Reduce voltage of 50V to 35V of Powersupply, regulator ?

[jbdivx]

Hi everybody , i'm sorry , i'am french 😀

I have a toroidal transfomer 500VA 2x37V

I will go to do an amplifier with TDA7293 or LM3875, LM3886...

But i have a problem : 37V * 1.41 = 50V approximatively

So, i need to reduce voltage of about 15V for supply a LM3875 for example.

So , can i use a regulator same the LM338T for reduce the voltage ?
http://www.datasheetcatalog.org/datasheet2/d/0kd9k2u8d4eukh8rypa0g60wc4wy.pdf

Is that using such a regulator can degrade the signal output of the amplifier and therefore have a sound worse ?

Sorry for my poor , bad , low , English.....

🙄

 

[pacificblue]

No need to be sorry for being French. 😉

The following link seems to be just, what you are looking for. http://www.romanblack.com/gainamp.htm

 

[drummer_Dave]

I am actually in the process of using a LT1084 (or LT1083) regulator
i will hopefully post my results as soon as i get the remaining parts in...

Dave

 

[jbdivx]

Thanks for you help

I just saw the page, but I have a problem , the assembly is too difficult for me ( utilisation of a PNP transistor )

I juste want , if it is possible to find a transistor which can do the job 😀

I finded the LM350 (3A max) and the LM338 (5A max) but both of them ,

when Vin - Vout = approximatively 20 V have a fall of Iout

So my idea is of take 2 x LM350 in parallel (it is already difficult for me)

So , do you think , it 's a good idea ?

 

[sasmit]

paralelling requlators by simply connecting them in paralell won't work..if thats what you're thinking 😉.. LM338 data sheet has many schematics on how to build a high current psu with multiple regulators.
If you're dropping too much voltage across the regulator you'll not be able to achieve the maximum current capacity of the regulator. If you're just gonna run 2*LM3886 .. 1 LM338 ( one per rail ) will be upto the job...but might run a bit warmer than usual. As regards sound quality there won't be muh difference, if your supply is adequately sized.

 

[Ted205]

is another transformer out of the question ?

 

[jbdivx]

I don't have other transfomer ..... 😀

sasmit, I want to use LM350 - 3-Amp Adjustable Regulator

http://www.national.com/mpf/LM/LM350.html

But , 2X LM350 for each canal ! 😀 in parralel....

Moreover , if i use a regulator , i must do this installation :

SoftStart => Transformer toroidal => Diode Bridge

=> Capacitor (1x15000uF) => Regulator => Capacitor (1x15000uF)

=> Capacitor MKP

I have only capacitor of 15000uF 63v at house ( I have 8 capacitors )

30 000uF for each canal for a TDA or a LM3875 is enough sufficient ?

 

[sasmit]

you do not need a very large cap after the requlator a 100 ~ 470uf shoud be there on the output of the regulator board ( close to the regulator output pins )and 2*1000uf ( one per rail ) on the amp board..( a small value 10uf or so and a film cap close to chip pins should already be there on the pcb )

Be sure to use the protection diodes for the regulators mentioned in the application note

on the 15000uf before the regulator solder a 0.1uf ( ceramic preferably ) directly onto the cap legs.

30000uf should be sufficient for both your channels ..

The MKP you mentioned at the end might already be present in form of a small bypass cap near the chip pins..( it should be a ceramic or tantalum rahter than film )

for bypassing powersupply a multilayer ceramic or even a disc type will perform better than MKP


🙂

 

[sasmit]

if you're going to use 2*LM3886 then 2 LM350's should just about do it, one on each rail i.e. one on +ve another on - ve.

If you're planning on paralelling you'll need 4 LM350... 2 per each rail and additional components like an opamp that will be used for paralelling the regulators.

 

[tryonziess]

You could always remove some of the secondary windings from the transformer. It might prove to be a huge ordeal, but will lower the voltage to what you need. The good thing is that you have a toroid type. These are much easier to unstack.
Tad

 

[Leglandu]

You can also add a lot of turns in series (in phase) with the primary ; try with 100 turns, measure what happens and calculate the whole numbers of turns. A long job however…

Or even some turns in opposite phase in series with your secondary. Try 10 turns, measure the voltage and deduct how much turns are needed to loose 15V. That is pretty unusual but it works and you’ll avoid to hurt your transformer.

