LM3886 cutting out. First places to look?

soma_hero

Member
2008-09-11 10:30 pm
Hello,

I've built an LM3886 x 2 channel amp following the project at ESP. It has a preamp stage driving the LM3886, 22k input resistor to ground, 22k feedback.

It has worked great for 2 years but one channel has recently started cutting out after being played for awhile at medium volumes. It just cuts out for about 3 seconds and then comes back in for a 30 seconds to a minute and then cuts out again.

The case is not hot, barely warm and the other channel is fine.

I just measured offsets with no cable connected to the input and no load connected.

Channel 1 : volume off 0.0mv volume up 0.4mv
Channel 2 : volume off -0.4mv volume up -100mv to -200mv. This channel cuts out.

+V is 28.8V, -V is -28.9V

I'm not totally convinced these are accurate with my meter since all offsets i've seen have been around 50mV. The channel that cuts out is drastically different than the normal channel however so this keys me in that something is amiss.

What should I look for? Should I play music through it with speakers connected and wait for the channel to cut out while measuring offsets?

The heatsink is grounded, and I have isolated the screws holding the chips from the chips themselves by wrapping the upper parts of the screw in electrical tape. I think maybe when one warms up, some tape maybe moving around and causing -V to short to Ground. Would this cause the amp to cutout? This should cause speaker damage i would imagine, if I'm correct that makes the output of the chip send full +V to the speaker? Speakers continue to work though....

THanks for any suggestions!
 
Sounds to me like one of the chips might be overheating because its thermal contact with the heatsink is somehow compromised - have you checked by putting your finger on the chip itself to see how hot it gets? Using electrical tape for insulation is not going to give good reliability, you're right there - better to get the correct bushes. If V- shorts to ground it will affect both channels though so that doesn't seem to be your problem.