Amplifier Reviews

Pass Labs Xs Preamplifier and Xs 300 Pure Class A Monoblock Power Amplifiers


The Design

Pass Labs has long been known for following their own path in designing preamplifiers and power amplifiers. Instead of just attempting to have the lowest distortion possible, they design by listening and let the spectra fall where they may.

The Xs series, which has just started shipping, was designed not only in that manner, but there were no budget constraints as to what components would be inside, or how many prototypes they would build before settling on the final circuits. This has been going on for at least a year, and my guess, is about 1 million dollars were spent in development.

The results are not simply a benchmark, but a landmark in design and sound quality.

I have never heard anything like these two products. Normally, I would divide the review into one for the preamplifier and one for the monoblock power amplifiers, but they were designed for each other, and that is the way I will review them.

The Xs Preamplifier is exquisite, to put it in one simple word. The industrial design (the looks) is superb, and the electronic circuit is a reflection. Two gain stages, biased into Pure Class A drive each channel of the dual mono configuration. The control circuit is in the top chassis, and the gain stages are in the bottom chassis.

There is a total of 200,000 µF of power supply capacitance, more than some power amplifiers have. As you will see in the bench tests, this preamplifier will drive any load, and the resulting spectrum is astonishing.

The front panel is spartan, with a Mute button, Mode (Panel Brightness, or Off), Input Selector buttons, Remote Control IR window, and the Volume Control (Spans - 89 dB to + 10 dB). The volume control is slightly offset with respect to the horizontal line of the top of the chassis. Very elegant.

The rear panel is full, and this is no small preamp chassis. Each input and output has a choice of XLR balanced or RCA single-ended connectors. The Outputs allow a Slave to follow a Master, and the Slave can be configured to have a different output level than the Master. This would allow the Slave to be used for bi-amping, where the second amplifier has a different input sensitivity, or its speaker has a different sensitivity. Very unique and very flexible.


The two chassis are connected together by two umbilical cords, one for each channel.

The remote control is the same as for other Pass Labs products. Solid metal, easy to use.

The Xs 300 is a massive beast at nearly 300 pounds for the two chassis. There are three gain stages. The input consists of a cascoded quad differential set of Toshiba Jfets, along with a quad set of Toshiba Mosfets that are part of the first and second stages. This provides the voltage gain for the balanced push-pull follower output stage, partially biased by single-ended constant current sources (the current sources are located in the bottom chassis). The output stage has 10 dB of negative feedback (Loop).

There are 36 pairs of transistors in the output stage (18 devices for each of the two push-pull phases), and 40 devices in the current sources (located in the bottom chassis). So, that is 112 transistors for a 300 watt amplifier. The current sources in the output stage are constant current sources, used solely to bias up the output stage with a single-ended character. The amplifier itself is a voltage source design.

A mass market power amplifier of 100 watts per channel might have 6 output transistors in each amplifier channel.

The front panel of the Xs 300 has an on/off button and a meter that reads straight up as long as the power output does not go above 300 watts into 8 ohms. If you are playing music loud, and which has extreme dynamic transients, the needle moves to the right, indicating that the amplifier is now into Class B output (600 watts peak). The amp is specified to output 600 watts into 4 ohms, but only the first 300 watts are Class A.


Here is a photo of the two Xs 300's placed between my speakers.


The Xs 300's rear panel has the umbilical connectors, an on/off toggle (when it is on, the unit is in 0.5 watt standby power - the front panel on/off button turns the amplifier on for use).  There are two sets of speaker binding posts, the most beautiful design I have ever seen. The opening into which you can insert banana plugs has a rubber gasket that keeps dirt from getting into the binding post. The post has a rubber covered surface for ease of gripping and turning, and it will tighten up to a certain point and then it will only make a clicking sound if you try to tighten it further (i.e., over-tightening the post is not possible).