MFOS Quad 3 Input Audio Mixer

Ray Wilson authored this content while he was actively running MFOS as the founder and resident genius.
We retain the content because it reflects a valuable point of view representing that time and place.

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.

Features

Four separate Three input mono Audio mixers
Each pair is switchable to become a Six input Audio mixer
Each mixer includes a selectable high gain channel
Single sided board layout for easy home production

Introduction

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.

Schematics

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

This is a single sided board that you can make at home.
This view shows the designators useful when trouble shooting.
All jumper indicators must be shorted with number 22 AWG solid wire or equivalent.

Parts Layout With Component Values

This is a single sided board that you can make at home.
This view shows the component values useful when populating the PC board.
All jumper indicators must be shorted with number 22 AWG solid wire or equivalent.

Bottom Copper

As viewed from the bottom of the PC board.

Silk Screen

As viewed from the top of the PC board.

All jumper indicators must be shorted with number 22 AWG solid wire or equivalent.

Bottom Copper X2

As viewed from the bottom of the PC board.

Silk Screen X2

As viewed from the top of the PC board.

All jumper indicators must be shorted with number 22 AWG solid wire or equivalent.

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