Pass Labs XA30.5 Stereo Power Amplifier
Won me over and plan to enjoy it for years to come.
Review By Dick Olsher
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  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
At 30 wpc (into 8 Ohm) the XA30.5 is officially the baby in the XA.5 lineup which culminates with the XA200.5 monoblocks. Yet, it's my impression that papa Nelson is proudest of the little guy. And I suspect such feelings are based on subjective performance rather than measurements. Since both of us have access to high-efficiency speakers, power is not an issue. As an advocate of the First Watt concept, Nelson probably discovered that the XA30.5 delivers the best sounding first watt in the lineup. The first watt is king. It sets the stage, so who cares about the other zillion watts if they're no better than the first. The trick here is coaxing the first watt in single-ended mode. The push-pull output stage consists of n-channel MOSFETs (positive going push side) and p-channel MOSFETs (negative going pull side). According to Nelson, the n-channel is biased at a higher level than the p, so that at very low power levels the output is effectively that of the n-channel functioning in single-ended mode. At higher levels, both n and p devices contribute to the output as a push-pull pair. Lower power also means less output devices, a positive in my book. And I imagine that the Super-Symmetry circuit Pass Labs is so proud of is most proficient in eliminating noise products when the number of output devices is minimized.

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.

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.

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
Some 1970s designers, most notably Jim Bongiorno, have shunned MOSFETs and stuck with bipolar output devices over the years. Bongiorno's Dynaco Stereo 400 was quite a beast in its day, and I've come to associate its androgenic character with that of muscle amps. It sonics were drenched in testosterone, a bit gruff and grainy, it sounded like it hadn't shaved in a couple of days. Yet it could kick butt while early MOSFET designs were softer and gentler lacking fire in their belly. It may be strictly a male thing, but BNP (before Nelson Pass) my feeling was that MOSFETs were too wimpy to be taken seriously – too much yin and not enough yang. Thank you Nelson for molding the MOSFET into a Samurai-like weapon with the ability to cut through the old paradigms of solid-state amplification. The XA30.5 offers a precise blend of power and harmonic purity – a balance of male and female attributes that I find most consonant with the needs of the music.

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
I'm basically a tube guy but try to stay amplification agnostic, keeping an open mind about what sounds best in a particular application. The Pass Labs XA30.5 won me over in several system contexts and I plan to continue enjoying it for years to come. It's one of only a handful of solid-state amps with musical heart and soul. A mandatory audition for anyone shopping around for an amplifier under $10,000.

Specifications
Type: Solid-state stereo amplifier
Power Output: 30 wpc into 8 Ohm, 60 wpc into 4 Ohm
Gain: 26 dB (dB)
Sensitivity: 0.77 V
Frequency Response: 1.5 Hz to 100 kHz (-2dB)
Distortion: 0.01% at 3 watts, 0.1% at 30 watts (measured at 1 kHz) 
Input Impedance: 30 kOhm balanced, 15 kOhm unbalanced
Damping Factor: 150 for an 8 Ohm nominal load
Maximum Output Current: 23 Ampere
Maximum Output Voltage: 18 Volts
Output Noise: 150 microvolt unweighted 20 Hz - 20 kHz
DC Offset: <50 mV
Slew rate: ±50 V/µs
Dimensions:19 x 7 x 19 (WxHxD in inches) 
Weight: 60 lbs.
Price:$5500

Company Information
Pass Laboratories
13395 New Airport Road
Suite G
Auburn, CA 95602

Voice: (530) 878-5350
Fax: (530) 367-2193
E-mail: info@passlabs.com
Website: www.PassLabs.com