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About Pass
Pass Laboratories, Inc. was founded in 1991 by Nelson Pass. Operating out of his shop at home, Pass developed prototypes of a single-ended Class A amplifier design. At the same time, he and Mike Burley were constructing an in-house machine shop with three home-built CNC milling machines with which to produce the amplifiers when the design was finished.

The first product, the Aleph 0, shipped in 1992. It was a mono-block design rated at 70 watts into 8 ohms, and was unique in using three gain stages of power Mosfets including a push-pull output stage biased by a big constant current source. The amplifier operated as single-ended Class A up to its 70 watt rating and in push pull Class A at higher wattage.

The amplifier was highly acclaimed, and was called the “Amplifier of the Decade” in Sterephile magazine.

At this time, Nelson hired his next-door neighbor, Elena Tong, to help with producing the amplifiers, and she manages much of the company to this day.

A stereo version called the Aleph 0s followed after, along with a preamp called the Aleph P. A larger version of the Aleph 0 called the Aleph 1 shipped the following year, and was a mono-block rated at 150 watts.

In 1994 Pass Labs released a newer Aleph design which simplified the circuit from three gain stages down to only two stages and used a newly patented power current source to bias the single-ended output stage. The new circuitry was incorporated into a revision of the Aleph 1, the 200 watt Aleph 1.2, and smaller models, the 100 watt Aleph 2.

In 1994 Nelson was joined by Wayne Colburn, who began working on improved version of the preamp and on a new phono stage. The company received two patents this year, one of them the design that became the SuperSymmetric circuit in use today.

In 1995, the 30 watt Aleph 3 and Aleph L preamp were released. Also in 1995 the Joseph Sammut left his position at Krell and became operations chief at Pass labs. The following year he was followed by designer Desmond Harrington.

In 1997 Wayne Colburn’s efforts bore fruit in the revised Aleph P, the Aleph Ono phono stage, and the D1 digital to analog converter.

By 1995 the company had outgrown Nelson’s buildings, and building was begun in a new location. In 1997 Pass Labs moved out of Nelson’s shop space and into a new facility in Foresthill. By then, even more space was needed, and was leased for storage elsewhere. Nelson’s original shop continues to house research.

In 1998 the company released the X1000, the first amplifier using the patented SuperSymmetric topology. The X1000 achieved 1000 watts with high performance, local feedback and only two gain stages: a cascoded differential pair of transistors and a large bank of complementary output followers. This ground breaking design has formed the amplifier products from Pass Labs to the present day.

The X1000 also introduced Desmond Harrington’s signature mechanical design work. The X1000 was followed by the mono-block X600, the stereo X350, and the stereo X250 and X150 models.

In 1998 and 1999, Wayne’s preamplifiers X0, X1, and Xono were released, and Joe Sammut became the President of Pass Labs, freeing Nelson to devote all his time to product.

In 2001 Pass Labs shipped the XVR1 active crossover and X2.5 preamplifier.

In 2002 the first XA amplifiers were released, combining the X and Aleph topologies into balanced single-ended Class A amplifiers with many of the characteristics of both the X and Aleph series. The XA200 was rated at 200 watts mono, the XA160 was 160 watts.

In 2003 the company produced the X150.5, the first major improvement of the X150 amplifier, and followed it with “.5” revisions to the other X amplifiers through 2003 and 2005.

In 2007 Pass Labs is applying the “.5” advances to the XA series of amplifiers, with the XA100.5, XA60.6, and XA30.5 shipping as of this writing. These revisions dramatically lower distortion and noise and also dramatically improved the performance of the amplifiers into low impedance loads.

Also in 2007, Desmond Harrington assumed responsibility as President of the company. Nelson and Wayne continue development work, Joe manages sales and marketing, and Elena and Kent English manage the rest of the company. All these people still like each other, and sit in the same office everyday. This way they can all can hear everything that’s going on, laughing about most of it.

For The True

These are Pass Laboratories ultimate expression of Power Amplifiers the perfect blend of science and visceral emotion. The successor to our already lavishly praised and well regarded Supersymmetry patent. Every technical effort has been made to assure exceptional specifications, among these:
  • Separate chassis for lower electromagnetic noise
  • Power supplies with greatly enhanced storage capacitance
  • Banks of redundantly parallel high speed / soft recovery rectifiers
  • Improved high frequency noise filters
  • Larger / quieter transformers
  • Improved passive decoupling
  • Lower power standby
  • Input impedances of 200 k-ohm balanced with negligible capacitance
These amplifiers will drive any load, while being driven from any source. If there’s a more emotionally satisfying amplifier anywhere, we’re not aware of it.
Point 8

For many years there has been considerable faith that if we simply keep improving the measurements of components such as amplifiers then they will sound better. Initially this was truly the case – equipment was sufficiently flawed from an objective standpoint that better measurements matched up with subjective experience.

At some level of objective quality there started to be a disconnect, and some audiophiles began to lose the faith. One of the responses to this was to examine more exotic sources of distortion in the equipment while some others simply worked to continue to reduce the flaws that were already understood. There's no doubt that some real progress resulted from these efforts, and now you can purchase products at reasonable prices which measure far better than the old stuff.

But the disconnect between the customer's perceptions and the measurements persists, and there have been cases of state-of-the-art engineering resulting in economic failure, apparently because people didn't care for the sound.

Well, of course you are dealing with people, and that will complicate any endeavor. The customer wants what the customer wants. I have heard arguments that audiophiles are irrational, that decisions are based on appearance or cost or advertising. Certainly there is plenty of that, and there have been plenty of blind tests that have demonstrated that “audiophiles can't hear the difference”, at least in the context of that test.

