Amp crackles and dies

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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
 
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.
 
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
 
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...
 
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.
 
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
 
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
 
well I wondered about that. I ignored the ground, and just connect the + input on the pcb to the + on the trasnformer and the same for the -.

AH so are you saying i should buy 2 12V 1A transformers, connect the - from one tranformer to the - on the PCB

the + from the other transformer to the + on the PCB

and then the remainging - and + from the 2 transformers, join them together and connect them to the ground?

So what i did earlier with the transformer was wrong then? Hopefully i didn't damage anything.

Thanks for the help
 
I think you didn't damaged anything...
If you decides to buy new transformer, then buy something quite good! you won't spend a lot buying a transformer that suites the amplifier;

Have a look here:
http://www.maplin.co.uk/Module.aspx?ModuleNo=3691&TabID=1&source=15&WorldID=3&doy=19m3

should you buy and wire by yourself remember to put right fuses and pay attention.

I think buying 2 new wall wart 12V and only 1A wouldn't be cheaper and would be sonically inferior.
You should use two separate transformers only if:
- you already have them and the quality and power you get is enough for your tastes
- you don't want to wire by yourself

If you already have two transforners hanging around even very small (few hundred mA) you could have a try connecting them as I said before for a test and in order to hear how much power you can get...
 
Thanks, that 50Va 12V one would suit it perfectly, its just i'm not too good with mains voltage.

If i was to purchase 2 wall wart transformers, then should they be regulated or is unregultaed ok, such as

http://www.rapidelectronics.co.uk/r...CAT_CODE=30351&STK_PROD_CODE=M29506&XPAGENO=1


Finally, if i was to use one of those 50Va transformers (I'd buy from rapid rather than maplins probably) could you tell me how i would wire it up.

http://www.rapidelectronics.co.uk/r...&CTL_CAT_CODE=&STK_PROD_CODE=M30994&XPAGENO=1

Thats the link for it there, there are 4 pins on the input 4 on the output, so i would presume the middle two pins on the input are birdged and the the outer ones connected to live and neutral

then on the output bridge the middle 2 and connect them to the ground, and the outer ones connect to + and - on the PCB.

Is that right?

Thanks for your time and help
 
unregulated is OK.

The 50VA type should be connected depending on the voltage in your country: if you have 240V then primary windings whould be in series; if you have 120V then they must be put in parallel.
The secondaries should go as you said.
Remember to put fuse with the mains wire (a 0,5A fuse).
If you prefer just go with the wall warts, they should be ok... and are already protected. you won't get much power but you can upgrade after...
 
240V so series then, so 4 pins if the outer two are for the live and neutral (+ and -) ill bridge the middle two.

So if i fuse the live wire on the input side, which would be fused by the plug on the end of the cable, but you suggest i use a 5A fuse rather than 13A?

I may stick with unregulated ones then thanks.
 
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