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I need to find a simple (preferably passive) way to take a stereo consumer line-level output, mix it to mono, and reduce it by 40dB to be used in a microphone level.

Following this answer: https://electronics.stackexchange.com/a/14982/1554 I see it's easy to use a pair of resistors to get down the levels I need. I've also stumbled across this schematic for mixing the two channels together:

enter image description here

(from http://jwsoundgroup.net/index.php?/topic/7015-passive-summing-stereo-to-mono/)

I can't help but notice how similar these two schematics are, and am wondering if I could combine them. Minimizing parts and size is important to me, as I would like to build the whole thing inside an XLR plug. What values of resistors do I need in this stereo-to-mono schematic to also bring the levels down by ~40dB? And, how is that calculated?

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Yeah, you should be able to. The linked answer has a single channel line to mic level attenuator, with a 10K series and a 100 ohm parallel resistor. If you want to make two inputs, change out the 10K for a 20K and put two of them in parallel, one for each input. So for the circuit you posted, just replace the 470 ohm resistors with 20K and the 20K with 100 ohms. It won't have the same impedance as a microphone, though - the impedance will be a bit less than 100 ohms. I believe a microphone is 600. So if that might be an issue, just multiply everything by 6 - use 120K and 600 ohm resistors.

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  • \$\begingroup\$ Great! Thanks for your input. Just to clarify, I'll be using 120k resistors in place of the 470 resistors (one on each channel), and then a 600 resistor in place of the 20k resistor in the diagram, tying the channels to ground. Is that correct? Thanks again... I'll have to go find some resistors tomorrow and give it a try. \$\endgroup\$ – Brad Nov 7 '13 at 4:20

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