[sdiy] G SSL clone RMS conversion

[mark verbos]

On May 6, 2014, at 1:23 PM, Justin Owen 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.

It's not RMS. However, it is feedback, not feed forward like a DBX so it is a whole different ballgame. The feedback is going to the "sidechain VCA" the fixed gain of which is the threshold. 

> 
> I also can't quite work out how the R pairs are arranged around the SW1/Ratio (following the two HWRs).

The resistors on the left change the gain of the sidechain signal, i.e. the ratio. The ones on the right move the threshold to make the changing ratio not seem to jump around. The diode do-dad is to make a knee I think.
> 
> 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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