the distortion as identical as possible on both polarities of the balanced output.
It is easier to make distortions in the two halves more identical than to remove them with feedback, and this is the operating principle of X amplifiers.
Since the X1000, Pass Labs products have continuously evolved toward higher performance. The Class AB “X” series was joined by the Class A “XA” amplifiers. Subsequent improvements to the Class AB “X” amplifiers resulted in the “X.5” products, and their phenomenal success has led to the Class A “XA.5” series.
As always the goal has been the best musical and objective performance possible with minimal parts in the signal path and minimal feedback. This process is not completely quantified, and many of the improvements are the result of trial and error and extensive listening.
Measurement of performance is important to us, and we feel that well- designed product sounds good and measures good. Apparently it is possible to make an amplifier which measures well but which does not please the ears, and so we let our ears be the final judge.
The X amplifiers deliver more output power per dollar, and the XA amplifiers have a better subjective performance. All of the amplifiers drive known loudspeaker loads without misbehaving, and all are unconditionally stable into low impedance and reactive loads.
All the usual performance metrics - power, distortion, noise, input impedance, damping factor and bandwidth have all been improved in this latest generation. The power supplies are larger, with fast rectifiers gating to larger capacitor banks. The increases in AC line noise worldwide have been addressed with heavier EMI filtering and dramatically quieter power transformers. In addition, through improved biasing regulation, the circuits give much more consistent performance under varying AC line voltages and over a variety of ambient temperatures.
All this is very nice, but only if it improves your listening experience. We listen very carefully and critically to our amplifiers with a wide variety of associated equipment and we take our time with it. Musical performance is most important, and is the basis of our success over the years.
It’s true that we also value the other things. The design and manufacturing decisions have to balance subjective performance against reliability, cost, specs, and ego (in approximately that order). That is why we make more than one amplifier, but all are built with the same commitment to quality.
The Pass Laboratories
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