Simple Mixer Schematics

A final  word on the capacitor

There are capacitors in two main circuit functions on the schematics above. the first is an electrolytic blocking capacitor. The idea is that a DC voltage can't got through a capacitor in series. What this means is that any DC offset voltage emanating from a preceding stage or source will be knocked on the head. only the AC voltage (The audio signal) will get through. The reason for this is simple. Suppose you had 10 sources each with a +1volt DC offset. This would add up in the mixer stage to be +10 volts. Not exactly desirable. It is therefore usual to use a blocking capacitor to stop this happening. This may not be so in all cases but is a rule of thumb for most audio circuits. The blocking capacitor is placed on the input near to where an unknown source is to enter the circuit. It is also usual to have one on the output stage which blocks any DC from leaving your circuit and propagating into any following equipment. The reason you need them on both input and output is simply that you never know what you might connect your circuit up to and there is no convention. If you are unsure of the polarity required for the blocking capacitor you can use a bi-polar electrolytic. Which is essentially two normal electrolytic capacitors back to back in the one package. The value of these capacitors are not important as long as it has no effect on the audio signal (IE accidently creates a lowpass filter) and the voltage rating is sufficient enough that it won't burn out. Usually 16 volt rating is sufficient. 25 volts to be on the safe side. 50 volts is called "over-engineering". The value of the capacitor can be anywhere between 0.1uF to 47uF but usually between 1.0uF and 10uF.

The other two capacitors, 27pF and 47pF are optional and for stability of the op-amps. Truth be known these were left in the schematic by accident because I simply modified the circuit from one I was working on at the time of writing. The original circuit was designed to closely approximate another commercial mixer as I was extending it's capabilities.

Out of interest these two capacitors cause the op-amps to behave as slight intergrator-filters limiting the top end response slightly above the audio bandwitdth. This is some times necessary where the op-amps used have such a high gain-bandwitch product that they tend to saturate with RF or at least HF signals. Thus becoming unstable in certain situations. Generally speaking these are largely irrelevant to the design.

Luis Fernando Cantor B.

Electronica de Estados Solidos

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