how to ground properly?

[Denis.BR]

I noticed that my LM3886 are not insulated, so the heatsink is "wired" to the -35V rail, so I won't use the 0v rail grounded to the chassis, but can I wire the chassis to the ground plug in the outlet?

Or do I need to insulate the LMs from the heatsink?

[sith]

It is possible not to insulate LM from heatsink if you're absolutely sure that heatsink is also physically insulated from other parts of device (chasis)... But, it is advicable to insulate LM from heatsink using washers and thermal grease

[abid_rehan]

I would go with Sith's advice.
It is advisible to insulate the chip from heatsink, coz if any time the heat sink comes in contact with the chasis (which is usually connected to ground) you will end up with either a fried PSU/Trafo or a fried chip.

[AndrewT]

Hi,
there are thermal advantages in the improved coupling of the chipamp direct to heatsink, remember to use a thermal compound to exclude all the air from the interface.

The risk of a short is high. Just a dropped fragment of copper or a carelessly used tool could blow the amp.

[!]

The risk of a short might be overstated, if you are dropping tools into your amp or there are copper fragments flying around, there are plenty of other opportunities for shorting.

It should go without saying that you can't drop tools into live equipment nor power it on with hunks of metal lying around, no?

If the amp has an undersized 'sink or is high output design, the improved thermal coupling may make the non-insulated package a reasonable advantage, but then the heatsink needs isolated from the case if it's metal. Assuming the case is metal and has line AC voltage going into it, the case should always be grounded but the 'sink with uninsulated package chip and no insulator, so it's at amp rail voltage, should never be grounded to AC/case ground.

If you want a metal case and hot 'sink with uninsulated package, you might consider heat resistant plastic or polycarbonate/etc, insulator blocks to mount the 'sink to the case. If you don't need the best thermal coupling you might just buy the insulation pads, since it is a negligable expense compared to having to devise a way to isolate the heatsink if you hadn't already planned ahead for that.