Soundcraftsmen PM840
A 22-pound MOSFET power amp that claims 50 amps of peak current for $499 - the number that made dealers sit up and take notice.
Overview
The PM840 is a small black box that makes a very large claim. Soundcraftsmen's own advertising billed it as a "tremendously high current" amplifier with "over 50 amps" of peak current capability, and the company leaned hard on the price: $499, presented as a "tremendous price breakthrough." That figure shows up in the period ads again and again. Audio (February 1986, p.11) ran the PM840 at the top of its Pro-System One spread. Audio (May 1986, p.45) repeated the same 410-watt, 499-dollar pitch alongside a matching tuner and preamp that brought the whole system in at just over a thousand dollars at retail. The spec sheet behind the pitch is more interesting than the slogan.
Start with the rated power, because this is where the PM840 separates itself from the usual budget amps. Soundcraftsmen rated it at 205 watts per channel into 8 ohms and 300 watts per channel into 4 ohms, both continuous RMS, 20 Hz to 20 kHz, at less than 0.05% THD. The 2-ohm figure is the one collectors tend to quote: 450 watts per channel, still continuous RMS, still across the full audio band. Those numbers come straight from the company's own catalog listings, and they stay the same across the 1986 and 1987 buyers'-guide entries. The PM840 shows up with the same 205/300/450 rating in Audio (February 1986, p.11) as it did a year earlier, which is more than can be said for a lot of small-shop amplifiers whose specs drifted with the brochure.
The engineering story is the "no-current-limiting power supply." Conventional amps protect their output stages by clamping current when the load gets demanding, and the audible result is compression or clipping right when a transient needs headroom. Soundcraftsmen's claim is that the MOSFET output stages and the supply together avoid that current limiting. That is why the amp can be rated down to 2 ohms at 450 watts per channel without the protection circuitry stepping in. The company's advertising described the MOSFET stages as "unlimited," which is marketing language. The underlying topology is a high-current supply feeding MOSFET outputs, and that is the reason this small chassis can be sold as a 2-ohm-capable amplifier.
The rest of the measured performance is respectable for the money. Frequency response is quoted at 20 Hz to 20,000 Hz, ±0.1 dB, which is a tight window and suggests the amp was designed to be flat rather than voiced. Signal-to-noise is listed at greater than 105 dB, slew rate at 50 V/microsecond, and damping factor at 200. Transient intermodulation distortion is described as unmeasurable. The total harmonic distortion figure of less than 0.05% is the same number quoted across the 8-, 4-, and 2-ohm ratings, a sign the amp was specified conservatively rather than tuned for one load. None of these are exotic numbers by the standards of high-end separates.
The PM840 is compact: 8½ inches wide, 5 inches tall and 12 inches deep, weighing 22 pounds. That footprint is deliberate. The amp was sold as a stackable or rack-mountable component, and Soundcraftsmen offered the PCX-1 rack-mount kit (optional, $49) to bolt it into a standard 19-inch rack, plus a PCX-2 kit for mounting two PM840s side by side. The front panel is a plain 8½-inch-wide face. You can stack these on a shelf one above the other or slide them into a furniture rack, and the company's advertising made a point of that flexibility, describing the choice between "separates or systems" as the reason to buy into the line.
One practical feature for anyone adding the PM840 to an existing setup is the PC-1 Power Coupler, a $39 accessory that lets you add the amp to a receiver-based system. You connect the PC-1 to your receiver's speaker terminals and run its cables into the PM840, which uses the receiver as a preamp and lets the outboard amp do the heavy lifting. It gave someone who wasn't ready to buy a full separates stack a way in, and it fits Soundcraftsmen positioning the PM840 as an entry point into the Pro-System line rather than a standalone flagship.
In the Soundcraftsmen lineup, the PM840 sat below the bigger Pro-Power models and the PM1600, which was rated at 600 watts per channel into 4 ohms and 375 into 8 ohms for $1,399. The PM840's 205 watts into 8 ohms and 300 into 4 ohms put it in the middle of the range, and the $499 price made it the affordable way into the company's high-current MOSFET design. The 2-ohm rating is the spec that counts for anyone driving electrostatic or low-impedance speakers, and it's the reason the PM840 still turns up in discussions of budget high-current amplifiers.
The PM840 is not a collector's trophy. It's a working amplifier, priced and built to be used, and the 22-pound chassis and plain front panel reflect that. The value is in the current delivery and the 2-ohm stability at a price that undercuts most of the competition, not the cosmetics.
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