Pass Labs XA.5 Series Amps

Sometimes you just see a very cool-looking product - or suite of products. We focused in on Pass Laboratories who introduced the impressive-looking XA.5 Series Power Amplifiers at the 2008 International Consumer Electronics Show, next month in Las Vegas. Their motto is "Designed for audiophiles by lifelong audiophiles". It's a good tagline, and the prices are unquestionably targeted towards audiophiles.

The XA.5 Series consists of five, mostly 2-channel, models — the XA200.5, the XA160.5, the XA100.5, the XA60.5 and the XA30.5 Power Amplifiers. The new models announced at CES are said to be significant improvements on their classic Pass Labs predecessors (but don't tell the owners of last year's models!)

In design terms, the XA.5 Series amps are designed to be stable regardless of speaker load. The idea is that they can handle transients commonly found in very high-quality, dynamic music and not break a sweat. These amps are made to really bring out the best in 24-bit mastered recordings. In terms of what's changed from prior models, the XA.5 Series amplifiers have tremendously larger output current capacity - more than five times the current, which is more than 25 times the power into very low impedance loads. This makes XA.5 amps entirely appropriate even for esoteric loudspeakers with very difficult loads.

The XA.5 amplifiers function via pure Class A operation. They also focus on attaining greater control over the large heavily biased follower output stages, which have low output impedance for consistently high damping under all load and signal conditions. According to Pass Labs, additional refinements include:

• Greater stability. XA.5 circuits do not compensate for any type of high frequency by using little frequency roll-offs, lag capacitors, output coils, Zobel networks, or other techniques. The amplifiers are stable because they are simple gain stages with minimal feedback.
• Lower noise. About one-tenth the noise from the audio to the radio frequency spectrum. The circuits are immune to environmental noise, and more immune than most to mechanical transformer noise from bad AC lines. (Looks like you won't require any esoteric, aftermarket power cables!)
  
• Better bias control. The amplifiers’ idling current is dramatically more constant against variations in temperature, line voltages, and signal transients. They also require less warm-up time to reach optimal performance.
• Higher efficiency and more power. The circuits operate 20 percent more efficiently, leading them to deliver approximately 20 percent more output power.
• Greater reliability. XA.5 amplifiers are harder to break, and easier to repair if they do.

For these amps Pass Labs replaced input differential MOSFETs with JFETs possessing much less noise, higher in­put impedance, and greater linearity. They added more power output devices to improve power handling and linearity. They developed a new bias generator that eliminated three sources of variation in the output stage bias current. They improved the power supplies by adding storage capacitance, larger and quieter power transformers, and fast-recovery rectifiers. They improved the wiring and circuit board layout in ways that lowered noise and distortion, and improved reliability and serviceability.

All the XA.5 amplifiers also employ the Super-Symmetry or “X” circuit developed by Nelson Pass, which improves performance by canceling distortion and noise. To the extent that distortion and noise can be made to appear identical by precisely matching characteristics of the two halves of a balanced amplifier, they will not be detected.

The New Models:

• The XA200.5 Amplifier drives 200 watts per channel at 8 ohms with a gain of 26db and 1 percent THD. Its input impedance is 30 Kohms balanced, 15 Kohms unbalanced; and its power consumption is approximately 700 watts, idle. It measures 19 x 11 x 28 inches (W/H/D), and weighs 180 Lbs.
  
• The XA160.5 Amplifier drives 160 wpc at 8 ohms with a gain of 26db and 1 percent THD. Its input impedance is 30 Kohms balanced, 15 Kohms unbalanced; and its power consumption is approximately 600 watts, idle. It measures 19 x 11 x 21.5 inches (W/H/D), and weighs 150 Lbs.
  
• The XA100.5 Amplifier drives 100 wpc at 8 ohms with a gain of 26db and 1 percent THD. Its input impedance is 30 Kohms balanced, 15 Kohms unbalanced; and its power consumption is approximately 300 watts, idle. It measures 19 x 7 x 19 inches (W/H/D), and weighs 110 Lbs.
  
• The XA60.5 Amplifier drives 60 wpc at 8 ohms with a gain of 26db and 1percent THD. Its input impedance is 30 Kohms balanced, 15 Kohms unbalanced; and its power consumption is approximately 200 watts, idle. It measures 19 x 7 x 19 inches (W/H/D), and weighs 75 Lbs.
  
• The XA30.5 Amplifier drives 30 wpc at 8 ohms with a gain of 26db and 1 percent THD. Its input impedance is 30 Kohms balanced, 15 Kohms unbalanced; and its power consumption is approximately 200 watts, idle. It measures 19 x 7 x 19 inches (W/H/D), and weighs 75 Lbs.

All five XA.5 Power Amplifiers are available now from authorized Pass Labs dealers at the following suggested prices:

For more information, please visit www.passlabs.com.

Unless otherwise indicated, this is a preview article for the featured product. A formal review may or may not follow in the future.