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

 

[deleteduser]

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.

man idk about all this man. I'm with OP as far as keep it simple , stupid, dual mono, whatever. I think you're on the right track op. Just build two identical cards and go for it. and if you want some sortof stereo linking capacity... just use two capacitors in series somewhere and keep it SIMPLE. cause dude. let me tell you how to match vactrols. if one is 15 ohms. and one is 17 ohms. well . get a two ohm resistor and put it in series with your vactrol or whatever. presto. now you have two 17 ohm vactrols or whatever. I dont even know what a vactrol is.

also. Op. in my opinion. if you're going to go through the trouble to build this thing, you might as well have independent controls in my opinion. not only is it probably more expensive and troublesome to do dual gang controls (potentially...) I think you're just throwing away a lot of functionality and possibility by doing that. having independent controls for LR is in my opinion essential. also. what if you want to run the channels in series and have two compressors in series on a vocal or bass or something. I would go with independent controls for my effort...

 

[ruffrecords]

man idk about all this man. I'm with OP as far as keep it simple , stupid, dual mono, whatever. I think you're on the right track op. Just build two identical cards and go for it. and if you want some sortof stereo linking capacity... just use two capacitors in series somewhere and keep it SIMPLE. cause dude. let me tell you how to match vactrols. if one is 15 ohms. and one is 17 ohms. well . get a two ohm resistor and put it in series with your vactrol or whatever. presto. now you have two 17 ohm vactrols or whatever. I dont even know what a vactrol is.

I agree with keeping it simple but if you want it to work in stereo you have to match the gain elements. Adding a resistor to one of them will not work. As they say in Star Trek " You cannae change the laws of physics"

also. Op. in my opinion. if you're going to go through the trouble to build this thing, you might as well have independent controls in my opinion. not only is it probably more expensive and troublesome to do dual gang controls (potentially...) I think you're just throwing away a lot of functionality and possibility by doing that. having independent controls for LR is in my opinion essential. also. what if you want to run the channels in series and have two compressors in series on a vocal or bass or something. I would go with independent controls for my effort...

I agree. Ganging controls is hard and limits the compressor to stereo only. It is not much effort to set the controls of both channels to the same settings.

Cheers

Ian

 

[cpsmusic]

Just to "throw the cat among the pigeons", one of the top mix engineers of all-time Chris Lord-Alge claims that he never stereo links compressors. He says that it narrows the stereo image and that he prefers the wider sound created when the L/R channels are unlinked.

I've actually experimented with this myself and haven't really noticed the image drift problem that is often referred to. This of course will depend on musical style. In most rock/pop the musical elements that activate the compressor are mostly panned to the centre. In other styles YMMV.

Cheers!