Melos SHA-1 headphone amplifier Measurements part 2

These distortion measurements were all taken from the front-panel ¼" 'phone jacks. Very similar results were obtained from the rear RCA jacks when they were loaded with pure resistive loads ranging from 600 ohms to 100k ohms. Corey notes above that the line outputs are not able to source high currents; as an experiment, I hooked up the Sennheisers to the RCA outputs and fed the Melos with the same 100mV, 50Hz signal. The output spectrum is shown in fig.6. Even though the Sennheisers don't drop below 200 ohms, the Melos is sweating to drive them from its line outputs, as witnessed by the huge increase in both the second harmonic (more than 100 times, to -44dB) and in the 180Hz power-supply component. The moral is simple: drive headphones from the correct socket, power amplifier inputs from the RCA jacks (which is what it says in the manual).

Fig.6 Melos SHA-1, spectrum of 50Hz sinewave, DC-1kHz, from RCA output jacks at 870mV into Sennheiser HD 560 headphones (linear frequency scale).

The burstproof nature of the correct headphone outputs is revealed by figs.7 and 8, which show the way the distortion changes with output voltage into a 100k resistive load and the HD560 headphones, respectively. The two overlaid traces in fig.7 are for frequencies of 100Hz and 1kHz, with the 1% "clipping" point reached at 3.85V. The third trace shows that slightly less output voltage is available at 10kHz, 3.6V. There is no significant change in performance when the Sennheiser 'phones are substituted for the 100k load (fig.8). Fig.9 shows the overload behavior of the RCA outputs into a 100k resistive load. Again, 3.9V is available for 1% THD at low and midrange frequencies, with a drop to 3.2V at 10kHz. This is easily enough overload margin, as all power amplifiers will be solidly into clipping before the Melos will.

Fig.7 Melos SHA-1, distortion (%) vs output voltage from headphone output into 100k ohms load, from bottom to top at 1V: 100Hz and 1kHz, 10kHz (top and left).

Fig.8 Melos SHA-1, distortion (%) vs output voltage from headphone output into Sennheiser HD 560 headphones, from bottom to top at 1V: 1kHz, 100Hz, and 10kHz.

Fig.9 Melos SHA-1, distortion (%) vs output voltage from RCA output into 100k ohms load, from bottom to top at 1V: 1kHz, 100Hz, and 10kHz.

Do these measurements reveal the Melos's excellent sound quality? Well, the volume-control dependency of the circuit's ultrasonic behavior probably doesn't result in subjective problems. You have low noise and subjectively benign distortion at very low levels, coupled with excellent drive capability and a very low output impedance. The signal path, too, is simple, with one device providing voltage gain (the tube) and one the current gain (the FET) for the line output, with an extra FET follower to beef up the current delivery for headphones. However, if I really knew why this straightforward little line amplifier sounded so good, I'd quit Stereophile to go into business as a really successful amplifier manufacturer. On the basis of the sound of the SHA-1, I believe that's about to happen to George "My Name's Not Gunther" Bischoff.—John Atkinson

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