RCA: Stereo to Mono?

[silentblackhat]

Does anyone have a schematic of a op-amp summing circuit or mixer that doesn't require a negative voltage from the power supply? Or any other simple mixer schematic for 2 ch
I keep finding schematics like this that require a negative voltage

[Perry Babin]

The problem with an electronic (op-amp based) circuit is ground loop noise (engine noise). Unless the power supply is isolated from chassis ground or the input is balanced, you will get engine noise.

For some simple circuits, you can allow the op-amp supply to ground through the shields (this was relativley common about 20 years ago) but the current passing through the shields can cause noise in other components that are driven off of the same RCAs.

The transformer based mixer is probably the best for what you're trying to do.

[silentblackhat]

Originally posted by Perry Babin
The problem with an electronic (op-amp based) circuit is ground loop noise (engine noise). Unless the power supply is isolated from chassis ground or the input is balanced, you will get engine noise.

i had this problem when i built a LM386 preamp and installed it in a car,. I got a power line filter that took care of the noise

[Perry Babin]

A power line filter won't work in most cases. The noise isn't caused by ripple on the power line. It's caused by a ground loop.