compressor sidechain question

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xazrules

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hi, i was wondering if it is possible to build a stereo compressor but with only a sidechain amp that drive both channels, because i'd like to build a fairchild 670 in a 3u rack to use mainly on the master.
maybe if is possible i can squeze the second signal amp somewhere inside the rack.
what do you think?

cheers
 

richiyobs

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i m trying to build a light version diode bridge...but i m not so knoledged in electronic. so i was looking for sidechain solution to...and found that varimu
 

Script

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You mean an external sidechain input to feed kick drum for example ?

Or do you mean a sidechain detector that folds program material from main audio input to mono to drive both channels ?

Heikki's comp does not have a sidechain input, which doesn't mean you couldn't add one.
 

xazrules

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Nono guys, I’ve already build a fairchild 660 but now I’d like to do a stereo version, but using just one sidechain amplifier to control the grid of the tubes of the right and left channel without the need to build two sidechain amps, that’s the most expensive thing in a 670 build.

cheers
 

xazrules

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You mean an external sidechain input to feed kick drum for example ?

Or do you mean a sidechain detector that folds program material from main audio input to mono to drive both channels ?

Heikki's comp does not have a sidechain input, which doesn't mean you couldn't add one.
Yes just one sidechain detector that sums the left and right channels and send the same control voltage to both for good stereo image
 

xazrules

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i m trying to build a light version diode bridge...but i m not so knoledged in electronic. so i was looking for sidechain solution to...and found that varimu
Richi if you wanna learn about compressor the easiest way is to build an altec 436/rs124 variation or better the poorman 670, it is very easy and very few components, and it teach you how every compressor works, so you can build whatever you need. My latest build has been an adr compex and I tell you that solid state comps have a lot of components so there are the possibility of more errors!
 

Tekay

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When you sum the signal before the threshold detectors signals that are present in both channels will be +6dB and out off phase signals will not hit the detector. We use to have a switch for single/dual detectors on our bus comp and it changed the stereo images.
 

moamps

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It should also be checked whether one SC circuit can control two sets of vari-mu triodes equally well. With one SC circuit, the possibility of MS dynamic processing is also lost.
 

xazrules

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I don’t really care about mid/side because I never use it, or if I really need I can split in the daw to two separate tracks. So if we send balanced L and R signals into a summing amp and then go into the sidechain are we not sending the same control voltage to both L and R amps? We are compressing everything in the same way, isn’t it? For example in a Fairchild comp could be done passively between the signal amp out trafo and the sidechain in trafo or we need a summing buffer between the two?
 

Matt Syson

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As TK commented, a mono sum of the incoming audio may have issues if there is out of phase material (so it cancels) and may under compress depending on panning. It would be more usual to have dual rectifier circuits and then possibly sum the DC from these into the attack/release/threshold controls. So while you CAN economise, you may not get the performance you will find most useful.
Matt S
 

abbey road d enfer

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I don’t really care about mid/side because I never use it, or if I really need I can split in the daw to two separate tracks. So if we send balanced L and R signals into a summing amp and then go into the sidechain are we not sending the same control voltage to both L and R amps?
Yes. however, this signal may not be correct, as has been mentioned earlier by Tekay. The best stereo comps use two detectors that are analog-OR'ed to the amp that drives the gain cells. That way, the CV is equal to each channel and is driven by the loudest signal.
We are compressing everything in the same way, isn’t it?
Not so sure. You would need to have gain-cell tubes perfectly matched.
 

FIX

Paul Wolff
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Feedback compressors are sometimes difficult to insert side chains, with the 660 if it's similar to the tube version with the transformer that follows the output and feeds the detector portion, could insert something at that point. Inserting a cheap peak compressor of graphic EQ would be interesting. Remember that the quality of the Comp or EQ doesn't matter much, as it never sees the audio signal path.
 
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