Pass Labs XA60.5 owner manual

XA60.5 Owner’s Manual

We start with simple FET circuits already having low distortion and noise, and arrange them in two symmetrical halves. The two halves of the amplifier channel are closely matched, eliminating a large portion of distortion and noise without feedback. A small amount of feedback is also applied, not so much for the purpose of reducing distortion but to make the distortion as identical as possible on both polarities of the balanced output.

It is easier to make distortions 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 understood, 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.

X.5 and XA.5 amplifiers utilize the same basic circuit topology and gain devices. They differ in the sizes of the power supplies, the chassis and heat sinks, and the number of output devices. The output stages of the X.5 series are operated in heavily biased Class AB, with an idling dissipation similar to the rated output. The XA amplifiers operate pure Class A with idling dissipation more than twice the rated output.

Within any given chassis size, the X and XA amplifiers are designed to dissipate about the same power, and it is not surprising that this leads to two channel amplifiers with half the output power per channel of comparable mono amplifiers, and Class A amplifiers with half the output of Class AB amplifiers.

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.

3