"gain make-up" on vari-mu compressors

[johnnytoobad]

Hi, i am working on a mutant vari-mu, i am planning to use a modified federal input/comp/sidechain(but balanced that means2 X 6SQU7) and sta-level output(12AT7/6V6), these "stages" will be coupled by 0,1u cap adn 500K, that makes a 3hz high pass, which is fairly good enough for bass... but i wasn't sure of using a pot there before the 12AT7  to use it as an output gain control, and the value of it, the sta level using 330K.
I was thinking about it, because this could be great to use with the output pad to get some crunch with heavy outpad and pot to max (lower R on signal path), or cleaner driving less the out stage padding less...
is it worth (the time, money, hole on the front panel...) adding this pot? wich value would you use???

 

[PRR]

> hole on the front panel

Breadboard it before you worry about pretty panels.

 

[johnnytoobad]

PRR, thank you for this great advice...
i guess that it means that it is not wasting time to try it... 
and it was just what i was asking "is it no use or not to try?"
so lets try... and there is not already pretty panel but second hand panel for prototyping, i was just talking about the effort to drill it with my bad driller or bringing it at friends place to use theirs but it would be best to go only once...

 

[Butterylicious]

Anything is worth trying.  If it doesn't work, it's not a failure, you've simply eliminated a variable.  What's that PRR saying, "plan to be wrong about something."  W/o know your circuit, it's hard to guess at a single pot value.  When in doubt, try a 100k because "in the dark, all resistors are 100k." (John Broskie)  At this stage, clip leads are a lot more valuable than drill bits.

 

[Rob Flinn]

I'm not sure exactly what your circuit is like being a hybrid, maybe you could post a pic ?

However, I do know the sta level a bit.    In the sta level the side chain is fed from the output of the 6v6 valves but before the output transformer.  If you have a pot prior to this point then you will be adjusting the operating point as well as the output.  In that particular case it is better to attenuate after the output transfomer, that way you won't be changing other parameters too.

not sure if this helps but .............

 

[johnnytoobad]

Rob, the side chain is taken before the ouptut stage, just like the Federal but with a balanced sidechain... only the output stage is almost like the one from the sta-level.
soon will come a schematic and pix from the beginning, but i got to do first metal works, as i am working full time and make this as a hobby it takes a little time... i need to make all construction documents before starting to make the thing itself... i learned it from my job, where we are allwys in the hurry and never think twice (but often make more than twice before it can work...), but i am not as rich as the company i'm working for...

 

[PRR]

You can breadboard with sockets nailed to a wood board and pots hanging loose. Yeah, it's awkward and will buzz. But you can try your plan and see if it has hope, before you invent in scrap-metal and drill-blood.

> i need to make all construction documents before starting

Some things are very-very hard to predict. Most innovative audio schemes won't work as expected the first time. It may just need a different cap. It may need a whole added stage.

I really am not following your text description. I think you need a pencil-sketch of the rough idea first. Especially before you seek inputs from others. Picture is worth 1K words? Don't do it finalized with ink on velum (or elaborate CAD). Cocktail napkins are traditional (and probably where the Federal started), though plain paper gives clearer scans.

 

[johnnytoobad]

here is the schematic but the side chain is now balanced...
PSU is checked and working, iwill try with a GZ34 for rectifiers (5U4 doesn't fit inside the 4U rack i am planning to use...) and check voltages with real loads as i tried simulating approximative loads with power resistors and it will rise a bit with the GZ34.
some components values (in the side chain) are not fixed and will be on prototype...
At my job i breadboard on a wood board with nails or thumbtack  for soldering components "redrawing" the schematics on the board. they do it that way for 50 years and this is the official way of breadboarding for the french railway security system...

 

[PRR]

> At my job i breadboard on a wood board with nails or thumbtack for soldering components "redrawing" on the board the schematics.

I'd love to see pictures of that.

 

[johnnytoobad]

I'm in holydays for 20 days so you'll have pictures soon when i'll get back, and hope to have time during these 20 days to breadboard the comp, but it is not sure at all, being away for 12 days...
here come the side-chain schematic for balanced compression, now highpass filter is 230Hz.