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I see that power supplies for guitar pedals, speakers, etc are always center negative. Is this a conscious design choice, or is it a marketing trap?

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  • \$\begingroup\$ What 'judgement'? \$\endgroup\$ – crocboy Jun 29 '14 at 17:26
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    \$\begingroup\$ It's all a conspiracy!!!! It's them damn space aliens again, and the government!!!! \$\endgroup\$ – Majenko Jun 29 '14 at 20:39
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It simply is what it is. There is no reason to expect either polarity. Probably someone early in this market picked center-negative arbitrarily, and others followed the trend.

It's a arbitrary choice, so neither is more wrong or right than the other.

You should, of course, never expect any particular piece of equipment to use a particular orientation or be compatible with a particular power supply unless it says so explicitly or the power supply comes with the product.

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  • \$\begingroup\$ It just seems to me like center positive is much safer, with the ground being connected to the barrel of the connector \$\endgroup\$ – crocboy Jun 29 '14 at 17:29
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    \$\begingroup\$ @croc: Since these power supplies are isolated from the power line, there should be no harm in touching either ONE of the terminals to something else. Again, the polarity is arbitrary and can be picked either way in the overall design. \$\endgroup\$ – Olin Lathrop Jun 29 '14 at 17:35
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    \$\begingroup\$ @crocboy: It really doesn't have anything to do with safety. However, in audio gear, the negative supply is often tied to the signal ground, which is often tied to other equipment. With a barrel connector, the center pin makes contact first, so making this the negative/ground connection will tend to introduce less noise and other problems into the system. In theory, it shoudn't matter; in practice, it sometimes does. \$\endgroup\$ – Dave Tweed Jun 29 '14 at 21:04
  • \$\begingroup\$ Super old post but this is not correct; it is not arbitrary. Having the center pin negative means you can connect it in a way such that when you unplug the jack, it closes an internal contact that connects the third pin on the plug to the center pin; this is usually used to connect the negative terminal of an internal 9V battery in lieu of the power jack \$\endgroup\$ – hatsunearu Feb 25 at 13:09

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