Calibrating your compressor is a crucial step to ensure its proper operation.
You’ll need a decent DMM that can read AC at 1KHz accurately. Usually, any meter over $30 can read 1KHz AC signals well enough.
When asked in the calibration steps to send a 1K at 0 dBu signal to the input, use your DMM to measure the AC voltage between the IN + and IN – at the input XLR (PINS 2 & 3). Adjust your signal generator until you see 0.775VAC between these points. When you see 0.775 VAC between IN + and IN -, you have 0 dBu at your input.
1 . Q Bias (All Revisions)
Set controls as follows:
Input = “24” mid rotation
Output = “24” mid rotation
Attack = full CCW (switched to off position)
Release = full CW
Compression ratio = 20:1
Meter mode = “GR”
Q-bias adjustment = full CW (Highest Output)
Apply a 1 KHz 0 dBu signal to the input and confirm with your DMM between IN + and IN – at the input XLR. Now move your DMM to the output XLR and measure AC between OUT + and OUT -. Adjust the output control to read +11dBu (2.75 VAC) on your DMM at the output. Slowly turn the Q-bias adjust (R59: Rev A /D and R81: Rev F ) CCW until a drop of 1 dB occurs, and your DMM reads +10 dBu (2.44 VAC). This places your gain reduction FET Q1 slightly into conduction.
2. Discrete Meter Circuit/Null Adjust (Rev A and D Only)
This step is only required for the Rev A and D. The Rev F uses an IC in the meter circuit and no Null adjust is required. If you have a Rev F skip to the next step.
Disconnect your signal generator. No signal is required for this step. Take R44 out of circuit by shunting the middle pin and pin not marked “normal operation”.
Set your DMM to measure DC volts and set it to measure the DC volts across TP10 and TP11 for the Rev A and across R74 for the Rev D.
1. Zero the compressors “GR” meter with the R71 zero adjust pot on the front panel.
2. Adjust pot R75 (null adjust) for 0.0 Volts across TP10/TP11 (Rev A) or R74 (Rev D).
3. Repeat 1 & 2 until both conditions are met.
Now set your shorting pin back to the “normal position” by connecting the two pins marked for “normal operation”. Your meter will most likely drop a few dB. Set it back to zero by using the zero adjust pot on the front panel (R71). DO NOT touch the null adjust, it was set in the last step.
3. Reduction Meter Tracking Adjustment (All Revisions)
This step sets your meter tracking so if your compressor is compressing 10dB, you see -10dB on the meter. Confirm your meter shows zero on the meter with GR OFF. If needed set it to zero using the zero adjust pot on the front panel (R71: Rev A /D and R55: Rev F ).
Set controls as follows:
Input = “24” mid rotation
Output = “24” mid rotation
Attack = full CW
Release = full CW
Compression ratio = 20:1
Meter mode = “GR”
1. Apply a 1 KHz, 0dBu signal, confirmed with your DMM at IN + and IN – at the input XLR (0.775 VAC).
2. Move your DMM to OUT + and OUT – at the output XLR.
3. Set output control for 0dBu (0.775 VAC) at the output.
4. Set attack full CCW (off position). Set input control for +10dBu (2.44 VAC) at your output XLR.
5. Turn the attack control ON (Fully CW) and readjust the output level control for “0dBu” if necessary.
6. Now let’s set the meter to display this 10dBu drop. When the attack is “off” (full CCW) and you see +10dBu at the output, set the meter to 0 by using the front panel zero adjust trimmer.
7. When the attack control is “on” (full CW) and you see 0dBu at the output, use the tracking adjust (R44: Rev A /D and R54: Rev F ) to set the meter to -10.
8. Repeat 6 & 7 until the output drops 10dBu whenever the attack control is turned ON. This can be a bit tricky. You’ll want to anticipate how one control will change the other.
4. Distortion Trim (Rev D and F Only)
To adjust the distortion trimmer (R86: Rev D and R16: Rev F ), you need some specialized equipment that can measure Total Harmonic Distortion (THD). Sometimes you’ll see it listed as THD+N which is the same measurement + noise. This calibration step is the least important. It might be the difference between having a THD measurement of 0.03% or 0.08%. If you can hear that, congratulations, you’re superhuman. It does have some effect on your Q1 GR FET, but again it’s minimal. If you don’t have a way to measure THD, just leave the trimmer centered. If you have hardware or software to measure THD, here is how you calibrate it.
