Pass XA25, Руководство пользователя, Страница: 3 / 8

INTRODUCTION
Entering their 22nd year of production, the X and XA power amplifiers have remained at the
cutting edge of audio through continuous innovation and refinement. 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. The 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 genera-
tion.
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. We still use 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 “personali-
ty” of the devices. The removal of this form of feedback is an important element in the per-
formance 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 evi-
denced 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.
Thank you for purchasing this product. Please enjoy.
Nelson Pass 12/20/16