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 two audio octaves. With undithered data representing a tone at exactly –90.31dBFS, the waveform was symmetrical, with negligible DC offset. The three DC voltage levels described by the data were clearly defined (fig.14). With undithered 24-bit data (fig.15), the H400 output a relatively clean sinewave.
As it had with the analog inputs, the Hegel's digital inputs produced very low levels of harmonic and intermodulation distortion. Fig.16 shows the output spectrum with an input signal of 1kHz at 0dBFS. The rise in the lower-frequency noisefloor shown in fig.13 is present, but the highest-level harmonic, the third, lies below –100dB. With an equal mix of 19 and 20kHz tones, each at –6dBFS, both the second-order intermodulation product at 1kHz and the higher-order products are all vanishingly low in level, and the aliased images of the primary tones at 24.1kHz and 25.1kHz were suppressed by 100dB and more (fig.17).
Fig.18 shows the spectrum of the H400's output when it was fed high-level 16-bit J-Test data via TosLink. The odd-order harmonics of the undithered low-frequency, LSB-level squarewave all lie at the correct levels, and the central spike that represents the high-level tone at one-quarter the sample rate (Fs/4) is extremely narrow. Repeating this analysis with USB data gave an identical result. With 24-bit J-Test data (fig.19), there were no data-related sidebands present.
Finally, I used the front-panel menu to enable the digital loopback function, which feeds the digital input data to the Hegel's coaxial digital output. With 16-bit Miller/Dunn J-Test data, the eye pattern from the amplifier's digital output was wide open (fig.20), and the jitter level was a low 535.1ps.
Hegel's H400 offered generally excellent measured performance with both analog and digital input signals.—John Atkinson
Footnote 1: Roon certification should have occurred by the time you are reading this review.
