FET Audio Mixer

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This simple circuit mixes two or more channels into one channel (eg. stereo into mono). The circuit can mix as many or as few channels as you like and consumes very little power. The mixer is shown with two inputs, but you can add as many as you want by just duplicating the "sections" which are clearly visible on the schematic.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
R1, R3	2	10K Pot
R2, R4	2	100K 1/4 W Resistor
R5	1	6.8K 1/4 W Resistor
C1, C2, C3	3	0.1uF Capacitor
Q1	1	2N3819 Junction FET
MISC	1	Wire, Shielded (Metal) Case, Phono Or Other Plug For Output

Notes

As many or as few channels as are required can be added to the mixer. Do this by just duplicating the input "sections" which are shown on the schematic.

A shielded case is probably needed to reduce hum and help stop oscillations.

The circuit can be powered by a single 9 volt battery.

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Comments

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anonymous FET Audio Mixer Saturday, January 20, 2018 6:56:27 AM

Neat little circuit. To better understand the circuit you need to know the 2N3819 FET operates in "depletion" mode which means current flows between source and drain when the gate is at 0 volts. This is the exact opposite of what you might normally think. Also the 0.1uf input coupling capacitor may be too small for you to effectively pass the lower bass frequencies. A non-polar 1uf electrolytic or MLCC would be a better choice. The 2N3819 is of older design but is still sourced by Central Semi and a very good choice for this application.

Alex FET Audio Mixer Wednesday, December 11, 2013 8:34:15 PM

Hello, can I feed the inputs of the mixer with synthetizer line out and also can I change the 9v to +15v? Do I have to canghe some value? Thanks

Crankcase FET Audio Mixer Thursday, March 07, 2013 9:51:10 PM

This circuit would not be ideal for guitars because the potentiometers will shunt the pickups, which defeats the object. A suitable design will have R1 and R3 replaced with fixed resistors 3Meg - 10Meg, the individual volume controls will need to be on the output of these input stages and then mixed into another stage, with the whole thing producing unity gain.

anonymous FET Audio Mixer Monday, March 19, 2012 10:13:08 AM

I just wanted to clarify. Me and my groupmates are trying to make a language laboratory so we need a 4-to-1 channel stereo mixers (which would mix four input from microphones) and a 1-to-4 audio splitters. about the possible noise or hum. is this good enough to produce very little hum? and do you have any idea for audio splitters?

anonymous FET Audio Mixer Wednesday, March 07, 2012 7:53:31 AM

can i use another transistor in place of 2N3819 Junction FET

anonymous FET Audio Mixer Wednesday, February 08, 2012 9:37:07 AM

Should the pot ideally be linear or logarithmic please?

anonymous FET Audio Mixer Saturday, March 05, 2011 12:14:36 AM

If you build this for guitar, the input impedance as shown is too low. I recommend replacing R1 and R3 with 10M resistors (instead of pots).

Clayton FET Audio Mixer Wednesday, October 13, 2010 12:57:56 PM

Would the FET audio mixer be recommended for running a midi sampler and a guitar into the same guitar amplifier input?

ciaran FET Audio Mixer Friday, September 03, 2010 7:55:30 PM

we are currently doing a very similar project in our school but instead of 2 inputs, we have 4...hope it works...

Nick FET Audio Mixer Friday, June 25, 2010 8:29:18 PM

Hey i was wondering if you put in more than 4 input sections would you need to put in a higer value R2/R4 because when all the pots are at zero there would be verry little resitance therefore shorting the FET For example If all the 10k pots are set a zero resitance to ground # of chans R2/R4 value 1 100 k ohm's 2 50 k ohm's 3 25 k ohm's 4 12.5 k ohm's And it goes on But as you can see. the restors in parallel rule defnetly comes into effect here So the question is do you have to put in a bigger restor to stop Q1 shorting I want to make shure befori go and build this Circuit Thanks

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anonymous	FET Audio Mixer	Saturday, January 20, 2018 6:56:27 AM
Neat little circuit. To better understand the circuit you need to know the 2N3819 FET operates in "depletion" mode which means current flows between source and drain when the gate is at 0 volts. This is the exact opposite of what you might normally think. Also the 0.1uf input coupling capacitor may be too small for you to effectively pass the lower bass frequencies. A non-polar 1uf electrolytic or MLCC would be a better choice. The 2N3819 is of older design but is still sourced by Central Semi and a very good choice for this application.
Alex	FET Audio Mixer	Wednesday, December 11, 2013 8:34:15 PM
Hello, can I feed the inputs of the mixer with synthetizer line out and also can I change the 9v to +15v? Do I have to canghe some value? Thanks
Crankcase	FET Audio Mixer	Thursday, March 07, 2013 9:51:10 PM
This circuit would not be ideal for guitars because the potentiometers will shunt the pickups, which defeats the object. A suitable design will have R1 and R3 replaced with fixed resistors 3Meg - 10Meg, the individual volume controls will need to be on the output of these input stages and then mixed into another stage, with the whole thing producing unity gain.
anonymous	FET Audio Mixer	Monday, March 19, 2012 10:13:08 AM
I just wanted to clarify. Me and my groupmates are trying to make a language laboratory so we need a 4-to-1 channel stereo mixers (which would mix four input from microphones) and a 1-to-4 audio splitters. about the possible noise or hum. is this good enough to produce very little hum? and do you have any idea for audio splitters?
anonymous	FET Audio Mixer	Wednesday, March 07, 2012 7:53:31 AM
can i use another transistor in place of 2N3819 Junction FET
anonymous	FET Audio Mixer	Wednesday, February 08, 2012 9:37:07 AM
Should the pot ideally be linear or logarithmic please?
anonymous	FET Audio Mixer	Saturday, March 05, 2011 12:14:36 AM
If you build this for guitar, the input impedance as shown is too low. I recommend replacing R1 and R3 with 10M resistors (instead of pots).
Clayton	FET Audio Mixer	Wednesday, October 13, 2010 12:57:56 PM
Would the FET audio mixer be recommended for running a midi sampler and a guitar into the same guitar amplifier input?
ciaran	FET Audio Mixer	Friday, September 03, 2010 7:55:30 PM
we are currently doing a very similar project in our school but instead of 2 inputs, we have 4...hope it works...
Nick	FET Audio Mixer	Friday, June 25, 2010 8:29:18 PM
Hey i was wondering if you put in more than 4 input sections would you need to put in a higer value R2/R4 because when all the pots are at zero there would be verry little resitance therefore shorting the FET For example If all the 10k pots are set a zero resitance to ground # of chans R2/R4 value 1 100 k ohm's 2 50 k ohm's 3 25 k ohm's 4 12.5 k ohm's And it goes on But as you can see. the restors in parallel rule defnetly comes into effect here So the question is do you have to put in a bigger restor to stop Q1 shorting I want to make shure befori go and build this Circuit Thanks