Input resistor problem(3886) please help

[avenger]

Hi Guys,
I have built a non-inv amp (LM3886), the issue is that I forgot to place a resistor from the non inverting input to ground. It worked but was really noise and the output was at a 35V potential(same as power supply Voltage). No mater what I did I couldn't figure out what was wrong specially with the very loud hum. Then I placed a 22k res. there and all is good!! no dc at all on the output.

what is the purpose of this resistor?
Now its very quiet but get a bit hot for my taste even at rest, I have a 3inch x4inch aluminum plate a a heatsink. Would this be normal?

Please reply with any thoughts at all.
Thank you very much in advance for yoru time.
God bless.

[Tube_Dude]

Originally posted by avenger
Then I placed a 22k res. there and all is good!! no dc at all on the output.

what is the purpose of this resistor?

Hi

The purpose of the resistor , is to provide a DC path, for bias the base of the transistor , of the differential pair, that is inside the ship.

Regards

[avenger]

so,
if this resistor is not there what would happen? Would we get the supplied voltage at the output like I got?


God bless.

[Tube_Dude]

Originally posted by avenger
so,
if this resistor is not there what would happen? Would we get the supplied voltage at the output like I got?


God bless.

Yes!....As one of the bases of the input pair is not biased ,only one "half" of the amp is working and the output will be at the voltage , of one of the rails.

God bless you to.

[avenger]

ok,
that makes perfect sense.
As far as the temperature goes...should it get hot at idle? Mine gets a little bit warm just sitting there.

Muito obrigado e continuacao de uma boa noite. Acho que e a' volta de uma da manha ai em portugal se nao me engano?

God bless.

[Tube_Dude]

Originally posted by avenger

As far as the temperature goes...should it get hot at idle? Mine gets a little bit warm just sitting there.

About the temperature issue...I have no experience with the LM 3886 .
Have you inspected with a scope , if there is no oscilation , at the output?
Have you used a Zobel?

Muito obrigado e continuacao de uma boa noite. Acho que e a' volta de uma da manha ai em portugal se nao me engano?

De nada! Escreves bem Português...pois são ,uma e meia da manhã...


Abraço

[Sherman]

Originally posted by avenger
ok,
that makes perfect sense.
As far as the temperature goes...should it get hot at idle? Mine gets a little bit warm just sitting there.

Muito obrigado e continuacao de uma boa noite. Acho que e a' volta de uma da manha ai em portugal se nao me engano?

God bless.

No experience with the 3886 but every 3875 I've built got hot even at low volumes during the first 10 hours or so. For whatever reason, after that "break in time" they all cooled off and operated at a much lower temp. Can't explain why but since it has happened over and over again I guess it is real.

Maybe something similar with the 3886?

[avenger]

wow, great,
at least mine is not the first to do that.

You had mentioned the oscilation, I will take a look with the scope, but what should it look like? what should I look out for?

Actually I think its oscilating because just now it was making a scrachy noise with the audio and it was a lot different than before and the temp. started to increase alot..I had to shut it off for a few minutes.

Thank you in advance.


Sim, eu sou portugues e vivo nos EUA. sou natural de Ponta Delgada, ilha de Sao Miguel nos Acores.

Continuacao de uma boa noite.

[Tube_Dude]

Originally posted by avenger


You had mentioned the oscilation, I will take a look with the scope, but what should it look like? what should I look out for?

You must look for a sinusoid at the output ,without any input signal.

Continuacao de uma boa noite

Para ti também.

[carlosfm]

How thick is that "aluminium plate" you use as a heatsink?
It it straight, no fins???