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I am trying to convert my old 9V battery operated Wah-wah pedal to a DC converted one, so I can use AC power. I bought a DC plug for it and was told that the center pin is for ground. However there are TWO additional pins. I only expected one since there's just a positive and negative wire. Which pin on the DC plug is for the positive wire? And is it true that the center pin is for the negative wire?enter image description here

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    \$\begingroup\$ Did you buy a plug, or jack? The picture looks more as jack, or receptacle. \$\endgroup\$ – Ale..chenski Sep 7 '17 at 23:29
  • \$\begingroup\$ There was a time when someone decided that centre negative was a good idea. Those times are gone, but your vintage equipment may be one of those bad ideas. A $5 DMM will be the best tool to verify. Since this a floating output, switching can occur on + or - . 3 pin jacks have a switch for disconnecting battery ground. \$\endgroup\$ – Sunnyskyguy EE75 Sep 7 '17 at 23:45
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    \$\begingroup\$ @TonyStewart.EEsince'75 the center-negative idea came from simplicity to switch between internal battery and external power, because it doesn't need any electronics, just a natural mechanical switch did the job. If the connector in on ground side and there is another ground in the system (like RS232 or USB cable, you need some resistors, diodes, and power transistor to make the switch. It takes too much thinking. \$\endgroup\$ – Ale..chenski Sep 7 '17 at 23:52
  • \$\begingroup\$ Isolation of the transformer means does not matter. Reducing the variations of walworts preesumes centre positive and uses regulated supplies instead of unregulated these days.(better idea) This makes field replacements easier. \$\endgroup\$ – Sunnyskyguy EE75 Sep 7 '17 at 23:53
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The pin with the tab on the bottom left is the center pin. The tab at the very top is the sleeve. The middle pin is a break contact on the sleeve connection.

Most old guitar pedals use center (-) - but not all. Check for continuity between the center pin of the connector and the battery connector.

You can either ignore the break connection or, if you still want to have the option of a battery inside the pedal, connect whatever lead corresponds to the sleeve to the break connection. This will disconnect the battery when external power is plugged into the pedal.

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For most barrel jack/plugs, the center pin is usually positive terminal, but there are exceptions, which cause severe confusion and a lot of burned gadgets who use this reversed polarity.

The plug polarity is usually marked on a body of AC-DC adapter, by approximately this sign,

enter image description here

The power barrel jacks usually have three pins, one for center, and two other form a normally-closed switch, see this illustration: enter image description here

When a plug goes in, it breaks the contact, so the wire connected to #3 gets disconnected from ground. This signal usually used in devices to switch between external and internal power sources. SO you need to check carefully the arrangement in your device before changing the jack.

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  • \$\begingroup\$ Older guitar effects pedals are usually center negative. Just got reminded of this a few weeks ago when helping design a new effects pedal and the client wanted to compatible with all of his other effects pedals. pbs.twimg.com/media/DHOQhueVwAAh2WH.jpg \$\endgroup\$ – Dwayne Reid Sep 8 '17 at 6:10
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The sleeve or outer contact often includes a switch which will open when the mating plug is inserted. This switch may be used to disconnect an internal battery when external power is used.

I think (but don't quote me!) that the center pin is usually positive, but both ploarities can be found. If in doubt, measure with a DVM or other voltmeter.

An ohmmeter or continuity checker can be used to determine which switch contact is which.

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