stereo to mono

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if it can't be done in software, then consider using an inverting opamp with two input resistors.

If you set R1in = 20k & R2in = 10k and Rf = 5k.
The output from the opamp will be V1*5k/20k + V2*5k/10k
Set all three resistors to the same value then the output = V1+V2, i.e. loud mono.
Set R1in = R2in = 20k and Rf = 10k, then output = 0.5 * {V1 +V1} = normal mono.
If you are sure your player hasn't a mono output option (because that is probably the best way to combine the channels), then what you want to do is place a resistor in series with both the left and right channels. Just splice them into each positive lead going to the amp, and connect the two resistor leads closest to the power amp. The resistors isolate the channels from each other while combining the two audio signals into one mono channel.

question, do the stereo to mono adapters sold at radio shack contain these resistors? or are they simple Y connectors?

i am getting a lot of noise using the radio shack adapters to drive a mono input from a stereo output. any idea why?

also, don't these resistors cause a volume level drop? and do they really isolate the channels? after all, a resister does not stop a signal, it just weakens it.

also, can these resisters cause some sort of change in impedance?

thanks again!
I do not know if the RS adapters contain resistors. They have worked well enough for me, though, the few times I've used them.
The noise might suggest the RS adapter is a simple Y, without resistors. Otherwise, I haven't a good guess to explain the noise.
If you consider the 1k resistor to be in series with the power amp input, which is likely tens of kilohms, then you see any level drop is fairly insignificant. And the resistors do isolate the channels from each other. "Isolate" in this instance may just mean that one channel doesn't try to drive the other channel.
The resistors will cause a change in impedance. Normally not enough to fuss about. But it is one of many reasons why using an op amp adder/mixer is often the preferred method of combining channels.
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