Hissing LM4766 circuit

[MaurI]

Hello I have a little problem!

I built an LM4766 based amp, using schematic from datasheet (auxiliary amplifier). Well you know that it has very few parts so it must be impossible to get it wrong but nevertheless when I powered it, everything worked but only that incredible hiss (I can't stand it). Well I am using +/-20V power supply (it has large capacitors and when I shut down the transformer, amp still works for about 2 seconds) So when I shut down the transformer and those capacitors start to discharge the hiss disapears. So I don't know is this hiss induced by Magnetic field from transformer (but I cant distinguish those 50Hz with my ears) or the problem is elsewhear. Another thing is that if I touch the input capacitor hiss almost disapears as well. So maybe my PCB is wrong, I really don't know where is the problem. Oh and I don't know about that 22uF capacitor - is it non-polarized electrolyt or what? Yes and there are slight differences in values of few resistors instead of 4,7 ohms I have an 5,9 ohms (I didn't want to go to the store to get those 4,7) and there is 22kohms instead of 20 plus there is no inductance and resistor between speaker and output of the chip. But I don't think that this is the real cause of the problem!

Maybe traces on my PCB are too slim?


Please give me some advice, I am not an expert so I don't have the necessary knowledge.

Oh and there is another thing When I use my TDA7294 based amp with the same power supply - then there is no hiss no nothing.

Thanks!

[wes-ninja250]

1. Is the noise there when the inputs are shorted?
2. Describe your power supply in further detail
3. Post pictures.
4. What values for Cs and Cm?

Others: http://www.national.com/ds/LM/LM4766.pdf -- page 7.

Wes

[sagarverma]

check solder joints.they should me shinny and round at all connections.
the prob u tellin is mostly bec of bad solder joints.if u have any prob in soldering then see the online guide on soldering on the webpage of a school.

[sagarverma]

check solder joints.they should me shinny and round at all connections.
the prob u tellin is mostly bec of bad solder joints.the thing u said that the hiss disappears when u push cap down surely indicates bad soldering.resolder the board agian.firstly desolder using solder pump then use thin tipped solder iron and flux cored solder wire which gives excellent joint in first shot itself.believer me its the root cause ,ihad same with my tda8563 amp.resoldering revealled the gem.so resoldering highly recomended.
if u have any prob in soldering then see the online guide on soldering on the webpage of a school(i dont rem. its name).it has very good tips on soldering.

[MaurI]

1) Noise is still there if input is shortened, but it is less audible!
2) My power supply is very simple - 200W transformer( it's not toroidal), two rectifiers, 2x 10000uF capacitors and stabilized output +/-20V
3) Cs=470uF, Cm=10uF
4) Ok pictures( I don't think that they will help in any way):
Transparencie that I used for for this:


The Board:


FInished( but with defects[hiss]):





So I start to think that this circuit catches some not wellcome signals, when there is no signal applied to the input I can distinguish some radio programme! Maybe some ground loops- I don't know!?

Plus I don't know about that Ci - what is it (non-polarizet electrolyt?)

Please - need at least some ideas!

[MaurI]

Sory for tihis bothering, but I fixed the problem, It was the Power Line!

And about that solodering - I do this since I was 9 (now I am 18) so I know how to soloder, My problem is lack of perfect theoretical knowledge!

Thanks for your help!

This Topic can be closed!

[wes-ninja250]

Good jobs fixing it -- but what was wrong with the power line?

Wes