A low cost four into one mixer unit.
Modern mixer designs are invariably based on an operational amplifier used in the configuration shown in Figure 1, and this design is no exception.
If we ignore input 2 and RB for the time being, the circuit is a straightforward inverting mode operational amplifier circuit. Due to a negative feedback action, the circuit stabilises the inverting (-) input at the same potential as the non-inverting (+) input. The latter is normally biased to the 0V rail in a circuit having dual balanced supply rails, or at half the supply voltage if a single supply is used. With no input signal, the output is at the same voltage as the two inputs, and this optimises the output voltage swing before the onset of clipping.
If RA and RC have the same value, an input voltage will produce an identical change in the output voltage, but a change of opposite polarity. For example, an input voltage of +2 volts would produce +1 volt at the inverting input if we assume no change in output voltage. This is caused by a simple potential divider action across RA and RC, and in practice it would result in the output swinging negative in order to balance the input potentials. This state of balance would be achieved with the output 2 volts negative of its quiescent level, since a potential divider action gives a potential at the inverting input which is halfway between the voltages at input 1 and the output. The circuit thus acts as an inverting buffer amplifier.
With an input to input 2 as well, the circuit operates in much the same way, but the output must now respond to the sum of the two input voltages. If input 2 is also at +2 volts, for example, the output would have to be 4 volts negative in order to counteract both input signals and maintain the balance of the input voltages by a potential divider action. With input 2 (say) 2 volts negative, the two inputs would counteract one another and the output voltage would remain at its quiescent level.
This configuration is known as the 'summing mode', and it obviously provides the required mixing action. Although only two inputs are shown in Figure 1, it is possible to have any desired number of inputs with an extra input resistor being used at each additional input. An important property of this circuit is the constant voltage produced at the inverting input, and what is termed a 'virtual earth' is formed here. This isolates the inputs from one another so tha changes at one input (such as connecting or disconnecting a piece of equipment) have no affect at the other inputs.
The full circuit diagram of the 'Quadra-mix' is shown in Figure 2, and has obvious similarities with the basic configuration of Figure 1. One obvious difference is that four inputs are provided in the practical circuit and this necessitates the use of four input resistors (R1 to R4). A DC blocking capacitor is also used at each input, and these are C2 to C5. The non-inverting input is biased by R5 and R6 since a single supply rail is used. C6 filters out any noise which might otherwise find its way to the non-inverting input due to stray coupling.
C7 provides DC blocking at the output and C1 is a supply decoupling capacitor. The circuit only has one control, and this is on/off switch S1.
A suitable housing for the unit is a diecast aluminium box having approximate outside dimensions of 120 by 65 by 40mm. The four input sockets are mounted on one side of the case with the output socket and on/off switch on the other. The positioning of these, especially the four input sockets, is quite critical as there is not a great deal of excess space inside the case. Figure 3 shows the positioning of the on/off switch and sockets, and it is strongly recommended that this layout is copied accurately. The mounting hole diameters are correct for the specified components, but note that other components might need mounting holes of slightly different diameters.
Details of the printed circuit board and wiring of the unit are shown in Figure 4. Construction of the board is quite simple, but it is easier if R2 and R3 are soldered into place before C2 to C5. Veropins are used at the points where off-board connections are made to the board.
Screened leads are used to connect the board to the input and output sockets. Provided the specified case is used, the completed printed circuit board is slotted into the lowest set of guide rails in the case, once all the wiring has been completed.
There are suitable spaces for mounting holes in the printed circuit board if a different case is used.
The battery fits into the space between S1 and the input sockets. A piece of foam material can be glued to the removeable base panel of the case so that the battery is held firmly in place when the base panel is screwed in place.
The unit is connected to the other components in the system using normal screened audio connecting cables. As the circuit has unity voltage gain, a fairly high input impedance, and a low output impedance, it should not produce any problems with incompatibility when it is added into a system. In fact, it can be used as a buffer amplifier in situations where a relatively high impedance signal source is driving a fairly low input impedance and loading effects are producing poor results.
As described here, the unit is only suitable for mono operation, but for stereo operation it is merely necessary to use two units, one in each stereo channel. If more than four inputs are required the circuit could easily be modified to have any desired number of inputs, as explained earlier. Alternatively, two 'Quadramix' units connected in series will accommodate seven inputs.
Feature by Robert Penfold
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