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Old 11th February 2004, 02:46 PM   #11
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after reading that pdf a couple of times, I have another question!

it says with bridged output that the +ve and -ve supply equal voltage, opposite in phase, thus quadrupling the output power to the speaker load. So...

If I leave the -ve wires dangling, won't this reduce the power to my headphones? Plus, you suggested dropping it even more with resisters in series with the +ve wires. Is this because headphones can't handle the same amount of power as two car speakers?

Can I calculate the appropriate resistance I need to place inline using the output power of the car stereo versus the input rating of my headphones?



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Old 11th February 2004, 04:35 PM   #12
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You are correct that doubling the voltage allows the power to quadruple and also (in a way) that the power available to your headphones will be reduced if you only use one side. However, as your headphones share a common connection you have no option other than to leave the -ve amp wire unconnected. Also, there is already more than many times enough voltage swing from just one side, so it's not an issue.

You could calculate the required resistance, but it also depends on how efficient your headphones are. It's much easier just to try a few different values Unless your headphones are really cheap nasty items at normal listening levels you will not run into the power limit. If you omit the resistor you are likely to cause damage to your headphones and you will probably find that because you have to have the volume set so low, even a small adjustment makes a lot of difference.
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Old 11th February 2004, 09:03 PM   #13
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okay, last post on this (maybe) to make sure I got it straight. I sketched up this picture. Is this correct?

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Old 12th February 2004, 09:35 AM   #14
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Looks good to me. You will want to connect the battery to the stereo as well though
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Old 12th February 2004, 10:19 PM   #15
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Just got off the phone with an authorized delco car radio repair guy. He said to wire the speaker commons together and mount that to the headphone common on the jack, then mount the left and right positive wires to the left and right on the jack. Period. Any other way would screw it up.
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Old 13th February 2004, 01:04 AM   #16
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I went back and read that pdf about bridged amps. It stated never connect two negative terminals or it would short it out. Goodness. Mr. Authorized Delco's instructions sound contrary to that paper.

Which way is up?
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Old 13th February 2004, 11:02 AM   #17
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Indeed, you are correct If the stereo uses a bridged output then shorting those -ve speaker wire will be a very nasty thing to do. However, perhaps Mr Delco knows that the stereo is an old one before the time of high-power output stereos and it does not use a bridged output, hence it's safe to connect those two wires together?

If you have an AC voltmeter, set it to read upto about 10 volts and play some music through the stereo. If the -ve speaker connection voltage changes a lot with respect to the 0V battery connection then it's a bridged output. If it never goes above 0.5V then you should be safe to connect them together and use that as the screen/ground connection for the jack.

Also, one other thought if the stereo uses a bridged output - it's probably quite likely that you will need to add a capacitor in series with the headphones as there will be DC present on the output which will need to be blocked from your headphones. I need to know the impedance of the headphones to work the value out.

If a bridged output isn't used then that would be the best solution giving the easiest method of connection.
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Old 13th February 2004, 02:46 PM   #18
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The radio is definitely bridged output. Mr. Delco said it was. And he said it put out 12 watts per channel.

But rather than trying to make the speaker wires fit the headphones, why not make the headphones fit the speaker wires? In other words, can't I simply rewire the headphones so that each ear has it's own pair of wires? Then I could route the radio's left and right wires to their respective RCA jacks, just like it was meant to work.
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Old 13th February 2004, 06:25 PM   #19
maylar is offline maylar  United States
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Yes you can do that if you want to mess with the headphone wiring.

My preference would be to wire a standard stereo phone jack with the series resistors and hook them to one speaker wire for each channel and ground. It'll be more than loud enough, and you can use any pair of headphones you want.
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Old 13th February 2004, 10:56 PM   #20
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Well, if Mr Delco said it was a bridged output and still said that you could short the -ve wires together, get him to try it and then explain the smoke that comes out of your stereo

Yes, you could rewire the headphones, but as I wrote earlier, there will be way more than enough volume even just using one side of the amp. It's up to you. All the headphones I've ever come across would be quite hard to modify for isolated operation; the wire and it's inner insulation is very thin, contacts are small and fragile.

I agree with maylar's comments.
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