Power Amplifiers

Pass Labs XA-100.5 Pure Class A Monoblock Power Amplifier


The Design

The XA-100.5 is as unique in appearance as it is in sound. The front panel has a standby/on switch (main on/off toggle is on the rear panel), and a meter that reads straight up when the amplifier is on. This is a measure of the current flowing, not watts. It indicates that the amplifier is drawing current even though you may not be playing any music. As long as the meter points straight up, you are running in Class A. If you are playing music loud, and there is a demanding transient, the meter will move a bit to the right, indicating Class AB operation (up to 175 watts output into 8 ohms).

The rear panel has a pair of handles to lift the amplifier with (I would seriously suggest two people), an XLR balanced and RCA unbalanced input (pins 1 and 3 are shorted with a U pin that you remove if you want to use the XLR jack), two sets of speaker binding posts, a 12 volt trigger connection, and a grounding post. The amplifier acts as a bridged circuit, so both speaker jacks are hot, and you should never ground the - post. A diagram of the output is shown below.


The heat sink fins are mounted in an unusual manner, horizontally, angled upward. More typical fins are mounted vertically, which act like chimneys for the heat.

The amplifier uses 40 Mosfet output devices (transistors). That's a lot of output devices for just 100 watts. In a mass market receiver, you might find its 100 watt power amplifier (one of the five or seven channels) having only 4 output devices.

Most power amplifiers these days use bipolar output transistors, and there is a long story as to why Nelson Pass uses Mosfets, which you can read in some of his white papers. They are well written and easy to understand, so I recommend them even if you don't care why he uses Mosfets, because there are good explanations of Class A, distortion, negative feedback, and other things.

There are only two gain stages in the XA-100.5 and there is no global negative feedback. Pass operates under the premise that the less gain stages, the better, because each gain stage adds distortion, and then one usually adds negative feedback to reduce that distortion. Pass also designs the circuits to produce mostly odd-ordered distortion (predominantly 3rd order) rather than even-ordered distortion. This probably surprises most of you. It surprised me. But, it turns out that when double blind studies are performed, using amplifiers that produce either mostly 2nd order or 3rd order distortion, about 1/3 prefer 2nd order, 1/3 prefer 3rd order, and the rest either like neither or both.

Here is a photo of the inside of the chassis. You can see all the output devices lined up against the inside of the heat sinks on the sides.