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Aphex 661 Expressor

Single-Channel Valve Compressor By Paul White
Published August 1996

Famous for their Aural Exciters, Aphex continue to diversify. Paul White takes a look at the US company's new valve‑based compressor.

Aphex have a solid reputation for designing and building innovative analogue signal processing products, and since their Aural Exciter, the company has gone on to build gates, mic preamps, compressors, limiters, and a number of other products, each of which has included some unique feature. The latest Aphex innovation is Tubessence, a low‑voltage valve‑based circuit that employs an ingenious solid‑state feedback circuit to make the valve behave and sound as though it is being run at several hundred volts. Tubessence has been included in a whole range of products, including the 661 Expressor reviewed here.

Front Panel Tour

The 661 Expressor is a single‑channel compressor which may be used on voice or instruments, and I get the impression that here is a compressor that wants you to know when something is being compressed! Anyone familiar with earlier Aphex designs may recognise the SPR (Spectral Phase Refraction) button just to the right of the Bypass button. SPR manipulates phase relationships in such a way as to enhance or clarify the bass end of a sound without using any EQ, but the effectiveness of the system depends very much on the signal you feed into it. Sometimes it sounds quite positive, while at other times it seems to do very little.

The compressor section is switchable for hard‑ or soft‑knee operation, and the Easyrider section provides fast or slow auto operation, relieving you of the responsibility of worrying about ratios or what knee to choose [dimpled? knobbly? — flippant Assistant Ed]. In effect, Threshold functions as a more/less compression control, and that's all you have to deal with.

In Manual mode, the usual Threshold, Attack and Release controls are available, and the user has controls of both input and output gain plus Threshold level at all times. Status LEDs are fitted to all switches, and external side‑chain access is via rear‑panel jacks. A switchable low‑cut filter is available in the side‑chain signal path to prevent excessive amounts of gain reduction being triggered by low‑frequency sounds. The signal inputs and outputs are electronically balanced, and are available on both jacks and XLRs, with a further TSR jack on the rear panel allowing two 661 Expressors to be linked for stereo operation.

After the compressor controls, there's the HFX section, which uses high‑frequency expansion to help preserve brightness when a lot of gain reduction is being applied. The user has control over the HFX ratio, and the frequency above which HFX takes effect (2kHz to 20kHz). There's no separate bypass switch for this section, so if you don't want to use it, you simply turn the Frequency control fully clockwise.

Both gain reduction and input/output level metering is on LED meters, and there are switches for stereo linking and for Slave mode. When two units are run as a pair, the unit set to Slave mode responds to the control settings of the master unit.

Studio Trials

In testing this unit, I tried just about every kind of voice and instrument I could lay my hands on, and it handled them all without complaint. Easyrider mode was particularly impressive, especially on bass instruments and vocals, though if you really want to get things pumping, I found it best to use Manual mode, set a fast attack combined with a reasonably fast release, and pile on the gain reduction at a fairly high ratio using the hard‑knee setting. If I had to characterise this compressor, I would say that it manages to make sounds punchy and upfront without compromising their original character. It doesn't make things sound muddy or squashed, and even without the HFX, the sound is quite transparent.

Adding HFX gives an edge to the sound that isn't dissimilar to what you might expect from an Exciter, even though the principle is quite different, and the SPR treatment at the bass end adds a subtle, rounded warmth to most bass sounds (though as mentioned already, there are some sounds on which it appears to make no difference at all).


On balance, this is a very useful and flexible compressor, with just the right amount of character, though I feel it would have benefited from an expander section to keep the pauses clean. It is also quite an expensive unit when you consider that it's a single‑channel device, and for less than twice as much, you could buy a dual‑channel valve compressor (the Drawmer 1960, for example). Where the 661 Expressor scores is in its flexibility, the operationally simple Easyrider mode and the HFX enhancement. It is a competent and very pleasant‑sounding compressor, with the quality of performance we've come to expect from an Aphex product. If you're in the market for something a little bit special, check it out.


  • Very versatile.
  • Warm, punchy sound.


  • Costly.


• A good all‑rounder, with lots of character for those prepared to pay the extra for something out of the ordinary.