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  1. #1
    New Member
    Join Date
    Sep 2009
    South Africa

    Car Audio wired to Stereo Audio Jack Plug

    Good Day

    I am a newbie and do not know much about electronics. I hope that I am at the correct forum.

    Can I wire the Front speaker wires of the Car audio to a Stereo Jack plug so that I can use my headset to listen to the radio while riding on a motorcycle?

    I am later going to make use of a bluetooth transmitter as I want to go wireless. I have already bought a Scala Rider headset which is bluetooth.

    Thanks up front for the advise.


  2. #2
    Hi Castos
    The audio output from your front speaker outputs is going to way too high to run phones!!! They WILL work but you need to have the volume control at bare minimum!! Turn up the volume and all you will do is blow the driver units in each headphone and they will be useless. You will have to attenuate the speaker outputs quite substantially to use them properly and I'm sure that you can get little boxes with speaker in-headphones out in SA. It's quite possible to make your own with a nice hefty resistor if you can find someone to do need an MAX of about 1 watt output at full volume not 50 watts!!!


  3. #3
    Several problems. (After you're on here a while you'll know I always throw cold water on everything.)

    First problem: Many, probably most, car stereos use a bridge-tied-load connection for the speakers. Long story short: neither wire for either speaker is at ground. All the wires are connected to amplifier outputs. Each speaker is driven by two amplifiers, out of phase, to effectively double the voltage you can get from a 12V supply.

    A headphone has two hot wires and a ground. You can make this work by just using one speaker wire from each channel and connecting the sleeve of the headphone jack to ground. But this brings up...

    Second problem: there is probably DC on the speaker outputs. If you connect a headphone between a speaker output and ground, you'll blow the headphones. You need to put an electrolytic capacitor in each of the speaker lines you're using, with the "+" side of the capacitor to the radio. (Actually, come to think of it, most capacitors only have the "-" side marked. That should be connected to the headphone.) Value is not critical; 200ufd to 500ufd would seem appropriate. The capacitor passes the signal but blocks the DC.

    Third problem: as Chris says, the audio output is too high. Besides the capacitor, you need a resistor in each line to reduce the power. 100 ohms is a commonly used value.

    So: For each channel, take one of the speaker lines. Connect to one end of the resistor. Connect the other end of the resistor to the "+" end of the capacitor. Connect the "-" end of the capacitor to the headphone. Unused speaker lines must be SEPARATELY insulated, not allowed to touch each other or ground.

    If you feel unsure about doing this, you shouldn't. Let an experienced tech do it for you.

  4. #4
    New Member
    Join Date
    Sep 2009
    South Africa
    Thanks for the info. I have a good understanding of what to do but will get an electronics engineer to assist.

    One thing I do not understand is what happens to the second wire (I presume it is the ground), will they be connected together to form the ground that goes to the headset?

    If I continue to read the forums, maybe (mighty big) I may be able to build one myself in the near future, but for now I just want to understand how it works.

    When you tell me to use a capacitor I look it up on wikipedia to understand what you are talking about.

    Once again thank you for the assistance.

  5. #5
    No, do NOT connect the extra speaker wires together, or to anything else. It is reasonable to assume there are four amplifiers in the radio, two for each speaker. You will only be using one amplifier from each channel; the other, unused amplifier for each channel will just idle. The two unused speaker wires should be separately insulated so they don't accidentally get connected to each other or anything else. If you connect them together, they will fight, and whichever amp is stronger will break the other one. If you connect them to ground, you'll blow both amps.

    The headphone common wire, which is connected to the shank of the plug, goes to DC ground.

  6. #6
    Karl, with due respect, where did you get the cap values, and why not use decoupling instead? If the idea is to eliminate DC from the output. Since most 12v system outputs use direct coupled as apposed to transformer coupled, and mosfet source current is sensitive to output Z wouldn't it make more sense to add a decoupling stage instead of just a filter? A .047 uF electolytic cap in paralel with a 10Kohm resistor will surely settle the power problem.l
    Last edited by penniesfromheaven; 26th Sep 2009 at 01:15.
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  7. #7
    They say a picture is worth 1,000 words. I went and actually did the capacitor calculation this time, and 47 microfarads is large enough. If you can't find the right size capacitor, get the next one larger.

    For the resistor, 100 ohms, and it doesn't have to be large - 1/4 watt is the standard size these days.

    The resistor has no polarity, but the capacitor does. You have to connect the plus and minus ends as shown.
    Attached Images Attached Images  

  8. #8
    Bass, I think there's some confusion here. We're talking a headphone circuit, not a line level circuit. A headphone is typically 32 ohms, and in series with a 100-ohm resistor, makes for a total load impedance of 132 ohms on each channel. (The radio doesn't mind; this just means it gets to loaf.)

    There is 6VDC on each speaker output wire. Normally, a speaker is connected across the two wires for each channel. Thus there is no net DC voltage across the speaker.

    A headphone has a common ground, so the normal speaker connection won't work. The sleeve of the headphone is connected to ground. This puts 6VDC across each side of the headphone. (Well, actually no, cuz there's a 100 ohm resistor in there, so it's actually about 1.5VDC, but that's still way too much.) Thus the capacitor, which blocks the DC but passes the audio signal.

    Of course, if the capacitor is too small, it will also partially block the AC signals which are nearest to DC; that is, low frequencies. A little pencil pushing shows that, in a 132-ohm circuit, a 47ufd cap is large enough to fully pass the whole audio band down to below 30Hz. A larger cap wouldn't hurt anything but would serve no purpose.

    I'm not sure where you intended to hook up the 10K/.047 parts. 10K ohms is high enough that it will have no effect in a 132-ohm circuit, and a .047ufd cap only gets down into the 100-ohm range at 30kHz, well above the audio band.

    It is true that 12V amps usually don't have transformer outputs. Actually, transformers are rare now. Only commercial amps (with 70V outputs) and tube amps have them.
    Last edited by karl eilers; 26th Sep 2009 at 20:14.


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