Resistor Mixers
(The Resistive Matrix)


Resistor Mixers are used in all types of Audio Circuitry. It can be found in Home Receivers. simple microphone mixers, High End Audio Systems, and the Broadcast Consoles, and DJ Mixers. In fact, it is the basic mixer and cheapest solution to combining two or more audio signals and providing basic isolation between the inputs.

The Resistor Mixer Matrix is just an economical solution to provide impedance and circuit loading continuity when selecting one or the other (even both or all) of the input channels.


The Simple Resistor Mixer
Using simple Resistors



Here is a common and simple resistive mixer that combines only two audio sources. You will notice the simplicity of this circuit.

This circuit is courtesy of Tom Engdahl, and is a "very" basic mixer design. It is also the mixer design that you will find in many of "those cheap two and three channel DJ Mixers" that are so popular.

Tom designed this circuit for one of his friends to be used as a small and "basic" Line mixer. The requirements were to provide for mixing two line level audio sources so that two tape decks could be faded in and out on one amp line input. The circuit is an audio mixer circuit in the simplest sense of the word. Essentially we have an over-simplified sound-on-sound circuit. 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 used as a mixer between two line level sources and one HIFI amplifier input. This circuit has been successfully used for mixing signals from two CD players or a computer soundcard and CD player. There are many situations where a simple mixer would be useful and commercially available mixer desks are too expensive and big.

This simple line mixer has two drawbacks: it has a lot of signal attenuation and the output impedance is quite high. The first problem can be solved by just turning a little more volume in the amplifier. The High Output impedance (the second drawback) is no problem when the mixer is connected to a high impedance consumer amplifier input with short wires (only a few meters maximum).

In the image is the entire schematic of the whole mixer circuit. The potentiometer slides which are actually two dual potentiometers are connected together using one line. Every input and output pin has corresponding ground signal on the right side of the signal line. Resistors R5, R6, R7 & R8 provide isolation between the respective attached/combined channel.

The mixer matrix is design to provide a 10K Component Line Impedance match for Consumer Stereo Components.


For all of you guitarists, and instrumentalists.

Here is an ultra-simple solution to combining "multiple" Instruments into a "single" amplifier input. This mixer is a classic example of the Tom Engdahl passive mixer design. CAUTION This circuit does not provide individual gain controls (but then guitars' have their own gain controls - don't they?

At any rate. This is an ultra simple solution to getting all those inputs into a single audio stream. If you desire a little amplification you can simply insert a buffer amplifier between the mixer and amplifier inputs ( How about one of the two 2N5088 Low Noise Preamps featured in the Transistor Preamp section? - But then you would have to put one or even two 9vDC Batteries in the mixer case.). If you would like individual gain controls, oh-well (there is always a trade off) you have two choices:

1st: You can insert the pots at the mixer inputs with the wipers feeding the 47K resistors of the mixer.

2nd: You can insert buffer amps between the instruments and the mixer inputs for each channel. The problem with this solution is that you will need power for the active circuit. This means you no longer have an ultra simple resistive mixer. NOW YOU HAVE AN ACTIVE MIXER. But hey - trade offs are trade offs.



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