[sdiy] G SSL clone RMS conversion

[Olivier Gillet]

I haven't had a close look, but could it be that it simply is an
envelope follower (with infinite attack and adjustable release) rather
than a RMS detector?

Olivier

On Tue, May 6, 2014 at 7:23 PM, Justin Owen <juzowen at gmail.com> wrote:
> Hello - I hope nobody minds me posting this here and I hope a compressor design/build question isn't too far out of the SDIY remit...
>
> I've been looking into various RMS to DC conversion schems including the one from the DBX 160, TI app note SNOA855 and a similar schem in the Sergio Franco book that Byron Jacquot mentioned recently (which is great BTW - thanks!).
>
> Someone also pointed me to this schematic of a clone of the SSL Bus Compressor:
>
> http://www.gyraf.dk/gy_pd/ssl/ssl_sch.gif
>
> I assumed the RMS conversion started with the 2nd and 3rd op amp after the sidechain VCA (third 'row' down) but it appears to be just two full wave rectifier circuits - I can't see the log/multiplier part that appears in the other RMS conversion schems I've seen that is normally done with transistors or a diode array.
>
> I also can't quite work out how the R pairs are arranged around the SW1/Ratio (following the two HWRs).
>
> I've tried to sim the circuit and it appears to put out a narrow pulse from the negative rail to 0V at the same frequency of the signal at the sidechain input - not what I expected from an RMS output.
>
> Can anyone offer any insight as to what this part of the circuit is actually doing or how it might be working or something that is similar that I can compare it with?
>
> Thanks,
>
> - J
>
>
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