Pass Labs XA30.5 power amplifier Measurements

Sidebar 4: Measurements

I examined the Pass Labs XA30.5's measured behavior with Stereophile's loaner sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and; for some tests, I also used my vintage Audio Precision System One Dual Domain.

Before performing any tests on an amplifier, I run it for 60 minutes at one-third its specified power into 8 ohms, which is thermally the worst case for an amplifier with a class-B or -AB output stage. Superficially, the XA30.5 appears to be a 30Wpc class-A design. But this is not the maximum output power. The specifications list the amplifier's maximum output voltage as ±35V, which, assuming this is the RMS voltage, is equivalent to 153W into 8 ohms. The XA30.5 thus transitions into class-B for the top 6dB of its dynamic-range capability. I therefore preconditioned the Pass amplifier by running it at 40Wpc into 8 ohms for an hour. The heatsinks were just over 60°C (140°F) at the end of that period; the distortion had dropped slightly, from 0.0192% to 0.0176%.

The XA30.5's voltage gain into 8 ohms was 26.65dB from both balanced and unbalanced inputs, and both sets of inputs reserved absolute polarity; ie, were non-inverting. (The XLR jacks are wired with pin 2 hot.) The input impedance of the balanced XLR jacks was 29k ohms, close to the specified 30k ohms; the input impedance of the single-ended RCA jacks was higher than specified, at 20k ohms. Both figures were constant across the audioband.

The output impedance was low, at 0.08 ohm at low and middle frequencies, rising inconsequentially to 0.1 ohm at 20kHz. As a result, the modification of the amplifier's frequency response by the usual Ohm's Law interaction between its source impedance and the modulus of the loudspeaker's impedance remained within ±0.01dB limits (fig.1, gray trace). This graph also reveals excellent channel matching, as well as a wide small-signal bandwidth—the –3dB point lies at around 125kHz. The XA30.5's reproduction of a 10kHz squarewave was consequently superbly square (fig.2), with short risetimes and no hint of overshoot or ringing. The 1kHz squarewave (fig.3) was similarly well formed.

Fig.1 Pass Labs XA30.5, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green). (1dB/vertical div.)

Fig.2 Pass Labs XA30.5, small-signal 10kHz squarewave into 8 ohms.

Fig.3 Pass Labs XA30.5, small-signal 1kHz squarewave into 8 ohms.

Channel separation was superb, at better than 110dB below 5kHz, and still 98dB (R–L) and 104dB (L–R) at 20kHz. The audioband unweighted signal/noise ratio, ref. 1W into 8 ohms, was an excellent 83.6dB, increasing to 87.2dB when A-weighted.

Fig.4 shows how the THD+noise percentage in the amplifier's output varies with output power into 8, 4, and 2 ohms. While the XA30.5 may give 30Wpc into 8 ohms in class-A (14.8dBW), the distortion at this power level is low, at 0.015%. The amplifier doesn't actually clip (defined as the THD reaching 1%) until a much higher power level: 130Wpc into 8 ohms (21.14dBW). Even higher powers were available into lower impedances before clipping: 195Wpc into 4 ohms with both channels driven (19.9dBW), and 332W into 2 ohms with one channel driven (19.2dBW). The shapes of the traces in fig.4 indicate that the distortion starts to rise out of the noise floor at output powers above a few watts, with then a gentle increase until the waveform starts to square off. I therefore examined how the THD+N percentage changed with frequency at a level of 10.5V, equivalent to 13.8W into 8 ohms. The results are shown in fig.5. While the distortion is very low in the low and middle frequencies, it does start to rise with increasing frequency above 500Hz, and is worse into 2 ohms (green trace) than into higher impedances.

Fig.4 Pass Labs XA30.5, distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 8, 4, 2 ohms.

Fig.5 Pass Labs XA30.5, THD+N (%) vs frequency at 10.5V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green).

What matters more than the absolute level of an amplifier's distortion is its spectral composition. Fig.6 reveals that the XA30.5's THD is almost pure third harmonic, even at a level where the amplifier's output stage is no longer running in pure class-A. And as you'd expect from the rich bias level, there is no hint of crossover distortion. FFT analysis indicates that while there is some second harmonic present as well as the third, it lies about 100dB down from the fundamental level (fig.7), and all higher-order harmonics are very low in level. The presence of sidebands at ±120Hz around the fundamental and third harmonic, however, suggests that the amplifier is working hard. Still, all power-supply–related spectral components are at or below –108dB, which is negligible.

Fig.6 Pass Labs XA30.5, 1kHz waveform at 50W into 8 ohms (top), 0.023% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.7 Pass Labs XA30.5, spectrum of 1kHz sinewave, DC–1kHz, at 37W into 8 ohms (left channel blue, right red; linear frequency scale).

Finally, the XA30.5's decreasing linearity in the top octaves led to a somewhat disappointing result on the high-power, high-frequency intermodulation test, particularly with the right channel (fig.8, red trace). (The power level chosen for this test was just below visible waveform clipping on the oscilloscope screen.) While the second-order difference component from the 19+20kHz test signal was respectably low, at –103dB left channel (0.0007%) and –94dB right (0.002%), higher-order products also made an appearance, with the 18 and 21kHz tones reaching –60dB (0.1%). This is very much a worst-case test, however.

Fig.8 Pass Labs XA30.5, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 75W peak into 8 ohms (linear frequency scale).

Pass Labs' XA30.5 is a Jekyll-and-Hyde amplifier. Ostensibly a 30Wpc class-A design, its measured performance reveals that it can actually deliver clipping-free peaks 6dB higher in power, while the fact that its distortion under those conditions is predominantly the subjectively innocuous third harmonic is commendable.—John Atkinson

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