The response of the Max to a 50Hz input at 2.5V output into 40 ohms is shown in fig.7: all distortion artifacts are below -90dB (0.003%). Harmonics of the power-line frequency are also clearly visible, but still more than 90dB down. Fig.8 shows the output spectrum resulting from an input signal consisting of an equal combination of 19 and 20kHz at an output of 2.5V into 40 ohms. The most significant artifacts here are at 18kHz and 21kHz (more than -66dB, or about 0.05%); the 1kHz difference tone is at -82.4dB, or about 0.008%.

Fig.7 HeadRoom Max, spectrum of 50Hz sinewave, DC-1kHz, at 2.5V into 40 ohms (linear frequency scale).

Fig.8 HeadRoom Max, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 2.5V into 40 ohms (linear frequency scale).

The 1kHz, THD+noise percentage vs output voltage curve shown in fig.9 was measured into loads of 150 and 40 ohms. As might be expected, the 40 ohm load reduces the available drive signal significantly. But the available output is more than adequate to drive any headphones to unpleasant—even dangerous—levels.

Fig.9 HeadRoom Max, distortion (%) vs output voltage into (from bottom to top at 10W): 150 ohms and 40 ohms.

The excellent test-bench results of the HeadRoom Max reinforce WP's fine opinion of the product.—Thomas J. Norton