Making a simple audio mixer with basic tone control is actually an easy task. According to our tutorial article on how to build an audio mixer, an audio mixer is actually a summing circuit, based on an operational amplifier and we can use additional circuitry for audio tonality control (tone control).
The circuit we present here is a basic audio mixer that also has additional circuitry for bass and treble control. The whole design is based a dual operating amplifier IC, the TL082. This mixer has three inputs but you could add more or remove any of them. The tone control mixer is also monophonic. For a stereo version you will need to make two identical circuits, one for the right (R) audio channel and another one for the left (L) audio channel and you will also need to use dual - stereo potentiometers.
The mixer consists of two stages. The first stage is a typical summing circuit based on IC1A. The gain of the summing circuit is set to be exactly equal to 1. This means that the summing circuit does not amplify at all. From the theory about the summing circuit, we know that the gain for each input channel depends on the ratio of the feedback resistance, R7, to the input resistance of each input channel (i.e. R4, R5, and R6 for the first, second and the third audio channel, respectively). Since all resistors are identical, their ratio is equal to 1 and therefore the voltage gain for each input is also set to 1. If you wish, of course, you may change the resistor values and adjust the voltage gain for each channel to a different level.
The C1 capacitor is used to limit the frequency response of the summing amplifier to the very high, not audible frequencies, in order for the noise level to be minimized.
Resistors R14 and R15 are used to DC-bias the non-inverting inputs of the operational amplifiers at a voltage level equal to the half of the supply voltage. This is essential, in order to the circuit being actually able to operate from a single supply voltage. C6 acts as a short circuit for the AC and thus, the non-inverting inputs of both op-amps are grounded on AC.
IC1B, along with a resistor - capacitor network, acts as a tone adjustment circuit. This circuit is based on Peter Baxandall's classic topology published in 1952 in Wireless World Magazine. The original design was based on electron tubes. In our circuit, of course, Baxandall's topology is implemented with an operational amplifier.
The operation of the Baxandall tone control circuit is based on altering the response of an inverting amplifier by altering its negative feedback. The R8 and R9 potentiometers, along with C7, C8 and C9, C10, allow the feedback to be altered for high (treble) and low frequencies (bass), respectively, in order to selectively vary the voltage gain on these two frequency bands. When the R8's wiper is at its terminal position towards C8, higher frequencies are amplified for about 20db. By the other hand, when the R8's wiper is at its terminal position towards C7, we get a cut-off of about 20db at the same high frequencies. The same thing also happens with lower audio frequencies (bass) when the R9's wiper is moving towards C10 or C9, respectively. In either case, treble and bass can be independently adjusted by ± 20db via R8 and R9. The frequency response becomes flat when both wipers are in their middle range position (assuming that both are linear potentiometers).
The supply voltage should be in the range of 12 to 30V according to the specifications of the TL082. Maximum supply voltage ensures greater dynamic range and better linearity.
The basic tone-control audio mixer circuit would be sufficient for most audio sources. However, for some special cases of audio sources, such as microphones or magnetic cartridges of turntables, you will need to add preamplifiers to the inputs.