Wiring Toroid 14V non center tapped for PSU

[civilmonkey]

I Apolgize if this has been covered, I've searched a on few forums with no luck.

I pulled a toroid transformer from a halogen light fixture. It's 105VA, 120 VAC in, 1 x 14 VAC out, not centre tapped.

I tired using two bridges, so I have two +/- 12VDC supplies. I tied the bridges together (one - to one +, connected to mains ground) to get a zero volt rail, but that didn't work and results in melted wires (no blown fuse though)!

If I use one bridge, and take two +/- 12 VDC supplies from it, can I use the mains ground as a zero volt wire?

My plan is to build a gainclone using LM1875 chips. I don't need a lot of volume.

Thanks in advance.

[cpemma]

The only way I know to do this is covered by Rod Elliot here for a preamp supply using a voltage doubler and regulators, but I don't think it's suitable for the current draw of even a modest power amp.

You really need a second similar transformer, or a pucka CT or dual secondary one.

[paulb]

Use a single-ended power supply version of the LM1875 circuit. See page 2 on the datasheet:
http://cache.national.com/ds/LM/LM1875.pdf

Never mind the wags who will tell you that an output capacitor is evil. So you can just use a single bridge rectifier to get about 20VDC. Works fine.

[cpemma]

Originally posted by paulb
Never mind the wags who will tell you that an output capacitor is evil. So you can just use a single bridge rectifier to get about 20VDC. Works fine.

Certainly OK with less high-end speakers that wouldn't know what to do with low bass if they got some. Was a time most small amps were single-rail...

Just out of curiosity, is there any problem using LM1875 in bridged mode on single-rail (with no output caps)? Never seen a circuit anywhere.

[paulb]

Even with a +/- supply, there are power supply caps effectively in the circuit with the speaker anyway. So you always have a capacitor somewhere. Bass response simply depends on the value of the capacitor.
LM1875 in bridge mode works fine, I actually have one sitting behind me at the moment. It uses op amps to drive it and split the phase. It's only useful if the speakers are 8 ohms or higher, as in bridge mode each amp sees half the impedance and the LM1875 is only spec'd for down to 4 ohms.

[paulb]

See attachment. I can run this amp from a single supply by just providing a reference ground at half the supply voltage (it doesn't need to provide much current).
This circuit was based on an article in, I think, Popular Electronics quite a few years ago. U2A is a servo feedback that keeps the output DC offset voltage at 0.

[civilmonkey]

Thanks for the ideas. That single supply schematic seems like it's worth a try.

Cheers

[paulb]

Yes, I built a single-supply version (right from the datasheet schematic) for a desktop speaker in a telephone case. Here's what my board looked like. I used verboard, with the strips of copper on one side. I used individual diodes (on the right) instead of a bridge rectifier. Ran from 16VAC.