Power Amplifiers

Pass Labs XA30.5 Stereo Power Amplifier


On the Bench

I measured the XA30.5 using my Roland Edirol UA-101 24 bit 192 kHz USB soundcard with Spectra Plus FFT analysis software. Measurements were made using the balanced input of the XA30.5, running into both 8 ohm and 4 ohm loads. Measurements presented here are into the 8 ohm load for simplicity. Results were qualitatively identical into the 4 ohm load.

Pass Labs XA30.5 Stereo Power Amplifier

The first three plots show the distortion spectrum of a 1 kHz sine wave at 1 watt, 15 watts and 30 watts RMS power outputs. The THD+N levels are measured at 0.08%, 0.04% and 0.13% respectively. More important is the distortion spectrum. Particularly at the 15 watts and 30 watts output levels, the distortion is dominated by the 2nd and 3rd harmonics. High-level harmonics are at least 10 dB down from the low order harmonics. This is the key to the midrange magic of class A amps. As I've said before, THD is an almost meaningless measurement. What's important is how that distortion is distributed in frequency space. Another clear indication of the XA30.5's overbuilt design: At the rated power output of 30 watts, the amp is nowhere near the 1% THD level where power output is usually defined. In fact, the XA30.5 put out about 110 watts of power into 8 ohms before it hit 1% THD+N.

THD+N versus frequency measured extremely flat, rising only moderately up to 48 kHz. This measurement was made at 15 watts RMS power output.

Frequency response was perfectly flat to 48 kHz as far as I could tell. The gentle rise towards 8 kHz then the gradual falloff towards 40 kHz is the frequency response of the soundcard, not the amp.

Intermodulation distortion products between 1 kHz and 100 Hz sine waves are more than 70 dB below the fundamental tones.

Time domain response is shown in this plot of a 1 kHz square wave, sampled at 96 kHz. Ringing is minimal, with good shape reproduction. I could only look at the 10 kHz sine wave on my standalone oscilloscope; the spectra plus software is not fast enough to resolve the square wave. On the scope, the performance was qualitatively similar to the 1 kHz square wave, but with a bit more ringing, and more slope on the tops of the voltage plateaus. Slew rate was about 50 V/uS as measured with the scope with a 1 kHz square wave.

Overall, the measurements of the XA30.5 were exceptional, and back up the real test, how the amp sounds.