My attempt to build a LDR compressor. Prototype done, some samples.

[abbey road d enfer]

It's there in a zip file.

Missing the AOP.lib!
I'll replace them with the standatd opamp, it shouldn't change much to the sim.

The log.exp circuit is there to set the slope of the compressor, and set the threshold.
If you directly send the output of the log circuit to the exp circuit. As you've seen tou'll get the voltage taht fed the log circuit. And the compressor should behave as a limiter (sort of).
If you put a voltage divider lts say 1kohm and 1kohm between the two, then the out put of the exp circuit should be the squareroot of the input.

Yes, that's what I see. The result is that the higher the signal, the lesser the compression ratio. Is it what you want?

the output should be something like v(out)= Vref2 * (Vin/Vthreshold)^(1-Ratio)
(wich i thin is the right law to drive the LDR)

I'm not so sure... Have you traced the Vout/Vin graphs?

Your final point was one of my question. Does all the frequency have to feed the sidechain or is it better to pu a little of Hi Pass filter?

It's a matter of taste, application and circumstance. For individual tracks, I generally don't use a HPF in the side-chain. For whole mixes, I use a "Thrust" filter (+3dB/octave). But it's just me, others would probably do different.

 

[ruffrecords]

Thank you, i have to think about it. I think i make a mistake when i think about it.
Something wrong like output take a value, system react and tell output to decrease, output decrease so system react and tell output to decrease etc, etc. Wich would lead to a limiter. This is an certainly obvious mistake, well i have to figure out where i'm mistaking.

That is not quite what I said. The question you need to ask is how much do I need to increase the input by in order for the output to increase by 1dB? To work out the answer the easiest way is to work back from the output as in my original reply. Otherwise you will certainly be confused by the fact it is a feedback circuit.

Cheers

Ian

 

[abbey road d enfer]

I have run your sim, with another opamp. It confirms my first thoughts. Whatever the input signal you chose, it just shows that the output is a scaled replica of the input signal. When I feed it with a full-wave rectified signal the output is also a full-wave rectified signal. In order to achieve rms conversion some timing must be applied between the log and antilog converters.
Now, your circuit works as a basic peak compressor where the attack and release times are governed by R27, R28 & C10, with some added quirks due to the log/antilog portion's non-linearities..