Why 3 pin power supply with two seemingly identical output pins?

[stoneattic]

I was hoping someone more knowledgeable than me (that shouldn't be difficult to find) can tell me why an external power supply would have two hot pins of the same voltage instead of just one.

I have a Samson MDR1688 mixer that uses an external 18VAC power supply with a 3 pin connector. It shows pins 1 and 3 as 18AC and pin 2 as common. Why would they use two seemingly identical hot pins instead of just one? I would understand if they were two different voltages or one was AC and the other DC, but why two of the same supply?

The reason I am asking is I would like to use a different power supply than the original. The form factor of the original is very "blocky" and I was hoping to find a flatter one like you see with flat top PCs. I can find 18VAC 1.5A power supplies but they have only two wires (18VAC & ground). Can I just jumper the other hot wire to feed the other hot pin?

Thanks!

[nosaj]

If you measure pins 1 and 2 and get 36v you would have 2 18vAC supplies. And 2 would be the center tap.
nosaj

[Enzo]

Because there are two 18vAC supplies in there, not just one with two holes. Think of it as a 36v center tapped transformer. Note it says right on it, 18v x 2.

This is a whole mixer, not a pedal. It wants to have +15 and -15 volt supplies for the op amps. One 18vAC supply can make only one of those at a time.

[g1]

If you measure from pin 1 to 3, it should read 36V.

[Helmholtz]

Why would they use two seemingly identical hot pins instead of just one? I would understand if they were two different voltages or one was AC and the other DC, but why two of the same supply?

I guess the two 18VAC voltages are not identical but actually out of phase (i.e. opposite polarity) wrt to common.

[stoneattic]

So I measured as suggested and it shows 18VAC between 1 & 2 and between 2 & 3, as expected. It does read 36VAC between 1 & 3. So essentially it is two 18VAC power supplies?

[Helmholtz]

So essentially it is two 18VAC power supplies?

More likely a single PT with a center tapped 36V secondary.

[tedmich]

Because there are two 18vAC supplies in there, not just one with two holes. Think of it as a 36v center tapped transformer. Note it says right on it, 18v x 2.

This is a whole mixer, not a pedal. It wants to have +15 and -15 volt supplies for the op amps. One 18vAC supply can make only one of those at a time.

As Enzo said, OP amps are often designed for dual supplies so they have the maximum voltage swing, compared to a single supply (among other reasons). This is similar to the reasoning for active guitar PUs using 2 x 9v batteries. Once the 2x18VAC makes it to the OP amps its regulated down to a nice clean +/- 15VDC, so 30VDC total

[stoneattic]

Thanks everyone. That explains it. I guess I'll stick with the original power supply. I figured there was a good reason.