The ‘dark’ switch I introduced on the suggestion of an engineer in a London studio; he said that the later SC2 V2 sounded ‘lighter’ than the original V1.05 and 1.07, so I had a look at the timings around the release control and introduced a change that could switch in the exact characteristic of the earlier versions…. but the difference is extremely small and not noticeable on most music material. I have repeated the changes in the new TFPRO538 stereo compressor, this one also has a ‘dark’ switch. The main difference is an extended release time and a slightly ‘heavy’ effect.
But the main characteristic of the sound of all these compressors is the use of a batch of opto devices that I bought back in 1988. I still have a good quantity left and while I changed to a more modern (and much faster acting) type for the P38EX, I have gone back to the original opto device for all new compressors including the TFPRO538 and also the new P38EX Mk2 that will be ready in October or November this year (2019).
Compressors are extraordinarily complex things!!