I want a 2-input summing amplifier where I can adjust the amount of signal from each source with a single potentiometer. I intend to hook up two microphones (Shure SM57 and Audio Technica ATR 1500, both with impedance matching transformers) to a single Orange guitar amplifier. The circuit I have currently built looks like this:
The single pot allows me to add some of one signal and simultaneously subtract from the other. The 330 \$\Omega\$ resistors are added so neither mic is ever completely silent. I am using unity buffers because each microphone has slightly different impedance and output levels, so it's much easier than trying to create some sort of impedance matching network for the two different microphones.
The two buffers are on the same LM358 chip while the summing amp is on a separate chip. I'm currently running the op amps off of a DC power supply, +7.5V, 0V, -7.5V, although I intend to ultimately run the circuit off either one or two 9V batteries.
The problem is that I'm getting some low frequency hum that I don't get when I just run a single mic into the guitar amp. So I figure I need to add a high pass filter somewhere but I'm not sure whether its better to put it before the buffers, before the summing amp, or after the summing amp. Any advice?