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INTRODUCTION
The
Ashiy
tEX-200
and
FET-500 represent an entirely
new
breed
of
power
amplifiers
which are destined to replace conventional amps in virtually every
application. Far from being just'another variation on well worn designs, these
amplifiers
have been engineered from the ground up to take full advantage
of
au
exciting
new
semiconductor
technology
which
has
recently
become
availabie_the
Power
MOS-FET
(metal-oxide-semiconductor,
field-effect-
ttausistor).
Combining
many of the desireable operating characteristics
of
vacuum
tubes with the efficiency of transistors,
the MOS-FET offers superior
audio
fidelity
and introduces an entirely new level
of
field
reliability.
Ashiy
power
amplifiers put all of these features together into
a
carefully
thought
out,
roadworthy,
user-oriented package which will provide years
of
maintenance—free service.
WHY MOS-FET'3?
coavtntiouai
power
amplifier design as typically produced
a
circuit
which
consists
of
two
parts;
1) the amplifier itself,
and 2)
a
collection
of
protective
circuits
which keep the amplifier from destroying itself and
the
speakers it is driving.
Some of the more common protective circuits
include
VI
iimitiug,
ciiermai
feedback,
detectors
that monitor DC
at
the
output
terminals,
short
circuit protection and turn-on delay circuits that keep the
speatcers from jumping out of their cabinets when power is applied.
The result
has
typically
been a compromise in
audio
performance,
greatly
compounded
electronic
and mechanical complexity,
and amplifiers which are intolerant of
less than ideal operating environments.
The
basic
weakness
of these amplifiers has always
been
the
power
output
transistors
themselves,
which
are prone to failure as a result of
"thermal
runaway .
This
breakdown occurs when the amplifier is delivering a
lot
of
power
and begins to heat up.
Increasing temperature produces an increase in
coiltator current in the output transistors, which in turn produces more heat.
If this cycle is unchecked, the transistor will be destroyed.
ProDaoiy tiic greatest advantage of power MOS-FET's for professional audio
use
is their complete immunity to thermal runaway.
Not only does this remove the
primary failure mechanism of the conventional amplifier and eliminate the need
for elaborate thermal protection circuits,
it also allows the design engineer
to
use
a
mucii
nigher quiescent current in the
amplifier's
output
stage,
thereby
insuring that the output devices are always operating in
their
most
lineal region.
This results in consistently lower distortion.
In addition, the simpler input drive requirements of the MOS-FET devices makes
possible
a
clean,
concise amplifier design.
The
resulting
reduction
in
component count and internal wiring provides for a compact design and enhanced
reliability.
THE ASHLY DESIGN
Our
goal in designing the FET—200 and FET—500 was to produce amplifiers
that
combine
the sonic excellence of a high-end stereo amp with the ruggedness and
stamina necessary in pro audio.
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