Solid State Power Amp Reviews

Before performing any measurements, I ran one of the Pass Labs XA60.5s (serial no. 26384) for an hour at one-third its specified maximum power of 60W into 8 ohms. This is thermally the worst case for an amplifier with a class-B or -AB output stage. A class-A amplifier, however, runs hottest when it is idling. By the end of the hour, the top panel was warm, at 107°F (41.8°C), and the side-mounted heatsinks were hot, at 120°F (49°C). With the amplifier idling for another hour, the heatsink temperature had risen to 124°F (51.1°C), the top panel to 108°F (42.3°C). (Temperatures were measured with a Mastercool infrared thermometer.)

I performed a full set of measurements using Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see www.ap.com and the January 2008 "As We See It"). The voltage gain at 1kHz into 8 ohms was very slightly higher than the specified 26dB, at 26.4dB balanced and 26.55dB unbalanced. (For unbalanced drive, pins 1 and 3 of the XLR jack were connected with a jumper.) The amplifier was non-inverting (ie, it preserved absolute polarity) with both input modes. The input impedance was 17k ohms, 20Hz–20kHz, for both balanced and unbalanced signals.

The XA60.5's output impedance, including 6' of speaker cable, was 0.1 ohm at 20Hz and 1kHz, rising to 0.2 ohm at 20kHz. The modulation of the amplifier's frequency response, due to the Ohm's law interaction between this source impedance and the impedance of our standard simulated loudspeaker, was a mild ±0.1dB (fig.1, gray trace). Into an 8 ohm resistive load (blue trace), the XA605's response was flat up to 20kHz, then rolled off to reach –3dB at 102kHz. The response rolled off a little earlier into lower impedances, but was still just 0.5dB down at 20kHz into 2 ohms (red trace). With this wide a small-signal bandwidth, the amplifier's reproduction of a 10kHz squarewave into 8 ohms featured short risetimes and a good square shape (fig.2).

The unweighted, wideband signal/noise ratio, ref. 1W into 8 ohms and taken with the input shorted to ground, was a good 79dB. This improved to 86.6dB when the measurement bandwidth was restricted to the audioband; and further still, to 88.7dB, when the measurement was A-weighted. Fig.3 indicates that both the odd and even harmonics of the 60Hz power-supply frequency were present, though these all lay at or below –100dB ref. 1W into 8 ohms.

The needle of the front-panel meter remained just to the left of 12:00 at 8-ohm powers below 60W. At 60W it moved slightly to the right, reaching 1:00 at 100W into 8 ohms and 1:30 at 120W. Specified as putting out 60W into 8 ohms (17.8dBW), the XA60.5 considerably exceeded that power, delivering, at 1% THD, 130W into 8 ohms (21.1dBW, fig.4), 210W into 4 ohms (20.2dBW, fig.5), and 330W into 2 ohms (19.2dBW, fig.6). The THD starts to rise above the noise floor at high powers, but is very low at powers of a few watts with a 1kHz signal, even into 2 ohms. The percentage of THD then slowly rises with increasing power, suggesting that the XA60.5 has only a small amount of corrective feedback (though I'm not sure how and where this feedback is applied in Nelson Pass's Supersymmetry circuit).

I examined how the percentage of THD+Noise changed with frequency at 9V, a level where I could be sure, from the earlier measurements, that I was looking at actual distortion rather than noise. The THD was extremely low in the midrange (fig.7) and hardly changed with load impedance, something that I conjecture is associated with good sound quality, as the amplifier's fundamental transfer function is not being modulated by the output current. However, the THD did rise linearly as the frequency rose—which, I conjecture, suggests that the circuit has a limited open-loop bandwidth. (With a conventional amplifier circuit, there is less gain margin between open- and closed-loop conditions, hence less corrective feedback available, at higher frequencies than at lower frequencies.)

Fortunately, the XA60.5's distortion is predominantly the subjectively innocuous third-harmonic in nature at both moderate (fig.8) and high (fig.9) powers, though the second, fifth, and seventh harmonics can also be seen in fig.9, all close to the –100dB level (0.001%). With the top-octave decrease in linearity seen in fig.7, it came as no surprise to see some high-order intermodulation products that were fairly high in level when the XA60.5 was asked to drive an equal mix of 19 and 20kHz tones at a level just below visible clipping on the oscilloscope screen (fig.10). However, the subjectively more objectionable second-order product at 1kHz is almost 50dB lower, at –100dB (0.001%).

The Pass Labs XA60.5's measured performance is very similar to that of the stereo XA30.5 (see ), which was favorably reviewed for Stereophile by Brian Damkroger in May 2009 and by Erick Lichte in December 2009. Both are well engineered, and each delivered more power than its modest specification would suggest.—John Atkinson