Mono summing cable

Hello,

I want to make a mono summing RCA cable to connect my audio interface/DAC to a mono amplifier. The input impedance of the amp is 30kΩ, the input sensitivity (2Ω, 100W, 1KHz) is 0.83V. The source max output level is +16dBu and i do not know its output impedance.

What value resistors should I use? Do I need a resistor on the summed output that goes to ground? What is the purpose of this resistor (can be seen in the schematic below)?

E3Aob.jpg


The amp is a wondom model WONDOM | STORE

The audio interface a Motu M4.
 
Would there be any benefit in using, lets say 1 kohm resistors, especially since the source is powerful enough that they wont decrease its output level? This shunt resistor effectively sets the "output" impedance of the whole source + summing cable combo ?
 
It's more like they set the maximum impedance the source "sees" for any difference between the L and R signal. In the worst case, the left channel for example may have a full output and the right channel zero (or even an out of phase signal, but not usually with a music source), and the left channel will "see" the summed impedance of these resistors. With 1k the minimum will be 2k. With 470 the minimum is around 1k, generally that's okay, but if the source cannot drive 1k then use larger resistors there.

They do also set the effective output impedance that the load "sees". So, like most things in audio design, it is a balancing act.
 
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How will i know if the source can drive 1k? from the sound? Also, isn't the purpose of these resistors to prevent the shorting of one channel to the other? Why would I want the source to be able to drive through them? Lastly, is the summed signal going to be hotter than the two summed ones? Effectively increasing the outputted voltage ?
 
If it struggles, you'll likely hear distortion. I doubt you'd hurt anything in the process. If that's the case just increase these resistors a bit.

It does prevent the channels shorting. Any difference will drive the summed impedance of these resistors, instead of a dead short.

The signal output from this passive cable will be about the same as one channel, assuming the source is mainly a voltage source, the load has a relatively high input impedance, and both channels are equal ie. a mono signal. Should be easy to demonstrate once it's made. One channel driven and one with no signal should result in around half the amplitude at the output, as it is effectively a 2:1 voltage divider.