Help with awesome sick idea!

[drewl]

So I have a late model trashed mixer I've been using for parts.
Well, it has the mic preamps mounted on little boards (1"x3") with 2 preamps on it, from 2- 4560 opamps, all surface mount components.
I figured out most of the pins and am going to make an amp/distortion pedal(s) with them....
Anyone have any mic preamp schems or links so I can get the general idea/topology?
In the meantime I'll draw it out....
so far I figured out how to power it and feed it a signal and get variable gain/distortion out of one of the stages.
It's got buffering transistors already mounted there, and I need to figure out what the second half of the opamp is doing. My guess is it uses 1/2 an amp for the each of the XLR inputs.
Thanks.

[Jobet]

Awww.

Okay, start with the unbalanced input instead. It will be easier than having to deal with XLR.

Then, adjust the input impedance of the mixer by changing the input shunt resistor. It could be a value between 1-2k for microphones. Replace that with a 1Mohm otherwise the input circuitry will load your pickups too much.

Then look for the feedback circuitry, more specifically a resistor that goes from the output back to the input. Lift that and place a potentiometer in series. This should allow you to raise or lower the gain. But wait...most mixers already have a gain knob. Mine do. So maybe you can scratch that.

Then look for the output of the stage, and locate if there is an output resistor. If none, you'll have to insert one, between 1-10k would work fine. After that, shunt in your clip stage going to ground. The clip stage will need that resistor to burn off the skimmed wavetops.

Modify further to taste, and you're all set. You've just converted a mixer channel into a guitar dirt circuit.

[Jobet]

Thinking a little further, if it's at the XLR inputs that you want to tap into, ground out the IC pin that XLR pin 3 is feeding. Then repeat what I just posted for the circuitry of XLR pin2.

Hey, I haven't done this before by the way. That's what's fun about thinking about it

[Jobet]

BTW you can check if the 2nd op-amp is used either as an output buffer or as a compensator for the tone control circuit.

4560's are dual op-amps BTW, so that means you're dealing with four gain stages.

[DDD]

There are some reports on the Russian guitar-related sites stating that connecting guitar instead of the magnetic head to the cassette tape recorder preamp gives very saturated and decent fuzz tone.
Worth a try?

[drewl]

BTW you can check if the 2nd op-amp is used either as an output buffer or as a compensator for the tone control circuit.

4560's are dual op-amps BTW, so that means you're dealing with four gain stages.

yes, I know they're dual amps....
each mini board is for 2-channels, that's why there's 2 opamps....each opamp has 2 inputs it looks like one for the XLR hot, one for XLR cold....
It's a multilayer board which is why it's a little hard to draw out, but like I said each opamp input has a buffer stage, coupling caps etc.
by reducing the resistance between 2 of the pins it increases the gain, so I assume this is where the gain pot was....with around 100mv in it will put out ~ 400mv.
All I need to do is add some clipping diodes and an output level pot and it can be an entire self contained distortion pedal!
Actually.... two of them!
Like I said, I just need to figure out what the 2nd stage is doing so I can turn it into a tone control.....think of a mixer's preamp....it's got a gain control and sometimes a low freq cut...anything else?...then it's sent to the line input jacks and then on the the tone contols....

[runmikeyrun]

i had an old two channel reel to reel that i used for distortion w/ great results.  The motor start capacitor died so i just daisy chained the channels together, man i got some really good "revolution" fuzz and just about everything in between.  Overdriving that first channel into the second really got things hot really fast, made for some good sounds.  So if you're dealing w/ two channels try a daisy chain option with a way to regulate the first channel's output into the 2nd's input, you might like it.