tube mic pre and compressor ideas?

[Michael A]

Hi All,

I'm toying with the idea of building a mic pre/compressor channel or two. Would the Altec 438A be a good plan? Any other suggestions? Tube is a must, point to point is preferred so I guess some simplicity figures in there too.

Thanks,
Michael

 

[rafafredd]

You could do a mic pre and include an opto (driven by a simple sidechain), in the feedback path - gain control circuit. Or you could use the opto in-between stages as an attenuator, with a no-overall-feedback design.

Otherwise, most VARIMUs can work as a mic preamp if you use a high ratio transformer at the input. I use the Gates Sta-level as a micpre/compressor.

 

[Michael A]

Sta-level looks like a good choice. 6V6 outputs? Did I read that right. You could drive a telephone pole with that. What about UA 177? Or Gates Level-Devil. I guess all the usual suspects are out there.

I have a few UTC A11 and maybe a couple of small Peerless that I thought I could use as input transformers. I was thinking Edcor for interstage and outputs if needed.

What else should I consider for parts? What are the favorites out there

Michael

 

[rafafredd]

The output iron of an STA-Level is not an easy quest. I think Sowter makes a replica???

Well, You need that power amp at the output to drive the sidechain well. Mine is all moded, with very fast attack and release settings. I do have a very different attenuator at the output also. A normal T attenuator instead of that lossy original one, so that I can use it with as very high gain mic preamp. You can set the threshold by controling input / output levels. Got it? I may have to darw the schematics, but I would have to get that beast out of the studio, cus I don´t really remenber all my mods.

 

[solder_city]

altec 438 has so much gain its almost a joke. too much! build an altec 436 with a good 'mic to push pull grids' input transfo (utc a-11?). lose the 50k dual pot input attenuator. run the bias voltage to the input transfo centertap. add a variable atten after the output transfo. theres your mic pre/compressor, plenty of gain. add a standard mic pad at input if needed.

i believe the 436 output is something like 20k:150/600 push pull, fairly standard item on the used market.

problem with an ambitious project like a sta level is that the parts are hard to source and 90% of people never get beyond the planning stage with something like that. a 436 copy is easier/cheaper to build therefor more feasible.

 

[Michael A]

I see what you mean about the gain on the 438. It would be nice to have a low noise 12AY7 in the front end though. What would happen if one substituted a lower gain tube for the 6CG7 in the 438 plan? Say something with a mu of 5 to 10 instead of 20? Would that change the compression scheme or not? I'm not stuck on copying a classic, which leaves some room to fool around. I would like a usable product though, which puts knowledgable advice at a premium.

Thanks,
Michael

 

[Michael A]

Looking at the schematic and manual it seems it would be nice to be able to separate the threshold and compression level controls. Or am I wrong? Any suggestions for how to go about this?

Thanks,
Michael

 

[emrr]

Threshold and ratio are one in the same in a simple system like this.