Chinese LM3886 Board having problems

[danielwritesbac]

The omission of the output cap isn't the source of the problem.

There are other, more elegant, speaker protection kits available. However, an output cap and a speaker fuse will get the basics covered.

I had meant to use the output cap as a temporary measure to protect the speakers while you troubleshoot the amplifier.

That will also allow you to make use of the amplifier. Perhaps selecting the right size DC rails fuse will help. Check the datasheet for mentions of amperage. The small speaker doesn't blow the rails fuses but the large speaker does? Well, the large speaker is making more bass.
That bass pulls more amperage, and pop goes the weasel! I mean fuse.
This will require a larger size fuse on the DC rails. When one blows without the other, you have 40v dc offset which will blow the speaker unless an output cap (or something more elegant) stops it. Suggestion: Make the AC and Speaker fuses whimpy but make the DC rails fuses just stout enough to outlast the other fuses.