How to do Single center tap/bridge regulated supplies?

[ChuckT]

Hi all,

I have a broken amp with a good preamp section and I want to replace the amp section with a gainclone.
It has a huge tranny with +/-40AC and only one center tap. This gives +/- 55V DC output. Is there any good circuit to drop the voltage down to below 35V?
Note I don't want to take apart the tranny to get 2 CT.

I had looked at Pedja site (Thanks Pedja for the info) and there is a nice discrete circuit. But I don't know if it will work with single bridge +/- supplies.

TIA.

[AndrewT]

Hi,
you are looking to drop >20V across the regulator. That is a lot of heat to get rid of.

Either buy the correct transformer to match the chipamp requirement or build an amp that suits the PSU voltage.

[danielwritesbac]

What are the auxilary windings that run your preamp section?
It should be possible to simply replace that center tap with another that has the right voltages for you. A 48vct with secondary windings for preamp, is a popular transformer.

[Ouroboros]

The sensible way is to make a couple of switch-mode buck regulators to drop the rails to whatever you need for your power-amp chip. You will need a different design for the positive and the negative rails of course as they will be sharing a common 0V.

[ChuckT]

Thank you, guys. I only know the basic electronics, so a switch mode is a bit out of the question.

The secondaries are 94Vct (1A), 74Vct (3.75A) and 15Vct (1A). The 15Vct is for the preamp section.


I wanted to use the LM3875 with +/-35V.
Guess I will use the LM3886 with +/-45V. Now that's only a 10v drop.

Can anybody tell me if Pedja discrete supply work with some modification. thanks.
http://www.pedjarogic.com/gc/supplies.htm

[AndrewT]

Originally posted by ChuckT
I wanted to use the LM3875 with +/-35V.
Guess I will use the LM3886 with +/-45V. Now that's only a 10v drop.

you can't run a 3886 on +-45Vdc unless the load impedance is near open circuit.

[ChuckT]

How about +/- 42v? What do u mean by open circuit?

[AndrewT]

open circuit=infinitely high impedance, i.e. no speaker connected.

[danielwritesbac]

Hypex modules can theoretically run on 42v

ABS Max continuous voltage for general audio amplifier applications:
LM3886TF, 37v
LM3875T, 40v
LM1875, 31v
And, at slightly beyond this point you can use fans and/or add about 4 ohms resistance in series with the speaker output.

These maximum voltages aren't something to do on purpose, but rather. . . just a note on "where" that "too far" might be.

Also, too-high voltage necessitates higher powered input or higher gain setting, and at some point either can be a mess.

Hey, is that 15v a single (15v AC) or is it a 15vct (as in 15+15 AC)?

[leadbelly]

IMHO you easiest solution is to use half-wave rectification on your existing transformer. If you add a regulator to that, plus you have the advantage of the excellent PSRR of the power amp chip, you will wind up with a good end product.