12au7 mastering limiter & 20dB gain

Blue Jinn

Blue Jinn

Rider of the ARPocalypse
I came across this project

http://www.diyfactory.com/projects/diy12au7comp/diy12au7comp.htm

I am in no way a tech, have never used an oscilloscope and barely know ohms law. I have built quite a few Paia kits successfully and this doesn't look all that difficult. And I managed to figure out and fix a power supply problem in an Otari 5050. Any thoughts?

BTW, it is designed for -10db not +4, looks like it can have a balanced input, but not a balanced output, thinking a Jensen xformer may be the ticket there.

I have been using an ART MDC2001 for mixdown. But I'd like to get a dedicated compressor limiter for that function, something w/o too many knobs. (Giving a lot of thought to the ART Leveler or dbx 262.)

But this unit claims a gain of 10:1

"The overall gain, when not compressing, is fixed at about 10:1 (20dB). The idea of a "unity gain" limiter is only valid when you hope to never use the limiter, and have plenty of headroom for the peaks you hope won't happen. When you plan to compress a lot of peaks, and don't have gobs of headroom like the old tube studio systems, a compressor/limiter needs to have gain. I assume that you have a level control on the device driving the compressor (usually your mastering mixer) where you can adjust "drive" and thus the amount of compression."

I'm not clear on what the designer is saying here.

Plus I would have thought that 20dB of gain from -10 puts that in the +4 range? (isn't that a 12dB gain?)

I'd like the option to use this with my DAT which is +4. (The OTari is selectable.)
 
The +20dB is for makeup gain after compression. That doesn't change the ultimate headroom of the circuit though. If you need +4dBu, you could either add a 1:4 step-up transformer on the output, or add another +12dB gain stage, whether tube or solid state. An easy way to get an extra +6dB and therefore close to +4dBu is add another 5532 in inverting configuration--that gets you electrically balanced output at the same time. That, and change R17 to 4k7 ought to get you there--although that could screw with the sidechain, so you might want to change the pad at R39/R40 to compensate.

This is a tested project with a verified PCB design (maybe still some for sale, check the Prodigy-Pro Black Market), so I'd say it's not too tough of a build. Parts count is modest.

Note that this is definitely a "color" box though.
 
Thanks!!!

I'll register over there and ask about the PCB. Thanks for clarifying the gain thing. I'm thinking I'll at some point give it a whack and then if it works out OK worrying about hte mods for balanced in.out and +4 later. Right now I'm just looking for parts especially the 1:1 xformers.

I have -10 to +4 converters. (Passive as well as active.)
 
I've been looking for transformers for this and some of the mod threads. As far as I can see, 1:1 on the input to use, looking at a 15k:15k EDCOR. Also, I've read some threads that suggest a 15k:-600 for the output rather than 1:1, any thoughts?
 
Actually, 600:600 Edcor, and 15K:600 on the output seems better. Having trouble deciphering hte layout for stereo, if I'm reading things correctly this is how it should look: any insight appreciated:
 

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