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Automatic Switchover from Car Speakers to Headphones

AngusBateman

Jun 7, 2016
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Hi,

I'm here and I'm no electronics expert although I'm reasonably handy

I want to create a circuit that includes:
- Four RCA speaker inputs from the back of my car stereo
- A switch, that switches the stereo output between either:
- Four outputs to my car speakers
- One 3.5mm headphone socket

This seems easier enough, but I am very conscious that I don't want to impact on the quality of the sound, and I understand that the outputs from my car stereo would fry my headphones. I'd also prefer that the change over between speakers and headphones occurred automatically when I plug my headphone jack into the socket (similar to most domestic HiFis).

SO if anyone could give me an idea of what I need to buy and a circuit diagram, I would be very grateful.

Thanks in advance.
Angus
 

shrtrnd

Jan 15, 2010
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Most systems that do what you want to do, just use the headphone jack itself (they're built to disconnect the main
speaker feed when you insert the headphone plug, using leaf switches).
The main issue, as you observed, is that the headphone connection is after the internal preamp, but before the power amplifier that drives the large speakers.
For us, we'd get into the audio circuit, and wire what we wanted. But you have to have some good electronics experience with that.
What you apparently want to do, is wire this externally from the outside of your audio device.
That'll take a headphone jack and some resistors, maybe some capacitors.
Because I wouldn't want to do it that way, and you are determined not to interfere with audio output quality,
don't you think you should check into buying a well-engineered commercial device to accomplish this? They'd
be properly shielded for quality sound output without inducing outside RF interference.
Consider checking with the manufacturer of your unit, and asking for THEIR recommendation to adding a
headphone interface to their gear. Because sound quality is important, it wouldn't hurt to ask for their advice
concerning their equipment.
 

BobK

Jan 5, 2010
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You can wire the headphones to speaker outputs with a suitable resistor in series. A 100Ω would be a good starting point. Try it with the volume turned down then turn it up slowly. If it gets to where it is distorting, up the resistor. If it doesn't go loud enough, lower the resistor.

Bob
 

AngusBateman

Jun 7, 2016
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Jun 7, 2016
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Thanks for the input.

Shrtrnd - I've tried finding a commercial solution, and was prepared to buy an audio amp with a headphone jack to achieve what I'm after. However I've yet to find something appropriate, hence my suggestion to build sometime.

BobK, thanks for the input...is it really that simple? if it is then I'm sorted.
 

shrtrnd

Jan 15, 2010
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BobK's pretty bright. I may have overcomplicated this, I've worked on a lot of sensitive systems and am maybe
thinking of issues that aren't applicable here.
BobK will probably be back on to straighten us out.
What kind of wattage are we talking about here?
 

BobK

Jan 5, 2010
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The only problem with connecting directly to speaker outputs is that they will be too high a voltage for the headphones. Headphones are typically 32Ω, so 100Ω will reduce the voltage by about a factor of 4.

Bob
 

AngusBateman

Jun 7, 2016
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Thank you for your help.

I'm not sure about the high voltages (I work in shipping where high voltage is 2000V :)). This speaker system is driven by the car stereo which works at 12 to 14V, and because I'm not pimping my stereo system with an amplifier, the stereo output to the speakers is only 4 x 45W. I think the speakers have impedance of about 4ohms, so I guess the output voltage to the speakers is about 13V.

My headphones have an impedance of about 25ohms and I think headphones operate at about 20mW (happy to be told otherwise), so I guess they operate on less than 0.1V.

(Happy to be told that I'm talking rubbish)

So would 100ohm resistor be enough? Would I be better off with a variable resistor?

Thanks again for your help.
 

AnalogKid

Jun 10, 2015
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Having switching contacts inside the headphone jack do all the work has one real and one potential problem. The potential problem is that the switch contacts might not be able to handle the current, will arc sometimes, and eventually not conduct. The real problem is that standard jacks do not have 4-pole switching built in. However, some do have independent switch contacts not related to the audio signals, so a stereo jack can control a 4 pole relay.

http://www.digikey.com/product-detail/en/cui-inc/SJ-3506-SMT-TR/CP-3506SJCT-ND/2625178

ak
 

AngusBateman

Jun 7, 2016
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Thanks AnalogKid, I was currently thinking of installing a switch on the dashboard that would flip the relay, kill power to the car speakers and supply power to the headphone socket.

I think this might be the easiest solution.

Thanks in advance.
 
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