One Transistor Audio Mixer

The simplest one transistor audio mixer circuit diagram on the net. It has only one transistor and can support as many audio signal as you can afford. BC109C is a very cheap transistor and with a few resistors and capacitors you can build quickly a mixer circuit diagram that you can use in your pirate radio or wherever you want, imagination is the final frontier. Please keep in mind not to use higher voltages than 22V as a power supply for this audio mixer.

Audio mixer circuit diagram

mixer circuit diagram


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  • Jim Keith

    What is your application? control of gain via remotely located DC signal? It can be done, but will take a little stretching of the grey matter.

    This may be accomplished via a JFET device that exhibits variable resistance as a function of gate to source voltage. This variable resistance may be applied as the shunt element in a two resistor attenuator. This may require substantial attenuation of the audio signal to keep distortion low–then the signal may be restored to the desired level via an op amp.

    • johny radio

      What you describe sounds like it might introduce a lot of noise. If i’m going to add an opamp anyway, then maybe i should just use an opamp circuit, instead of the transistor.

      I’m seeking an extremely-low-parts-count voltage-controlled amplifier. It does not have to be a mixer (ie, it does not have to accept multiple audio inputs). It should use basic off-the-shelf parts, not a VCA on a chip. It’s a project for a low-budget community electronics workshop ( No time or budget to order special parts.

      It should be at-home in an analog music synthesizer. Control voltage would range from 0 to 10v. Audio should be low-noise, low-distortion.

      A voltage-controlled resistor or voltage-controlled potentiometer seems an easy way to add voltage-control to any parameter, of any circuit (amp, filter, oscillator). Except for the issue you described. And, only if the VCP/VCR is also very low parts count.

      Thanks for your circuits and feedback!

  • johny radio

    Hi, is there a simple way to add voltage-control? -thx!

  • Chinmoy Mitra

    Hi, this mixer circuit is very interesting. I would like to incorporate six inputs for this mixer. Suppose I feed signal to one input, and all the other volume controls are at minimum position, there will be a drop in the gain in the active input (which is actually the loading effect of all the volume controls). Can you please give some figures in dB to specify this loading? In virtual earth mixers using opamps, there is practically no loading effect at all. But since this circuit is so simple and cheap, I would definitely like to try it out. Thanks.

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