Lm4780 problem, 3.5V on output when power on...

Hi!
Just finished my stereo gainclone using two lm4780 chips each running in parallell mode. Before powerup everything semms nice and there is no short circuit and the powersuply works fine delivering 36.7 volts unloaded.
But when I turn the power on I read 0,1 Volts across one channel (out gnd --- out), a bit high but no problem? The other channel reads 3.7 Volts and the chip gets really hot!
Does anybody have an idea of where I sholud start looking? Will try to post some pictures later on...
 

ttan98

Member
2006-04-04 11:24 am
Melb
my experience, mine was working fine ie o/p=26mV then after using for a little while o/p goes to 1v, why,

fault: the i/p wiring from the pot to i/p was broken.

I suggest

1. check the input is tie low ie low resistance to gnd.
2. check the i/p to gnd resistor is properly soldered in.

cheers.

you are using the chipamp as parallel pair, make sure the gains on both amps are very close to each other, otherwise you have problems down the track.

cheers.

btw, 0.1 o/p is too high on the other channel as well.
 
Wait a second, I havent tried it with a source yet.
All my measurments have been without any source. To me it sounds strange that the chip would get very hot when the power is on but there's no input but maybe that's the way it is and my amp is actually working just fine? If it's so I shouldnt have my amp on when there's no source?
 
jaste said:
Wait a second, I havent tried it with a source yet.
All my measurments have been without any source. To me it sounds strange that the chip would get very hot when the power is on but there's no input but maybe that's the way it is and my amp is actually working just fine? If it's so I shouldnt have my amp on when there's no source?

I'm gonna bet that without any input source your LM4780 is oscillating. It would be helpful if you had a scope. You might be picking up some RFI, or oscillating of its own volition.

FWIW, I have been able to get the LM4780 to oscillate as high as 1.5 MegaHertz -- and it gets very unhappy.