An audio mixer is a circuit for combining two or more analog signals so that they do not interfere with each other or cause loading on the source signals. They are mostly used to mix signals from microphones and other music sources like in recording studios, karaoke sets, etc.
A professional-level mixer may have dozens of channels and settings for each channel, including pre-gain presets, PFL (pre-fade-level), four-band tone controls, and mixer level. It also has built-in effects such as reverb, has a graphic equalizer, monitor system controls, and output amplifiers, to name a few. But for this article, we are going to look at something a lot more humble.
The basic idea behind a mixer is this: different inputs are fed into a common summing point, and these are added and then multiplied by the gain of the summing amplifier. Shown below are some input scenarios you may see on the internet. But I have avoided them for a couple of reasons.
The circuit on the left will have a varying input impedance depending on the slider setting, and the circuit on the right will cause a change in level on the adjacent channels when the slider is right down low. The circuit on the right does have the advantage of dealing with extreme input levels.
For this tutorial, we will build a mixer of our own design. Our mixer will have two stages: an input pre-amplifier per channel and an output summing amplifier.
There are many examples of mixers, such as that mix at the front end of the mixer. I can’t entirely agree with it as there is always a need to deal with powerful signals, say, from a line feed or sub mixer output. I want the input impedance presented to each input to be constant and not a function of the gain setting.
So here, the input impedance will be 47k to match most audio sources and dynamic microphones. There will be three stereo channels. The mixer will run on a single supply of a 9-12V battery (this imposes a severe constraint on the range of signal headroom).
I used a common LM324 (not the best for low noise and some low noise quad op-amps might be quieter, but I had some lying around). The LM324 is designed for single-supply operation and has four separate op-amps. A 1M is in parallel with the level sliders R3, 6, and 9. The purpose of this is to keep a constant DC bias on the op-amp and prevent slider noise. The caps C1, 3, and 5 tailors the upper-frequency response to 20kHz, while the input caps C24 and 6 tailors the lower frequency response to 20Hz. The switches S1, 2, and 3 turn the channels off. R15 and 16 divide the supply rail in half and provide a 6V bias voltage for all the non-inverting inputs. R11 is the main volume control. Note that all pots should be of the LOG type (audio taper).
The circuit above is for a three-channel mixer with a master volume. To make this a stereo mixer, simply duplicate it for the other channel. They can share the same battery.
How it works
U1.1 to U1.3 are the input pre-amplifiers. Their function is to provide a suitable input impedance to the microphones and this is set by R2, 4, and 7 at 47k which is standard for high-impedance circuits and Hi-Fi. R1 sets the gain of the input amplifier and R3 is only there to prevent scratchy noise as the pot is varied. This pot is setting the actual level of each input as it will be presented to the summing amplifier.
C1 provides a high-frequency roll-off to keep out RF problems and inaudible oscillations. After this pre-amplification, S1, 2, and 3 will then allow us to mute any section. R14, 10, and 13 are the actual mixer part as pin 13 of U1.4 is a virtual earth which causes the virtual voltage at this point to be the sum of all three inputs. This is amplified 10 times by U1.4 because of the ratio of R12 to any one of the input resistors. R11 sets the final level as it will be sent to the PA or amplifier.
The gain of the summing amplifier is expressed as A = R12/R14 or 10X and the output of the combined inputs is Vout = -(A*V1+A*V2+ A*V3).
This can be simplified into -A(V1+V2+v3), thus called a summing amplifier.
Note that there is a minus sign there as the op-amp is working in its inverting mode.