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Speaker to Headphone conversion, powering speakers?

Have 3 questions I hope hardware- audio people can help me on -

1.  What type of resistor is needed to cut standard 8 ohm speaker output down for a headphone jack, and is this done on the 2 stereo live line, or also the ground? in nother words, how many resistors, where.

2.  Is there any simple way to make mini-amplifier circuit for 8 ohm speakers that need power, when coming out of a MB that has not enough power.  You know, you see these built into small computer speakers, with black DC converter, any easy way to make this DC circuit oneself?

3.  How to power off a set of speakers when you plug in an extension speaker.  Typical example is a TV, with two small built in speakers.  Want to add extension speakers and when plugged in, cut off built in ones.

I am trying to add headphone jack and extension powered speakers to TV with only two tiny built in speakers.  Budget for this about $0, keep that in mind. Please tell me briefly rather than links, I spend all day searching the net, dont need to do it any more.  Thanks for your help.
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scrathcyboy
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scrathcyboy
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5 Solutions
 
ridCommented:
1. Depends on the headphones (their sensitivity and impedance) . Sometimes no resistor is required, sometimes you'd need one 100 ohm resistor per channel. Resistors in the signal lines only, not ground/common lead. You may have to experiment some... or better, get a dual potentiometer (or "rheostat") type resistor and adjust it to get suitable output level. A 2X100 ohm one should be fine in many cases.

2. Small mini-amps are usually available, even as DIY kits. from electronics stores. Normally they don't include a power unit (AC-DC converter), but those things aren't too expensive. If the budget is $$0.00, you may be in a tight spot, unless you can use some leftover from another appliance. Such a setup would be what you need for #3 as well.
/RID
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garycaseCommented:
rid addressed #1 and #2 well -- the impedance matching simply depends on what amplifier you're feeding the input to; and the least expensive amp will be a small hobbyist unit.

For #3 you simply need a small DPDT mini-jack => wire the speakers through the normally closed terminals; and when a plug is inserted it will disconnect the speakers and feed the output to the plug.   You could wire the impedance matching resistor/potentiometer on this jack so it was only in the circuit when the external plug was inserted as well.
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scrathcyboyAuthor Commented:
thank you both.  The headphones are typical mini-stereo, like on walkman, nothing special.  So I need 2 resistors, 100 ohm each, on input, none on ground?

Should have mentioned, have lots of AC adapters in 3-18 volt range, that is not a problem, but have never seen a mini-amp kit.  What kind should I ask for?  The speakers I want to use are not computer speakers, they are 3 inch 8 ohm 20W speakers like you see on audio equipment, with the black-red terminals on back where you push in 2 speaker wires, usual audio setup, am sure you are familiar with this.  You put these on weak channel, get almost no sound of of them, so maybe 5W amp is enough?

So for item 3, I could break the line going to the built in speakers in the TV, then put this DPDT jack across that?  Then when I put in a stereo plug for either the headphones, or the extension speakers, I would like it to disconnect the built in speakers, but seems to me it would make the connectiion to the built in, not break it.  Maybe I dont understand how the DPPT jack works.  Looking for -

TV --->  jack for speakers ---->  jack for headphones ----> internal speakers
              (tiny low W amp)                 |
                      |                              headphones
                ext. speakers

thank you
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ridCommented:
Doesn't the headphone jack cut out the internal speakers? If it does, you could use that jack... Otherwise, most of these plugs have braking contacts included that will cut out whatever is "beyond" the connector when a plug is inserted. 3.5 mm or 6 mm "telephone" jack type assumed.

2x5 W is plenty, unless you are in a very large room or the speakers are of a very hard-to-drive model.
/RID
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scrathcyboyAuthor Commented:
There are no jacks in the TV right now, only internal speakers, I would have to wire any jacks in.
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garycaseCommented:
If you can't find a simple DPDT jack that breaks the internal connection when plugged in (this should be readily available at Radio Shack, but I didn't see one on their web site), you could use a simple DPDT switch to switch between the internal and external speakers -- like one of these:
http://www.radioshack.com/family/index.jsp?categoryId=2032278&cp=2032058.2032230&allCount=68&fbc=1&f=PAD%2FProduct+Type%2FDPDT&fbn=Type%2FDPDT

and then use a jack like this for the external connection:
http://www.radioshack.com/product/index.jsp?productId=2103452&cp=2032058.2032231.2032280&pg=13&parentPage=family

I'd ask at Radio Shack, however, or any other electronics store that caters to hobbyists -- they should have a simple jack that has an automatic disconnect for an internal connection.
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scrathcyboyAuthor Commented:
Aha!  So DPDT simply means double pole double throw, on either up or down to different connects with center off, now I see, yes I already have one of those.  I will try RS tomorrow for the mini-amp kit and the jacks, Im sure I can get the jacks, as to whether I can get/make the miniamp kit, that is another story.
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garycaseCommented:
If you're a bit handy and have modest soldering skills you can make a simple little amp that will work just fine for this:  http://www.electronics123.com/s.nl/sc.8/category.33/it.A/id.462/.f


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scrathcyboyAuthor Commented:
Terrific, thats what I was looking for, soldering is not my forte, I can melt anything, but I will give it a try, also, all the other links you gave.  Thank you much garycase, and also rid for your ideas too.  regards SB.
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