Soundcraftsmen RA7502 (1981)
Thirty-five pounds of Class-H muscle with a rack-mount face and LED ladders that count the watts as they go by.
Overview
The second thing is the front panel. It has a pair of vertical LED banks, twenty per channel, calibrated from −40 to +3 dB. Wattage scales are inscribed along the rows for mono or stereo operation into 16-, 8-, or 4-ohm loads.
The company billed the RA7502 as a professional amplifier, aimed at touring musical groups and other professionals who needed high-power sound reinforcement amplification. Behind the rack ears sits a Class-H circuit. The design is credited to Soundcraftsmen chief engineer Paul Rolfes, who had put it in an earlier Soundcraftsmen amplifier roughly four years before this model appeared. Audio (July 1981, p.54) noted that at least two other companies had since used variations on the Class-H theme. Soundcraftsmen ended up with patents but comparatively little of the credit.
Here is what Class-H does. The amplifier carries two positive and two negative power supplies, with the low-voltage rail a fraction of the high-voltage one. At low listening levels it behaves like a conventional Class-AB amp running off the low rails. As the signal approaches the limit of those rails, what Soundcraftsmen calls its "Vari-Portional" circuit anticipates the waveform's approach and raises the B+ voltage to buy extra headroom, then lets it fall back. The B− rail does the same in the negative direction to prevent downward clipping. So the amp spends most of its life on the low rails, which saves energy and cuts internal dissipation substantially. It only jumps to full rail voltage for the brief moments when peaks demand it. That is why the RA7502 can run fanless at these power levels.
Soundcraftsmen rated the RA7502 at 250 watts per channel into 8 ohms, 375 watts per channel into 4 ohms, and 750 watts bridged mono into 8 ohms, all across 20 Hz to 20 kHz. Audio (July 1981, p.54) put the amplifier on the bench and found it delivered in excess of 300 watts per channel into 8-ohm loads and nearly 450 watts per channel into 4 ohms at its rated 0.09 percent distortion for a 1-kHz signal. Backed off to the rated 250 watts per channel, the magazine measured THD of 0.015 percent and SMPTE-IM of just 0.035 percent. Dynamic headroom came in at 1.94 dB, meaning short-term peaks as high as 390 watts per channel into 8 ohms in stereo mode. Bridged, the reviewer pushed output voltage to roughly 240 volts peak-to-peak across an 8-ohm load - an average power of around 900 watts before final clipping set in.
The rest of the measured numbers hold up. Damping factor measured in excess of 200 in stereo, about as high as the reviewer's test rig could resolve. The factory claim is greater than 250 at 50 Hz in stereo and greater than 125 bridged. Frequency response measured flat within −1.0 dB from 5 Hz to 50 kHz, comfortably inside the ±0.25 dB, 20 Hz to 20 kHz manufacturer spec. Signal-to-noise, measured the standards-conscious way at 1 watt out with 0.5 volt input, came to 85 dB. Measured the way Soundcraftsmen and others quote it, with level controls wide open and referenced to rated output, it read 113 dB, well past the claimed 105 dB. CCIF twin-tone IM measured a very low 0.0097 percent, and IHF-IM only 0.06 percent. Slew rate is specified at 50 V/µS for the amplifier in stereo mode and greater than 90 V/µS for the supply. Input impedance is 32 kilohms stereo and 16 kilohms bridged. Input sensitivity is 1.2 V stereo and 1.0 V bridged for rated output.
Around the back, stereo or bridged mono operation is selectable. The panel carries ¼-inch phone-jack inputs wired so that either balanced or unbalanced sources can be used, depending on how the mating plugs are wired. Output terminals are five-way binding posts on standard ¾-inch centers. The amplifier is specified stable with any load of two ohms or above, and it carries four kinds of protection. There is a dual-purpose auto-crowbar short-circuit circuit with automatic reset and front-panel LED indication. There is a thermal sensing circuit with automatic reset and front-panel indicator. There is external fusing. And there is a patent-pending "Auto-Buffer" circuit designed to provide automatic internal electronic compensation for continuous 2-ohm operation without tripping protection or current limiting. Audio (July 1981, p.54) confirmed the Auto-Buffer worked when the load was dropped to 2 ohms briefly and the amp pushed into overload. Additional per-channel LEDs light when the Vari-Portional circuitry engages and when actual clipping occurs.
Each amp shipped with a parchment-like individualized certificate of performance. It notes the serial number and the major measured specs for that actual unit, and it is signed off by the inspector who tested it.
Two related models are worth knowing about. The RA7502 is the professional version of the lower-priced Model PA5001A, a basic power amp at $749.00. The PA5001A lacks the front-panel displays, mono bridging, and balanced input capabilities of the pro version, but shares the same performance specs and circuitry. Per HiFi-Stereo (October 1981, p.36), the Model RA7501 is identical except for omitting the spectrum analyzer, at a price of $949. That is the same figure Soundcraftsmen quoted for the RA7502 itself in its published specifications. Anyone comparing the two should confirm which panel features a given unit actually carries.
Audio (July 1981, p.54) had no reservations recommending the RA7502 for use in any high-quality home music system, and came away impressed that even at peak levels that lit the 0-dB LEDs repeatedly the amplifier stayed unusually cool — confirming the fanless Class-H claim under difficult conditions. The reviewer's summary was blunt: don't let the industrial look fool you, it's a true high fidelity product. Soundcraftsmen also marketed the amp, along with others in its line, as "digital/PCM ready" — the argument being that forthcoming digital sources would carry so much dynamic range that listeners would need more power and more headroom to reproduce them without compression or clipping. That's marketing framing, but the measured 1.94 dB of dynamic headroom and the near-400-watt stereo peaks give it some substance.
Expect the usual caveats for a 1980s pro amp. The LED display banks and their calibration are the distinctive feature and worth verifying on any candidate unit. The rear-panel mode switching and phone-jack input wiring need a careful once-over. The 55-pound shipping weight makes these expensive to move. The individualized performance certificate is a nice thing to find in the box but is frequently long gone. Beyond that, the documented record on this model is thin. Production years, current market pricing, and known failure points aren't established in the available sources, so treat any specific claims about run length or resale value with caution.
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