richfiddler11

It might be better to run an SPST switch between the L and R channels of one input, but *after* the resistors.

With the switch open, you will have a normal stereo input. With the switch closed, the mono signal will be fed to the L and R output mix.

The way you were thinking about would work too, but if you plug a stereo input in while the switch is in mono mode, the stereo output won't be too happy.

Hmmm... now that I think about it, you are absolutely correct, the scheme I suggested will make the whole output mono because it will tie the L and R output mixes together.

I have to think most stereo devices can tolerate having their L and R channels tied directly together (I know you're talking about mono sources, but have to look at the worst case).

If this is the case, then the SPST (e.g. simple open/close switch) between L and R channel before the resistors like you suggest would work fine.

To be totally safe, you could do something like this with the input:

>--R---+----R--> to mix right
             |
    SPST Switch
             |
>--R---+----R-> to mix left

But again, this might be overkill.

Good catch!

Rich