Quick Amp Mod Question

Hi guys, have an Alesis RA150 here. It has phono and jack inputs and I would like to use both inputs at the same time but the manual states

"Only one input jack of a channel should be used at a time:  don't use the 1/4"/XLR input jack and phono input  jack  simultaneously.    Do  not  plug  two different  sources  into  the  same  amplifier  channel. While  this  will  not  harm  the  amplifier,  the  sources may interfere with each other causing distortion."

I'm thinking if I just replace R107 with something like a 1k I should be good, can someone confirm please.  Thanks

1k Ohm is still a little low if two inputs get shorted together.

That schematic symbol looks like a switching input, but I've never seen phono jacks with switching.

Using 2k for R107 will cause about 1dB signal drop, but be a little easier to drive and not load down (if two inputs are indeed shorted together).

JR 

No, that won't work. You would have to also have 1k in series with the phono input. But even that would be crude.

Although I don't really understand this schematic. Is that a normaling contact on the phono jack? How is that mechanically possible? If yes, they why would the manual state that you would get distortion? If no, how would the circuit work at all since it would effectively be grounded through 47R. Doesn't make sense.

If the schematic is vaguely correct, you could make a cable for the 1/4" / XLR input where you deliver one signal to the + term and one signal to - term. That would effectively mix them together (albeit with one signal inverted but if the signals are not correlated, it shouldn't matter).

.

Thanks chaps. Just checked and the phono socket does indeed seem to be switching. Plugging a phono cable in whilst the jack input is being driven cuts the signal. I Didn't even notice that on the schematic, my bad. That's that plan shot down then. Cheers!

Don't give up so fast.
You should need to cut some traces and replace some resistors but it can be done very easily.

Unfortunately it's very much easier said than done. Can't reach the tracks without stripping the whole thing down, will be less effort to just make a little summing box, though I appreciate your effort. Thanks all.