MEASUREMENTS
Iperformed a full set of measurements on the Hegel H400 using my Audio Precision SYS2722 system.1 I used the front-panel controls, the remote control, and the Hegel Music Systems app, which I installed on my iPad mini after connecting the amplifier’s Ethernet port to my network. Not all settings are available with the app. The amplifier is specified as having a maximum output power of 250W into 8 ohms; I preconditioned the H400 before the measurements by following the CEA’s recommendation of running it at one-eighth of the full specified power into 8 ohms for 30 minutes. Following that period, the temperature of the top panel was 109.5°F (43.1°C). That of the side-mounted heatsinks was much higher, at 173.5°F (78.6°C). It’s a good thing the heatsinks are covered by panels.
Looking first at the H400’s line inputs, the amplifier preserved absolute polarity at all its outputs with both balanced and unbalanced input signals. The volume control operated in accurate 0.5dB steps. With the volume control set to the maximum, “100,” the voltage gain at 1kHz from the loudspeaker outputs into 8 ohms was 32.55dB with both the balanced and unbalanced inputs; it was 5dB at the variable Preamplifier output and –0.13dB at the fixed preamplifier output. The balanced input impedance measured 9.3k ohms across the audioband; the unbalanced input impedance was 14k ohms at 20Hz and 1kHz, 12.8k ohms at 20kHz.
The Preamp Out output impedance was 1k ohm from 20Hz to 20kHz. The loudspeaker output impedance was extremely low at just 0.02 ohm. Consequently, the modulation of the H400’s frequency response due to the Ohm’s Law interaction between this impedance and the impedance of our standard simulated loudspeaker was negligible (fig.1, gray trace). The
amplifier’s response into resistive loads was flat in the audioband with its output into 8 ohms (blue and red traces) down by 2dB just below 200kHz and that into 2 ohms down by 2.5dB at 100kHz. Both the very close channel balance and the overall response were preserved at lower settings of the volume control and from the Preamplifier outputs. Fig.1 was taken with the balanced inputs; the response into 8 ohms with the unbalanced inputs was also flat in the audioband, and down by 2dB at 110kHz. With either input type, the H400’s reproduction of a 10kHz squarewave (fig.2) had short risetimes and no overshoot or ringing.
Channel separation was excellent, at >80dB across the audioband R–L and >90dB, R–L below 9kHz, and still 83dB at 20kHz. The wideband, unweighted signal/ noise ratio, taken with the unbalanced input shorted to ground and the volume control set to its maximum, was an okay 63.6dB left and 62dB right, ref. 2.83V, which is equivalent to 1W into 8 ohms. These ratios improved to 89.5dB and 84.5dB respectively when the measurement bandwidth was restricted to the audioband, and to 92.8dB and 87.8dB when A-weighted. The blue and red traces in fig.3 show the H400’s low-frequency noisefloor at 1Wpc into 8 ohms with the volume control set to its maximum; the green and gray traces show the noisefloor spectrum at the control set to –20dB and with the input signal increased by the same 20dB so that the output level remains at 2.83V. The levels of the random noise components are similar with both volume control settings, but the power supply–related spuriae are lowered by up to 10dB at the control’s –20dB setting.
Fig.4 plots how the THD+noise percentage in the Hegel’s output varies with power
into 8 ohms with both channels driven. At our usual definition of clipping, which is when the THD+N reaches 1%, the H400 did not meet its specified output power of 250W into 8 ohms (24dBW), clipping at 225Wpc (23.52dBW). Hegel notes that the specified power into 8 ohms is “DualMono,” so I repeated this test with just one channel driven. The H400 now just missed its specified power, by a negligible 0.1dB, clipping at 247W (23.9dBW). It is important to note, however, that I performed the measurements on one of the hottest days in July, when there was much demand for AC power. Even though I had our air conditioning turned off, the AC wall voltage in the test lab was 116.5V with the amplifier idling and dropped to 115V with the amplifier clipping into 8 ohms and to 112.9V with it clipping into 4 ohms, both with both channels driven.
Hegel doesn’t specify the maximum power into 4 ohms. Fig.5 was taken with both channels driven into 4 ohms; the amplifier clips at 340Wpc (22.3dBW) into that load.
Fig.6 shows how the H400’s THD+N percentage changed with frequency at 20V, equivalent to 50W into 8 ohms and 100W into 4 ohms. The distortion into 8 ohms (blue and red traces) is very low. The right channel’s THD+N (gray trace) rises more than the left’s (green trace) into 4 ohms, though still to a relatively low level. It also rises in the top two audio octaves into both impedances; this will be due to the amplifier’s limited open-loop bandwidth.
The THD+N waveform, taken at 50W into 8 ohms (fig.7), appeared to comprise second and third harmonics; this was confirmed by spectral analysis (fig.8). Both harmonics lie close to –90dB (0.003%), and while higher-order harmonics are present, these all lie at even lower levels. Intermodulation distortion was low in level even into 4 ohms (fig.9): The second-order difference product with an equal mix of 19kHz and 20kHz tones lay at just –94dB (0.002%) ref. the peak signal level, and the higher-order products at 18kHz and 21kHz lay at –80dB (0.01%).
To examine the H400’s digital performance, I used the coaxial and optical S/
PDIF inputs—the optical input only accepted data sampled at rates up to 96kHz—and USB data sourced from my MacBook Pro. (While Roon recognized the Ethernetconnected H400, it reported that “Unfortunately, the manufacturer has not yet completed certification for use with Roon.”) The USB Prober utility identified the H400 as “Hegel USB” from “Hegel Music Systems,” with the serial number string “2 (none),” and indicated that the USB port operated in the optimal isochronous asynchronous mode. Apple’s AudioMIDI utility showed that the Hegel accepts 16-bit, 24-bit, and 32-bit integer data via USB sampled at all rates from 44.1kHz to 768kHz.
The Hegel’s digital inputs preserved absolute polarity from all the outputs. With the volume control set to the maximum, the output level with a 1kHz tone at –20dBFS was 449.5mV from the variable Preamplifier output, 248.8mV from the fixed Preamplifier output, and 10.72V from the speaker output into 8 ohms. This last is 12dB below the clipping voltage into that load, so the H400’s DAC offers 8dB more gain than is strictly necessary. To avoid overloading the amplifier’s output stage, I performed all subsequent digital input testing at the fixed Preamplifier output, with the volume control set to “0.”
The Hegel H400 offers a single reconstruction filter for PCM data. Fig.10 shows the filter’s impulse response with 44.1kHz data and indicates that the filter is a long, minimum-phase type with all the ringing following the single high sample. The magenta and red traces in fig.11 show the FIR Long filter’s ultrasonic rolloff with data sampled at 44.1kHz. They reach full stop-band attenuation just above half the sample rate (indicated by the vertical green line), with the aliased image at 25kHz of a 19.1kHz tone at 0dBFS (cyan, blue) suppressed by almost 110dB. The harmonics associated with the 19.1kHz tone all lie at or below –100dB (0.001%), with the second harmonic highest in level.
Fig.12 shows the frequency response with data sampled at 44.1kHz, 96kHz, and 192kHz. The audioband response is flat with all three sample rates, but with then a gentle rise above 20kHz, interrupted by a sharp decline just below half of each sample rate.
An increase in bit depth from 16 to 24, with dithered data representing a 1kHz tone at –90dBFS, dropped the H400’s high-frequency noisefloor by up to 18dB, less at lower frequencies (fig.13). This behavior is unusual and implies a measured resolution between 16 and 17 bits below 1kHz and close to 18 bits in the top