Ashly FET-200 - Introduction

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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