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Pass Labs X-150 Power Amplifier

The X-150 power amplifier is the stereo version of a relatively new series of Nelson Pass amps that incorporate his latest design topology, Supersymmetry.

The X-150 power amplifier is the stereo version of a relatively new series of Nelson Pass amps that incorporate his latest design topology, Supersymmetry.
Product PointsApplications: Studio monitoring

Key Features: Two-channel (150 W/channel at 8 ohms) Supersymetry topology; balanced Class A circuitry; RCA and XLR inputs; five-way binding post speaker outputs

Price: $4,000

Contact: Pass Laboratories at 530-367-3690;

The X series consists of 600 and 1,000 W monoblocks, 350 and 150 W per channel stereo units, as well as three- and five-channel amplifiers for use with surround sound.

The Supersymmetry circuit topology is said to improve the performance of a balanced amplifier by matching the characteristics of the two balanced halves. As long as distortion and noise can be made identical on the two outputs, they will not be seen at the balanced output.

The X-150 ($4,000) uses matched components in a simple balanced Class A circuit containing only two gain stages: a balanced Class A voltage stage driving a bank of 20 high-power MOSFETS operate as followers with a minimal amount of feedback around the output stage.

The X-150 accepts both single-ended and balanced inputs and is fully DC-coupled with no capacitors in the signal path. It is rated at 150 W/channel into 8 ohms and 300 W/channel into 4 ohms. The output stage is biased at an idle dissipation of 100 W/channel, operating Class A up to 50 W and Class AB at higher power levels. The heat sinks won’t burn you, but holding onto them for more than 3 seconds or so is definitely uncomfortable.

The power supply uses a high-current toroidal transformer driving the secondary output stages. Four large 31,000 uF filter capacitors are said to reduce power supply ripple to approximately 1/10 V. The front-end circuitry is powered by a separate supply, with an additional 20 VDC to drive the output stage rail-to-rail without distortion.

The appearance of the X-150 is similar to the larger amplifiers in the series. The faceplate is machined from thick aluminum stock and then anodized a light gray color. The handles are the same extrusion as used on the larger amplifiers and are anodized a darker gray. The main power switch is located on the rear of the amplifier, and a stainless steel pushbutton standby switch is located on the front panel.

Front-panel LEDs indicate the status of the amplifier. The input, output and AC connectors are located on the rear of the amplifier, with enough space between them to install even the fattest of speaker cables. Single-ended inputs are via RCA connectors, and balanced inputs are XLR connectors. Speaker connections are made via five-way binding posts. The amplifier is 19 inches wide (but not rackmountable), 7 inches high and 20 inches deep from front handle to rear handle. The X150 weighs in at approximately 70 pounds.

In use

Source equipment for the listening tests consisted of DSD masters played from hard disc through a Meitner D2A converter, as well as CDs using a Meitner Bidat D/A, each in the balanced mode. Both DACs were feeding the new EMM Labs’ Switchman Mk 2 multichannel preamp/switcher.

The balanced outputs of the Switchman fed the balanced inputs of the X-150, keeping the entire signal chain balanced right up to the SLS S8R ribbon speakers.

I played the Steely Dan Two Against Nature CD and heard things I had never heard before – the inner detail was astounding. This record sounded like it was mastered on this very monitor system.

Next I played the DSD master of the new Bob Mintzer Big Band Homage to Count Basie. Here the X-150’s finesse and detail let you get your head into the intricacies of Mintzer’s arrangements, making it possible to hear the inner harmonic voicings. A slight edge was noted on the trumpets, which is common for an amplifier with this degree of inner detail.

Dennis Erwin’s gut string acoustic bass sounded just like it did when I was standing a few feet in front the bass while he was setting up – big and fat, without any boominess. The articulation of the X-150 on the bottom end is very good and is probably due, at least in part, to the amplifier’s high damping factor.

For some time now I have been touting the advantages of powered speakers, but after listening to the X-150 for a while, I’ve changed my tune a bit. No powered speakers that I am aware of contain amplifiers in the same league, qualitywise, as the Pass Labs X-150.

Furthermore, a speaker enclosure is not an ideal environment for a sophisticated piece of electronic equipment with all the vibration and resonance. That said, passive crossovers are still a huge limitation, making it difficult to realize what a really good amplifier can do. Okay, so what I really want is a speaker with dedicated external electronics, with zero phase shift and high bit-rate digital crossovers, feeding Meitner 1-bit converters into amplifiers like the Pass X-150 – maybe then, the speaker won’t be such a weak link.


The Pass Labs X-150 stereo power amplifier is well made with the highest quality components – it is gorgeous to look at and sounds great. Unfortunately, it is a bit pricey by pro audio standards. For the pro market, it may be wise for Pass to offer the X series amplifiers without the cosmetics at a more affordable cost – I’m that sure many producers and engineers would appreciate the delicacy and detail of this amplifier.