Concerning the cable to be used, I would say 0.8 mm of diameter for the primary or 1.5 for the secondary… to be confirmed but not so critical as the trans seems oversized for the amp.

 

[jbdivx]

Thanks for all !

I do go to remove some of the secondary windings from the transformer .. too dangerous For Me 😀

But I think to use 2 x LM 338 in serie for reduce voltage of 20V

10V for each LM338

But I have a problem , do you know a regulator ( LM ... or other) in negative voltage which can debite for than 3A ?

Thanks

 

[pinkmouse]

I don't know of any high current single devices - the only way is to use the regulator to control more transistors, as in the schematics you have already rejected.

However have you considered that the cost of a regulated power supply combined with the huge heatsinks it will need is likely to be more than the cost of a new transformer?

 

[pacificblue]

But I think to use 2 x LM 338 in serie for reduce voltage of 20V

Is it easier to add one regulator, two resistors and two capacitors than to add one transistor and one resistor?

But I have a problem , do you know a regulator ( LM ... or other) in negative voltage which can debite for than 3A ?

LM133 or LM333, but I think they are both obsolete. You will have to use a positive regulator for the negative rail, like in the link from post #2.

 

[jbdivx]

Toroid Transformator cost money.

Espacialy a 500VA trasnformer..

When you listen music , you use only 1 Amplifier

So, i have the idea to use only 1 Power supply that i can install on all of my Amplifiers ( For example a TDA 7923, a LM3886 , or a PASS amplifier "Zen")

I would rather put my money in assembling TDA7293 or LM3386 or transistor (for pass amplifier ) than in miscellaneous toroidal transformers

+

 

[pinkmouse]

Toroid Transformator cost money.

Yes, they do. But how much do you think a variable PSU capable of dissipating 200w or more will cost?

 

[tpsorin]

Thanks for all !

I do go to remove some of the secondary windings from the transformer .. too dangerous For Me 😀

But I think to use 2 x LM 338 in serie for reduce voltage of 20V

10V for each LM338

But I have a problem , do you know a regulator ( LM ... or other) in negative voltage which can debite for than 3A ?

Thanks

What is the danger? You have to remove the windings with transformer not connected 🙂)

1.Measure the output.
2.Remove 10/20 turns
3.Measure the output voltage
4.Calculate how many turns you have to remove to get 35V

It is the most logical way to reduce 15V. Otherwise you loose 15/50 = 30% of power

 

[pacificblue]

So, i have the idea to use only 1 Power supply that i can install on all of my Amplifiers ( For example a TDA 7923, a LM3886 , or a PASS amplifier "Zen")

TDA7293 and LM3886 have similar demands, so it would make sense to buy an adequate transformer for them.

The Zen amplifiers usually come with a very different approach. If I remember correctly, Mr. Pass recommends PI-filters, which means you should be prepared to spend more money on inductors than on the entire rest of the amplifier including the rest of the power supply.

 

[AndrewT]

Toroid Transformer cost money.

When you listen music , you use only 1 Amplifier

How important is your hobby/pastime to you?
How do you decide what budget to allocate? Make a decision.

If you listen to a one amplifier system then the best you can manage is monophonic. Stereo/two channel is not available.


My more complicated setup uses ~ ten amplifiers, each with their own transformers.
If I ever get my active system up and running I expect about twenty amplifiers to drive the system.

 

[jbdivx]

yes, you're right,

That why , this afternoon , I haved remove some of the secondary windings from the transformer.

From the moment , on the secondary I have 27 in place of 37V

Probably I'll have to remove a little more turns, But I waiting to receive my MUR860 to see how many volts I have, after the bridge

Probably I will made , with this transformer , if I can 😀 a :

TDA7293, LM3875 (LM3886) , and a Mini Aleph ( With a regulator , for the voltage )