Hi,
I've just built an LM3886 based audio amp, the circuit is pretty much the one from the datasheet minus the inductor on the output and with a couple of other things modified. The supply is unregulated +/-17V and for filtering I have two 40V 10,000uF caps and two 100nF caps.
The problem is tricky to explain but I'll do my best.
So the center point of the AC output should be at 0V, obviously. After the amp has been on for a few seconds (it works perfectly in this time) I can see, on the scope, the output slowly dropping down towards the -17V rail (DC coupling used on the scope). After about 30 seconds the output starts to hit the negative rail and clipping occurs. Eventually the output just goes down below the rail and disappears.
I've tried a few things. The only one that made a difference was disconnecting the filter cap between the 0V and -17V rails, but leaving the other one in. This prevented the output from dropping but was, of course, filled with the mains crap.
Removing the filter cap from the 0V and +17V rails made no difference.
I've tried two different types of capacitors on this and the change made no difference either. I'm a bit confused on this one so if anyone could give me some suggestions it would be greatly appreciated.
IO