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Introduction:
Entering their 22nd year of production, the X and XA power amplifi ers have remained
at the cutting edge of audio through continuous innovation and refi nement. At
Pass we build amplifi ers with excessive output stages, huge heat sinks and massive
power supplies. We use 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 simplifi ed 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. We still use the same “New Old Stock” of small power JFETs and
cascoded MOSFET, in the classic “CFA” voltage gain circuit. The amplifi er 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 stabilizing 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 its subjective qualities are most special. In an
industry where products with great specs are frequently found to be dry and boring,
this amplifi er breathes life into the music.
Thank you for purchasing this product. Please enjoy.
Nelson Pass 12/20/16
