TDA2040 Amp Transformer

[novalic]

Hello, this is my first post...
Ive been searching posts about this topic, but after reading, I dont really get my question answered. So Ill make it concicious:

Ive done a 20W amp with a tda2040 IC, just as its posted in general guitar gadgets (bah, with some modifications, i just added a pot for the volume and a jack for input). We tested the amp at university, with an ocilloscope and a big variable power supply, and it works fine.
My questions are: what transformer do I need for supplying enough power to this amp? It says i need 18v bipolar power. I plan on putting a 50w, 4ohm speaker as output.
I read that these bipolar power supplies in general guitar gadgets wouldnt work fine to supply the amp. So should i get a transformer and solder the secondary directly to the circuit?
What secondary should I buy? And how much Ampere should i look for?
Any other recommendations? (on the speaker / transformer).
Thanks!

[Enzo]

Any power that goes to your speaker ultimatels comes from the power supply.  The power amp IC, the TDA2040, is really just a valve that controls how much current flows from power supply to speaker.

You need +/-18v supplies.   When rectified and filtered, the DC voltage in a basic supply is more or less the peak voltage of the incoming AC.  SO calculating what RMS voltage peaks at 18v, I get about 13v.   SO for full wave, we'd want something like 13-0-13 VAC.  That would be 26v center tapped or 26vCT.   But 26v is an odd amount, so you'd probably look at 24v or 28v or 30v.  All with center tap.

You then need to run the AC through a rectifier bridge (either an actual bridge, or four discrete diodes, doesn't matter) and add filter cap to smooth it.

No, you would not just wire AC to the circuit, unless you had a power supply circuit already built.

[J M Fahey]

Agree and add:
if you want to know what to order you will need a 12+12 o 14+14VAC 60 to 80 VA transformer + 4 1N5402 diodes or a 3 to 6 A rectifier bridge + 2  4700 uFx25V capacitors.

[novalic]

hey, thanks a lot for the replies. just another little thing, if i wanted to build a diode bridge, would some 1N4007 diodes work?
thanks! :duh

[novalic]

i guess not as they work with a max of 1A

[Enzo]

4 1N5402 diodes or a 3 to 6 A rectifier bridge

That is why Juan suggested the above earlier.

[novalic]

im curious on how to calculate the VA i need for such things...

[phatt]

Google in .5 sec

http://en.wikipedia.org/wiki/Volt-ampere

This little pic might be of more use if you don't like maths
Phil.

[J M Fahey]

Agree and add:
as a "rule of thumb", the V.A. you must order will be between 1.5 to 2 times the RMS rating of the SS amplifier (Tube amps need much more).

So if you have a 25W amplifier, you will need between:
1.5 x 25W=37.5 V.A. (this *will* work, but you have not extra margin)
2x25W=50 V.A. which will be comfortable.

If you have more V.A. available, fine, it will work cooler, etc.
Sometimes there's some special sale or you can get surplus stuff for cheap, and a "larger" one costs less than the standard one.
Go for it
Only problem maybe that sometimes it's too large or heavy for the chassis you have, but nothing else.

[novalic]

thanks a lot guys.
i want to add up that i know nothing about electronics, i study computers. i started a few months ago as a hobby that i always liked, some things (of the very little i know) are still confusing. thats why i ask these things that are, maybe, a bit dumb.

about the size of the trafo, im planning to do a big box with 2x 12'' speakers, so it wouldnt be a problem. i will give some real power to this )

thanks!