Adding LED gain reduction meter to optical comp.

[blearyeyes]

I built 4 Demeter based optical clones off of Osh Park and haven't boxed them all. Ideally the meter would register the gain reduction of the comp. Unfortunately the Chinese are a bit shy on documents and it's been several years since I bought these meter kits but the circuit looks pretty simple. I built one up last night and it functions attached directly to the output of the pedal.

Now the challenge is to pick the control signal off of the Compressor circuit and have it work with the meter. I'll post links to what I have information wise if anyone's interested in it.

Oh and cutting a slot for the LEDs in the enclosure..

here's the compressor docs:

http://www.shattuckforensics.com/music/Compressor.pdf

[EBK]

Yes, links would be good.

[thesmokingman]

I've done a vu meter on the output of a pedal before ... I suppose it would be neat to have pre and post for comparison.

[blearyeyes]

I've done a vu meter on the output of a pedal before ... I suppose it would be neat to have pre and post for comparison.

Ideally it would be gain reduction in a reversed meter. Got used to setting my old LA-4. Lets you see how much compression at a glance.

[EBK]

I've done a vu meter on the output of a pedal before ... I suppose it would be neat to have pre and post for comparison.

Dan and I have conversed a bit through PM about this. 

I was trying to convince ("convince" isnt really right....) him that you can display something potentially more useful than pre or post: the difference of the two.  This tells you what the compressor is doing without muddying up the presentation by having it jump all around with the dynamics themselves.

In a vactrol-based compressor, the difference signal will be some transformed version of the LED current (or voltage, I suppose).  The trick would be to properly map it to a scale that has some resemblance to dB (dBu?) attenuation (I believe the Demeter is a single-ended compressor that attenuates the highs but doesn't amplify the lows, but I could be wrong).

[blearyeyes]

I posted the compressor docs in the original post with a picture of the meter PCB which has the component values in the silkscreen.
After my discussion with EBK I let the details percolate in my poor ADHD brain and realized that an op-amp outputting the difference between the input and the output might be the way to get basic gain reduction readings.  That is probably what EBK was talking about. Too many details and my brain pulls a Bart simpson.  So EBK eloquently described how he added a meter and I heard blah blah blah...then after a nights sleep I had the brilliant idea of using a differential op-amp between the in and out.  I'm sure I just stole his idea.. 

Can you take the control signal directly and make anything useful out of it?

[EBK]

If you just outright take the output signal and subtract the input, you are not going to get a representation of the gain.... You would need to divide rather subtract.
I may have oversimplified a bit by merely saying "difference".  I meant the difference of dB levels.

That's why I was suggesting basing your measurement on the thing that affects gain itself.

[blearyeyes]

It would be so cool if I knew what I was doing.

[EBK]

It would be so cool if I knew what I was doing.

No worries. Learn what you can as you go along.

I was looking at that compressor schematic, and I may be able to give you a workable solution. 
Here's the idea:
That circuit uses a vactrol with a slow-looking turn off time (maybe all vactrols are like this *shrug*), so the LED current would not necessarily be the best measurement to use.   Instead, we can take advantage of the fact that your meter circuit handles dB conversion for us, and we can feed it a copy of the signal portion that flows through the LDR.  I'll make a quick drawing in a few minutes to show this better.  We will need an op amp.

[EBK]

Edit:  I need to rethink this a bit....

This is so much easier in a VCA-based compressor.  Sorry.

[blearyeyes]

No Sweat. don't worry about it. If you think of anything let me know.