A big old transformer from a stereo amp, 130 volt center tapped, 65-0-65.

120 v AC primary.

I want to make a power supply that will put out 30-0-30 v DC for a homemade stereo amp.

Problem: Once rectified to DC what kind of circuit do I need to bring that high DC voltage down to the required 30-0-30? I can’t find any regulators that will take that high of input voltage, most top out at 40 DC. The split rail part is easy enough I just need to drop the DC down to where I can use it.

 

The other way is to explore taking a few turns off.
If lucky, it will be wound bi-filar which is both windings together, so you can unwind both equally.
First wind on 10 turns and measure the volts to find the turns-per-volt.
Or take 10 turns off and measure the difference.
If you don't need tight regulation, just a rectifier/capacitor supply could suffice.

 

The other way is to explore taking a few turns off.
If lucky, it will be wound bi-filar which is both windings together, so you can unwind both equally.
First wind on 10 turns and measure the volts to find the turns-per-volt.
Or take 10 turns off and measure the difference.
If you don't need tight regulation, just a rectifier/capacitor supply could suffice.

Well a good idea for sure Max but I do not have the means or resources or desire to rewind it. I have done that before but in a shop that could do it easily. I am working from home in a limited workshop.

 

Don't you want an amplifier with a ±100V supply? It's only 10dB louder!
One alternative would be to make an amplifier with a capacitively coupled output than runs on a 0-100V supply. That's getting closer to your target.

 

What wattage or VA is the transformer, or what amps are you hoping for at 30v?

 

It will cost you more to reuse that old transformer and assemble the remaining pieces than it will cost for a proper SMPS for your application. Especially working from home with a limited workshop.

 

Thanks for the reply Michal.
It is for a stereo amplifier so not a constant load.
I don't understand how a capacitor in series with the primary would bring the output of the secondary down to a level that a common voltage regulator could handle.

 

I agree with Papabravo, by the time you've got a couple of 10A bridge rectifiers and a couple of quality low-esr 4700u 100v electrolytic caps, you're into $60-80. A couple of 120AC to 36v DC 360W SMPS will cost about the same..

 

Well a good idea for sure Max but I do not have the means or resources or desire to rewind it. I have done that before but in a shop that could do it easily. I am working from home in a limited workshop.

You don't have to rewind it if you take turns off to achieve the lower voltage.

 

if you have two of them you could cascade them to get 30-0-30.
but seriously, sell it and get another transformer.

 

An all discrete linear regulator circuit for that voltage and current is not particularly complex, but it will generate a *lot* of heat. There are ways around that, such as switching pre-regulators, but because your transformer is center-tapped, as opposed to two separate secondary windings, ground management gets messy.

As above, consider that what you have is the wrong part for the job.

Update: Does the transformer havwe terminals or wires for the secondary connections? If terminals, inspect closely the wires going to them. If there are single wires to the ends and two wires to the center-tap terminal, you might be able to separate them so you have two independent secondaries.

ak