Power Amplifiers

Pass Labs XA-100.5 Pure Class A Monoblock Power Amplifier

On the Bench

Distortion measurements were performed within an 80 kHz bandwidth, using a Carver Mark IV ribbon speaker as the load, except where indicated.

At 1 kHz, the 3rd an 5th harmonics were predominant, along with a small 7th harmonic. This is all odd-ordered distortion, which was the designer's (Nelson Pass) intention. I had always thought that odd-ordered harmonics would sound bad, but that is not the case, as long as they are primarily the lower numbered harmonics (3rd and 5th). In any case, the total amount of distortion was less than 0.01%.

Using 19 kHz and 20 kHz sine waves, the B-A peak at 1 kHz was 102 dB below the fundamentals. This is excellent.

IMD, using the SMPTE standard of 60 Hz and 7 kHz sine waves, was only 0.009%.

THD+N vs. Frequency was less than 0.1% out to 8 kHz, then rose to 0.2% at 20 kHz. The Impedance/Phase graph for the Carver Mark IV ribbon speaker is shown in the second illustration.

The measured frequency response was 20 Hz - 60 kHz, - 1 dB.

Power output at 8 Ohms was far above the specification. The sharp knee was at 150 watts, and clipping (1% THD+N) was at 175 watts. From 0 to 100 watts, operation is in Class A, and above that, it goes into Class AB operation.

At 4 Ohms, the sharp knee occurred at 250 watts, with clipping at 290 watts.