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Operating Manual - FTX-1001, FTX-1501, FTX-2001, CFT-1800 Power Amplifier
9.3 Bridged Mono Operation
(Normal/bridging switch in, channel 1 input used, channel 1 level control used, BRIDGED output used)
10. DESIGN THEORY
Ashly FTX Series III amplifier circuits are totally
discrete using high-voltage, wide bandwidth electronics.
The input section is a complimentary Darlington differen-
tial amplifier stage for high common-mode rejection, low
noise and low distortion. All voltage gain stages in the
amplifier operate class A for low distortion. The compli-
mentary MOSFET output stage has almost infinite cur-
rent gain, therefore the output driver stages are nearly
lossless. All circuitry within the feedback loop is discrete
which allows the amplifier to recover instantly and cleanly
without any spikes or glitches from a clipped condition.
This overall design approach inherently assures low noise,
low distortion, and excellent transient and overload re-
sponse. Inputs are bridging and active balanced, and all
input connections can operate as balanced or unbalanced.
The MOSFET output devices themselves do not require
short circuit protection and V.I. limiting circuitry; how-
ever, output relays are employed for turn-on delay and
speaker protection from fault conditions. The output re-
lays will open if DC voltage, ultrasonic signals, a shorted
output, or an internal amplifier failure is detected. Any of
these fault conditions will illuminate the protect mode
LEDs on the front panel. Each amplifier channel operates
completely independent of the other so that one channel
in protect mode will not affect the other channel. Designed
into the FTX Series III and CFT amplifiers is the hard-
ware necessary for computer control and monitoring as
such systems become available. All electronic components
are mounted to rugged plated-through glass-epoxy circuit
boards. Large computer-grade electrolytic capacitors as-
sure long service.
The FTX Series III amplifiers are packaged in a
rugged 16-gauge steel chasses. The power transformer is
located at the front center of the chassis to minimize front
panel torsion. Each channel’s electronics and heat sink
are identical yet completely independent and modular,
each with a single plug-in connector for ease of servic-
ing. Large custom black-anodized extruded aluminum
heat sinks with fan cooling provide plenty of thermal head-
room.
The CFT-1800 utilizes quiet convection cooling
for studio control rooms, home hi-fi, and other applica-
tions where low ambient noise is essential. Like the FTX
Series III, the CFT-1800 is a modular design for ease of
servicing. Because of the passive cooling in the CFT-
1800, thermal headroom is somewhat reduced from that
of forced air cooling, thus making it inadvisable to drive
4 ohm loads at continuous high power with this ampli-
fier. It is also important to allow for free air circulation
above and below the amplifier to achieve optimum cool-
ing action.