Circut for mixing rca signals from headunit?

[Bjohn118]

I am wanting to add a in dash eq in my car for my sound system. I have 5 rca outputs on my radio. 2 for front, 2 for rear, and 1 for subwoffer, my eq only has rca inputs for 1 left and 1 right. I need to build a circut that that takes either combines front and rear for the left and right and then adds the sub signal to the combined of both, or alternatively just take the sub signal to the left and right front channels and not include the rear. Any ideas or suggestions? I don't need or want level control in this circut because the eq will do that and also outputs the seperate sets of rcas for left right and Subwoofer. I just need to feed it the full range signal.

 

[rjenkinsgb]

All you need is a resistor from each source signal to the next input, and grounds/screens directly linked.
The resistor value depends on the input impedance of the device they are feeding, but for low level audio try 1K to start with; if that causes distortion, try 10K.

The signal level will be reduced to some extent in any passive mixer.

Example from google, for two channel stereo - ignore the one with the question mark.

 

[Bjohn118]

Thankyou very much! I had seen using just resistors from other research but didn't quite grasp the concept due to the fact that it was 2 completely different frequency ranges. I don't so much care if the signal level is down a little because the eq also has a build in level control that can boost the output signal back up going to the trunk. My only concern is that I don't want to loose frequencies, or hurt sound quality. My scope is currently in storage but I have alot of readily available components for projects and I have a distortion detector as well. I saw previously that the resistor method was a quick and dirty way and others were saying to use opamps or other circuts that were made to constantly change the mixing. Would an op amp circut be better or cleaner than the resistor method?

 

[Bjohn118]

Also per the schematic you have shown my 5th channel is just a single rca since subbass is mono. In this case if I changed nothing from the schematic except for the 5th channel would I use 2 5k resistors one going to left and one going to right, or would I make the split to left and right after the 10k resistor?

 

[danadak]

Right now, because of high R values, the various sources do not interact
with each other. But you do lose signal because of this. If you use an OpAmp
summer you can eliminate crosstalk between sources while preserving, and
/ or amplifying each channel.

 

[Nigel Goodwin]

Have you tried just feeding from front left and front right?, the rear two are usually identical, and they would usually include the sub audio as well - the sub output is just a mono version of the stereo signal (L+R), and 'may' be filtered to remove high frequencies, or that could even be done in the sub-amp itself, so could well be full frequency mono as well.

You might well be over complicating this?.