Amp crackles and dies

[DD_Davo]

Hi

I built the Velleman K4003 amp and for testing it i'm using two PP3 batterys. When i connect the amp up to speakers and an input it plays ok for the first few seconds then crackles and dies, with the occasional pop and crackle. If i unplug one one of the batteries and wait 10 seconds, plug it back in i usually get another 3 seconds of amplification before it cuts the sound again.

The main IC the TDA1521 gets warm so i presume something inside it is working, but the first time i did it i foolishly put the IC in the incorrect way and put power through the amp (it didn't work), and wondered if i could have damaged it.

If the sound does stay on it tends to crackle unbelievably and you can't make out what the music is.

Do you think it is the IC, which i should replace or could it be something like the capacitors. I doubt the diodes are casusing any problem as there working as a rectifier if i use AC current however i'm using DC.

Thanks for any help

[Davide82]

the problem is with the small batteries you're using.
The 9 volt battery can't give all the power you need, they are simply way too small!
If you want to run batteries, put together some alkaline AA (at least).

If you wait some time unplugging the batteries you let them to "recharge" a little bit (for some reason which I don't explain here) and they can run a for some seconds again.

If what I say is true and you want to test before changing the batteries, you could probably reduce the problem a little putting big capacitors (2200, 4700 uF) in parallel with batteries.

[DD_Davo]

Ah ok thanks, well i've been messing with it for past few hours and i got it running ok for 10 mins eventually but when i turned the input volume up it just distored like mad. However i think the IC is probably ok, so if i try a bigger supply. The amp requries a maximum of 4A 24V but i may try and find an old transformer which will deliver 12V at 1A and see if thats any better.

Would you suggest anything inparticular as i don't fancy wiring my own 50Va transformer up.

Thanks

[Stocker]

It's really not that difficult to come into possession of a 24v 4A supply. If you have a 12V 1A wall wart, go for it, but if you are going to spend money, do some more looking.

[DD_Davo]

Its a 24V 4A transofmer i can't seem to find, or less i buy just a transformer that i wire up to my circuit, but i'd prefer a plastic encased and tested one, that i could just chop the end of and connect the wires to my PCB.

Thanks

[Davide82]

simple small plastic encapsulated transformers are difficult to find above 12V - 1A. I think it isn't even enough because (looking on datasheet) the chip needs at least +-7,5V which is 15V asymmetrical PSU!

I would buy a symmetrical 12-0-12 transformer and wire by yourself (but pay attention to power line!).
If you don't need the whole power, the transformer has only to be rated for 2 x 1 A (so it is about 25W total: cheap). If you want to go for loud music and better fidelity you can go for larger transformer (say 2 x 3A?)...
You also need, of course, bridge diode rectifier rated 3 A at least and two capacitors about 4700 uF 25V...

[Davide82]

I forgot something:
if you want to go really cheap there's another way: you can get TWO 12 V 1 A cheap wall wart and use them as symmetrical PSU.
It will not sound GREAT but it works, you've only to decide what you're looking for...
Should you choose this way, it would really improve the sound increasing the capacitors in parallel with the two transformers, say 2 x 4700 uF.

[DD_Davo]

great thanks, well the amp has a built in rectifer and it has 2 * 4700micro farad capacitors if thats what you mean.

http://www.velleman.be/Downloads/0/Manual_K4003.pdf

That link there is for the manual of this amp, shows the circuit diagram. What i'm after is something reasonably loud with ok sound quality, i realise i won't get the full power usage out of the amp or less i use the 2*12V 2A transformer required (50Va)

Can you explain more about the two 12V 1A transformers please, also what is wall wart transfomers? But if i can buy something cheap thats already wired that i only have to connect to my amp to deliver reasonable volume at reasonable quality that woul;d be great.

Thanks for your help

[DD_Davo]

This morning i wired up a 12V Unregulated transfromer and the amp worked fine for about 20 seconds then just cut out again. I think this transformer only delivers about 360ma anyways, so about 1/3 of an amp, but it was working. After thjat though the speakers only emmited the sound of 'flowing current' like an electrical hum, and no audio could be heard.

Do you think it still is power issues or that the IC or electrolytic capacitors have been damaged, i know these capacitors could be damaged if they don't fully discharge and therefore have a 'memory' which cuases them not to function fully next time.

Thanks

[Davide82]

I wonder how you connected the transformer; basically you need two PSUs (or a center tapped transformer) just as written in the manual (you see the two batteries connected?). Of course a normal transformer with only two wires coming out is not suited...
tell me more...