HeadRoom Max headphone amplifier Measurements

Sidebar 5: Measurements

All measurements were taken from the HeadRoom Max's front-panel headphone jacks. With its very high input impedance (484k ohms) and respectably low output impedance (0.5 ohms at 20Hz and 1kHz, 0.55 ohms at 20kHz), the Max's performance is unlikely to be compromised by interface problems with associated equipment. Maximum voltage gain measured 12.3dB. DC offset was a highish 15mV in the left channel, 14.8mV in the right. S/N Ratio (ref. 1V out) measured 73.8dB at the maximum setting of the gain control, but improved to 84.2dB in the 12:00 position. The Max is noninverting from its inputs to its outputs. Its volume control tracking was fair: the right channel was -0.17dB relative to the left at full volume, but +0.18dB at 12:00 and +0.79 at 9:00.

Figs.1 and 2 show the Max's frequency response at 1V output into a 100k ohm load. These curves show the response with various combinations of the filter and processor switches. With both out of the loop, there is still a considerable HF rolloff at -2.3dB at 20kHz. Fig.3 shows the left-channel frequency response with both processor and filter on, with the left and right inputs in and out of phase and shows the maximum processing applied to signals that are different in both channels. The unusual responses in figs.1-3 are deliberate, and intended to optimize the incoming stereo signal for headphone listening. (The philosophy behind this is discussed in more detail in JA's HeadRoom amplifier review in Vol.17 No.1.)

Fig.1 HeadRoom Max, frequency response at 1V into 100k ohms with (from top to bottom at 500Hz): filter & processor on; filter & processor off (right channel dashed, 1dB/vertical div.).

Fig.2 HeadRoom Max, frequency response at 1V into 100k ohms with (from top to bottom at 500Hz): filter off & processor on; filter on & processor off (right channel dashed, 1dB/vertical div.).

Fig.3 HeadRoom Max, frequency response at 1V into 100k ohms with filter & processor on and (from top to bottom at 500Hz): channel signals in-phase; out of phase (2dB/vertical div.).

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