Calibration, the quick quick quick version. Assuming you've pretrimmed all of the pots as instructed, there will be little to do. This was an exercise in gross overkill, and I'm actually removing half of the pots on my production version, majority are not needed. Anyways:
Engage the "External In" Button. This acts as a soft bypass, and makes compression impossible (unless of course you have something plugged into the TRS that's providing an external signal". Put Blend knob full counterclockwise (Fully Dry). Put threshold fully clockwise. Set Gain to 0. Set filters to "off". Set attack to "30". Set release to ".3" Leave everything like this until instucted otherwise:
Now, feed the compressor a 1K signal at about +4dbu (probably somewhere around -15dbfs in your daw) and route it back into your daw. Actual signal level isn't critical, just make sure it's somewhere around there.
Unity Gain (VR1L/R) - Toggle between bypass and comp in. Adjust VR1L/R for unity gain (ie - same levels when the compressor is engaged as it is when it is in bypass)
DTrim & Magic - Now put the blend knob to fully clockwise (wet). Look at the 1Khz signal coming back into your DAW with a frequency analyzer (I recommed blue cat's as it goes down to -120dbfs & it's free):
http://www.bluecataudio.com/Products/Product_FreqAnalyst/
You'll see the 2nd harmonic distortion node at 2Khz. Adjust DTRIM (VR3L/R) for minimal distortion at 2Khz. (While you're here, this would be a good time to hit the grind button so you can see/hear it working, just don't forget to turn it off before continuing with the calibration)
Now you need to crank the gain on the compressor to max (+18db). So turn down the input signal going to the compressor a little so you don't clip your A/D when you do this. I usually turn it down so my DAW meters read -22dbfs or so. So anyways, crank the gain to max, adjust magic pots (VR2L/R) so there is minimal peak at 2K (you'll probably need to turn it counterclockwise quite a bit). Repeat these 2 steps (DTRIM and MAGIC) until distortion won't go any lower.
Meter - With "external in" still engaged, look at the signal level coming back into your DAW and take not of it. Now disengage "external in", and turn the thrshold knob until your level is 8db lower than the level I just said to take note of. Adjust meter pot (VR201) until the analog meter says 8db.
Gain Knob - VR203(+Gain) & VR204(-Gain) allow to trim the gain knob to for accurate gain levels. In other words, they will allow you to make sure when gain is at max (+18db), that you get exactly 18db of gain, and when gain is at minimal (-6db), that you get exactly 6db of attenuation. Tread carefully here, as one will affect the other, and turning either of these pots will move your 0db point, so it is recommended that neither of these pots be turned more than a couple rotations.
Export a 1Khz test tone and make sure that the level is low enough (usually -22dbfs or so here) that it will not clip your A/D coming back in when you crank the gain to max 18db. Make sure blend is in wet position, engage "external in" button so compression isn't possible.
VR203 (+Gain) - The amount of gain at max should already be pretty accurate (typically 17.7db - 18.3db), so there will be very little adjustment to do on this one. With the compressor hard bypassed, look at the level coming back into your DAW. Now engage the compressor and crank the gain to max. Adjust VR203 so that there is exactly 18db of gain.
VR204 (-Gain) - The "what" here is up to you, but I will tell you the "how". Because of variance of linearity of the pot, tolerance of the pot (this is why I said to use the one closest to 10K here) as well as numerous other factors, if you trim VR204 so that you have exactly 6db of attenuation with gain full counterclockwise, then when you put the knob to "0" on the silkscreen, you will likely not have exactly 0db of gain. For this reason I will usually put the gain knob at "0" on the silkscreen, and adjust VR204 so that I have exactly 0db (unity gain) when toggling the compressor in & out of hard bypass. This however means that when you put the knob fully counterclockwise, it will likely read anywhere between 5-6db of attenuation, not exactly -6db. It's a compromise here, but I would much rather have "0" be "0", and any gain above "0" to be accurate (for makeup purposes), then have exactly 6db of attenuation when knob is at "-6" on the silkscreen.
That's it for calibration. Assuming you have pretrimmed VR4L/R & VR203 then there are no adjustments to be made for the sidechain or ratios. Ratio's should be accurate worst case within 0.3 : 1 tolerance, (example 4:1 may be anywhere form 3.7:1 to 4.3:1). This is normal. There are too many outside variables here for all of them to ever be exact, and any attempt to adjust one will affect the other.
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