Xs Preamp Release

Xs Preamp Release

There are people who do not know that Wayne Colburn has designed the Pass Labs preamplifier products for the last 20 years.  Wayne's modesty has tended to keep him out of the spotlight, and he does not like to write, and so it falls on me to trumpet his latest achievement.  It follows in a smooth progression of successful preamplifiers and phono stages, each recognized as offering a genuine improvement in sound quality.

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.

Naturally the comparison will be made with its predecessor, the XP-30 line stage preamp, which achieved incremental performance improvements over the XP-20 and XP-10.  The most obvious difference is that the Xs comes in only two chassis, where the XP-30 had at least three chassis, one for supply and control, and one chassis per channel for as many channels as you might reasonably want.  The Xs is designed solely for two-channel operation, and it fits in two (larger) boxes.

The XP-30 had a few unique features, among them independently adjustable dual-output sets for use in bi-amping and the high-voltage output swing capable of driving unity-gain power amplifiers such as the First Watt F4.

The Xs preamp expands on these features and adds some nice new touches, such as remote control of the relative levels of the dual outputs, and absolute phase switching.

The volume controls are improved in performance with 0.5 dB steps, expanded dynamic range and lower noise.  The harmonic content of the 0.001% distortion figure of the volume control has been tweaked to favor 2nd harmonic over 3rd.  The buffer stage for the volume control has been eliminated, giving an even simpler gain path.  Improved circuit boards layout puts the cherry on top.

The gain stages still use matched complementary Toshiba JFET inputs, bipolar cascodes, and MOSFET outputs.  Pass Labs has some of the few remaining stocks of these largely unavailable parts.  Some changes have been made – the Class-A bias has been doubled, accompanied by much larger heat sinks, and the circuit will now drive any reasonable load (I have successfully used it to drive my 16 ohm Lowthers).

The circuits are now completely DC-coupled throughout (no capacitors), and the frequency response is flat to 100 Khz.

The XP-30 could swing enough voltage to drive a balanced follower output stage to 80 watts rms.  The Xs increases that figure to over 150 watts.

Some of the materials have gotten more exotic – the gain stages are mounted on ceramic circuit boards with gold plating.  The channel motherboards are made of Panasonic Megtron with immersion gold, which performs about as well as Teflon or Polyamide but without the adhesion properties.  The power supplies  use 4-ounce, plated-through high temp FR406.

While the two channels share a chassis, they are isolated on separate board systems with distance between them, so the crosstalk figures between channels are as good as the XP-30’s separate chassis.  Each channel and the digital control circuits have their own isolated supplies, each mounted on it's own board in a separate chassis.

The three separate power supplies use the lowest noise transformers available from Plitron and they incorporate EMI filters on the AC line input, secondary output, fast/soft recovery rectifiers, large storage capacitance and extensive regulation.  The active regulation is both series and shunt types, and is followed by passive filtering using polypropylene capacitors.

All this in machined aluminum cases.  If, like me, your eyesight is not what it once was, you will also appreciate the larger display on the control panel.

It does not automatically follow that more money and more exotic parts and such will result in a better sounding product.  Indeed, some of the things Wayne tried did not work as hoped, although they measured well enough.  Fortunately, there was lots of time available, and the back-and-forth process between Wayne and the four other listeners assured the finest product we could possibly make.

At this point, I can only say that if you are on a restricted budget you might be wise to avoid borrowing one of these.

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