© 2013 Benden Sound Technology

Made by Serif

Background

Gyraf Audio's clone of the mix bus compressor found in certain vintage analogue mixing desks is a popular DIY project and many custom parts are now available off-the-shelf to assist with building a professional-looking unit.

The original design information can be found here, and there are extensive discussions about every aspect of the G-SSL here.

I was asked by a client to build and test two units, and the results can be seen in the photographs.

Observations

Neither of the two units are standard G-SSLs! There are several documented enhancements to the basic design, perhaps the most popular being various flavours of external/extended sidechain. Both these units featured the following sidechain options:

Standard (unmodified).

External.

Highpass filtered at one of four different frequencies (to compress bass frequencies less).

Thr*st High and Medium strength (more complex sidechain frequency responses designed to add punch).

A known shortcoming of the G-SSL is that the compression amount varies with heavily panned input signals. This is because the mono sidechain simply sums the left and right channels rather than taking whichever channel has the highest amplitude at a given instant. Clearly, then, a signal present on both input channels would be compressed more heavily than one of the same amplitude present, say, on only the left input channel.

To put this right requires a second sidechain to be built which, in the case of the extended sidechain described above, calls for quite a bit of extra circuitry. Nonetheless, this was done for one of the two units; the other has the standard, mono sidechain.

Cases came from Purusha via the Prodigy-Pro forum. They were drilled to suit the extended sidechain mods but not the second sidechain jack, which I had to add. The round meters are modified Behringer units which can be ought online - they are inexpensive, but not terribly linear and should be taken as a guide only.

When testing, I found that signal distortion in bypass mode (not a true, hard bypass) was rather high. The solution, found buried in the forums, is to ensure that the bypass switch grounds the 620K makeup gain mix resistor rather than leaving it floating. Otherwise, noise coupling into this high impedance point raises the distortion.

Also, quite a bit of manual trimming was required to achieve the specified compression ratios. I suspect this is caused by opamp offsets in the sidechain circuitry and would investigate further before building further units.

Rear view of G-SSL compressor. This unit has a stereo sidechain so requires an extra jack.

Internal view of G-SSL compressor. Sidechain here is stereo with multiple filtering options.

G-SSL compressor main PCB. This contains the basic compressor function but not  the sidechain mods.

Extra board containing stereo sidechain with multiple filter options.

Internal view of second G-SSL compressor. Sidechain here is mono with multiple filtering options.

Extra board containing mono sidechain with multiple filter options.

G-SSL compressor front panel. Note extra switch for additional sidechain options.