Carver PM-900
A pro-rack workhorse whose power transformer only wakes up when the music demands it.
Overview
The PM-900 is a professional amplifier built around Carver's patented Magnetic Field Amplifier circuitry, and the power supply is the whole story. A conventional amplifier keeps its power transformer energized the entire time it's switched on. The PM-900 puts its transformer inside a voltage regulator's feedback loop, so it's only fully on when the amplifier needs full power output. The rest of the time it does just enough work to keep the main filter capacitors charged to plus and minus 97 volts. That one design decision explains the small size and light weight. The transformer can be much smaller because it isn't being asked to loaf at full mains potential all day.
Behind that supply sits a triple-rail output stage. By minimizing the voltage dropped across each output transistor, the design cuts heat dissipation, which lets Carver shrink the heat sinks. The magnetic field supply and the high-efficiency output stage together give the PM-900 its high power-to-weight ratio. That counts for something when you're loading a rack into a truck.
The feature set reads like a tour amp and an install amp got together. The period trade press noted that flexibility - Sound and Communication (August 1990, p.27) grouped dual-channel and low-frequency designs among the amplifier categories installers were shopping for that year. There's direct 70V output capability. In bridging mode the PM-900 produces enough output voltage to drive a 70-volt distribution system without a step-up transformer at the amplifier, delivering 625 watts to the system. (Transformers are still required at each loudspeaker, as with any 70V system, and recalculating the tap ratings on those step-down transformers lets the amp deliver additional power beyond 625 watts.) There's a clipping eliminator that senses the onset of clipping and pulls the input level down to hold distortion under one percent THD with up to 8 dB of overdrive. It's a limiter in spirit, and it responds to clipping from any cause: excessive input drive, power line sag, low load impedances. And there's remote/manual sequential power on/off, so an entire rack of Carver amplifiers can be brought up in sequence without tripping breakers or causing surges.
The front panel carries 11-step left and right level attenuators, marked in dB, plus a seven-LED-per-channel output status display. Green means power-on at the bottom, five red LEDs show output power level in dB relative to maximum, and yellow LEDs at the top show the onset of clipping and double as protection indicators. Around back you get parallel-connected barrier strip terminals, female XLR connectors, and 1/4-inch tip-ring-sleeve jacks for inputs. The speakers get 5-way binding posts. There's a ground-lift jumper to break ground loops, and the sequencer terminals - a RECEIVE terminal that accepts +5 to +15 volts DC to turn the amp on, and a SEND terminal that passes +11.4 volts to the next amplifier in the chain. A removable accessory panel hides the clipping-eliminator and mono-mode push switches and is meant for Carver's add-on modules.
Carver's own literature names the PMX 2-way Electronic Crossover, the PG-2 Dual-Zone Paging Module, and the PMT-2 Balanced Input as available add-ons, along with external 70.7-volt transformers. In mono mode the amp uses only the LEFT input connector and level control, and Carver specifies that mode for 8-ohm-or-greater loads and for 70V direct operation.
The protection package is thorough. The PM-900 includes input RFI filtering, power line filtering, load protection from DC output, amplifier protection from sustained current limiting caused by severe overdrive or abnormally low load impedances, and thermal overload protection. The CLIP/PROTECT LEDs light when any of it engages. Carver's manual is candid about what distortion usually means in practice: excessive loss in the input controls when the preceding mixer, equalizer, or crossover can't produce enough output, overdriving that results in output clipping, or current limiting from excessively low load impedances. If the yellow protection LED is lit, the manual points first at speaker connections. Stray wire strands shorting across terminals are the classic culprit.
The 10-ampere slow-blow fuse on the AC line is a Schurter SPT 001.2534, Littlefuse 3AB 326010, or Bussman MDA10 for 120V operation, according to Carver. Repeated fuse blowing is a sign of internal trouble, not a reason to try a bigger fuse.
The PM-900's manual is a service-oriented document, and its specification section says only that features and specifications are subject to change without notice. The electrical figures aren't reproduced in the excerpts available here. Power output beyond the 625-watt 70V figure, frequency response, THD at rated power, damping factor, and the amp's physical dimensions and weight are not documented in the material on hand. Treat any number quoted elsewhere with suspicion until it's checked against a factory spec sheet for the exact revision.
The manual does make clear who the amp was for. Bob Carver's introduction describes the PM-900 as ideal for touring and permanent installations thanks to its lightweight, rugged construction, and pitches the growing range of add-on modules as the path to versatility. It was built for people who move amplifiers and need them to survive it: sequenced power-up, transformerless 70V drive, a limiter that saves high-frequency drivers, and protection circuits that assume the load will misbehave eventually.
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