Operating Manual - FTX-1001, FTX-1501, FTX-2001, CFT-1800 Power Amplifier
9.3Bridged 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.
