Simple Mixer Schematics

Two more variations

The third circuit shows a Mixer with input attenuation. This is a fairly simple concept. A potentiometer is placed in the signal's path between the source and the summing resistor. When the wiper of the POT is at the top it simply represents a 10K load to the source. 10K is a pretty high value and most line level devices can easily drive this load. With the wiper at the other end of the pot it still represents a 10K load to the source but the input is effectively at ground (Shorted out) so no signal gets through. With the wiper in mid way position the input loading is still 10K, however the signal has to flow through a 5K resistance and is also dumped to ground by 5K. Halving the potential reaching the input resistor/summing node.

The fourth and final circuit shows a full on stereo mixer. Two new types of input networks are shown. the first is a stereo-in with balance. Similar to your stereo amplifier etc. A dual gain pot is use for volume whilst balance is single. Note that following the volume pot is a 10K resistor connected to one end of the balance pot. With the wiper in the centre position and connected to ground as it is, means that the incoming audio is virtually running through a 22.5K resistor to ground. That is 10K +(1/2 of 25K) = 22.5K Because of this attenuation the feedback resistor around the virtual earth op-amp is increased to 33K to compensate. This is not exactly unity gain but it comes awfully close. A very slight and probably un-noticeable gain.

The other input is MONO in but is pannable between left and right. The same deal as above applies here except that the first two 10K resistors are joined together so that the signal is split across two paths. Strictly speaking the first two 10K resistors in the stereo input are not necessary but are needed for the mono circuit so that the pan pot does not short out the signal when at either extremes of travel. They are included in the stereo input simply to compensate for unity gain over all. This input scheme is the basis for 99% of all large mixing consoles.

Epilogue 

Well hopefully I've provided enough information so you could go out and roll your own designs. And hopefully I've been able to work it in such a way that it's relatively understandable. If there are any mistakes, errors or omissions, please feel free to point them out. But Please no nit-picking. I'm only doing this because of the number of questions asked on this subject and the relative interest for people to design their own.

No responsibility is taken for any damages or any other shortcomings if you actually use this information. If you start out building one of my designs and end up wiring yourself to the national grid, it's you're problem.

Luis Fernando Cantor B.

Electronica de Estados Solidos

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