Summing L/R pre-outs for sub

[TB Rich]

Has anyone used one of these cables successfully to connect from stereo pre-outs to a mono subwoofer input?
(I don't want to connect only the L or R before anyone suggests).
Summing Cable

This cable is sold and positioned as being needed for use with an Antimode when sharing a Hifi amp and AVR:

I suspect it'll work fine as other than the fact I'm connecting to a miniDSP and not an Antimode - I'm using it as intended. However not being entirely literate on the engineering side of things I just want to do a bit of due-diligence and make sure it won't damage anything!

My basic understanding of the subject is line level outputs are low impedance, whereas inputs are high - and you therefore can not use a basic Y cable as it would effectively tie the 2 outputs together.
What I'm not sure on is this: given a 10k resistor on each channel, does that make it an effective 20k? And then given the miniDSP input is 10k - is that then a problem?

Thanks

 

[noiseboy72]

We used to build these summing cables all the time to run stereo sources into mono PA systems and never had a problem. The chances are that the input impedance on the DSP is more than 10K and quite honestly the drop in level will be able to be made up in the DSP in any case.

 

[TB Rich]

We used to build these summing cables all the time to run stereo sources into mono PA systems and never had a problem. The chances are that the input impedance on the DSP is more than 10K and quite honestly the drop in level will be able to be made up in the DSP in any case.

Thanks, so does the impedance of both the cable and the input of the DSP effectively add together in this scenario? So that it would present a 30k load to the pre-outs (20k cable (if 10k each channel), and 10k DSP)?
I think it's generally considered no bad thing having high impedance on an input, but it's not clear to me why or if the cables impedance level matters in the chain at all?

Level wise however it won't matter there's a slight drop as I'll be likely taking some gain out within the DSP anyway, I run the sub fairly hot for AVR purposes as my main listening level is low-ish and my AVR doesn't have dynamic range compensation.

 

[noiseboy72]

Yes, I think so. You are driving a higher impedance load, but this really won't cause a problem, as the pre-amp will be able to drive a few 10s of Ohms without issue.

 

[TB Rich]

Cheers, I think I'll give it a go then.

I don't use my sub for Hifi at all and tbh it doesn't really need it for 99% of music content, however since moving the sub it's now in a far better position so I feel like there would be no downsides in having it added (and integrated via the miniDSP on a 2nd config) to just provide some extension below 30hz where the mains start to drop off.