But I don't think that's the whole story. My experience is that under the right conditions the customer can often hear the difference, and his observations are not to be ignored.

First, it has to be acknowledged that the science of cognitive perception is still in its infancy, and the ear is not really a microphone and the brain is not a tape recorder. The data we do have on this subject is a little like quantum mechanics – we have to change our view to make sense of the illusions and paradoxes that accompany the interpretation of sensory input.

With regard to amplifiers at least, I think we already have the measurement data in hand. The problem is in our failure to interpret it with respect to human perception.

This puts some emphasis back on listening tests - extensive long-term tests with reliable listeners and familiar equipment and environments. This is not a cheap and easy procedure. Even assuming that the listener(s) really can hear, we still have the vagaries of individual taste, and not a large population of qualified listeners and systems.

In the end, even if we can design around the perceptions of a small qualified listening panel, we are still inevitably aiming at a minority audience in the real world. That's OK with me – One percent of this market is still a lot of amplifiers.

Point 5

When introduced, the XA.5 series incorporated all of the improvements listed below and also operate pure Class A. The distortion is even lower, the control is even better.

More importantly, the XA.5 amplifiers have tremendously larger output current capacity over their corresponding XA models – more than five times the current – which is more than 25 times the power into very low impedance loads. Very difficult loudspeakers are now very appropriate loads for XA.5’s.

Sonically the result is:

Greater power and control – No matter what your loudspeaker, the X.5 and XA.5 power amplifiers have the power and stability essential to deliver the music without strain. They have the dynamic range to do justice to the 24 bit recordings of the 21st Century and the elegance to retrieve the micro dynamics offered by the most sensitive loudspeakers. This simplicity and power moves easily from total silence to explosive transients with ease and without information loss.

More music - These are not simply good high power amplifiers. They have been designed by lifelong audiophiles to deliver the warmth, sweetness, depth and space that are the hallmarks of fine audio products. Most importantly in this regard, the X.5 and XA.5 amplifiers are significant improvements on their classic predecessors.

The next generation of X series amplifiers: the Pass X.5 series.

Pass Labs continues to improve its power amplifiers through on going R&D. After five years, these improvements acquired enough mass to warrant a change in designation, and in 2003 the X.5 series was announced. The improvements are:

Cascode JFET Symmetric front end – input differential MOSFETs have been replaced by the same JFETs we use for the input devices on our MM and MC phono stages. They have very much less noise, higher input impedance, and greater linearity. They operate self-biased in a symmetric quad topology, each device cascoded and level shifted to form the full voltage gain stage of the amplifier.

More power output devices – Greater quantities of matched complementary power Mosfets operated as followers, giving more current and power handling, and also improved linearity.

Single-ended Class A bias – The output stage not only uses a heavy bias current for push-pull Class A operation to a large portion of the rated power, but also has a single-ended bias source for single-ended Class A at low wattage.

New bias circuit – A newly developed bias generator has eliminated three sources of variation in bias current in the output stage. It also allows approximately 20% greater power output for a given supply voltage.

Lower distortion – The X.5 circuits achieve lower distortion at all power levels and into all types of loads, particularly in the all-important audio midrange.

Greater reliability – These amplifiers are harder to break, and are easier to repair if they do.

Pre Amplifiers
Xs Preamps
When we began development of the Xs power amplifier five years ago, we knew that we would also need a companion piece. Of course, back then we thought we could have this done in a year or so... Wayne was given carte blanche on the design and an unlimited budget, and as progress on the amplifier was very slow, he had lots of time to acquire and play with many exotic parts and materials. And spend money. The result so far is the Xs Preamp which will be followed by the Xs Phono in 2014.
XP Line Level
The Pass Laboratories XP-10, XP-20 and XP-30 preamplifiers are the natural descendants to the long lived, highly acclaimed and successful X series of Pass Laboratories line-stages.

The X series pre-amplifiers received many highly respected and well-deserved accolades over the years…. justifiably so. They provide a solid foundation for development of the XP series line-stage preamplifiers.

The XP preamplifiers sound more dynamic, bass lines are more clearly, cleanly and accurately delineated; instruments and voice are presented in a greatly refined spatial envelope and with better spectral balance. Fine nuances and detail, now become apparent even in loud and highly complex musical passages.

These are simply the best pre-amplifiers Pass Laboratories has ever built! Audition the new Pass Labs preamplifiers and rekindle your love affair with music!
XP Phono
Since 1997 Pass Laboratories has been respected for building some the finest phone stages in the audio industry. The transition from Aleph Ono, through X-0no and now to the XP-15 and XP-25 represents a sophistication and maturation of both thinking and component evolution.

Each new design of that evolution has brought a steady improvement in measured metrics such as diminished background noise and has contributed to a sonic image presented with improved resolution and additional sonic ease.

The popular advent of the moving coil cartridge demanded all this as well as higher available gain.

Each successive generation of Pass Laboratories phono-stage has addressed the special needs of these cartridges, while maintaining the ability to bring out the best in more conventional high output moving magnet cartridges as well.

The single chassis XP-15 easily bests the sonic performance of it’s predecessor the X-ono.

The twin chassis XP-25 eclipses the already exceptional performance of the XP-15. The XP-25 boasts adjustable gain, two separate inputs, a low cut filter, mute feature, resistive and capacitive loading all on the front panel for easy access.

With the XP-25 you will discover a whole new dimension of dynamics, inner detail and spectral richness.
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