Stereo to Mono preamp output

[gui_tarzan]

I need a sanity check. I found this stereo to mono drawing online and need to know if my drawing with the DPDT switch is correct. The original wiring on the stereo all-tube preamp I'm working with was biffed something awful and I want to make sure I'm thinking right.

[uneumann]

Looks fine except that the 47k on the Right output will slightly decrease the R channel output. You could add a 47k to the Left output to compensate and make the levels equal.

[Helmholtz]

Looks fine except that the 47k on the Right output will slightly decrease the R channel output. You could add a 47k to the Left output to compensate and make the levels equal.

The schematic looks symmetrical to me.

[uneumann]

With the switch in the stereo position, the R side has a 4.7k to 47k divider that attenuates the R output. The Left side doesn't. If you add a 47k to the L output, they'll both be attenuated by the same amount (which is very small, admittedly).

[Helmholtz]

So use a 100k from each 4.7k to ground on each channel before the switch.

[gui_tarzan]

The next question then would be - why are there resistors in that circuit anyway?

[Helmholtz]

The next question then would be - why are there resistors in that circuit anyway?

Because it's no good idea to directly connect two low impedance outputs.
If the resistor(s) to ground are necessary depends on the input impedance of the amp.

[gui_tarzan]

That makes sense. Thanks for all the help!

[uneumann]

Oh - one more thing I should have mentioned... you can also just remove the 47k on the R output. That will work too without introducing any imbalance. I guess the point is that having a resistor to ground on one output is best to be avoided. Either have them on both outputs or remove them both. The series resistors (4.7k) are still a good idea as Helmholtz states - to avoid damaging the signal sources.