A few LM3886 layout tips I wish I'd known on my first board

I built my first gainclone about six years back and made every grounding mistake posible in one board. The hum was bad enough that my wife complained from the next room. What finally fixed it for me wasn't the layout on the PCB itself, it was the input jack. I had the RCA ground bonded to the chassis at the rear panel and the signal ground joined somewhere else, so the shield was carrying current it had no business carrying. Lifted that bond and soldered a small wire from the input ground straight to the main ground point and the hum just vanished. Went from audible at the listening chair to dead quiet with my ear on the tweeter.

Since then I've done three more boards and I pour the ground fill like you describe, but I keep the sensitive input traces routed over that pour rather than next to it, if that makes sense. Seems to work fine.

The oscillation thing bit me once too. I had the feedback resistor loop sitting right above where the speaker terminals came back into the board, and it would sing whenever I connected a certain speaker cable with high capacitance. Moved the output run to the board edge and never saw it again. prototype boards are cheap, I say spin a second revision rather than talk yourself into the first one being good enough.

The mute resistor bit me on my second board. Sent the files off thinking I'd double checked everything, boards came back, and the amp powered up dead. Spent an evening poking around the supply rails before I actually looked at the datasheet properly. Ten cent resistor, whole amp offline. One other thing that helped me, I put a row of test points along the bottom edge of my last layout. Made scoping the supply pins so much easier than poking at cap legs and slipping off onto the next trace.

Another thing worth sweating on the 3886 is how much copper you actually give the tab. My first board had a skimpy little pad under it and the chip would hit thermal shutdown maybe twenty minutes into a loud session. Current boards I flood a good sized area on both layers with a dozen or so vias stitching them together, and I can run the thing flat out all evening without it backing off. Also worth leaving a bit of slack in the tab holes, my drilled holes always end up a hair off and forcing the part in stresses the legs.

Update: went back and looked at my last board with the input jack thing in mind. The RCA shell is tied straight into my ground pour right at the connector, which kind of defeats the whole separated-grounds plan I was preaching. Ended up cutting a slot around that corner and running the shield over to the signal ground point on its own trace. Hum floor is noticeably lower now, so the wife-test comment was onto something.