Input = Full CW
Output = to number “18” on the front panel
Attack = full CCW (switched to off position
Release = Full CW
Compression Ratio = 20:1
Meter mode = “GR”
Apply an input signal of 500 Hz at -30 dBu (0.024 VAC) and measure THD of the resulting output signal. Adjust trimmer until the minimum amount of distortion is achieved.
If all of the steps went well it’s highly likely your compressor is working properly. Congrats! Now go, plug it in, and record some music!
FET/RACK and FET/500 Input Mod
However, the revision A’s input is hotter by design, a full 6 dB. Some users find they have a hard time getting light compression. Here is a very simple mod you can do to the 3-deck potentiometer that will result in a 6dBish “slug” on the input. You can do this mod on any revision of FET/RACK or FET/500 and it’s simple.
Newer FET/RACK PCBs have a R-OPT next to the t-pad for this resistor. If you have a FET/500 or older FET/RACK you can easily still add the mod. Looking from the bottom (rev A Rack pictured) locate the side middle lug that attaches to pin 1 of the input transformer. Solder a 100Ω resistor (anything above 1/8W is fine) from that terminal to the front middle terminal. The front is where the shaft is located.
FET/500 Calibration
Step 1 – Qbias Adjustment. In this step, we’re going to set the FET slightly into conduction.
Set Input and Output to midway
Set Attack and Release fully clockwise
Ratio at 20:1
Put the unit in GR Off mode
Apply a 0dBu (0.775 VAC) 1KHz signal to the FET/500 input
Observe the output level and turn the Qbias trimmer counter-clockwise until the output level reaches a maximum value and no longer increases. The FET is now in “pinch off”
Adjust the Output control so the FET/500 output reads +11 dBu (2.75 VAC)
Rotate the Qbias clockwise until your output drops by 1 decibel to +10dBu (2.45 VAC), it will happen quickly
Step 2 – VU Meter Calibration:
Set Input and Output to midway
Set Attack and Release fully clockwise
Ratio at 20:1
Put the unit in GR Off mode
Apply a 0dBu (0.775 VAC) 1KHz signal to the FET/500 input
Adjust the Output control so the FET/500 output level reads +16 dBu (4.88 VAC)
Adjust the VR1 “Gain Trim” on the meter board so the RED “Peak” LED starts to illuminate.
Adjust the Output control so the FET/500 output level reads -6 dBu (0.388 VAC)
Adjust the VR2 “VU Trim” until the -10 LED LED starts to illuminate.
Remember that this meter reads 0dBu = +4dBu like the original units. So the -10 dBu LED is equal to -6dBu.
Step 3 – Gain Reduction Tracking
Set Input and Output to midway
Set Attack and Release fully clockwise
Ratio at 20:1
Deselect GR Off so your unit is now in GR mode
Apply a 0dBu (0.775 VAC) 1KHz signal to the FET/500 input
Adjust the Output control so the FET/500 output reads +0 dBu (0.775 VAC)
Put the unit in GR Off mode
Adjust the Input control so the output reads +9 dBu (2.18 VAC)
Repeat the last 4 bullet points until you see an output of +9dBu in GR OFF mode and an output of 0 dBu in GR ON mode
With the unit in GR OFF (9 dBu output) adjust the “0 Adj” trimmer located on the front panel near the input knob so the 0 illuminates
With the unit in GR ON (0 dBu output) adjust the “Tracking Adj” located on the main PCB and accessible through the meter PCB so the -9 LED illuminates
Repeat the last 2 bullet points until you see GR meter dropping between 0 and -9 just like your output level.
Step 4 – Signal Preamp Linearity (Rev D and F Only)
Set Input fully CW and Output to “18”
Set Attack and Release fully clockwise
Ratio at 20:1
Put the unit in GR Off mode
Apply a -30dBu (0.0245 VAC) 500Hz signal to the FET/500 input
Adjust the “Dist Adj” located on the main PCB and accessible through the meter PCB for lowest THD