How can I sum a stereo RCA signal into a mono mic imput easily?

[simon5]

Hi,

I want to know how can I sum a stereo RCA signal into a mono mic imput?

The CD player on my girlfriend small stereo is broken.
She got a old large CD player that plays very well.
It haves the usual left and right RCA outputs.
I want to sum them into a mono signal to drive the mono mic imput.

I temporarly plugged both RCA + wires together with the mono + wire and I left the mic imput ground floating. I connected both RCA grounds together.

I'm almost sure that's not a great way to do that.
But... it works.

Also, if I hold the mic imput ground between my fingers, I get much better gain (loudness), less distortion and less humming (lower noise floor). Maybe it's a bit of a Placebo effect since I get so much more gain. What's causing this?

I also tried Left RCA + with mono + and ground with ground.
I didn't plug the Right RCA at all.
I was getting very bad distortion, so I used the above method.

[Tube_Dude]

Put two 1k Ohm resistor in series with the center pin of each RCA.

Connect the other side of the resistors together...and bingo...you have the two stereo channels ....in mono.

[simon5]

Can I connect the mic ground with RCA ground after that? I guess it's making a stupid ground loop problem.

[gmphadte]

Mic input requires a small signal and the RCA outputs are much higher, of the order of 100-200mV.
U need to connect two equal value resistors (4K7 to 10K each) to the two RCA outputs and join one 560 ohm resistor to both and ground. This will make the signal mono and attenuate.
All this should be constructed near the Mic input.

RCA ground will be connected to Mic ground, in case of two units, i.e. signal source and amp.

Gajanan Phadte

[simon5]

Thank you very much !

If I understand correctly I connect a 10k resistor in series with each RCA output and I connect both outputs together, then connect that to the mic imput and add a 560 ohms resistor that will be in parallel with the mic imput, so between imput and ground. After that, I connect every ground together.

Can someone explain me this part please ?

If I hold the mic imput ground between my fingers, I get much better gain (loudness), less distortion and less humming (lower noise floor).