I recently got an old dbx 166 compressor which only has a 2 prong power cord. I noticed that it’s creating a ground hum (or what sounds like a ground hum). I have it plugged into my power conditioner and none of my other gear (all with ground plugs) have no hum. I’ve tried multiple options with no luck, but I’m wondering if this may be due to the lack of ground prong? Any thoughts would be appreciated.

Did you try turning the plug 180* in the AC socket? If that didn't help, try taking a piece of wire, any wire, bare both ends and touch one end to the DBX and another to the chassis on another 3-prong grounded unit. Try and find a chassis screw or some other unpainted spot on each device so you make contact with the actual chassis of the units, not blocked by powder-coat of paint. Make an electrical connection.

Be aware that in a two-prong plug, likely that one side of the plug, and thus the chassis might be directly tied to wall AC. Don't get shocked.

Good luck.

If that doesn't work, you could try not grounding one end of the interconnects. TRS into the DBX? TS? XLR?

D.

It is also possible that the power supply in a piece of gear that old is dieing, sorry... If you power it up and plug some headphones into the output with nothing on the inputs does it still hum?

Had the same thing on a now-ancient Mackie MS1202, and putting in new PSU filter caps got rid of it entirely; quiet as new now.

Power supply caps. Good call!

D.

dbx used small mains PSU caps, 4x 470 uf.

I use 4 x3300uf 35v Panasonic HD caps, then they really quiet down. Post regulator caps are 2200/25v HD.