How to lower down PSU's voltage (42v->24v)

Juntuin · 2018-01-16 5:44 am

Hi,

I have case and PSU leftover from my previous Aleph 5 - build. I was looking for something with a little less power, F5 for example, but my rail voltage is way too high for that (42v). Is there any idea to lower rail voltage with some ebay(?) DC step-down regulator? Or should I just build something that takes higher rails or just buy suitable trafo for F5?

Psu is:
30v x 2 secondaries
545va
264 000uF CRC

I actually have two of these..

Thanks!

tomchr · 2018-01-16 5:49 am

I'm assuming you're dealing with a toroidal transformer. The secondary is usually wound on the outside of the transformer. You could tap into the secondary about half way down the winding. In the past, I've used a pin to poke through the insulation and into the secondary winding. Measure the voltage between the end of the secondary and the pin. Move the pin until you get the voltage you want. Then solder a wire onto that point of the secondary.
Be really careful that you don't accidentally stab the primary winding. Sparks will fly and you'll put your life in danger.

It makes a mess of the transformer, so I guess whether this is an acceptable solution to you depends on how much you like that transformer and how desperate you are to reuse it.

I seem to recall that many of Pass's projects can be run from a +24 V supply delivering a few amps. How about finding a laptop charger or buying one of Mean Well's desktop supplies? They perform well and are really quite inexpensive.

Tom

famousmockingbird · 2018-01-16 6:17 am

If you have room you can add a 12v bucking transformer and get the 30v down to 18.

traderbam · 2018-01-16 7:28 am

Buy new transformers. Selll the old ones on eBay.
Or
Put your two transformer primaries in series to hallve their secondary voltage.

FauxFrench · 2018-01-16 8:01 am

I have seen one Buck converter that can handle 42V on the input from "out east" where most are produced anyway. Will 200W do?
How fast the regulation is I do not know.

If you connect the secondaries in parallel, use separate bridge rectifiers for each winding.

200W 15A DC DC 8 60V TO 1 36V 12V voltage power Buck Converter Step down module-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group

200W 15A DC DC 8 55V to 1 36V Adjustable Buck Converter Step Down Module Voltage Power Supply 48V to 24V 12V 5V Aluminum Shell-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group

15A 200W Adjustable DC DC Step Down Converter Buck Module 60V48V adjustable voltage stabilized synchronous rectification module-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group

AndrewT · 2018-01-16 10:45 am

Don't use a bucking winding.
It will ruin the regulation of a perfectly good toroid.

Either use a low power version of an F5t, or buy the correct transformer.
An F5 with the cascode front end of the F5t will work on your higher supply voltage.
You only use the power your ears need.

The F5 can be set up with full ClassA bias. But because it is a push pull amplifier it can be set up with any value of bias current and the topology allows the amplifier to transistion from ClassA to ClassB whenever the current demand goes outside your ClassA limit.
You choose how hot to run your heatsinks and listen to hear if you can detect that transition from ClassA to ClassB.

Juntuin · 2018-01-16 11:37 am

Hello, thanks guys for your time to help

so it seems that best move would be new trafo. I'm tempted also to try smps, but maybe not this time.

Well, I can use one of these recent trafos to power my upcoming ovation-nx

chermann · 2018-01-16 8:14 pm

AndrewT said:
Don't use a bucking winding.
......

Why? Buck is to noisy. Limit of the coil? To slow regulation?

Sorry my noob questions

FauxFrench · 2018-01-16 9:30 pm

A Buck regulation loop has to cope with the important delay from the LC output filter and the corresponding phase-lag. The more you increase the output capacitor to get less ripple and more charge reserves for transients, the worse it gets.

AndrewT · 2018-01-17 9:00 am

chermann said:
Why? Buck is to noisy. Limit of the coil? To slow regulation?.............

AndrewT said:
Don't use a bucking winding.
It will ruin the regulation of a perfectly good toroid............

chermann · 2018-01-20 7:02 am

FauxFrench said:
A Buck regulation loop has to cope with the important delay from the LC output filter and the corresponding phase-lag. The more you increase the output capacitor to get less ripple and more charge reserves for transients, the worse it gets.

Thx.

I have to read this 10 times but i am Not sure i understand

chermann · 2018-01-20 7:19 am

I am nearly in the same position. I have trafo which are bigger e.g. 36vac. But i need 34 dc.

My idea is to use some Caps in series and Pick up the One Voltage of 1 cap line.
Yes for sure its wasting some caps....but cheaper Solution???

FauxFrench · 2018-01-20 2:37 pm

Capacitive reduction is useless unless for very low power.

You may rectify your 36Vac and get some 47Vdc across the buffer capacitors. Then you can "Buck" it down to 34Vdc with the step-down converter (200W) proposed in an earlier posting.

AndrewT · 2018-01-20 3:20 pm

FauxFrench said:
Capacitive reduction is useless unless for very low power.

You may rectify your 36Vac and get some 47Vdc across the buffer capacitors. Then you can "Buck" it down to 34Vdc with the step-down converter (200W) proposed in an earlier posting.

I agree a step down switching regulator might be a good solution. It will waste less heat.
Then add on lots of RF attenuation for all the high frequency grunge that accompanies the output.

lexx21 · 2018-01-20 3:55 pm

Use a couple of resistors on the ac out of the transformer prior to the rectifier. High wattage non-inductive. Play with the value until you get what you want.

chermann · 2018-01-20 6:37 pm

FauxFrench said:
Capacitive reduction is useless unless for very low power.

You may rectify your 36Vac and get some 47Vdc across the buffer capacitors. Then you can "Buck" it down to 34Vdc with the step-down converter (200W) proposed in an earlier posting.

thanks, but i want to evaluate more solution....nooby questions..sorry.. :wave:

thx

chermann · 2018-01-20 6:38 pm

lexx21 said:
Use a couple of resistors on the ac out of the transformer prior to the rectifier. High wattage non-inductive. Play with the value until you get what you want.

...hmm...that what could be a burning heat low noise solution

thx

lexx21 · 2018-01-20 7:57 pm

Not really. I do this on the 5v ac supply to 4v heaters of az1 rectifier tubes and its quite happy. .5R on each side of the heater gives the 4v needed. I used 20w thick film resistors.

Use the aluminum clad power resistors and mount them to the case, heat problem solved provided you have enough surface ares to sink the heat.

chermann · 2018-01-20 8:00 pm

lexx21 said:
Not really. I do this on the 5v ac supply to 4v heaters of az1 rectifier tubes and its quite happy. .5R on each side of the heater gives the 4v needed. I used 20w thick film resistors.

Use the aluminum clad power resistors and mount them to the case, heat problem solved provided you have enough surface ares to sink the heat.

thx...yes i have some of these big brothers...8 ohms 50Watt...housing is an old amplifier e.g. denon AVR 1803

thx...

dotneck335 · 2018-01-23 8:23 am

This will do it:
High current dual rail regulator kit for power amplifier or bench power supply ! | eBay

How to lower down PSU's voltage (42v->24v)

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