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PantSeatFlyer 8th February 2004 04:08 PM

converting car speaker wires to headphone jack
I have an 80s Delco AM/FM car stereo that I want to play with for home audio. I want to set it up so I can listen to it with my headphones.

Any suggestions on how to hook up a headphone jack to the speaker wires? It's not just a matter of soldering them to the jack, is it? On another board, someone suggested I would need to match the radio's speaker output with the headphones and that it could be done with a resistor divider, as follows:


This is easily done with a resistor divider... I'd put a load
for the speaker, too. This would look like an 8-ohm 20W
resistor (RatShak sells 'em) across the speaker leads, with
a 500ohm pot across that... your h-phones are tapped
from ground to the center wiper of the pot. Adjust for taste.
Since I'm strictly an amateur with electronics, I'm not sure how to build this. Can someone help (a picture, for example) or is there an 'off-the-shelf' solution?


maylar 9th February 2004 02:22 PM

The standard way to use headphones from a speaker output is to just put 220 ohm 2-watt resistors in series with the headphones on each channel. Radio Shack used to have an adapter for that but I don't see it on their web site.

Since headphones have a common connection between channels, you will have to hook the common to the radio's ground wire and use only the + speaker wire from each channel.

PantSeatFlyer 9th February 2004 04:11 PM

thanks for the info, dave.

Why 220 ohm/2 watt resistors?

My headphones are rated 300ohm. Do I need to match that up with the radio's output?

maylar 9th February 2004 05:23 PM

I honestly don't know where the resistor value came from... but that's my recollection of the resistor used in the Radio Shack adapters (I used to sell them back in the 70's). 220 ohms is also typical of what's used inside most amps with a headphone jack (if they don't have a dedicated driver).

You can hook them direct without a resistor, but the volume adjustment will be touchy.

wooferhound 10th February 2004 12:18 AM

If your headphones are rated at 300 ohms, then you can hook your speaker output directly to your headphone jack with no resisters and the volume will be about right.

You would probably want to add 8 or 16 ohm resisters from the left & right wire to the ground wire to load the amp down enough to sound clear.

PantSeatFlyer 10th February 2004 02:16 AM

thanks for the info. 8 or 16 ohm resisters to the left and right (-) wires, then to the ground, you say?

so this means the (-) wires for each channel are grounding wires for the speakers? Is there not a place for ground in the headphone jack? Should I also have a ground wire going to the jack?

richie00boy 11th February 2004 08:05 AM

Don't connect the speaker -ve to anything on the jack! Car stereos have what is termed a bridged output where both connections move around with respect to ground as it maximises voltage swing. So if you either connect these -ve wires together or to ground you will fry your stereo.

You probably don't need to load the output with anything, but if you think it sounds harsh then connect a 10 ohm resistor of 10 watts or so across the +ve and -ve speaker wires. Connect the +ve wire to the jack 'signal' connection via a resistor in the range 47 to 220 ohms, this will allow you to turn the amp up to at least a little bit before it blows your ears out. Use the 0V/ground supply star point as the ground/shield for the jack.

PantSeatFlyer 11th February 2004 12:50 PM

What is "ve"? I'm an amateur when it comes to electronics and I think in terms of (+) and (-). Whenever I connect speakers to the two channels, I connect the (+) to the (+) post and (-) to the (-) post, left and right. That's my simple understanding of it.

So are you saying I can connect both (+) wires to the headphone jack (for left and right), but leave the (-) wires dangling, not connected to ground, not connected to anything? If so, don't I need to ground something? What about the radio itself? Do I need to ground the headphone jack with a separate wire of it's own? And if I do ground something, what do I ground it to? A copper rod outside in the ground? I apologize if I sound dense, but I don't clearly understand.

I know it would be a lot simpler just to buy a ready made stereo, complete with headphone jack, but that's no fun.

richie00boy 11th February 2004 01:18 PM

+ve is simply shorthand for positive, hence -ve is negative!!!

Yes, just leave the speaker -ve wire dangling, insulated of course :)

As I mentioned before (but maybe not clearly enough), the 0V/ground/screen connection for the jack is taken from the battery/PSU 0V connection.

That's it. No extra earthing or anything required. Just remember to insert some resistors maybe 47 to 220 ohms, one or two watts, in series with the jack connection, unless you want to suffer deafeningly loud levels even at very low volume settings.

PantSeatFlyer 11th February 2004 01:24 PM

nice. thanks for clearing it up.

I also searched on the term "bridged output", since you mentioned it. I found this helpful pdf document, which explains it nicely I think. It also has pictures, which I tend to think in.

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