Sidebar 3: Measurements
I performed a full set of measurements on the Hegel H400 using my Audio Precision SYS2722 system. 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.
Fig.1 Hegel H400, line input, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (1dB/vertical div.).
Fig.2 Hegel H400, line input, small-signal, 10kHz squarewave into 8 ohms.
The Preamp Out output impedance was 1k ohm from 20Hz to 20kHz. The loud speaker 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.
Fig.3 Hegel H400, line input, spectrum of 1kHz sinewave, DC–1kHz, at 1Wpc into 8 ohms with volume control set to the maximum (left channel blue, right red), and to –20dB (left green, right gray) (linear frequency scale).
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 Hegel H400, line input, THD+N (%) vs 1kHz continuous output power into 8 ohms.
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 "Dual-Mono," 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.
Fig.5 Hegel H400, line input, THD+N (%) vs 1kHz continuous output power into 4 ohms.
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.
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Fig.6 Hegel H400, line input, THD+N (%) vs frequency at 20V into: 8 ohms (left channel blue, right red), 4 ohms (left green, right gray).
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%).
Fig.7 Hegel H400, line input, 1kHz waveform at 50W into 8 ohms, 0.0057% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.8 Hegel H400, line input, spectrum of 1kHz sinewave, DC–10kHz, at 50Wpc into 8 ohms (left channel blue, right red, linear frequency scale).
Fig.9 Hegel H400, line input, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100Wpc peak into 4 ohms (left channel blue, right red, linear frequency scale).
