Creating a stereo version of an existing circuit

[snaper]

Hi Guys,

I really like the gentle compression / limiting of the optical things, like the LA.
After some research, I've built the LA-Light, as a fun little effect box.
Believe it or not, I love it.
I'd like to make a stereo version out of it.
The question is that if I want to turn it to stereo, is it "enough" to create two boards, match every possible components and use dual gang pots?

 

[snaper]

btw, here is the schemo.
As you can see, it starts with a balanced ot unbalanced converter, then the limiter.
I think I'll use a simple 1:1, 600:600 transformer at the end.
Could you recommend something "crispy"?

 

[Mictester]

Hi Guys,

I really like the gentle compression / limiting of the optical things, like the LA.
After some research, I've built the LA-Light, as a fun little effect box.
Believe it or not, I love it.
I'd like to make a stereo version out of it.
The question is that if I want to turn it to stereo, is it "enough" to create two boards, match every possible components and use dual gang pots?

Not really. If you do that, you'll get some really weird stereo imaging problems - a loud sound in one channel won't reduce the gain in both channels, so the image will "wander". My approach (and that used by most stereo processors) is to use a rectifier per channel and sum the outputs of the rectifiers to (effectively) a mono control signal. This control signal then controls both attenuators equally (assuming you've matched the parts), and the stereo image remains intact.

 

[ruffrecords]

First thing you need to do is change the labels of the gain and threshold controls on your schematic. The one you have labelled gain is in fact the threshold control and vice versa.

To make it work in stereo you need to do two things.

1. Create a control signal based on the sum of the left and right inputs and feed it to both left and right Vactrols
2. Match a pair of Vactrols. This is not too difficult but you may need a lot of Vactrols to get a similar pair. The method I use is to feed a constant current to the Vactrol LED and measure the output resistance with a meter. The resistance you want is whatever would give about 20dB of attenuation. If you can get a pair that have a resistance within a few percent of each other at 20dB attenuation then they will track well enough for stereo. For the circuit you posted, the resistance value you need is 4K.

Cheers

Ian