First I did Fine Tuning Step 1, then didn’t touch that again. I looked at it both on the scope and recording the drum track back into Pro Tools to double check. After that I tried doing Step 2 per the original instructions of adjusting R22 and R56 (mine are older V2.1 boards with trimmers I installed in those resistor spots) with the 1k tone, but the compression was all out of whack, both on the meters and to my ears, after the THD sounded right. With the power off those trimmers can be measured in place, so it was easy to reset and try again a few different ways, but I never could get it to where I was totally satisfied doing it that way. I also noticed when listening to the compression with the stock values of 12k (R22) and 1k (R56) before touching the THD tuning that Ch1 would drop out when the signal got low enough. It sounded like a gate cutting off early. I then decided since compression was more important to me than THD precision that I’d just worry about that and let the THD fall however it ended up. I remembered a couple of mentions in here about lowering R56 to 200R and starting there, so that’s what I eventually did to begin the process again. I also decided to loop a nice sounding, fairly basic and slow, uncompressed mono drum overhead track that had a good decaying crash at the end instead of 1k tone, so I could hear if it was still cutting out. I multed that to both channels so I could also listen for symmetry (panned L and R on my console). Because I could measure the trimmers in place (power off!), I could raise and lower them the same amounts on both channels as I went along. I set R56 to 200R, listened, and compression was good but had no THD. I then raised both channels (R56 only) 100R at a time until the THD became nice and audible but the compression was still clean and mostly symmetrical from fastest to slowest attack and release. I then lowered Ch1’s R56 very slightly to get the compression symmetry of that last drum hit and cymbal decay to match visually on the Pro Tools meters and to my ears. With the multed tracks panned L and R it now sounded like a mono track down the middle, with no more L to R back and forth warble. After that I double checked with a low to high volume (from no compression to -16db) sweep of 1k and it looked and sounded good. I didn’t run it on a graph to check for perfection, but it sounds very right to me. The way the THD ended up for me is plenty enough of it at 12 o’clock input with program material, and 2-3 o’clock with a single track of drums or guitar, and output set for unity gain between Bypass and THD. I hope this helps!Hey folks, I've spent hours experimenting with adjusting these circuits but I'm still struggling with the calibration on this. The compression curves refuse to match between channels.
Is anyone able to explain exactly what each of the trimmers does? I've not added a trimmer for FT2A as the general consensus is that it's not necessary, but I'm starting to wonder if it will help.
I've attached graphs of my attempts at matching the response curves between channels using a 1kHz sine wave (note colours are not consistent between graphs). So far I've only tried making adjustments in compression mode:
- Graph 1 shows the response when both channels are set with every trimmer at the same resistance and unlinked.
- Graph 2 shows the effects of adjusting FT2B 1 turn and 1.5 turns clockwise, and adjusting the input of R to compensate for gain changes (-1 means one step back from full).
- Graph 3 shows the effects of adjusting FT1 and R input to compensate.
@thelivingroom your build looks great. Hoping I can get there soon... Any chance you could explain your calibration process in more detail?
I should also mention that I matched every resistor exact (to whatever decimal as far as my dmm would read) between the two channels, including the attack switches. I also matched the caps on the sc hpf.
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