It seems that some power transformers have so little leakage reactance
that they are closely coupled to the power line.  First, put about 0.01
uFd directly across the transformer secondary.  Then place about 0.001
across each power diode and another directly on the DC side to ground.
    In really bad cases, place a resistor in series with the power diode
as large as can be tolerated say 4.7 ohms and duplicate the suggestions
above.  The goal is is to slow down the switching transients created by
the 0.6 volt drop across the power diodes.  73  Cliff K7RR

On Thu, 18 Nov 1999, Kevin Ravenhill wrote:

Hi all,

This may well have been mentioned before on this reflector, so apologies to
those concerned if it's going back over old ground.

For anyone plagued by power line "hash" when receiving on LF, particularly
when using wire antennas, it's worth checking the cleanliness of the DC
power supply which is being used to power the receiving setup.

I have found out the hard way that the rectifier diodes in some power
supplies can generate truly enormous amounts of harmonic hash at these
frequencies. I have not checked too closely but I assume this is simply
being re-radiated through the transformer and into the mains wiring. There
is EC EMC legislation relevant to this aspect of power supply design, but
I'm not sure how effective this is in "special cases" such as our very weak
signal conditions.

I have tried several power units ranging from simple regulated "plug-top"
types to more complex heavy current 13.8V PSUs specifically designed for
powering radio gear. I currently use a small double-insulated (i.e. no mains
earth connection) plug-top PSU to power my receiver. This was originally so
bad that it completely wiped out reception on my AKD HF3 below about 2MHz
(note that this PSU was actually intended for this receiver!). In this case
the problem was cured by the simple expedient of fitting a 0.1uF ceramic
capacitor across the transformer secondary feed to the bridge rectifier, but
this is not necessarily a cure for all situations. Lots of lossy ferrite
etc. on the mains side may be called for!

My Daiwa 13.8V 12A unit is much better in this respect but does still
introduce a small level of extra noise. This is not really significant
enough to warrant modification though. A slightly cheaper grade of 13.8V 5A
PSU was tested and this proved too noisy to be usable without extensive
modification.

Finally, I have a multi-output variable voltage lab-type PSU for bench work
which has also proved too noisy to power the LF gear as it stands.

I suppose the surest way to eliminate this kind of noise would be to run the
receiving gear from batteries, which would also get round the problem of
noise directly coupled from the mains. I would be interested to know how
many other people have encountered similar problems with LF reception.

73

Kevin, G1HDQ