Some time ago I was asked to solve the following problem: a small choir plays a number of four fixed microphones. The audio signals from these four microphones had to be amplified, mixed and the resulting signal had to be applied to an audio power amplifier.Because I couldn't find a tool to do this, I built one.
That's what it's all about.

Supplies

Personally, I had all the components and materials in my own workshop, from recoveries.

Electronic components can be found on AliExpress at low prices.
Mechanical materials (metal and plastic) are recommended to be recovered for example from old LED or LCD TVs.

There are also some materials such as: screws, nuts, spacers, self-adhesive foil that can be purchased from DIY stores.

Step 1: Schematic Diagram

The low level signals (1-10mV) from the 4 microphones are applied to the inputs In1 ... In4
through the 4 female jacks diameter 6mm.

Following 4 low signal and low noise amplifiers, one on each channel, made with Q101 ... Q402.

All transistors are low noise (eg BC413) and resistors with metal film.

There is a positive feedback loop (C106, C107, R107 on the first channel)
which increase amplification at high frequencies.

There is also a negative reaction in cc. with R106,R101 which thermally stabilizes the stages.

Next, a volume adjustment is made on each channel with P100 ... P400.

Next on each channel is an amplifier with 10, made with U1-TL074 operational amplifier with Tz JFET at the input and low noise.

The mixing of the signals is done on R412, which implies a small attenuation of the signals on the 4 branches.

At the output OUT you can easy get 1 Vef. enough to excite an final power amplifier.

The power supply starts from the Tr. transformer which has two separate secondary .

The first gives 16V / 50mA and the second 24V / 100mA.

The voltage of 16Vca. is rectified with D502, filtered with C504 and stabilized with D505. The voltage of -9V is thus obtained.

24Vac voltage is rectified with D501, filtered with C501. An unstabilized voltage of + 33V is obtained.

This voltage is lowered and stabilized at + 20V by U501 LM317.

D504 offers a voltage of + 9V.

Voltages of +/- 9V are required to supply U1.

The presence of the supply voltage is indicated by the LED lighting.

Schematic diagram.pdf
Download

Step 2: List of Components, Materials, Tools.

All resistors are 0.125W or 0.5W, metal film.

120-1pc.
470-2pc.
680-4pcs.
1K-9pcs.
2k2--1pc.
10K-5pcs.
39K-4pcs.
47K-4pcs.
220k-8pcs.
390K-4pcs.
470K-4pcs.

Potentiometer 10K-4pcs.
Knobs for potentiometers-4 pcs.

Radial electrolytic capacitors

2.2uF/16V-4pcs.
4.7uF/25V-8pcs.
10uF/16V-2pcs.
47uF/16V10pcs.
100uF/16V-4pcs.
1000uF/25V-1pc.
1000uF/35V-1pc.

Capacitors 330pF/35V-4pcs.

Semiconductors:

Transistors type: BC413, BC173C, BC109C, 2N930- NPN low noise-8pcs.
IC: TL074-1pc.

IC:LM317-1pc.

Rectifier bridge: 1PM1(100v/1A)-2pcs.
Zener diodes: PL9V1-2pcs.
LED 5mm. blue-1pc.

Others:

PCB for power supply module (ExpressPCB project) -1 pcs.

PCB for mixer preamplifier module (ExpressPCB project) -1 pcs.

Tr: trafo. with 2 secondary: 16V/50mA and 22..24V/100mA.-1pc.
Network connection cable.
Jack female 6mm-5pcs.

Pin headers 25pcs.

Wires

Solder tin

Tools for soldering

Pliers for cut the component terminals
Heat shrinkable varnish 2.5mm diameter-50cm.
Digital multimeter (any type).
Aluminum plate 165X235 mm and the thickness of 1.5 mm. for the bottom of the device(photo 1 )
Aluminum plate 190X20 mm and the thickness of 1.5 mm. for support of potentiometers.
Semi-transparent plastic plate 165X235 mm and thickness of 3 mm. for the top panel of the device
(photo 1 )
Screws, nuts, spacers (photo 1 ).
Screwdrivers.
Instruments for measuring mechanical dimensions.
Tools for metal and plastic drilling, filing, metal and plastic cutting for mechanical processing of the device (you have to be friends with them to do the job).
Self-adhesive foil for labels and ornaments.
Lust for work.

Step 3: Making and Assembling PCBs.

For mixer, the PCB is made of 1.5 mm thick FR4 ,double sided. No metallic holes.
The crossings are made with uninsulated wire.

For power supply, the PCB is made of 1.5 mm thick FR4 ,single side.

After echting and drilling, cover with tin, manually. We check with the digital multimeter the continuity of the routes and the possible short circuits between them. PCB is designed in ExpressPCB, a program that can be used freely. It can be downloaded freely from the Internet.

Photos 2,3,4,5 show the assembled PCBs.

At the address:

https://github.com/StoicaT/4-Microphones-Mixer-Pre...

there is the design of the PCBs device and other details of project. You can download the PCB design that contains all the elements necessary for execution, of course if you have ExpressPCB installed on PC / laptop.

Step 4: General Assembly

Starting from the mechanical parts in photo 1, the mechanical assemblies will be made successively according to photos 6, 7,8, 9, 10,11.

The aluminum plate will have the dimensions of 165X235 mm and the thickness of 1.5 mm.

The semi-transparent plastic plate has the same dimensions and thickness of 3 mm.

When you work to this board, it must to remember that the plastic melts at relatively low tool temperatures.

It is recommended to use hand tools, not electric ones.

Step 5: Wiring and Putting in Function

The wiring is done according to the schematic diagram and photo 12.

Heat-shrinkable varnishes will be used after soldering the wires to the pins.

Putting in function is done by measuring the voltages according to the
schematic diagram with the digital multimeter. Check that they are correct.

Step 6: Final Assembly and Use.

If everything is OK, mount the plastic plate, using the jack plug nuts, then the potentiometer knobs, see photo13.

Self-adhesive labels will be applied and for aesthetic reasons, self-adhesive foils ornaments can be added ,see photos 14,15. Here, the artist in us can express himself freely.

The labels are made with the Inkskape program and can be found together with PCBs.

Of course, the Inkscape program must be installed on your PC / laptop. It is a program that can be downloaded and used freely.

The microphones are inserted in the MIC1..MIC4 jacks.
The volume adjustment is done separately for each microphone, at the desired level.

The output of the mixer is connected to the input of the power amplifier with a 6mm jack- jack cable.

Connect device to the AC network.

And that's it!