Power supply Question

[Adam M.]

I am building a second amplifier... and the amp is together ok, but untested. The power supply doesn't perform as expected.
It is a 15VAC wall wart rated 1t 2.4A, and when running it through a bride-on-a-chip (rated at 25A) I get about +-12v out. I have checked the wiring, and it seems correct. I measured the no load voltage of the transformer, it is almost 20 volts. Is it possible there could be that much voltage sag? What can I check?
-Adam

[Tube_Dude]

Have you put the filter capacitors , at the output of the bridge?

[Adam M.]

They are soldered directly to the output pins of the bridge:

+out---------------------------
|
+
Cap
-
|
0V
|
+
Cap
-
|
-OUT-------------------------------

Please excuse the poor ascii art.... this is the setup I am using.

[Tube_Dude]

Is the transformer 15 V --0--15 V AC ?

[Adam M.]

I don't think so... It is just a surplus wall transformer that spits out 15VAC. A moment of weakness from allelectronics.com http://www.allelectronics.com/cgi-bi...152&type=store

I don't see any way it could be 15-0-15.

[Tube_Dude]

Originally posted by Adam M.
I don't think so... It is just a surplus wall transformer that spits out 15VAC. A moment of weakness from allelectronics.com http://www.allelectronics.com/cgi-bi...152&type=store

I don't see any way it could be 15-0-15.

So everything is OK , what you have is 12 X2 = 24 volts.

You don't have + - 12 volts relative to ground , for that you must use a transformer with middle point.

In your case , you must not use the caps in series but in parallel because there are no middle point ground.

What voltage you need for your amp?...Only one rail or a negative and positive rail referenced both to ground?

[carlosfm]

You need another one of those.
You need two of those trafos, if you want to use them with the amp.

[Adam M.]

I see with respect to the voltage now...
I am using an LM1875, so I need a +- rail. I planned on using the middle point between the caps as the ground reference for the circuit, and keeping the "ground" seperate from actual ground, since it doesn't sit at 0v. What would the schematic for a series connection of the caps look like, and can you give a brief description of the theory?

THe last amp I built used a 25v + 25V transformer, the only difference between that circuit and this one is the reference to 0v directly on the transformer. How does this affect things in comparison to what I am working on now?

[Adam M.]

Originally posted by carlosfm
You need another one of those.
You need two of those trafos, if you want to use them with the amp.

Yuck... how would that work?

[Tube_Dude]

If you use two transformers you can use the layout of fig. 3 at

http://home.no.net/andiha/articles/audio/claclassa.htm

The way you are trying to obtain the zero point , without a middle point transformer , doesn't work...