Figure 1 is a block diagram of a four input version of the Mixer. A signal connected to one of the input channels first passes to a buffer amplifier stage. The buffer amplifier has a high impedance input to match the signal source. Its output impedance is very low so that it can drive the next stage without distortion. Another function built into the buffer amplifier is an input gain control. This control allows the channel's contribution to the final output mix to be varied smoothly from zero to maximum.

Following the buffer amplifier in each channel is an equalization stage. The circuit used here is a parametric equalizer. This allows the frequency response of the channel to be tailored by the insertion of a single peak or dip in the gain of the channel anywhere in the audio band. Two equalization controls are provided, one sets the centre frequency, the other sets the degree of peak or dip present. See Fig. 2.

After equalization the signal is passed to the mixer amplifiers via a pan pot. The pan pot operates in the same way as the more familiar balance control found in most stereo amplifiers. It allows the signal to be distributed between the two output channels. Use of the four pan pots allows each channel to be positioned anywhere in the stereo image of the output.

An additional control on each input channel allows some of the signal to be sent to a third output mixer amplifier, the 'effects send' amplifier. All three output mixer amplifiers operate in the same way. Each receives four signals, one from each input channel. These inputs are added together or mixed to produce a single low impedance output. The outputs from the two main mixer amplifiers drive the output sockets via individual output channel level controls.

The final block is the 'effects return' amplifier. This acts as a very simple input channel, consisting of a buffer amplifier and a pan pot. It receives its input from an external effects unit (if one is being used) and drives the two main mixer amplifiers. The pan pot allows the 'effects' signal to be mixed between the two main output channels.