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February / March 2010
Pass Labs XA30.5 Stereo Power Amplifier
The XA.5 series of power amplifiers, released in late 2008, represents Pass Labs' new flagship line of power amplification. The common denominator is pure Class A operation. Significant enhancements are said to have been implemented, including a cascode JFET symmetric front end, single-ended Class A bias at low wattage, improved push-pull bias circuits and output stage topology, more MOSFET output devices, bigger power supplies and better circuit layout. The Super-Symmetry circuit, the star of the X series has been retained, and the cosmetics are similar. I have to admit that the metered fascia of the XA30.5 gives me a reassuring impression of strength and elegance. But it can't entirely hide a bevy of heat sinks which run toasty warm (around 50 degrees C) as the amp dissipates about 225 watts.
Technical Details But this amp is far from being a wimp. It can generate almost 60 watts peak in pure class A and is capable of being driven hard well beyond 100 watts peak as it shifts into Class AB operation. As there are so many loudspeakers on the market with impedance minima around 3 to 4 Ohm, it's good to know that the XA30.5 doubles its power output into a 4 Ohm load; 60 wpc of pure Class A is substantial. There was a time when 15 wpc was considered ideal for domestic use. Of course, that was before the advent of inefficient bookshelf speakers in the mid 1950s. So exactly what sort of loudspeaker sensitivity would the XA30.5 be comfortable with? For various reasons this review had an unusually long gestation period. The positive aspect of that was the additional time to experiment with a variety of speaker loads.
A couple more details. You'll note an AC power switch on the back plate and a switch on the front plate that toggles the amp from Standby mode to Operate mode. The intent is to leave the amp powered in Standby mode while not in use. For best sound, it needs to warm up until the heat sink temperatures are hot to the touch and that may take about an hour. It's surprising how much more sweetly this amp sings when it's fully cooking at between 50 and 55 degrees C.
Sonic Impressions Let me spell it out: this is not the sort of amplifier to impose a particular personality over the soundstage; it maintains a neutral balance. Yet is reveals itself as being in the room Zen-like - by the things it does not do. It does not limit bandwidth, obscure detail, veil the soundstage, or mellow harmonic textures. It's possible to change the subjective balance by simply limiting an amplifier's bandwidth. Transformer-coupled single-ended triode amps are very good at that. With a power bandwidth of maybe 20 kHz, harmonic textures turn soft and liquid, and the tonal center of gravity is shifted toward the midrange. Instead, the XA30.5 dazzled with its transient speed a function of its wide bandwidth and excellent slew rate. A high damping factor was responsible for marvelous bass control. And the soundstage remained transparent and cohesive. Transparency, the ability to see the inner recesses of the soundstage is a very good thing, but just as important and much more difficult to pull off is cohesiveness. Pulling the left, right and center regions of the soundstage into a convincing organic whole is something few amps can do well. The XA30.5 was able to convincingly "connect the dots" thus joining a select company. However, image outlines were not as palpable as with the Audio Space Reference 3.1 (300B) integrated power amplifier which is quite a magical amplifier in its own right. Fully fleshed out image outlines seems to be a tube phenomenon. Tube based power amps are better at this than solid-state designs and the explanation may well lie with inherent differences in distortion spectra. I would be fine accepting tube distortion as the perceptual mechanism for a more spatial delineation of the soundstage. What matters to me the most is reproducing the most convincing illusion of the real thing, and in this case the means justify the end. To paraphrase the late Jason Bloom of Apogee Acoustics fame, I don't care if the amplifying devices are even made of Swiss cheese! For me, in addition to tonal weight, major realism triggers are timbre accuracy, textural purity, low distortion and unbridled musical drama. The Pass amp scored high in each of these categories. Distortion products were kept in check even when it was driven hard. There was no grit, grain, or electronic glaze to interfere with the ebb and flow of harmonic textures. Microdynamics together with pitch and rhythmic inflections constitute the music's emotional foundation. J Gordon Holt's famous goose bump test was about being startled by nuances, dramatic tension bubbling beneath the surface. It was not about mindless audiophile delights such as being able to reproduce the 1812 Overture's cannon shots at gut shaking volume levels. With the Pass amp the magic was in the whispers.
Conclusion
Specifications
Company Information
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The
XA30.5 loved my BassZilla Platinum Edition Mk2 DIY speakers. The BassZilla's
in-room sensitivity of about 97 dB makes 30 wpc into 8 Ohm look like a
colossal power reserve. It was also evident at the other sensitivity
extreme that the Analysis Audio Omega planar/ribbon speaker at a
sensitivity of 86dB/W/m is probably already a tad below a reasonable lower
threshold. The Omega is known to be a current hog, and while the playback
of baroque music did not present any particular problem, there were
occasions with more dynamic source material when I wished for more
headroom. On the other hand, the Reference 3A Episode, rated at 91dB/W/m,
was more than satisfied by the XA30.5 in my medium-sized listening room.
My guidance is to stick with a speaker sensitivity of at least 89 dB in
order to maintain sufficient headroom.
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