Tannoy SR-840

A rack-mount studio power amp that promised 250 watts a side — and, in the reviewer's hands, very nearly delivered, right up until it hummed.

Overview

The Tannoy SR-840 is a stereo power amplifier built primarily to drive studio monitor loudspeakers — a fact the period reviewer did not miss, noting dryly that the design seemed "clearly an adjunct to marketing Tannoy loudspeakers." The unit carries the Tannoy/Tresham name, and the manufacturer is listed as Tannoy Tresham Ltd of West Norwood, London. It is a rack-mount design, 3U high, with a nicely finished alloy front panel machined with slots for a standard 19-inch rack. Substantial handles flank each side, protecting a front panel that was already well protected.

The power figures are the headline. The manufacturer's provisional specification rates the amp at a minimum 250 W RMS per channel into 8 Ω with both channels driven, 450 W into 4 Ω, and 550 W into 2 Ω — the last figure flagged as an absolute minimum load impedance, not for continuous full-power operation. Bridged mono, it was rated at 875 W into 8 Ω. Distortion was specified conservatively: total harmonic distortion and noise of 0.05% worst case across 10 Hz to 20 kHz at any power up to rated maximum, typically 0.01% at 200 W into 8 Ω. Intermodulation distortion came in at 0.03%, and the amp boasted a damping factor above 200, a slew rate of 100 V/µs, and a rise time of 1.5 µs. Full-power bandwidth was quoted at 15 Hz to 40 kHz, +0/−0.5 dB.

Around the front panel, the layout is purposeful. To the right sits a large locking pushbutton for mains power — with the unusual quirk that a beeper sounds between switch-on and the engagement of the internal protection relays. Two red LEDs sit nearby: one for power-on, the other to light on thermal shutdown. To the left are two vertical rows of six LEDs forming level indicators for the two channels, offering 3 dB increments below full power, which is represented by red LEDs. The rear panel uses large-diameter output terminals — impressive at first glance, though the reviewer reckoned their only real advantage over smaller terminals was the large knob making tightening easier.

The review itself, by Hugh Ford in Studio Sound (August 1984, p.79), is a genuinely mixed verdict, and worth reading closely. Ford found much to like — the rise time of the level indicators was reasonably fast, crosstalk between channels was minimal, phase shift was small, and the amp ran acoustically quiet since it needed no forced-air cooling. The heatsinks, he noted, at times ran too hot to touch, but the thermal overload never operated during the review even when the amp was driven hard at high levels and high frequencies. The problem was noise. Ford's summary was blunt: while many aspects were good or very good, the unit was "severely let down by a variable and sometimes poor noise performance." He attributed this not to the amplifier modules themselves — whose noise performance was good — but to insufficient attention to the performance as configured for the final product. The offending noise was entirely harmonics of the power-line frequency, picked up by the amplifier modules from the mains transformer.

Tannoy's response matters here. Following the review, the manufacturer told Ford the noise and hum were not typical of production, and a second sample showed a distinct improvement. That caveat — pre-production unit, problems acknowledged and reportedly addressed — is the honest frame for the SR-840. It is a well-made amplifier with distinct promise as a studio monitor amp, in Ford's words, once the manufacturer attended to the minor problems found in that early sample.

For collectors, the SR-840 occupies a curious spot: a Tannoy-branded amplifier rather than a Tannoy loudspeaker, rack-mounted, MOSFET-based according to a later trade note describing a Millbrook Sound install that paired Tannoy SFM double-15 monitors with a "Tannoy SR840 class A MOSFET power amplifier." A North American listing in Sound and Communication (March 1988, p.39) priced the SR840 at $3,495.00, listed at 500W/250W, with a 5.25" × 19" × 19" chassis. It shows up in studio installs of the period rather than in audiophile living rooms, which is exactly where a 3U rack amp with big handles belongs. Survivors should be checked for the mains-transformer hum Ford documented — and buyers should confirm which production sample they're hearing before judging the design by its noisiest example.

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