I use a metal cased Motorola mobile speaker from the Motrac days. It
doesn't pass much high pitched hiss. And I round off more with the Pass
band tuning.
There have been some passive low pass filters suited to speaker lines
published in QST and recent ARRL handbooks.
I've built a couple of my own designs that are switchable for cut off
frequency, they are 4 ohms in and out and have at least 4 different
cutoff frequencies in essentially a 2:1 step from about 450 Hz going up.
They clean up hiss and sidetone distortion on most any radio. A CW
signal coming through a narrow filter then with the pitch tuned to about
400 Hz with that step switched in on the speaker, there's nothing there
but CW, very little noise. My reproducible design has been printed in a
local club newsletter so I probably could scan something from it and
send it out. It needs a bunch of switch poles, a bunch of capacitors,
three large RF chokes (available from digi-key) and a couple audio
transformers that are available from Hosfelt or Radio Shack. A 7th order
Chebychev low pass takes away a lot of unwanted filter leak around and
IF and audio circuit noise.
73, Jerry, K0CQ
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