Transformerless Vari Mu Compressor build thread

[manulaudic]

Regarding C9, C10, C11 and C12, i a was told when building the first build that alternatively i could use 10uf caps... What would be the difference?
I have some 22uf but i can't seem to get them close enough to match.

 

[scott2000]

Regarding C9, C10, C11 and C12, i a was told when building the first build that alternatively i could use 10uf caps... What would be the difference?

a highpass.
a little down @ 20hz...think it was .5db

 

[Heikki]

On main board C13 should be matched with C14 and C15 with C16.

Cap that might be good to match between the channels is C2 on the sidechain board.

I wouldn’t bother matching matching C9, C10, C11 and C12. Those 22u caps are expensive. like Scott said if 10u caps are used there the frequency response is down about 0.5 dB at 20 Hz. -0.5 dB at 20 Hz doesn’t bother me because I doubt any of my speakers can reproduce 20 Hz and I’m not recording pipe organ with somebody going hard on the lowest pedal. For mastering use it is probably best to use 22u just in case.

 

[manulaudic]

On main board C13 should be matched with C14 and C15 with C16.

Cap that might be good to match between the channels is C2 on the sidechain board.

I wouldn’t bother matching matching C9, C10, C11 and C12. Those 22u caps are expensive. like Scott said if 10u caps are used there the frequency response is down about 0.5 dB at 20 Hz. -0.5 dB at 20 Hz doesn’t bother me because I doubt any of my speakers can reproduce 20 Hz and I’m not recording pipe organ with somebody going hard on the lowest pedal. For mastering use it is probably best to use 22u just in case.

Thank you both for answering, the issue i have is with matching the 22u, the ones i was able to find are 10% and the values are quite far from each other so making it impossible t to get same highpass filter frequency from both channels.

 

[Script]

"RC filter calculator online"

Could calculate and see what the difference roughly is. Need eight. So two out of the ten (for price break) that I ordered didn't make it.

If still bothered about some difference, could put smaller value (but same voltage rating) in parallel. But would need to sit on underside of board.

 

[paolomunarini]

"RC filter calculator online"

Could calculate and see what the difference roughly is. Need eight. So two out of the ten (for price break) that I ordered didn't make it.

If still bothered about some difference, could put smaller value (but same voltage rating) in parallel. But would need to sit on underside of board.

Or maybe make a small perfboard daughter board with two pin headers and 2x caps in parallel. That's what I'm doing in my build, using 2*10uF in parallel for each input stage.

 

[manulaudic]

Right, the closest I was able to get is a pair or 21,85uF and 21,41 and 21,45 uF. It's not too bad and I am thinking I should match c9 with c10 or c9 with c11 ? meaning maybe c9 would be 21.85uF and c10 21.41 uF and c11 21.85uF and c12 21.45uF?
Would that be a good idea?

 

[Script]

I'd say that's close enough in any case. Most likely there'll be more noticeable variance (in gain) coming from the tubes.
Will ever use unit unbalanced ? If so, maybe match caps sitting on hot (+) input pins first. But really marginal.

 

[TomKelly]

Finally finished my build, and am now testing. I'm getting very different voltages at the two reference points (stepped sidechain boards) before trimming. One board is measuring 4.1V and the other 5.4V. Consequently one of my meters is lit to 1dB and the other to 3dB permanently. All the other voltages check out within +-0.1V.

I've tried swapping tubes - both swapping the left set for the right set and then swapping out all of the tubes for a new set. No change to the CV and the meter LEDs. When I first powered up the voltages and meters were much more similar, if that means anything... Any suggestions gratefully received!

 

[scott2000]

I'm getting very different voltages at the two reference points (stepped sidechain boards) before trimming

Post #37 may have your answer

 

[TomKelly]

I think I may have discovered the problem - changing RV1 doesn't appear to change the voltage at all, so I'm guessing RV1 is defective.

 

[scott2000]

I'm guessing RV1 is defective.

Did you try the full range of the trimmer to see if it eventually affects the voltage?

You can test the trimmer with power off if you suspect it's goofy.

 

[TomKelly]

Did you try the full range of the trimmer to see if it eventually affects the voltage?

You can test the trimmer with power off if you suspect it's goofy.

Yes, full range from stop to stop - no change at all. Will take the board out of the box and test with power off as you suggest next

 

[scott2000]

Yes, full range from stop to stop - no change at all. Will take the board out of the box and test with power off as you suggest next

Also.....If it's not the trimmer,
...Trying to remember a similar issue I had and my link switch was wired backwards/flipped iirc.

 

[TomKelly]

Also.....If it's not the trimmer,
...Trying to remember a similar issue I had and my link switch was wired backwards/flipped iirc.

Ah, ok. How would that affect the CVs? If I turn the link on/off on mine the voltage for the faulty channel changes from 5.4 to 5.2v, but there's no change to the meter

 

[Heikki]

The link combines the CV voltages by connecting ratio pots/rotary switches parallel. When making adjustments to RV1 it better not to have the link on.

Did you confirm if a faulty trimmer is causing the problem?

 

[TomKelly]

So I've swapped out RV1 for a confirmed working trimmer - nothing has changed, sadly. When I turn RV1 (all the way from one position to the other) the voltage doesn't change on channel 1, whereas it does on channel 2. Is there anything else I could test?

 

[Heikki]

Turn ratio to max and release to slow and measure from the junction where R37 and R48 connect together and does the voltage change there when you turn the trimmer.

 

[TomKelly]

Sorry - I don't understand. Measure the voltage between R37 and R48, or R37 (or R48) against ground?

 

[Heikki]

Voltage between ground and R37 or R48.