Idea for optical compressor OR expander

[earthtonesaudio]

How does this look?  Obviously it's just a snippet, but I think the idea is sound.
I was thinking about putting this before an autowah, for more control over the envelope.

[Paul Perry (Frostwave)]

You can see how it works, if you consider the four extreme cases:

1. VR1 fully left, photocell resistance infinite.
2. VR1 fully left, photocell resistance zero.

3. VR1 fully right, photocell resistance infinite
4 VR1 fully right,photocell resistande zero.

In case 1 & 2, the gain of U1 is set only by R2 , and gain of U2 is set by the ratio of R4 to the resistance formed by R3 in series with the parallel resistance of VR1 and the LDR.

In case 3&4, the gain of U2 is fixed, but the gain of U1 is set by the ratio of R1 to the resistance of R2 in series with the LDR paralleled by the total resistance of VR1.

It is very ingenious. i don't know whether it is possible to choose the resistances to give an 'even' gain over the whole range of VR1.

[mac]

Looks like a dod envelope follower without the pot. Interesting. I´m going to experiment with it.

mac

[earthtonesaudio]

It is very ingenious. i don't know whether it is possible to choose the resistances to give an 'even' gain over the whole range of VR1.

Thanks!  ...Right, I was thinking the same thing about trying to keep the overall gain relatively stable so there isn't a big volume boost when you turn the pot to one side.  I need to experiment with different resistor values to see how close I can get.

Looks like a dod envelope follower without the pot. Interesting. I´m going to experiment with it.

mac

I'll have to check out that DOD.  Please post your findings!   

[earthtonesaudio]

Alright I did a little quick table to test some values on paper, and here's some examples:

VR1=1M, all fixed resistors=10k:

1. VR1 fully left, photocell resistance infinite.  ...Gain=.01
2. VR1 fully left, photocell resistance zero.  Gain=1

3. VR1 fully right, photocell resistance infinite  ...Gain=101
4 VR1 fully right,photocell resistande zero.   ...Gain=1

If you make VR1 closer in value to the fixed resistors, the range is not so dramatic.  Anyway, as long as all the fixed resistors are equal, the gain is 1 whenever the photocell is zero.

Oh yeah, and if VR1 is set to halfway, the gain is 1 no matter what the photocell is doing.


This could make a neat control.  You could label it "dynamics" and have "more" or "less" depending on which way it's turned.

[Johan]

...it's a clever idea you got there( ..wish I thought of that... ) ..but if you use two LDRs instead, on from each outer leg of the pot, meeting at the viper, you probably get much more predictable results when the viper is anywhere but at at the endpositions...with some diodes in the second stage, it could be a continiously variable compressor/overdrive...cool

johan

[mac]

http://www.generalguitargadgets.com/pdf/ggg_ef440_sc.pdf

BTW, not sure if dod also has an optical compressor.

mac

[arawn]

so your basically talking about a Vactrol right?? if so Interesting. If not, then I have missed something! but I sure would like to understand 

[earthtonesaudio]

Yeah, a Vactrol, or the homemade type with the LED and photocell wrapped in heat-shrink. 

[arawn]

cool that helps I think I get it