 data.

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 H400. We see that the digital reconstruction filter has no pre-ringing and sustained post-ringing.

J-Test (coaxial)

The chart above shows the results of the “J-test” test for the coaxial digital input measured at the fixed line-level outputs of the H400 where 0dBFS yielded about 2.6Vrms. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least-significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc.). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

Here we see a strong J-Test result, with virtually no peaks in the audioband above the -150dBFS noise floor. The peaks at very low frequencies at -110 to -120dBFS are due to power-supply related harmonics (60/120/240Hz, etc.). This is an indication that the H400 DAC should have strong jitter immunity through this input.

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 outputs of the H400. The optical input yielded essentially the same results compared to the coaxial input.

J-Test (coaxial, 10ns jitter)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level outputs of the H400, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. There are no peaks visible at the telltale 10kHz and 12kHz positions.

J-Test (coaxial, 100ns jitter)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the H400, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. Here we see the telltale peaks at 10kHz and 12kHz, but at the very low -135dBFS level. More evidence of the strong jitter immunity in the H400 DAC.

J-Test (optical, 100ns jitter)

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the H400, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The optical input yielded essentially the same results compared to the coaxial input.

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 H400’s fixed line-level outputs with white noise at -4dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows the brick-wall-type behavior of the reconstruction filter. There are no aliased image peaks within the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is highly suppressed at -110dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are near the same level.

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

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 50kHz. 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 see that the deviations between no load and 4 ohms are extremely small at roughly 0.03dB. This is a strong result and an indication of a very low output impedance, or very high damping factor. With a real speaker load, deviations measured at roughly the same level.

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

The chart above shows THD ratios at the speaker-level outputs 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 220W (near the rated output of 250W). The power was varied using the H400’s volume control. The 1W THD ratios were the lowest, ranging from 0.003% from 20Hz to 3kHz, then up to roughly 0.01% at 20kHz. The 10W THD ratios were only about 5dB higher than the 1W data. At 220W, THD ratios ranged from 0.01-0.02% from 20Hz to 1kHz, then up to 0.2% at 20kHz.

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

The chart above shows THD ratios measured at the speaker-level outputs of the H400 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). THD ratios into 4 and 8 ohms are remarkably close (within 3-5dB). For the 8-ohm load, THD ratios ranged from 0.002% at 50mW, up to 0.015% at the “knee” at roughly 220W, then up to the 1% THD mark at 259W. For the 4-ohm load, THD ratios ranged from 0.003% at 100mW, up to 0.015% at the “knee” at roughly 380W, then up to the 1% THD mark at 429W.

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

The chart above shows THD+N ratios measured at the speaker-level outputs of the H400 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). THD+N ratios into 4 and 8 ohms are remarkably close (with 2-3dB). They range from 0.02% at 50mW, down to 0.004% in the 10 to 50W range.

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 H400 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 50W at the output into 8 ohms (and roughly 100W into 4 ohms, and 200W 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. The 8- and 4-ohm data are extremely close, ranging from 0.005% from 20Hz to 2kHz, then up to 0.03% at 20kHz. Remarkably, the 2-ohm THD data is only roughly 3-5dB above these levels. This is a very strong result, and shows that Hegel has prioritized a robust power supply and output stage in the H400.

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 H400 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 2-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a 3-way speaker (Paradigm Founder Series 100F, measurements can be found here). Generally, THD ratios into the real speakers were similar (higher at some frequencies, lower at others) to those measured across the resistive dummy load. The differences were generally within the 5dB range, and the results were generally in the 0.002% to 0.02% range. This is also a very strong result and shows that the H400 is largely speaker load invariant.

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 H400 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). We find that all three IMD traces are close to one another, with the three-way speaker yielding 5dB higher results at 20kHz, but 5dB lower IMD results at 3 to 4kHz. Generally, the IMD results ranged from 0.003 to 0.02%.

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 H400 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). We find very similar IMD ratios into all three loads, between 0.02 and 0.005% across the sweep. Another strong result.

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 analog line-level balanced input. We see that the signal’s second (2kHz) and third (3kHz) harmonics dominate at roughly -90dBrA, or 0.003% (-100dBrA for the left channel at 2kHz). There are subsequent signal harmonics visible between -100dBrA, or 0.001%, and -120dBrA, or 0.0001%. On the left side of the signal peak, we find power-supply related noise peaks (60/120/180/240/300Hz etc) at the -110dBrA, or 0.0003%, and below level. Overall, this is an average FFT result for a modern solid-state amplifier.

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 analog line-level unbalanced input. We see effectively the same result as with 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. We see effectively the same result as compared to the analog input FFT above, but for a higher noise floor due to the 16-bit 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. We see essentially the same result as with the 16/44.1 FFT above, but with a lower noise floor due to the increased bit depth.

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, with no signal harmonics above the -135dBrA noise floor, and power-supply-related noise peaks at -105dBrA, or 0.0006%, 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, with no signal harmonics above the -140dBrA noise floor, and power-supply-related noise peaks at -105dBrA, or 0.0006%, 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. The most predominant (non-signal) peaks are the second (100Hz) and third (3kHz) signal harmonics at -90dBrA, or 0.003%. Other peaks (both signal harmonics and power-supply noise related harmonics) can be seen at -100dBrA and below.

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 -105/-90dBrA (left/right), or 0.0006/0.003%, while the third-order modulation products, at 17kHz and 20kHz are at -90dBrA, or 0.003%. Like the 1kHz FFT, this is an average IMD result.

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

Shown above is the FFT of the speaker-level output of the H400 with the APx 32-tone signal applied to the analog 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 are at and below the -110dBrA, or 0.0003%, level.

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

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 16/44.1 (-1dBFS). We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -110/-95dBrA (left/right), or 0.0003/0.002%, while the third-order modulation products, at 17kHz and 20kHz, are at -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 (-1dBFS). We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -110/-95dBrA (left/right), or 0.0003/0.002%, while the third-order modulation products, at 17kHz and 20kHz, are at -90dBrA, or 0.003%.

Squarewave 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 H400’s slew-rate performance. Rather, it should be seen as a qualitative representation of the H400’s relatively high bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. In this case, we find clean corners with only very mild softening and no over/undershoot.

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

The final graph above shows the damping factor as a function of frequency. We can see damping factors ranging from about 600 to 200 for the left channel and 400 to 100 for the right channel. This is a very strong result for an integrated solid-state amplifier.

Diego Estan
Electronics Measurement Specialist