This clever design rethinks many aspects of compressors that we've long taken for granted.
Arguably, the last ground-breaking development in analogue dynamics processor design was the employment of silicone transistor VCAs as the gain-reduction element — that was half a century ago. We've seen technical specs improve, and facilities such as side-chain filters and wet/dry mix controls become commonplace, but at heart most 'new' dynamics processors reinterpret existing concepts. If anyone were to find a way of doing things differently, it would be Jakob Erland, the man behind the Danish high-end hardware brand Gyraf. He has consistently applied radical, outside-the-box thinking to deliver a truly special range of analogue hardware, and in the gloriously titled Gyratec XXIV Passive Aggressive Cadmium Compressor ('G24' for short), he's created something genuinely new — I believe this design has pushed the envelope further than any other modern dynamics processor.
Erland's primary design aim was to create a dynamics processor as transparent-sounding as Gyraf's latest EQs. This line of thinking first spawned the G21 Magneto-Dynamic Infundibulum (if you read my SOS September 2013 review of this, you'll know just how much respect I have for Gyraf) and although still based on common topologies, the G21's entire design concept was genuinely forward-looking; it's not a 'compressor' in the common sense of the word, but it achieves a certain amount of peak control through saturation of its audio transformers and was an important milestone en route to the G24. A secondary aim for the G24 was to create a cadmium-based optical compressor that would not fall foul of the new EU rules prohibiting hazardous substances (hence 'Passive Aggressive').
The G24 is an optical compressor with a unique side-chain section, and it's unusual in that it boasts a choice of two audio paths which you can select with the flick of a switch. One path is entirely passive, and the other active. The latter employs a very transparent-sounding transistor-based buffer amplifier in the output stage.
Gyraf conceived this 2U rackmountable processor primarily for use in bus processing and mastering, hence both the focus on sonic transparency and the control layout. The controls you'd expect to find on a typical compressor are all present (Threshold, Ratio, Attack, Release and Output) and as it's a true stereo processor there aren't separate controls for each channel; instead, the G24 employs carefully matched (<0.2dB) dual potentiometers, with detents for easy recall. Unusually, though, there are two separate controls for each parameter, the reason being the G24's two different side-chains, which can be selected individually or blended together using the A/B Control potentiometer. The effect is not unlike having access to two completely different compressors — one could use this feature to quickly jump back and forth between two different compression settings, for example, with one in the hard left position and the other in the hard right. But, of course, the mix of the two side-chains could be anywhere between these extremes.
Many engineers familiar with the more delicate dynamic control tasks required during audio mastering employ a technique where no single compressor is asked to shoulder the whole burden of gain reduction, instead sharing the load between two or more processors. With the G24, this principle can be achieved with more transparency, because only the side-chain is doubled — not the actual audio path, with its inherent colorations. Another consideration is which of the two compressors should come first. This is a very important question, because often a faster peak limiting compressor is combined with a slower, more gentle leveller, and the order in which the program is processed through these units plays a significant role for the outcome. This is why each of the G24's side-chain sections has its own continuously variable blend control to move between feedback and feedforward modes — these blend controls not only define the compression character of the A and B sections, but also the order in which the program is affected by both side-chains.
Although it's this novel feature which most...