Transformerless Vari Mu Compressor build thread

[Script]

Can anyone confirm that the BJT alternative to the sidechain transformer works ok?

I had intended to go BJT in the Diode 8 simply for cost reasons, but then never did -- also cos I had run out of real estate in the case.
Heikki's dual comp does without SC input xfrm. But THR is implemented differently there.

 

[C H]

The 107N is 1.5dB down at 300Hz. Data sheet doesn't say what happens in the three octaves below that. Should we care? Doesn't this mean there's a high pass filter in the sidechain whether we choose it or not?

 

[JMan]

There is a HPF at around 20Hz by default

 

[C H]

Indeed, but doesn't the transformer also impose a high pass at a much higher frequency?

 

[thelivingroom]

The 107N is 1.5dB down at 300Hz. Data sheet doesn't say what happens in the three octaves below that. Should we care? Doesn't this mean there's a high pass filter in the sidechain whether we choose it or not?

The 107N and 107V data sheets show the frequency plot. Somewhere in my messages Heikki recommended the 107V over the 107N.

 

[JMan]

Indeed, but doesn't the transformer also impose a high pass at a much higher frequency?

I do see your reasoning here and I can't speak to it on a technical level. Just going by ear, my unit does seem to be responding as I would expect well down into the low frequencies when I do not have the added HPF engaged. But I haven't verified by running any sweeps or the like.

 

[C H]

The 107N and 107V data sheets show the frequency plot. Somewhere in my messages Heikki recommended the 107V over the 107N.

View attachment 146213

View attachment 146214

Ah OK, that's not too bad. Still, wouldn't bjts be better (and a lot cheaper)?
Does the transformer step up, or down? The pinouts on schematic and data sheet don't match.

 

[scott2000]

wouldn't bjts be better (and a lot cheaper)?

I'm not having any issues with the Hammonds. Can use any transformer too obviously..
from Heikki when I was talking with him on the subject
"Resistance from R35 & R37 and the sidechain transformer inductance form a high pass filter for the sidechain. This is done on purpose. In my opinion in a backward acting vari-mu compressor there should always be a high pass filter on the sidechain. I think I have written more about this on groupdiy"

Better? Who knows.. Cheaper?
Would be cheaper to replace R6 on the sidechain boards with a jumper... if you wanted faster attack times... But then we're getting into maybe more subjective design choices.. Mods...
but again, Heikki's view..
"In my experience when limiting/compressing for effect 2 ms is about the fastest attack needed. With faster attack times you would want to make sure the tubes are in near perfect balance. Because when the “thump” gets to be high enough frequency it is hard to keep it out from the sidechain and from making that overcompression effect which can be seen in the Manley example, even though the “thump” itself might be inaudible. In my opinion ultra fast tube compressor should have a feedforward topology like the Gyraf vari-mu. Feedforward vari-mu compressors have their own unique problems and probably thats why feedforward topology is rarely used in vari-mu compressors. "


These units are laughingly good for what they cost. When I was in Bob Katz studio testing it along side some other very impressive units, it had zero issues being right at home and was preferable to some. And this was before the mastering version being made. Of course that's program dependent and the Leapwing plugin bested all the hardware he had on certain material...

Does the transformer step up, or down

In the compressor they step up
"With 107N the output levels where compression starts will shift few dB up"

The pinouts on schematic and data sheet don't match.

I think it was an error that was never corrected. Pin 1 on transformer goes to the square pad on the pcb.