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To All from PA0SE
On LF I use a simple audio signal clipper that
brings all signals in the headphones to the same level. It may not improve
readibility but makes listening much less tiring by limiting all sorts
of crashes and other impulsive noises.
I have the clipper always in use and would not do
without anymore.
The clipper consists of two germanium diodes type
OA85 connected anti-parallel over the output of the receiver. To be effective
both the voltage and the impedance at the point were the diodes are
connected must be sufficiently high. I use passive audio bandpass
filters between receiver and headphones and the diodes are connected over
the first tuned circuit of the filters where impedance and voltage are
high enough. (Do not put the clipper behind the filter.)
When you use low resistance headphones the system
will not work. Perhaps you can find a suitable point for the diodes in the audio
section of the receiver: for instance the voltage over the volume
control may be high enough for the purpose.
If you find in your junkbox a pair of
old valve-type audio output transformers (for example with a ratio of 5000
to 8 ohms or thereabouts) you could connect the high impedance
windings together and put the diodes there as well. The low impedance
windings are connected to the receiver output and the headphones
respectively.
I also tried anti-parallel diodes in
series with the receiver output as advocated by Jan Smeets,
ON4ASZ/EA3DPD ("Technical Topics", RadCom, March 1998) hoping they
would improve readability of weak signals in noise. The idea was that
the output level of the receiver would be adjusted so that signal +
noise would just overcome the threshold voltage of the diodes and the noise
alone not. But it did not work. Understandable in retrospect, considering the
wide amplitude variations of noise.
73, Dick, PA0SE
JO22GD
V.d. Marckstraat 5
2352 RA Leiderdorp
The Netherlands
Tel. +31 71 589 27 34
E-mail: [email protected] (new address, but this may change soon again as the new
provider Freeler makes a mess of it)
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