lm3886 +25V out with hizz and hum!

[demogorgon]

hi!

I know i'v seen a post where a guy had a simular problem some time ago, but i cant find it, so i'm making a new one.

as the topic says i'v been making a lm3886 amp and finally got it finnished.

as i try it out it blows the membrane of my speaker almost up to the roof! (ok, now i'm overesaturating)
i measure +25V of the board and when the trusty old test speaker is connected the membrane was pushed as far as it would go and i head hum and hissing.
the thing is made with kostasthegrate's pcb, etched it myself.
there is no apparrent short circuts anywhere eiter.

i have absolutly no clue as to what could be wrong with it, every thing should be correct!

the trafo is a 160w transductor 2*17.5V

i'l post some pics of the amp and the scematics if its needed in finding the problem.

thanks a lot!

regards
marius

[carlosfm]

Originally posted by demogorgon
as i try it out it blows the membrane of my speaker almost up to the roof! (ok, now i'm overesaturating)
i measure +25V of the board and when the trusty old test speaker is connected the membrane was pushed as far as it would go and i head hum and hissing.

I'm sorry, but I'm here laughing for some 5 minutes...

Originally posted by demogorgon
i'l post some pics of the amp and the scematics if its needed in finding the problem.

Show us the PCB too, some mistake you made there.

[demogorgon]

hey, glad at least one of us is in a good mood :P

this is what diy is all about. trying and failing, trying again and failing...again.. almost like my life's story.

well, pictures was requested.

[IMG]www.home.no/demogorgon/over.JPG[/IMG]
[IMG]www.home.no/demogorgon/under.JPG[/IMG]
[IMG]www.home.no/demogorgon/scematics.JPG[/IMG]

hope this helps..

regards
marius

[carlosfm]

Originally posted by demogorgon
well, pictures was requested.

[IMG]www.home.no/demogorgon/over.JPG[/IMG]
[IMG]www.home.no/demogorgon/under.JPG[/IMG]
[IMG]www.home.no/demogorgon/scematics.JPG[/IMG]

hope this helps..

regards
marius

"Not Found
The requested URL /forums/www.home.no/demogorgon/scematics.JPG was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request."

[demogorgon]

well, then let's do it the hard way:

you know the filenames from above, sp just hit the link without the picture name.

www.home.no/demogorgon/

it's working for me.

edit: i dont have an index file yet, so you'l se the index.
just find the files
above.JPG
under.JPG
scematichs.JPG (misspelled)

[Rudy]

your feedbackresistor is connected to the -supply, and not to the output signal like it is supposed to be

Rudy

/edit after looking at it again, the feedbackcircuit is totally wrong

[demogorgon]

i take it then that th feedback resistor is the 220kohm one?

oh.. the pcb or just my relentless soldering?
a pointer in the right direction would be nice..


regards
marius

[Rudy]

yes i mean the 220K resistor, from witch schematic have you drown the pcb ?

/edit actually, it could work if you just connect the resistor to the output, but before connecting your speakers again, measure dc on the output.

good luck

rudy

[carlosfm]

Originally posted by Rudy
your feedbackresistor is connected to the -supply, and not to the output signal like it is supposed to be

Yes, that's the problem.

It's regulating at 25V.

[demogorgon]

the schematic found here: Simplified LM3886 amp with PCB

i did something stupid i must admit.

I etched, drilled and solderd, but never questioned or read or studied.
so i really dont know anything of what u are talking about..
well, maby its not too late. i'l sit down with my adobe and lm3886 spec sheet i guess.