Stereo Overdrive for Synth

[ChrisPolin]

Hi all,

I've had an interesting request for a stereo overdrive, for use with synths and drum machines. The L and R signals are separate, and so in the absence of a better plan, I'm going to keep it simple and go for a 'dual mono' approach, with two identical and separate boards, with common gang-pots, switch and 9V supply.

I have these questions two:

1) Is this separate signal approach the best way forward? I can't think of a simple means by which I can combine the signals and separate them again after the overdrive circuit.

2) I was going to go with a tube screamer layout, because having never made a pedal for a synth I'm just falling back on an old faithful. Any better suggestions welcome, although the guy who wants it is after that sort of sound.

If anyone has any experience with something like this, or suggestions, I'd be interested to hear them!

Cheers

[cylens]

my advice on stereo overdrive is a bit wierd at first but it sounds so much better to me.

I'm often using slight distorsion on stereo mixes (i'm a mastering engineer), and my take on it is that if you distort separately L and R it messes a lot the stereo image (which is not too important for synths, but crucial for mixes)

SO

the key is to perform stereo distorsion in the MS domain (mid/side), to do this you have to first convert L/R to M/S, then apply distorsion, then convert back from M/S to L/R

MS to LR conversion back and forth is pretty straightforward in theory, it's something (stereo distorsion in MS mode) that I'm planning to work on for a pedal in the not too far future...

as for which circuit would be fine, I've built mono distorsions for keyboards and after trying many different "classic" circuits, I've cut it down to 2: RAT (for drum machines) and Green Citrus (search this forum) for synths.

[ChrisPolin]

This is interesting! I've never heard of MS domain before. Presumably you're taking a portion of each signal for the 'mid' component, and the rest comprises the 'side'? How do you propose to do that, and get the original L/R signals back again?

[ChrisPolin]

Ah a quick google has revealed, 'mid' is the sum of the two signals and 'side' is the difference between them. So presumably you'd invert one of the signals for the side?

[ChrisPolin]

https://www.etsy.com/uk/listing/239662236/ms-matrix-mid-side-encoder-decoder-kit?ref=related-1

Something like this, perhaps. So (forgive my ignorance, I've never strayed out of mono space before), does one apply distortion to both M and S signals before decoding them to L/R again? And so would there still need to be two separate circuits for this?

[cylens]

yes, that's it!

M=L+R (which is the mono signal)
S=L-R

and then decoding is almost the same but there's a 1/2 factor

L=(M+S)/2
R=(M-S)/2

should be doable with opamp buffers and inverting opamp buffers

it's well known in the audio engineering domain but never seen it (maybe of the large majority of mono effects).

i'll make some quick audio clips just to demonstrate what it changes. once again, might be overkill for synths...