ESS Model 200

A power amplifier from the Heil-era ESS stable — documented largely through the pages of the magazines that measured it, and through the corrections they later had to print.

Overview

The ESS Model 200 is a stereo power amplifier from the firm best remembered for the Heil Air Motion Transformer, and in the mid-1970s it was the power stage that sat beneath the company's preamplifier in a matched ESS separates pairing. The record of it is unusually magazine-bound: the amplifier was profiled by Audio in May 1974 (p.48) alongside the ESS preamplifier, and the same magazine ran a correction that July (p.51) after an incorrect graph slipped into the original profile — a small episode that says less about the amplifier than about how thin the paper trail has become since. HiFi/Stereo Review flagged the Model 200 in its new-products column in March 1974 (p.18), describing it as sharing performance characteristics with the original design it succeeded.

What the surviving documentation establishes with confidence is the damping factor: at 20 Hz the damping factor is 100, according to the HiFi/Stereo Review new-products notice (March 1974, p.18), a figure that speaks to how firmly the amp would grip a woofer's back-EMF and keep low-frequency reproduction under control. That number is the amplifier's most reliably reported electrical characteristic in the period press, and it appears in more than one place in the record.

The circuit itself, as Audio described it in the May 1974 profile (p.52), ran a full-wave rectifier bridge feeding a 500 µF filter capacitor, with a high-gain emitter-follower regulator whose base is fed from a voltage divider across the input filter cap; a 50 µF capacitor from base to ground knocks input ripple down by a factor of roughly 375:1, and the high-cut filter introduces a very sharp cutoff above 9,000 Hz (HiFi/Stereo Review, September 1973, p.24).

That is a conventional topology executed with attention to supply cleanliness, and the magazine's own summary of the phono-equalizer and output-amp circuit was that it was "not startlingly new" but that ESS had worked out the parameters for very low distortion and wide bandwidth. Treat that as the honest contemporary verdict: competent engineering rather than revelation.

The July 1974 addendum is worth knowing about for anyone chasing down a used example or a scan of the original review. Audio printed a corrected total-harmonic-distortion-versus-power and intermodulation-distortion-versus-power graph (p.51), noting that the right channel's IM distortion fell to the left channel's levels after a small bias-pot adjustment. That detail is a useful diagnostic hint for present-day owners: the bias trim on these amps is adjustable and worth checking, and channel-to-channel IM asymmetry can often be brought back into line with a careful tweak. It also means that anyone relying on the May 1974 profile as printed is looking at the wrong curve.

Collectors should treat the Model 200 as a document-poor piece. There is no service manual, no owner's manual, and no factory specification sheet in the surviving record that this archive has been able to locate, and the amplifier's reputation rests almost entirely on the two 1974 magazine profiles and the new-products notices. What that means in practice is that anyone buying one is buying an amplifier whose measured performance is known from a single review and a single correction, not from a manufacturer's data sheet — and should price it accordingly. The ESS name carries weight among AMT enthusiasts; the Model 200 does not yet carry the same weight among amplifier collectors, and the thin documentation is the reason. It is a piece to buy because it completes an ESS separates stack or because the circuit interests you, not because the market has decided it is important.