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I salvaged two large speakers from an old iPod player/alarm clock device. Each of the speakers have three wires tied to it. There is a positive and negative and a third which I assume is ground since it is tied to the thick metal casing.

full picture wire connection details

I understand how a two-wire speaker works and is used but I haven't encountered a three-wire speaker before. I don't see the point to the third wire besides it being a ground.

Phyllostachys
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    It's my opinion that this would be a perfectly good question if the original poster had written it better-- and included pictures. There's a learning opportunity here with component identification and the proper use of a DMM. –  Jan 03 '12 at 20:43
  • @DavidKessner The thing that stuck out to me was that there seems to be no desire to learn/understand what is going on here, they just want to know how to hook it up. Question could be very good if written from the perspective of wanting to understand the electronics involved with a speaker. – Kellenjb Jan 03 '12 at 21:22
  • @Kellenjb Fair enough. Hey, at least it wasn't like some people who just copy their homework assignment over as a question. –  Jan 03 '12 at 21:47
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    This is an entirely reasonable question, with a textual description sufficient that no pictures are required. It's quite likely that the speaker will work fine without the ground wire - perhaps it played some role in reducing interference. – Chris Stratton Jan 04 '12 at 05:09
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    I already know HOW a speaker works generally and don't care to much for component identification since it is a speaker... how varied can they get? I've just never seen one that has three instead of two wires. I suppose I'll have to wait til classes start again and use a DMM and function generator to check it out (Comp. Eng student). – Phyllostachys Jan 04 '12 at 16:43
  • The recent edits have greatly changed the nature of the question, and it seems suitable for reopening. Thank you for making the effort to fix this problem! – Kevin Vermeer Jan 04 '12 at 18:13
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    I'm closing this question since the original images are no longer available, and they weren't uploaded to the StackExchange service – W5VO Apr 07 '15 at 02:58
  • I've debated about deleting it. I still have access to the images but didn't know about StackExchange's image server (still don't actually). It may not have existed when I first posted this question. – Phyllostachys Apr 13 '15 at 15:55

3 Answers3

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The size difference and rough casing on the center wire indicate that it's probably shielded cable. This takes the form of a braided wrap on the two speaker wires, and prevents the speaker from receiving interference, for instance from 60Hz mains wiring or transformers in the iPod player device.

The speaker itself is basically an electromagnet, or, more simply, a coil of wire. Current goes in one end, and out the other, and that's about it. This coil should be isolated from the speaker casing. The speaker is driven with AC, and probably capacitively isolated from ground. Shorting to ground could damage the amplifier, depending on the configuration, so shielding is not generally used on speaker wire.

However, in an alarm clock (plugged into mains), which is also used to play audio and charge iPod batteries (which are significant power draws), there's probably a sizeable transformer in the enclosure, which is probably pretty close to this speaker and the speaker wire. This could produce a hum, so the designers of the device decided that the speaker needed to be shielded. The risk of the speaker wire shorting out and damaging the amplifier is minimal, because the speaker wire never needs to flex within the alarm clock.

If you do connect these speakers to something else, you can probably ignore the third connection. If you reuse the cable, simply leave the shield at the other end floating.

Kevin Vermeer
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  • I wonder how well the differential cable already deals with the noise issue. Maybe it makes help make a complete case as in the alarm clock case and provides part of the shielding for EMI reasons on a device. – Kortuk Jan 04 '12 at 22:49
  • What do you mean by "capacitively isolated from ground"? If you place a cap from the signal to ground then the driving AC will be removed, and if you couple the AC signal into the coil then I wouldn't desribe that as being isolated from ground. – sherrellbc Jul 29 '14 at 13:49
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It is likely for shielding purposes. Speaker leads in genernal are notorious for picking up RF, including being an antenna from which it can couple back into the driving amplifier.

You mentioned that this was out of something intended for use with an ipod; if it is also intended to work with an iphone GSM radios tend to be very good at causing thumping/pulsing sounds in cheap amplified speakers during certain network operations. With my previous phone, I'd hear this unintended "notification" of an incoming call in my PC's speakers as the nearby phone acknowledged itself to the network, a second or two before the actual ringtone began.

Note also that the speakers are magnetically shielded, probably to limit scan distortion if they are placed near a CRT monitor - wonder how long they will continue bothering with that!

Chris Stratton
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Ha ha ha! That is so funny! The speaker is a standard speaker. Nothing special. What is funny is that someone soldered a wire to the rivet that holds the speaker terminals to the frame! I'm an EE in the pro-audio industry and I've never seen this done.

The only conceivable use for that third wire is as a shield-- although I doubt that it is very effective at that. You could certainly disconnect or ignore that wire with no bad effects.

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    Any chance this could have been for wire stress relief? – Kellenjb Jan 04 '12 at 22:24
  • @Kellenjb If it is, then it's for no good reason. A thicker gauge wire would cost the same and have similar strength, but have cheaper labor costs to assemble and would probably sound better. –  Jan 04 '12 at 22:47
  • Well it was in a cheapie one hung low iPod player so that doesn't seem too surprising. – Phyllostachys Jan 06 '12 at 15:51