Why are AC/DC adapters for musical instruemnts tip negative?

[Frankentronics]

After many years of wondering about this, I finally decided to Google the question and was unable to find the answer.

I see that all musical instrument accessories which use wall warts require the input for be tip negative and thus the coax cable to be shield positive. Why is that, please?

Thanks...

[Ian.M]

My first guess would be its a legacy of early Germanium BJT effects boxes.  Back in the early solid state era, as Germanium PNP devices were far more available and had better specs than NPN ones, circuits almost invariably had a positive ground.   Knowing that roadies aren't particularity inclined to read the small print on equipment labels when setting up,  It would take a brave manufacturer to change the convention once established.

[drussell]

The usual answer goes something like this:

Most electronic devices and their power supplies have center-positive polarity, so why do pedals have center-negative polarity?

Because many pedals work with either a battery or a power supply; and, to preserve the life of the battery, it is disconnected from the circuit when either (1) a power plug is inserted or (2) the input plug is removed. This is accomplished by connecting the battery's negative terminal to the input jack's ring contact and the battery's positive terminal to the sleeve shunt on the power jack. When the input plug is inserted, it completes the circuit for the negative battery terminal. When a power plug is inserted, it breaks the circuit to the positive terminal of the battery. Power jacks do not have a shunt for the center (pin) contact, so a center-positive power supply would not work in this type of circuit.

See EEVblog #1015

[Ian.M]

The reason why common usage swapped over to center positive  is NOT the battery switching.  There is *NOTHING* to stop you switching the negative side of the battery using the sleeve shunt of a center positive power jack.

If, however you have a PSU with a three pin grounded mains plug, and the output is grounded, then putting 0V/Ground on the center contact with the outside 'hot' at whatever the DC output voltage is would be F&*%ing *STUPID* - the first time its sleeve touches another ground (e.g. a grounded case), it would short out the PSU.

As unwanted ground loops are a major problem for music kit, you pretty much never see a grounded PSU for an effects box or pedal so there has been no good reason to change over to center positive, and the vast number of center negative devices out there are an excellent reason *NOT* to change over.

[darrellg]

Dave did a video about that:

[Frankentronics]

...See EEVblog #1015

Thanks for pointing me in that direction. I now watched that video and I also saw there was a forum link to a debate https://www.eevblog.com/forum/blog/eevblog-1015-beware-evil-(but-clever)-dc-jacks/?all. I can also see there's speculation and disagreements.

In any case, I read through that other debate and it pretty much satisfies my curiosity.

Thanks...