LM3886 problems...

[tasos_e]

This last month I have been trying to build an amplifier using LM3886 chip.
This is my first try in the complementry supply. Till now I have been "playing" with single supply chip like TDA7375.


Well I have used the circuit and component values National suggest at the first page of it's datasheet. You can see the circuit board I made (red lines are short wirrings, blue capacitors and green resistors.)


Well here is the real problem.
I put everything on that board including the heat sink and then supply it.
what I get? +40 volt at the output...
with no input signal.

The supply I use is +, - 41.8Volt.
Ok there was a miss calculation somewhere there and got a 2x30Volt 150Watt toroidal for my 2x50Watt amplifier.
(Wana be amplifier).
But I imagine, if that was the problem there would be no output at all (power protection) and not +40Volt.

Ok I know it is a bit high (at the limit of the chip). When the supply is open air I measure +41.8 and -41.8 but then when I connect it to the board and measure the supply voltage at pin 1, 5, 4 it is +38V and -38V

It is strange to me that I hear everyone say how good that chip "work" and all I get is +supply at the output (pin #3), having nothing connected as input signal. Thank god I measured the output before I connect it to the speaker, or I would have them in my hand

I also get that +40 volt at the output, even if the mute pin has no curect drown from it.
Is it realy that power supply I use? or something else?

Then I think, I might have used a wrong feedback resistor and the amplifier work as a comparator, but checking them I find that the values I used are the correct.


Hope for a reply and even better a solution
Thanks for your time
Tasos.

[teemuk]

I'm afraid I'm not very experienced with chip amps - even a greater problem is that for some reason I can't no longer view any attached pictures of posts on my browser. This forum soft has definitely some problems... Anyway, do a double check on your wiring and board layout. Seriously. You could have a small mistake there. I believe the human error is the most common "fault" scenario with chip amps. Check that you haven't shorted anything with solder bridges.

Also, your supply voltage also seems rather high. If i'm correct these amps are usually run from +-25 to 35V rails - not with +-40V! Yeah, I know: Datasheet says supply max 84V (+-42V) but I don't know if anyone really goes that far as it indeed is the maximum.

What could I say more without actually seeing any of your schematics or attachments... Well, check that your input is indeed referenced to ground and DC coupled. Fit a capacitor to feedback path (for unity DC gain) - assuming there isn't one already.

[tasos_e]

Ok here it goes again

First of all thanks for the reply.

Well I now use a +35 -35 supply.
Also checked the printed circuit. There are no short there.
There are no capisitor to the feedback, but am using the circuit that National has at the first page of the datasheet.

Have even try to short the output with the input to set it to 1 gain.

Still get the same +35 at the output, like there is no negative suppy. but THERE IS.

Am having a hart time here, it is the first time I use complementary supply ( i say that again) and well, am a bit confused.

Thanks again for any help

[teemuk]

If you want, you can email me the schematic diagram and layout to teemukyttala[nospam]@gmail.com (remove the nospam portion) and I will have a check on them when I have some time. Without seeing them I will just have too many "maybe"s and assumptions to offer.

Am having a hart time here, it is the first time I use complementary supply ( i say that again) and well, am a bit confused.

Your supply seems to be fine according to your measurements. If in doubt, here is an example of a dual supply for reference:

http://www.coolcircuit.com/circuit/powersupply/basic_dual_power_supply.GIF

There is a chance that the chip you use has for some reason blown and is now showing up this fault. You should really try it with another chip - after you have made sure there really isn't any problems left that could have possible ruined the first one. Otherwise you likely just blow up the other chip as well - and a PCB has a limit on how many resolderings it can virtually last.