REV G 1176 kit - calibration problems

hi, i have a calibration problem with my hairball REV G compressor.

I did the calibration step by step like its written in this calibration manual:

http://www.masonaudio.org/ma_files/G1176_Calibration.pdf

Q bias : OK
Dist. Trim : OK
GR Meter Zero: OK

GR Meter Tracking: Problem!!!

I put a 1 kHz, 50mV signal, Set the input level 12oclock and set output level to the same 50mV signal like the input.
Than I switched to GR mode. and turned up the level of my input signal at the signal source...
both input and ouput signals are increasing 1:1... so if I turned up the INput signal at the source, the Input and the Output signal was always at the same value... no 6dB gain reduction!!! I tried to trim the tracking ADJ R54 on PCB, only 2db result on the VU - Meter...

I tried to put in a audio signal, no Reduction, no compression,...

maybe I did something wrong but I can`t find the problem on the PCB or on the wiring...

Can someone help me? Anyone heart about this problem allready???


thankfully!
  twhanny

Hi,

the Gain Reduction Meter tracking calibration is pretty straightforward. The idea is simply to get a given gain reduction in VU (+4) mode and then have the meter track accordingly and show the exact same gain reduction in GR mode.

I've calibrated mine with a -10 dBu signal as advised in http://www.axtsystems.com/index.php?option=com_content&view=article&id=56:1176lncalibration&catid=34:1176ln&Itemid=62

A good idea is to have a precise metering, I've used Spectrafoo, which is absolutely first class in this domain.

So you want to zero your meter with no signal in GR mode.

Then switch to VU mode, 20:1 ratio, fast release, bypass gain reduction, input the test signal, back your output to read O VU.
You should now have signal in = signal out on your metering, so that when gain reduction will be happening you can confirm it is the 10 dB you are expecting.

Switch gain reduction on, you should see a little drop in your output signal. If not switch back to no gain reduction, turn your input up 3 dB, reduce your output to read again 0 VU, so 3 dB down. Switch to gain reduction, observe the drop.
What you want, as the 1176 is a fixed threshold compressor is to have 10 dB of gain reduction happening. To achieve this you need to input enough signal. Repeat these steps  a couple of times, increasing your input and reducing your output so that in gain reduction you compress your signal 10 db.

Confirm with your metering that you have signal in = signal in bypass, and then signal in = -10 dB signal out.

Then do not touch your in/out pots as it would alter the state you just created.

Switch to GR mode, you should have a 0 VU reading in bypass and a minus something reading in gain reduction. Adjust VR55 to read 0 VU in bypass, switch to gain reduction, adjust VR54 to read -10 dB in compression. Repeat these steps switching between bypass and gain reduction so that both readings are correct.

Confirm everything on your external metering, soundcard in my case.

Now your compressor should compress 10 db and your meter should show it !!!

Cheers, Piotr.

http://www.sengpielaudio.com/MeterComparisonTable.htm

Hi,

you can also check the original manual that contains a lot of informations, both on calibration and the unit itself:

http://www.jblproservice.com/pdf/vintage%20jbl-urei%20electronics/urei-1176lnmanual.pdf

Piotr.

(please use (and read) the dedicated 1176 help thread)