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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.