Passive Audio Mixer

Posted on Aug 9, 2012

I designed this circuit for one friend of mine to be used as a small portable DJ mixer. The circuit is an audio mixer circuit so simple as it can be. There are two dual logarithmic potentiometers in the circuit to adjust the input signal levels and some resistors to do the actual mixing. The circuit is totally passive, so no power supply is needed. The circuit is suitable to be uses as a mixer between two line level sources and one HIFI amplifier input. This circuit have been successfully used for mixing signals form two CD players or computer soundcard and CD players. There are many situations where simple mixer would be useful and commercially available mixer desks are too expensive and big.

Passive Audio Mixer
Click here to download the full size of the above Circuit.

This simple line mixer has two drawbacks: it attenuates the signal all the time (even sliders set to maximum) and the output impedance is quite high. The first problem can be solved by just turning a little more volume in the amplifier. High ouput impedance is no problem when connected to high impedance amplifier input with short wires (few meters). That line means that those potentiometers are mechanically connected together so that they move together (that line does not mean any electrical connection because it is not connected to those potentiometers). This kind of mechanical connection is done in dual potentiometers. If you use two separate linear potentiometers then you can connect the sliders of them together using some suitable method. If you prefer totally separate left and right channel volume controls then you don't need to do this mechanical connection between the potentiometers. Could I use 1/4" phone jacks as opposed to RCA jacks? Yes. No problems. You can use any line level signal connectors in the place of those RCA connectors when you wire them correctly to the circuit. On each input, there are obviously two connections. To which part of the jack does each connect? For RCA connectors: Outer part in RCA connector goes to the common ground (right tap in schematic connections). The center of the RCA connector is connected to the resistor (left tap in schematic). For PHONE jacks: The body of...

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