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. RS
carries 3 regulated warts which would cover that:

273-1680 3-12 VDC @ 1000mA

273-1662 1.5-12 VDC @ 300 mA

273-1667 3-12 VDC @ 800 mA

Which one would be the best choice to see if I get any improvement?

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!

Hum very often comes from ground loops, in which case an isolation
transformer in the audio line fixes it. Radio Shack 270-054A
about $15.

It will come in handy for other things, even if it doesn't fix this.

I need them all over, between the radios and the computer.

Batteries are not very efficient at 12v; my own wall wart solution
is a switching power supply in the basement, powering about 30
12v devices each of which came with a wall wart. If you go this
route, fuse every line at below its capacity (eg. 7a for the
main 12 line, and 1/4 and 1/2 and up amps on each individual device's
line from that). Use fast blow because slow blow gets quite hot.

Normally the internal resistance of a wall wart prevents starting
fires in the power cord, but a big 12v supply is fully capable of
turning a short into a fire without overloading itself.

--
Ron Hardin


On the internet, nobody knows you're a jerk.

If you can solder and want to add a filter to your wall warts-this is
simple to build.I have built several of these & they can solve your
regular hum problem unless you need a ground loop isolator.

http://www.dxing.info/equipment/wall_warts_bryant.dx

Kim

On Feb 4, 9:01 pm, "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. RS
carries 3 regulated warts which would cover that:

273-1680 3-12 VDC @ 1000mA

273-1662 1.5-12 VDC @ 300 mA

273-1667 3-12 VDC @ 800 mA

Which one would be the best choice to see if I get any improvement?

I can't tell you which if any of the units you list will be suitable.
I have found great variability amoung units with the same part number.

One thing that might help is to take a portable SW, I use a DX398
becuase
I have one, and power up each of the units and see if any are RF
quiet.

Regulation is one aspect, but modern diodes turn off so fast that they
can cause serious RF noise.

This is one viewpoint.
http://home.computer.net/~pritch/shortwav.htm

I only use wall warts operation away from home or for those odd
devices
that don't operate from +12V. I tend to build LM317 step down
regulators
to allow +3V, +6V or +9V devices to operate from my 12V rail.

Some devices require AC and some are positve ground. I try to avoid
the
later if at all possible.

Good luck.

Terry