Ampro Input Cards

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Nov 16, 2021
New York USA
Came across an ebay lot of late 70's (i think?) Ampro broadcast console input modules. They were cheap enough to gamble on and they have input transformers. I couldn't find a schematic so drew this up the other night. I think it's right, but i'm not so great at reading color codes so unsure of some of these values.

Anyone familiar with these? So far I've figured out that it's an lm709 with single supply, dc coupled output, fixed gain. I got one working on the bench and measured 20x on the output. Next is to check it out with a mic and see how it sounds.

My hope is to rack them up and use them as mic pres with phantom power and phase invert. So my questions are:

-can I add variable gain to this? Replace R4 with a pot?
-replace 709 with 5534? I found a thread about upgrading Auditronics 709 to 5534 that might apply?
-ideas for balancing/driving output? replace output cap with a 1:1 transformer?
-just take the input transformers and go with a dual supply design?
-anything I'm missing or grossly misunderstanding? Still very much learning here and I can say with confidence that there's lots here that I still don't understand.

Here's the only info I could find. Has some block diagrams at the end (the cards I have are stereo channels):


  • ampro input card.pdf
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