Features

Four mono input channels each with Panning, Effect 1 Send and Effect 2 Send.
Effect 1 and Effect 2 return can be used as additional stereo line inputs.
Two stereo line inputs.
Stereo Line Out
Integral Stereo Headphone Amp
Easy to add additional mono channels with another board.

Introduction

A versatile stereo mixer for use with your synth modules. You can easily change gains on the mono input channels to accomodate high-Z microphones (not balanced input mics), electric guitars or other instruments. By adding another board and populating only the mono inputs (or mono inputs and stereo input components) you can add more channels to the audio and effect busses of the first fully populated board. More details on this configuration can be found below.

This is an intermediate to advanced project and I do not recommend it as a first project if you are unable to build projects of this nature on your own or are just getting started in synths or electronics. Only the circuit and some explanation are shown here. A lot of project building experience and electronics knowledge and equipment ownership (scope, meters, etc.) is taken for granted. If you are interested in building this project please read the entire page before ordering PC boards to ensure that the information provided is thorough enough for you to complete the project successfully.

In the Stereo Mixer there are four mono input channels and four sets of stereo line inputs. Each mono input consists of a level pot, capacitively coupled to a buffer amplifier. Channel one for example has R2 (100K audio taper pot) as the input level control. The wiper is connected via C1 and R3 to U1 (TL071 low noise op amp) which provides a gain of 10. Feedback resistors in any of the mono input channels can be independently adjusted to provide more or less gain for any channel. The formula for gain is very simple (feedback resistor value/input resistor value). Each channel is shown with 47K input resistor and a 470K feedback resistor (giving a gain of 10 (470,000/47,000)). The small capacitor across the feedback resistor is there to prevent oscillation since the op-amps are not set for a gain of one. The channel 1 buffer feeds its panning circuit R5,R6,R7,R15,R8 which is used to send the signal to the right and left audio buss. The pan pot R7 brings the junction of R5,R6 or R15,R8 closer to or further from ground which causes the signal through either path to be attenuated when the wiper connected to ground is brought close to either one. This cause the signals being fed to the right and left audio buss to appear to be biased either left or right in the stereo field. The channel buffer output also feeds the effect send pots for the channel. Each effect send pot has two resistors connected to its wiper that are fed to the effect buffer busses. The effect buffers for Effect Send 1 are U6-A and U6-B (Tl072 dual low noise op amp). The effect buffers for Effect Send 2 are U7-A and U7-B (Tl072 dual low noise op amp). The audio buss buffers are U5-A and U5-B (Tl072 dual low noise op amp). The output of the audio buss buffers are used to feed headphone level pot R76 (dual audio taper 100K pot) whose wiper feed the headphone amps (U8 and U9 LM386 low voltage audio power amplifiers) via attenuators (R75 & R77 and R78 & R79). The effect returns and stereo line inputs are fed to the audio buss via the pots used as attenuators for each of the input pairs and the summing resistors. This is a very simple design and gains can be changed anywhere in the circuit by changing the values of individual summing resistors or feedback resistors in any of the op amp circuits.

If you want more input channels you can build another board but do not add the audio buss buffers, effect send buffers, or headphone amplifiers. Connect the second boards busses to the completely assembled board and it will provide the buffers for all channels. See part layouts highlighting what I mean below. It goes without saying (but obviously I'm saying it) that the second board will also need to be connected to the power supply.

To expand to eight mono input channels you use another board and only populate the mono input sections. You can populate the stereo line input components too if you want more stereo line inputs. Both boards must be powered from the supply and ground. Run jumpers from the partially populated (expander) board to the fully populated board to bring the mono channels to the stereo buss, effect busses and headphone amplifiers. Of course you need another set of pots and jacks for the four mono input channels. They should be wired in the same manner as the original four channels. I don't know a better way to explain this but if I have not described using another board for expansion clearly enough please consult one of your electronically oriented friends to see if they can help. Good luck.

You can see how the one board is fully populated with components but the second board only has the components to support the additional input channels.

I find this view useful when I'm populating the board. I don't have to go back and forth from the designator to the value. It speeds up construction. Click the "Larger GIF" link and print the image as landscape.

This wiring layout and front panel are only suggestions. The wiring drawing is meant to show how the front panel is connected to the wiring points on the PC board. Whether you use coax cable is up to you for the panel to board wiring. Don't write to me about this. You can also shield the unit by building it into an aluminum box.

Stereo Mixer Project Parts List

Qty.	Description	Value	Designators
4	TL071 Op Amp(s)	TL071	U1, U2, U3, U4
3	TL072 Dual Op Amp(s)	TL072	U5, U6, U7
2	LM386N-4 Low Voltage Audio Power Amp(s)	LM386N-4	U8, U9
1	LED	LED	LED1
2	Capacitor Electrolytic(s)	10uF	C19, C15
2	Capacitor Electrolytic(s)	1uF	C28, C39
2	Capacitor Electrolytic(s)	220uF	C29, C38
12	Capacitor Electrolytic(s)	47uF	C9, C12, C37, C24, C36, C27, C26, C34, C35, C25, C10, C13
2	Ceramic Capacitor(s)	.047uF	C40, C30
14	Ceramic Capacitor(s)	.1uF	C1, C3, C5, C7, C16, C20, C17, C21, C18, C22, C23, C33, C44, C43
10	Ceramic Capacitor(s)	10pF	C2, C4, C6, C8, C32, C42, C41, C31, C14, C11
5	Dual Gang Pot(s) Audio Taper	100K	R63, R70, R65, R72, R76
12	Potentiometer(s) Audio Taper	100K	R2, R9, R13, R17, R24, R28, R32, R39, R43, R47, R54, R58
4	Potentiometer(s) Linear Taper	100K	R7, R22, R37, R52
21	Knobs		For all pots
2	Resistor 1/4 Watt 5%(s)	10 ohm	R82, R85
2	Resistor 1/4 Watt 5%(s)	100K	R77, R78
16	Resistor 1/4 Watt 5%(s)	120K	R5, R6, R15, R8, R20, R21, R30, R23, R35, R36, R45, R38, R50, R51, R60, R53
6	Resistor 1/4 Watt 5%(s)	150K	R61, R68, R64, R71, R66, R62
4	Resistor 1/4 Watt 5%(s)	15K	R80, R83, R81, R84
16	Resistor 1/4 Watt 5%(s)	20K	R10, R12, R11, R14, R25, R27, R26, R29, R40, R42, R41, R44, R55, R57, R56, R59
2	Resistor 1/4 Watt 5%(s)	300K	R67, R74
3	Resistor 1/4 Watt 5%(s)	4.7K	R79, R75, R86
4	Resistor 1/4 Watt 5%(s)	470K	R4, R19, R34, R49
6	Resistor 1/4 Watt 5%(s)	47K	R3, R18, R33, R48, R69, R73
1	Stereo Headphone Jack	Stereo Jack	J17
18	1/4" Phone Jack(s)	Phone Jack	J2, J3, J4, J1, J7, J14, J15, J18, J16, J19, J5, J8, J10, J12, J6, J9, J11, J13

Stereo Mixer (+/-12V to +/-15V)

Article by Ray Wilson

Features

Introduction

Stereo Mixer Project Parts List

Stereo Mixer Project Parts List