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Testing 1176 Audio Compressor
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Testing 1176 Audio Compressor
yan6
Hey Guys I have an 1176 opto compressor that seems to have stopped working and I would like to start to troubleshoot it. Is it possible to test a compressor with test equipment, ie signal generators and scopes. In the past Ive had to feed a recorded audio signal into it; record it and compare the two waveforms to see if its compressing. It would be much better if I could see it realtime on a two channel scope.

I hope this isn't a dumb question, as I'm typing it I'm thinking outloud that I could put an audio signal into it on ch1 of the scope and ch2 of the scope on the output. Problem being that you cant get a nice trigger on a complex wave to really measure it hmmm.....
tarandfeathers
You don't need a complex waveform to test, you can do it with a sine wave, which is how I've calibrated every 1176 I've built, using multiple channels to look at multiple points in the signal path.

For reference, the 1176 does not have any optical element, it uses a FET as a variable resistor to control gain.

For looking at actual audio audio on a scope, roll mode is your friend.
m.o
Indeed, all you really need for basic test is a constant sine wave in and a db / vu meter on the output of the compressor.
Start with threshold all the way up (no compression, just pass through), make a note of the db value coming out.
Start turning down the threshold, you should see the db value drop as the compression starts.

You can use this setup for checking compression ratios too;
Set a certain volume into the compressor, turn threshold untill compression just starts.
For example to check 4:1 compression increase the level of the sine going in by 4 db, the output should only have increased by 1 db.
(This is how I remember it without looking it up).

I just used my soundcard and my DAW (reaper) with a sine generator plugin and monitoring on the input channel.
DAW is usefull to get db values (as opposed to measuring the waveform on an oscilloscope).
clorax hurd
yan6 wrote:
Hey Guys I have an 1176 opto compressor


1176 is not an opto compressor. It uses FET for gain reduction. It's back from the old days, when user manuals looked like service manuals look today. You can find lot of info in there: http://www.studiomanuals.com/docs/urei/1176ln_manual.pdf
You perhaps (probably) have some 1176 clone, but most of the info in that original's manual applies to most of the clones too.
Calibration steps and performance checks starting @ page 11
yan6
Thanks for the support all, its up and running again.

It is a clone and it has some kind of blend knob with and off switch, which I never use. It looks like the off switch was not engaging causing the calibration to be way off on one channel.

The link to that manual is great, its going to get printed and placed with my other important build notes thumbs up
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