It is organised as building blocks,
This is what Monika wanted
-> one microphone preamp,
-> one (passive) stereo line input,
-> one channel output amplifier,
-> stabilizer circuit
... and its elements follow (with certain variations):
but before the nice drawings download there is time for a brief 'basics' course!
and now the nice drawings must have downloaded!
the alu frontplate should be connected only once to ground -> at a microphone input (the most sensitive one),
there is a possibility for a 'phantom' microphone supply - the red resistor; attention: there are electret microphones which have an additional ring -> like the stereo mini jacks - these get their 'phantom' power at the ring (get a stereo input socket!) - connect the red resistor on the ring;
Micro preamp is the simplest operational amplifier non-inverting design, therefore a good op-amp is needed (TL 071, NE 5534, CA 3140); the design is for single voltage supply (and op-amps are not), therefore half_the_supply voltage line is needed everywhere.
also: very close to every op-amp a couple of 'blocking' capacitors (0,1microfarad 50V ceramic) are needed (to block the noise from the two voltage supply lines).
The elements for send submix are in yellow colour. If you don't need send submix you can eliminate all the yellow elements. If you need more submix groups just replicate same ' yellow' configurations.
Double potentiometers are necessary because we don't use 'buffers' for the line inputs (-> look up ).
After faders (through equal value resistors) we connect (for the stereo inputs) all left to the left sum common strip (bus), all right to the right sum common strip and for the mono/ microphone to both (one resistor goes to left sum strip, one resistor goes to right sum strip).
We need two/ three kinds -> for the prelisten/ phones, for the main mixer output, for the send submix output (if needed).
Again they are build in the exactly the same manner as the microphone preamps - except the feedback resistor (going from pin 6 to 2) is about 10 times smaller -> 10Kohms. But for phones amplifier it should be larger -> 100Kohms. Experiment with this (feedback) resistor!
All connections to grounds should go separately to common ground point in stabilizer (see above) and all the voltage supply lines should also go separately to stabilizer (and again: blocking capacitors should be used with each op-amp).
The best would be to think of the ground connections which go to op-amps pins 4 as voltage supply lines - complementary to the plus voltage supply line - these two leads should be 'twisted' together to prevent any 'hum' induction.
Grounds which go from the potentiometers are (same as input and output grounds) - 'signal' grounds. Acting in this manner you prevent any possibility of 'buzz' or 'hum'.
And the minus pole of this capacitor should act as the common ground point for all the blocks!
We get power from an outside 'adapter' of 12V to 24V DC secondary voltage (100mA is OK), which lets us play safely because we don't deal with lethal tensions anymore! Just get the right socket, connect its + socket via a forward biased diode and it will be impossible to ruin the mixer with different adapters.
The stabilizer circuit is the most common MC 78xx (xx = 12 or 18, which is the stabilized output voltage). To make use of the larger dynamic range (before clipping) of output signal - find an adapter of 18-24V DC and use MC 7818 (and MC 7809 as you'll find in the next paragraph).
And that's all (for now). In future you may expact some image files for PCBs, because the building on the perforated board seems too much time & energy consuming.