Almost Finished!

[Devil_07]

Hey all,

I've been working on this project at work for a few weeks now, as a side project, and it's almost finished!!



These microphone preamps came from an old RCA BC-2B. There's a BC-2B sitting on the desk in this pic. They are RCA MI-11241 pre's.



The box didn't have any holes pre-drilled, so it was like working with a blank canvas.

Front



Back



Here's a close up of the gain control and the switches on the front.
The switch with the resistors is the Pad.



The 2-channel mic-pre with the PSU.



I socket mounted the power for the mic-pres and put an IEC on for the A/C. It was just a terminal(death) strip before. Touch that thing in the wrong place and you and the circuit breaker go boom.

I still have some cosmetic work to finish up before it's officially finished. Like labeling and adding a pilot light to the box and the PSU so you can see when its on or off.

I'll post another pic when everything is final.

[tINY]

So, is it a two stage, single stage with an active load or diff-amp configuration?




-tINY

[Devil_07]

I'm not really an electronics engineer, more of a mechanical type. I'm gonna venture a guess and say it's a 2-stage because of the two parts to the 12AY7 tube. The audio path internal to the pre is unbalanced and uses center tapped transformers on the input and output. Probably makes it a differential amplifier.

The fixed value resistor in the negative feedback loop was changed out for a pot of the same value to vary the amount of negative feedback for gain control.

after initial testing, it sounds great. nice and clean with a nice bright top-end.

EDIT: if I can find a drawing online I'll add it to the first post, or I'll see if I can get my scanner to work when I get home tonight.