Conductive standoffs, common grounds, etc.

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Hey,

I couldn't really find a topic about this.

I'm building an amplifier out of e-bay components.

I have two power supplies which supply 36v and 12v. Can these power supplies have their negatives connected together?

The reason for this is that the standoff holes that I mount my boards with have ground rings. This would essentially connect power ground + signal (audio) ground to the chassis, which is connected to mains earth and the power supply ground leads.
 
I'm already running into ground loops between my pc and the amp. Got myself a pair of 1:1 isolating transformers that I'll put after the input switch.

I also just got the idea that I might be better off getting a buck converter and using that tapping off of the 36v. That'd eliminate one case of possible ground loop right?
 
A picture/drawing of the layout would definitely help to see where potential problems may be. A simple rule to begin with would be to keep all grounds (signal and power) as separate as possible, and all signal and power pairs close coupled, that is twisted together or use coax in the case of signal wires
 
I know it's a bit too much but for 3 dollars it's a good deal imo. I'll be screwing the plastic posts into a metal casing, so I need taps...........
screwing a self tapper and particularly a self tapper with a "cut" to resemble a thread tap will screw into a plastic spacer. but do take care not to split the spacer. you may have to drill the spacer so that the screw does not have to cut as deep.

I tap my plastic spacers and use a machine screw. That works reliably.
If the plastic spacer is tapped straight through I find the machine screw is very tight. It is vibration proof.
Instead I squeeze the side of the plastic spacer with flat nosed pliers as I tap and then move round to the next sides of the hex and squeeze again as I remove the tap. This give a "slack" fit for the machine screw, if that is what I want/need.
 
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