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Adding hardwired auxiliary input to car radio amp

john2k

Jun 13, 2012
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Looking for some help and advice. My car radio unit has a built in amp and has 8 wires going to 4 pair of speakers. It also supports a 3.5mm audio aux input. However, the 3.5mm aux input is treated as a source and I can only hear it when I'm in that source itself. So if I'm listening to the radio or any other source then I cannot hear the sound from the aux device. I need to somehow hardwire my aux input directly into the amp so that it mixes straight into the radio regardless of what channel I am in. Can anyone help advising me how I can even begin such a task?

Would really appreciate some help.

Thank you
 

Bluejets

Oct 5, 2014
5,881
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Oct 5, 2014
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5,881
However, the 3.5mm aux input is treated as a source and I can only hear it when I'm in that source itself.

That's pretty much how it is supposed to work.
If you have a schematic on the particular radio it might be possible to "hack" but it could present all sorts of problems.
Without it, you would be flying blind and any suggestions would be "generalised" at best.

If you want to give it a hack bash, try putting your signal ground to radio ground, and your signal via a small capicitor to the top end of the volume control.
No promises on what will happen though.
 

john2k

Jun 13, 2012
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Another possible solution I'm thinking is could I potentially add a mini compact 12v amp somewhere (one of those really compact ones) and somehow have my compact amp also feed audio out to the front two speakers? Because the auxiliary input is really for navigation instructions and other notifications so it doesn't need to be super loud. That would mean two amps (car amp plus the compact amp) will be connected to the front speakers. Not sure if that's even possible without damaging the speakers. Plus I'm sure some sort of auto switching between the two amps needs to happen
 

john2k

Jun 13, 2012
188
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Jun 13, 2012
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That's pretty much how it is supposed to work.
If you have a schematic on the particular radio it might be possible to "hack" but it could present all sorts of problems.
Without it, you would be flying blind and any suggestions would be "generalised" at best.

If you want to give it a hack bash, try putting your signal ground to radio ground, and your signal via a small capicitor to the top end of the volume control.
No promises on what will happen though.

I've been trying to locate service manual for the radio and no luck. I have the manufacturers schematics but that shows how it's all hooked up and which connector pins are what. It doesn't show the internals of the radio itself.
 

Bluejets

Oct 5, 2014
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5,881
add a mini compact 12v amp somewhere (one of those really compact ones) and somehow have my compact amp also feed audio out to the front two speakers?

That would be disasterous....
 

john2k

Jun 13, 2012
188
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That would be disasterous....

lol, so only other option would be to have some seperate spare tweeter speakers or something hooked up to a mini amp just for the aux announcements and leave the main system alone?
 

Bluejets

Oct 5, 2014
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Well, I guess, but that flys in the face of what you originally asked.

As it stands, I guess it is a sound mixer of sorts.

Biggest problem of course is the whole idea is in your head and we have no idea why you would need to do it.
There may be better ways to attack the project but unless you describe it fully, no one really knows.
 

Frankchie

Nov 14, 2017
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Some radios will automatically switch to the aux input when something is plugged into it. I suspect that's done through mechanical activation of contacts at the aux jack. It might be possible to disconnect the appropriate wires from those jack contacts and use a circuit that emulates those same contacts when it detects that your external source produces some audio. Your external source would always be physically plugged in to the radio aux jack, but the radio would not detect it because the aforementioned wires would be disconnected from the jack.

It's not a trivial project. If your external source happens to have an indicator like an LED that indicates audio is playing that would make the project a little easier.
 
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