Audio Mixer with LM381

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Audio Mixer

The circuit schematic below represents one method for the design on an audio mixer. The active component is an LM318, although basically any operational amplifier would work in its place. The circuit is the classic design for an Op Amp summing amplifier. Four individual audio channels are shown and may be mixed together. Channel D is really intended to show the addition of more channels and not used in the equation to calculate gain.

Audio Mixer

Component Values

Capacitor Selection

The capacitors [C] are not used in the mixing function, but are there to DC block one channel from effecting another channel. The value of the capacitor should be selected so that it passes the signal of interest will little or no attenuation. In the case of an audio mixer the value should be selected so that it passes the audio frequency range. The value of each capacitor should be identical for each channel, otherwise an off-set in gain will occur between the channels. A common value found in one data sheet indicates a 1uF capacitor would work. Refer to Companies making Capacitors. A 0.01uF capacitor is connected between the minus input and ground.

Potentiometer Values

The value of the potentiometer or adjustable resistor depends on the range of mixing desired and the values used for the other resistors in the circuit. Common potentiometer values include 100k and 500k ohms. A data sheet used 500k ohm values for each potentiometer. A decision also has to made regarding the style of tapper; a linear tapper or audio tapper. Refer to the Audio Amplifier topic for more information of using an audio tapper. The value and style of potentiometer should be same for each channel. Refer to Companies making Potentiometers.
Note that most commercial mixing boards use a linear potentiometer which moves back and worth in a straight line, which is not the same thing as a linear tapper. Read more about the difference in the Resistor dictionary; Linear Tapper vs. Linear Potentiometer.

Resistor Values

The resistors are used to set the gain of the Op Amp. So the values may be adjusted to produce the desired gain of the circuit, using the equation shown in the graphic. Starting values that could be used include: R4 = 22k, R5 = 2.4k, Rf = 790 ohms. Note that the gain calculation only includes channels a, b & c, while channel 'd' is only meant to show that any number of channels may be added to the circuit.

Operational Amplifier

An LM381 is shown in the schematic, but may be substituted for one that is easier to procure. The LM381 is a Low Noise Dual Preamplifier, although only one section is being used in the schematic. The LM381 is only produced in a 14 pin DIP, but any package style works depending on the amplifier used. Although not indicated in the schematic, the Op Amp design uses a 24 volt Vcc [pin 9], with pin 4 grounded. The LM381 is also internally compensated, so no frequency compensation is shown in the diagram either. As with all operational amplifiers a by-pass capacitor of 0.01uF should be placed between Vcc and ground.
Related Audio Circuits;
The pages provide schematic diagrams for both passive and active circuits, and in some cases a passive circuit that is also used with an active circuit [operational amplifier].
Audio Base Control, standard passive low frequency adjustment.
Treble Control standard passive high frequency adjustment.
Tone Control, for all in-one systems.
Midrange Adjustment Control, for more control.
Loudness Adjustment Circuit, for low volume bass.
Audio Amplifier Circuit, Operational Amplifier Schematic.
Audio Dual Channel Amplifier Circuit, Operational Amplifier Schematic.
Audio Cross-over Network Circuit, Passive filter.