power supply set up help

[Axtman]

I have an amplifier circuit that I want to test. It is uses a split supply bridge rectifier circuit. The transformer has a center tap that is grounded. The two outer secondary taps then go to a bridge rectifier and smoothing capacitors.

I want to bypass the transformer, rectifier and capacitors and hook the circuit directly up to a power supply. The rails of the circuit are +43v and -43v. (Those votages are then knocked down to 12v and 15v.)

I have a variable voltage/amp power supply that has three posts. Red for postitive. Black for negative. Green for ground.

How do I set up the power supply? When I turn it up to 43 volts I only get 43 volts between red and black. I want positive and negative voltages.

Thanks!

[TimFox]

The third output post (ground) is not connected to the actual power supply.  It is there to give a convenient way to connect either the red (positive) or black (negative) output to "ground" (i.e., protective ground, the third wire on the line cord, and the chassis of the power supply).
You cannot use this supply to provide +43 and -43 V to your amplifier.  You need a second power supply to get the negative rail.
There are slightly complex circuits ("rail splitters", q.v.) that can provide a "center tap" to your single 43 V power supply to get +21.5 and -21.5 V, if that suffices, but they require a power device to handle the difference in current (which may be small) between the loads on the + rail and - rail.

[Axtman]

Thanks TimFox.

So how would I set up two power supplies?

Below is what I want to do.

[TimFox]

Connect the negative of one supply to the positive of the other supply.  That connection is your "0" point.  The positive of the first supply is your "+" rail, and the negative of the second supply is your "-" rail.  You may want to connect the ground posts of the two supplies to the common "0" point.  Obviously, you need to adjust each supply to the same absolute value.  Sometimes, it is convenient to use an AC power strip to supply mains to the two supplies so that you can turn them on and off together with the switch on the power strip.