The ADR Compex F760X-RS ($2,750) is a classic FET based compressor/limiter/expander/gate and has been a go-to box for many engineers over the past several decades. The vintage units continue to garner top dollar from those in the know. With the cooperation and full support of ADR, Tim Mead’s Q2 Audio has brought the elaborately designed Compex back to life. I should add that while the audio signal path and side-chain processing circuitry is completely faithful to the ADR’s version, some features have been updated to improve long-term reliability and operational flexibility, including the addition of external side-chain access which is configurable in the compressor or gate side-chains or via internal jumpers.
The limiter is activated via a three-position toggle switch (Off, On, or On with Pre-emphasis). It has a fixed threshold and ratio (100:1), so the input level setting determines the amount of limiting. Since the circuit was designed for the broadcast industry, it has an extremely fast attack (below 250 μs) with marginal overshoot and a fixed 250ms release time.
The compressor controls include threshold, ratio (1:1, 2:1, 3:1, 5:1, 10:1 and 20:1), attack time (250 μs, 2.5 ms, 25 ms) and release time (ranging from 25 ms to 3.2 s, plus an “auto” setting). The compressor’s auto release setting, which I found to work extremely well, employs a dual time-constant in which transient peaks more than 6 dB above the threshold have an extremely fast recovery while lower level signals have a substantially slower recovery time, thus minimizing pumping artifacts.
The expander has a fixed 1:2 slope which increases to 1:20 when switched to the gate mode. The range is adjustable from 0 to 20 dB. The threshold ranges from Hi to Lo and release from Fast to Slow. I’m not sure exactly what the numerical relationship is to these settings, but the gate/expander needs to be set by ear, so I don’t think it matters. The attack time can be set to 20 μs, 2 ms or 40 ms. A three-position toggle switch selects between Off, Gate and Expander modes.
In addition to 13 independent controls (eight knobs and five switches) for each channel, the box has independent Stereo Input and Stereo Output controls. The stereo-coupling mode switch puts the Compex in stereo mode, which combines the control voltages produced by the compressor and expander side-chains. This prevents unwanted image shifting by providing identical gain-reduction in both channels. Since the momentary attenuation of individual transients doesn’t typically create perceptible image shifts, the limiter side-chains remain independent in the stereo-coupling mode.
Kick and snare drum record wonderfully through the Compex circuit. Adjusting the compressor’s attack and release make it easy to fine tune the kick drum’s sustain and the crack of the snare. The expander also works well with percussive instruments.
The Compex works exceptionally well with vocals, too. It’s the only device that I’ve encountered that allows the simultaneous use of compression and expansion and extremely dynamic vocal performances can benefit greatly from this feature, especially when recorded in a project studio with a higher than desirable noise floor.
Most compressors are either colored or clean, and the thing I like best about the Compex is that it can go extreme in either direction or anywhere in between. Subtle stereo buss compression beautifully glues a mix together, while extreme bass, drum or vocal squashing can add a ton of character to an otherwise boring performance.
Rivaled only by George Massenburg’s 8900 Dynamic Range Controller in complexity, the ADR Compex F760X-RS’s controls are initially a bit of a challenge to get around. But it’s worth figuring out because the box sounds wonderful on virtually any sound source and it is amazingly powerful, allowing massive sonic control.
Russ Long lives and works in Nashville, engineering and producing a wide variety of music and film projects.