Pass Lab’s build amplifiers with excessive output stages, huge heat sinks and massive power supplies. Using the highest quality components in simple linear topologies, assembled and tested with great care in Auburn, California. They measure well and sound great. They are heavy, reliable and expensive.
The new XA25 offers this at slightly lower power, without frills, at a lower price. It embodies new components and innovative circuits coupled with the best elements of the previous generation. The signal path from input to output has been simplified to fewer components. Degeneration, “the other form of feedback” has been eliminated. The output stage consists of a single pair of 800 watt new generation output transistors operating push-pull Class A. The same “New Old Stock” of small power JFETs and cascoded MOSFET, in the classic “CFA” voltage gain circuit. The amplifier is still DC coupled and has no frequency compensation.
The result is faster, lower distortion, lower noise, higher damping and larger Class A operating envelope into low impedance loudspeakers, all in a simple 3 stage circuit with total of three pair of push-pull gain transistors.
In their ideal state, matched push-pull FETs give perfect “square law” cancellation of distortion, an effect somewhat spoiled by degenerative Source resistance used to constrain the “personality” of the devices. The removal of this form of feedback is an important element in the performance
of the XA25 and is accomplished by new approaches to stabilising gain and bias of push-pull FETs.
This gives a lower output impedance, more gain, and lower spectral content of distortion, as evidenced by measurements (see the specs section), but the more important factor is the difference it makes to the music.
We observed this difference some years ago in listening tests. Degeneration removed some of the organic quality to the sound and so became the subject of further development work.
The XA25 measures superbly, but it’s subjective qualities are most special. In an industry where products with great specs are frequently found to be dry and boring, this amplifier breathes life into the music.