On Sun, 04 Feb 2007 13:01:46 -0800, willismat wrote:

I have a preamp which came with a wall wart causing hum. Doing research
into past posts here brought a comparison between RS wall warts back in
1998, saying that catalog # 273-1662 was unregulated and caused hum while
#273-1664 was regulated and did not. Having just checked RS's website,
273-1664 is no longer available, but 273-1662 is listed as being regulated
(perhaps upgraded over time?). The power requirement of my preamp is 12 to
13.8 VDC @ 100mA.

Try using batteries first before unting supplies. After that, I would
suggest using a linear (big transformer) isolated supply with no ground
reference. Use a ground lifter. But make sure your antenna and radio are
grounded elsewhere first!