[sdiy] G SSL clone RMS conversion

[Mattias Rickardsson]

On 6 May 2014 19:23, Justin Owen <juzowen at gmail.com> wrote:
>
> 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.

No RMS, as mentioned already by others.

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

They are a bit tricky. The diode there also makes the rectifier react
differently to positive and negative signals, which I don't really
understand the point of.

> 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.

Possibly some error in the simulation? I simulated it once in LTspice,
even built it with 2164s, and in both worlds it behaved well!

/mr