stereo to mono before speaker amp

[JW]

This is probably a pretty basic thing for most folks here, but I'm wondering if someone can tell me if this is correct.

I'm going from 50 ohm output (also 600 ohm transformer sometimes) to an Adcom 535 speaker amp, assume unbalanced (90's era?) inputs.

Anyway, just trying to add a little sum to mono switch prior to the the amp. Please see my hand drawn schemo using 680 ohm resistors. Could be a little higher I guess, but might be in the ballpark?

???

 

[joaquins]

  I'd use higher value for the input and shorted outputs, maybe a 100Ω for the outputs just in case... For the inputs I'd guess something about 4k7 would be fine. The output resistors are there just to avoid shorting the two output together, while it shouldn't be a problem, as they are much smaller than the inputs shouldn't affect much either. The inputs would do the summing and you might need to experiment with them, I'g guess you'll find the optimal value between 1k and 10k.

  If it's too low you load too much the sources and have losses (level and LF) and potentially distortion, if it's too high the losses are driving the inputs, mostly level,  not so much freq issues or distortion, unless transformer inputs or other special cases. Compare the level between driving directly and with the network, use only one channel source for this and leave the other connected but without signal, if you feed the same signal from both sources you will get higher level. With low Z sources and hi Z loads you could archive pretty low insertion losses, worst case you should get 6dB losses, anything over that you have your values off.

JS

 

[JW]

Thanks

I actually went ahead a built in a switch before you had replied. I used 4 matched 1K resistors. It works fine. The level is just slightly different. Not enough to matter much. The input impedance of the amp (Adcom 535) is insanely high. Over 90K.

So, with that much load, I think I'm okay.