Versatile mixer design based on a low-noise op-amp suitable for microphones and instruments

When it is required to apply two or more signals to a single input, it is unlikely that good results will be obtained by simply connecting all the signals direct to the input. This action is more likely to give rise to problems such as a low impedance signal dominating the others, which become virtually short circuited by its low source impedance, and affects the levels of the others.

A simple mixer is needed in order to avoid these problems, and the design described here can be built with up to four channels. There is a high input impedance (about 800k) at all four inputs so that there is minimal loading of the signal sources applied to the unit. The output impedance of the unit is low, and it can be employed with any normal amplifier or tape recorder without loading of the output causing any problems. The maximum voltage gain from any input to the output is about 7dB (a little over two).

The Circuit

The mixer design is based on an op-amp, and Figure 1 shows its circuit diagram. IC1 is used as an inverting amplifier but there are four input resistors connected to the inverting input, one for each input. The output therefore responds to the sum of the input signals rather than to just a single input, and the required mixing action is produced. R7 is the feedback resistor and has a higher value than the input resistors (R1-4) giving a small amount of voltage gain. RV1 to RV4 are controls which enable the amplitudes of the input signals to be adjusted individually. C2 provides DC blocking at the input to IC1. A 'virtual earth' is formed at the inverting input (pin 2) of IC1, this effectively isolates the four inputs and their potentiometers from each other so that there is no interaction.

R5 and R6 are used to bias the non-inverting input of IC1 to half the supply voltage, this gives a quiescent output voltage which is also at this potential. This permits an output signal level greater than 2 volts RMS before the output signal becomes clipped and seriously distorted. C1 prevents stray feedback to the non-inverting input at high frequencies which would cause instability. The current consumption of the circuit is approximately 2mA, and a PP3 size, 9 volt battery gives many hours of operation.

Construction

The Veroboard layout and the wiring details of the unit are shown in Figure 2. The component panel is straightforward but note that there are thirteen breaks to be made in the copper strips before soldering in the components, and three wire links on the component side of the board. Rotary potentiometers and 3.5mm jack sockets were used on the prototype but slider types and/or alternative connectors can be used, if preferred.

The unit should be housed in a metal box to minimise stray pickup.

If a three channel mixer is required, omit RV4, JK4 and R4. Similarly, if a two-channel mixer is needed, RV3, JK3 and R3 should also be omitted.