MFOS Quad 3 Input Audio Mixer
Article by Ray Wilson
This is an advanced project requiring understanding of electronics and electronic building and trouble shooting skills and equipment. Access to a good oscilloscope, DVM, bench power supply, soldering station and hand tools will be required. Only the schematic, circuit description, and suggested panel layout are provided. Please read over the entire project before ordering PC boards or other products.



This Quad Audio mono mixer can be used to create sub-mixes in your modular or to amplify an instrument whose signal you want to experiment with. It is basically four three input audio mixers. Each pair of mixers can be switched to produce two six input mono mixers.


Schematic Page 1

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Op amp U3 and associated components comprise mono mixer 1. Op amp U3-A provides the switchable high-gain channel for mixer 1. The signal applied to J9 is fed via 1uF non-polarized aluminum cap C11 to one end of R29 (100K input level pot) used as a variable voltage divider (the other end is grounded). The adjusted signal on the wiper of R29 is fed to R30, 20K resistor which is connected to the non-inverting input of U3-A. U3-A's feedback resistor R37 is paired via switch S4 with either 2K (resistor R36) to ground (S4 is closed, gain is 50) or 472K (R35 470K series with R36 2K) (S4 is open gain is 1.21). Capacitor C14 suppresses unwanted op amp oscillation.

Op amp U3-B is used as an inverting mixer. Both of the adjustable signals coming from J11 and J13 and the output of U3-A are mixed via resistors R31 (100K), R42 (33K) and R48 (33K). The signals from J11 and J13 are capable of up to a gain of three. The optionally amplified U3-A signal sees a gain of one through U3-B. The inverted output of U3-B is fed to the switching network which facilitates changing each mixer pair's function to 6x1 instead 3x2. Switch S6 either sends the inverted outputs of U3-A and U4-B to their respective output inverter buffer or to both of them.

Op amp U4-B and associated componented work in exactly the same way to form the second mixer of this pair. The circuitry on page two functions in the same way as that on page one. Again the designators have been changed to protect the innocent. Lots of bypass caps throughout to keep the circuitry quiet.


Schematic Page 2

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See Schematic Page 1 for Circuit Description


PC Board Information

Parts Layout With Component Designators


Parts Layout With Component Values


Bottom Copper

As viewed from the bottom of the PC board.


Silk Screen

As viewed from the top of the PC board.


Bottom Copper X2

As viewed from the bottom of the PC board.


Silk Screen X2

As viewed from the top of the PC board.


Front Panel Information

Suggested Front Panel Layout

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Suggested Front Panel Wiring Diagram

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MFOS Quad 3 Input Audio Mixer Project Parts List

Component Notes

All cap voltages should be 35V or more. All resistors are 1/4W 5% (1% will work fine). MFOS PC boards use dual inline IC packages. MFOS PC boards usually use 5mm (0.2") cap lead spacing unless it is obvious from the board's legend that it should be larger or smaller. I often specify axial leads for any caps mounted on the front panel but you can use radial as well by adding some wire to the leads.

Project Parts List

Qty. Description Value Designators
4  TL074 Quad Op Amp  TL074  U1, U2, U3, U4  
12  Potentiometer  100K  R1, R4, R13, R16, R21, R24, R29, R32, R41, R44, R47, R49  
24  Resistor 1/4 Watt 5%  100K  R3, R6, R9, R12, R15, R18, R22, R23, R25, R26, R27, R28, R31, R34, R37, R40, R43, R46, R51, R52, R53, R54, R55, R56  
4  Resistor 1/4 Watt 5%  20K  R2, R5, R30, R33  
4  Resistor 1/4 Watt 5%  2K  R8, R11, R36, R39  
8  Resistor 1/4 Watt 5%  33K  R14, R17, R19, R20, R42, R45, R48, R50  
4  Resistor 1/4 Watt 5%  470K  R7, R10, R35, R38  
4  Capacitor Aluminum Bipolar (non polarized)  10uF  C9, C10, C19, C20  
12  Capacitor Aluminum Bipolar (non polarized)  1uF  C1, C2, C3, C5, C7, C8, C11, C12, C13, C15, C17, C18  
8  Capacitor Ceramic  .1uF  C21, C22, C23, C24, C25, C26, C27, C28  
4  Capacitor Ceramic  47pF  C4, C6, C14, C16  
2  Capacitor Electrolytic  10uF  C29, C30  
2  Switch DPST  DPST  S3, S6  
4  Switch SPST  SPST  S1, S2, S4, S5  
16  1/4" Phone Jack  .  J1, J2, J3, J4, J5, J6, J7, J8, J9, J10, J11, J12, J13, J14, J15, J16