Calibrating / Adjusting the SL-1176-KIT

You will need an oscillator, a voltmeter and an accurate external VU meter to make these adjustments. If you do not have an oscillator, a test-tone CD will work, or generate a sine wave from your DAW. A console or other outboard device with analog VU meters makes these calibrations quite easy to set. Using a DVM to measure voltage and some math to convert voltage ratios into dB is not quite so easy — but that's a good exercise if you're inclined... 

In a nutshell, you will be using an external reference such as a console's VU meter to show how much gain reduction is actually taking place. Keep in mind that the "Bypass" switch disables the unit's gain reduction ability; it's not a hardwire bypass. For example, with the bypass switch on (bypassed), adjust the 1176 for a 0 VU console reading, then engage the Gain reduction by turning the bypass switch off. The VU meters on the console might read –5dB. That’s your external reference. The compressor is actually doing 5 dB of gain reduction. You then match the 1176's GR meter by adjusting the tracking adjust pot RT-3 CW to go down, or CCW to go up the meter scale.

It takes many small adjustments between RT-3 and RT-2 to make both read properly. This is because the pots interact with each other. More on this later. 

Q-bias of the gain reduction FET

  1. Set the oscillator to output a 1KHz sine wave at 0 dBu (.775 Vrms).
  2. Connect the oscillator to the 1176 input and connect the output of the 1176 to a device with an accurate VU meter such as a console's input channel.
  3. Set the 1176's "Bypass" switch to the off position. 
  4. Set the 1176's Output pot to the 12 o’clock position. 
  5. Adjust the 1176's input pot until you get a reading of 0VU on your console VU meters.
  6. Adjust RT-1 (CCW moves VU lower) on the Side-chain board until you see the console VU meter move to the left to indicate less level. Then turn RT-1 the opposite direction while watching the console VU meter. It may take several turns of RT1 to see a change. The goal here is find the point where the gain stops increasing thereby moving the FET out of its conductive range. Once you have verified there is no more level increase, (if the level keeps going beyond the scale of your meters, back off the 1176 input until you reach a readable scale with some headroom), turn RT-1 in the opposite direction until the console VU drops 1dB in level. You have just biased the FET to the start of its conductive range.
Distortion Trim Adjustment

This is controlled by RT-1 on the input board. It can only be set properly with the use of a distortion analyzer. Fortunately, the adjustment is so minor that it doesn’t have much effect on the distortion of the unit. If you do not have a distortion analyzer, simply set it in the center of its range and you will be fine. There is really no audible effect of setting RT-1 in any position. I included it to be true to the original.

If you do have a distortion analyzer, bypass the unit and send 1kHZ @ .775Vrms into the 1176. Adjust input and output for unity gain. Adjust RT-1 for minimum distortion.

Setting "0" on the gain reduction meter
 

With no signal at input, adjust RT-2 on the side-chain board until the meter reads zero on the scale. CCW moves the meter up on the scale. Again, it may take several turns of the pot to see movement. You will then be ready to adjust the tracking of the meter to follow the actual gain reduction of the FET designated Q5.

Meter tracking for gain reduction
Getting the Gain Reduction meterto accurately track gain reduction involves many small adjustments between the meter zero adjustment, RT-2, and the tracking adjustment, RT-3. It helps to have an external analog VU meter such as on a mixing console to quickly verify changes in gain.

  1. Bypass the unit. Select a 20:1 ratio via switch. 
  2. Apply a 1kHz signal to the input. Set the input pot to 12 o’clock. 
  3. Adjust the output for a 0VU reading on the console's VU meter. 
  4. Engage gain Reduction by turning bypass off. Watch for a gain reduction at the console's VU meter. If you don’t see any level reduction there, turn up the input pot on the 1176 to apply more gain reduction at the FET. You will then need to zero out the console's meter with the 1176 bypassed to get an easy reading of gain reduction when the Gain Reduction is engaged. You want to match the gain reduction on the 1176 GR meter to what is seen on the console's VU meter by adjusting the tracking adjust, RT-3. CW movement moves the needle down in scale. 
Once the GR Meter accurately tracks, as reflected by the external reference meter, bypass the Gain Reduction and note the movement of the GR meter pointer. It probably moved in the direction that RT-3 was adjusted. 
  1. Re-adjust RT-2 to re-set the meter pointer to "0 VU," (no gain reduction bypass on). 
  2. Re-engage the Gain Reduction (bypass off). The tracking probably moved a little.
  3. Re-adjust RT-3 to show actual gain reduction. Bypass the unit and re-adjust RT-2 So that the pointer is again at 0VU. Each time you step through the procedure, the meter should move less off its mark than in the previous attempt at calibration. It is not uncommon to have to repeat the steps 5-6 times before they both lock in and stay where they should, just like a stock 1176 GR meter calibration.
Once the meter's are accurately tracking, change the amount of gain reduction by turning the input pot up or down. You will need to adjust the output pot to set a zero reference on the external VU meter again while the 1176 is bypassed. Turn the bypass switch on and off and see if different amounts of GR track correctly on the 1176 meter. The meter on the 1176 is usually accurate up to 10 dB of GR. Above that, it can be off by a few dB. This is common on all 1176’s and will vary with the type of meter actually used.