Simple multi-channel mixer

It is a very simple construction consisted of few well-known applications of operational amplifiers.

The input stage:



The inputs are symmetric and it is not possible to connect directly a condenser microphone. Only dynamic microphones or other low source of symmetric or not symmetric signal can be connected.
At the beginning the signal goes to the high pass filter 6dB/oct. and F3 about 50Hz (C1, C2, R1, R2). C3 attenuates high, over acoustic audio signal components, capacitors of a similar value can be mounted in parallel to the R5 and R6 for the same purpose.
Then the signal is amplified approximately 20 times (what excludes fed the signal to the signal input of line level (0dB – 0,707Vrms) or higher).
The second from the top operational amplifier on the scheme has no function in this system.
The third from the top operational amplifier with elements R9-13, C4, PNP (of low power, e.g. BC556) and LED (plus from the transistor) form a pseudo distortion indicator. It does not indicate a distortion, but only the increase of the signal above the level set by the divider R10, R11.
The last operational amplifier with R7 and P1 form another step that amplifies the signal, giving as adjustable amplifies a result (from 0 to approximately 66dB at R7=3K, but it can be decreased even to 1K which will give a maximum amplify 200dB). In this project, R7 has a value of 1,5K.
C7 separates the constant component which in this case can be quite significant for the values of amplify.
R8 is part of the next stage ie the adder, or „proper mixer”.

The output stage/adder:



You can connect any number of input stages to the input „IN” of this stage by connecting together outputs „OUT” of the input stages.
P1 is a potentiometer of logarithmic characteristic and adjusts the signal level at the output of the mixer.
The first operational amplifier with R1 and „R8” form the adder. The second operational amplifier with R2, R3 form the inverter, which gives the symmetric signal to the output.

The whole system requires a stabilized symmetric power supply +/-12V, or a different value if you only take into account the maximum and minimum supply voltage of operational amplifiers and adjust the value of resistor R13 that limits the current of LED.

Housing:
It is a housing from ATX amplifier, cut and bent.

Elements used:

• operational amplifier x5
• potentiometer with knob x5
• socket x6

Some pictures:







Link to original thread (useful attachment) – Prosty mikser wielokanałowy