spdif

1176 compressor problem
« on: March 07, 2019, 09:19:08 AM »
Hello everyone! I am new here and came with a faith that my problem would be solved by some of you genius out there!  :)

So, I am building an 1176 compressor. It is not from Hairball. I downloaded the schematics and etching files from Mnats website. The version is RevH.

The build went smooth and it worked flawlessly for a few days. But recently 2, 3 days back I got a strange problem. The gain reduction according to the ratio buttons is not working. The JFET is "JUST" stable on the quicent bias. It doesn't vary at all. I can provide all the voltage details if someone comes around for the help.

Thanx in advance.

spdif



dmp

Re: 1176 compressor problem
« Reply #1 on: March 07, 2019, 10:47:22 AM »
Sounds like the control voltage from the sidechain has a problem.
Look for bad solder connections.
 

gyraf

Re: 1176 compressor problem
« Reply #2 on: March 07, 2019, 10:48:57 AM »
please use (and read!) the dedicated help thread: https://groupdiy.com/index.php?topic=646.0

Jakob E.
..note to self: don't let Harman run your company..

spdif

Re: 1176 compressor problem
« Reply #3 on: March 07, 2019, 11:28:36 AM »
Sounds like the control voltage from the sidechain has a problem.
Look for bad solder connections.

Thanx for the input. I too believe that something is surely wrong with the GR control amp.  If I remove the Audio FET and feed in a 0db sine wave then across the rectifier diodes I get corresponding DC voltage. But after connecting the FET (with just a quecent bias) I see the quecent bias only across the rectifier diodes. Hence the FET dosent work as a Variable resistor.

spdif

Re: 1176 compressor problem
« Reply #4 on: March 07, 2019, 11:31:20 AM »
please use (and read!) the dedicated help thread: https://groupdiy.com/index.php?topic=646.0

Jakob E.


Jakob Sir, thanx for the input. I will look into the thread.

spdif

squarewave

Re: 1176 compressor problem
« Reply #5 on: March 07, 2019, 11:44:36 PM »
Thanx for the input. I too believe that something is surely wrong with the GR control amp.  If I remove the Audio FET and feed in a 0db sine wave then across the rectifier diodes I get corresponding DC voltage. But after connecting the FET (with just a quecent bias) I see the quecent bias only across the rectifier diodes. Hence the FET dosent work as a Variable resistor.
There is a feedback loop: the GR control amp (aka sidechain) takes it's input from the output of the signal preamp. When the signal is attenuated, the GR control amp get's less signal and thus attenuates less. This feedback loop linearizes the gain reduction. So the control voltage (CV) will behave differently if you take the JFET out and break that feedback loop. I would leave it in when testing.

When testing compressors, I find it useful to make a sound sample (using something like Audacity or Wavepad) that consists of bursts of tones (like 100ms every 1s for 10s), play that into the compressor while recording the output in another track and then visually compare waveforms. You should see the signal level change in reaction to each tone burst more or less, faster or slower, etc.

spdif

Re: 1176 compressor problem
« Reply #6 on: March 08, 2019, 08:11:36 PM »
There is a feedback loop: the GR control amp (aka sidechain) takes it's input from the output of the signal preamp. When the signal is attenuated, the GR control amp get's less signal and thus attenuates less. This feedback loop linearizes the gain reduction. So the control voltage (CV) will behave differently if you take the JFET out and break that feedback loop. I would leave it in when testing.

When testing compressors, I find it useful to make a sound sample (using something like Audacity or Wavepad) that consists of bursts of tones (like 100ms every 1s for 10s), play that into the compressor while recording the output in another track and then visually compare waveforms. You should see the signal level change in reaction to each tone burst more or less, faster or slower, etc.

You are absolutely right! Yesterday while I was messing around, I put the FET in place and gave a sine wave signal at 0db. And before doing this I removed the ATTACK pot connections. Now at this time I was able to see the "corresponding DC" voltage on the rectifier diodes in GR amp. But as soon as I connect the ATTACK pot, the Qbias voltage ie -0.5v in my case comes on the diodes . I don't see any corresponding DC on the diodes which could be subtracted from the Qbias (as the manual says) looks like my attack pot is short or something OR maybe my GR amp is not amplifieng the signal to "THAT" level which is required and that's why the voltage on the diodes dosent go positive... :-\

radardoug

Re: 1176 compressor problem
« Reply #7 on: March 09, 2019, 12:27:59 AM »
Short to ground on the pot?

spdif

Re: 1176 compressor problem
« Reply #8 on: March 09, 2019, 05:00:57 AM »
Short to ground on the pot?

I didn't get that... :o

 

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