Power Amplifiers

Pass Labs XA30.5 Stereo Power Amplifier

ARTICLE INDEX
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Introduction

What do a 30 watt stereo solid-state amplifier and a 500 watt solid-state monoblock have in common? A lot, it turns out. The seemingly diminutive Pass Labs XA30.5 weighs 75 pounds, has an enormous toroidal power transformer, and uses 20 output transistors to deliver 30 watts of pure class A power. It turns out these specs are similar, or in some cases, better than my recently reviewed Emotiva XPA-1 monoblocks. The XA30.5 proves that rated power is by no means everything. In fact, if you have speakers of even moderately high sensitivity, the XA30.5 renders any discussion of power output irrelevant. When paired with the right speakers, this may be one of the best amplifiers there is, regardless of price.

Specifications

  • Design: Two Channel Power Amplifier
  • Power Output: 30 Watts RMS x 2 into 8 Ohms, Class A
  • MFR: 1.5 Hz - 100 kHz
  • Sensitivity: 0.77 V for Full Output
  • THD+N: 1% at 1 kHz, Full Output
  • Input Impedance: 30 kOhms
  • Damping Factor: 150
  • Slew Rate: 50 V/uS
  • Power Consumption: 200 Watts
  • Dimensions: 7" H x 19" W x 19" D
  • Weight: 75 Pounds
  • MSRP: $5,500
  • Pass Labs

Nelson Pass is a legend in the field of audio electronics. He started Threshold in the 1970's and has been responsible for dozens of landmark audio amplifiers since then. He holds over half a dozen patents for audio amplifier circuits. He also is active in the DIY audio arena, publishing many of his designs for audio hobbyists to build on their own. Pass has always been a proponent of class-A amplifiers, where the bias current through the output device is high enough that the transistor is always conducting. These designs operate the transistors in their optimal, most linear operating regime, but require a tremendous amount of power and dissipate a large amount of heat for a given power output. This means the whole amplifier has to be massively overbuilt as opposed to a typical class AB design where a much smaller bias current is used.

In a sense, a class AB amplifier and a class A push-pull amplifier like the XA30.5 are the same. A class AB amp is designed to operate in class A for the first few watts of output power, then transition to class B where one transistor handles the positive part of the waveform, and another the negative. The "class A" portion is the small overlap between the two. Class A amplifiers, if of a push-pull design with a pair of transistors (or several pairs) handling the positive and negative portions of the waveform, simply have a much larger overlap region. They're designed to operate with both transistors fully capable of amplifying the entire signal, both positive and negative voltage swings, over a large part of their power output.

Pass Labs, the latest company headed by Nelson Pass, has built both single ended class A amplifers and push-pull amplifers of both high bias class AB design and class A design (similar except for the level of transistor bias). The single ended amps begin to clip when they reach their class A limit, but the push pull class A amps can keep going. Pass Labs currently offers both a series of high bias class AB designs (the X series, with rated powers of 150W to 1000W per channel) and the XA series amps, which deliver their rated power output entirely in class A.

Both series of amps make use of at least three of Nelson Pass' patents. Both use the "supersymmetry" input state, the "Aleph" output stage and a single ended class A constant current bias supply. The supersymmetry input state is a patented fully differential amplifier topology that uses the cancellation offered by the balanced design to eliminate the need for large amounts of feedback to reduce noise and maintain stability. This is a key Pass technology that allows the design of low feedback amps with lots of power. The supersymmetry input stage drives a pair of Aleph output stages, one for each leg of the balanced signal. Each Aleph output stage also has a single ended constant current bias supply, so that the first watt or so of output power is delivered in single ended class A. This topology is an absolutely zero compromise approach to audio amplifier design. All of Pass' current designs use this topology. They differ in the number of output transistors needed to achieve the rated power output, and the amount of bias on the transistors.

The XA amps are all biased to dissipate three times their rated power output at idle. The X series amps are biased to dissipate about 1.5 times their rated power output. All XA amps are rated by the power they deliver into class A, not their power at 1% THD. The XA amps all deliver about 20% the power of the similarly sized X series amp (30W vs. 150W, 60W vs. 300W all the way up to 200W vs. 1000W. This is somewhat misleading, though as all the XA amps will deliver about three times their class A power in class AB mode at 1% THD. The XA30.5 is the smallest of the XA series amps, and is outwardly very much the same as the X150.5 amp. The XA30.5 is biased to deliver 30W of class A power, while the X150.5 is biased to deliver the first 5W RMS in class A. The XA30.5 (as we'll later see) hits the 1% THD point at 100W RMS, while the X150.5 can make it to 150W RMS. In this sense you trade more class A power for less maximum power.

One point here that Nelson Pass made when I spoke with him at the beginning of the review: There is no "clunk" or any other audible sign when any of these amps transition from class A to class AB. It is a completely seamless transition both in terms of measurement and audibility. Once the amp does transition into class AB, the distortion spectrum will begin to change subtly, with higher order distortion harmonics beginning to increase. Even this change is not sudden, though. In all cases, the Pass low feedback designs have their distortion biased toward low order harmonics. When operating in class A, high order distortion is further reduced. Unlike many high feedback amps, the distortion versus power output curve rises very gently as the 1% THD point is reached, rather than rocketing up as the amp begins to clip like with high feedback designs. This allows Pass amps to operate much closer to their rated power output while maintaining excellent sound quality.