[AMRadio] Low-pass audio filter for transmitting

D. Chester k4kyv at charter.net
Sat Oct 27 12:16:18 EDT 2007

> Don/K4KYV runs a -great- signal at 3.1kc, when he's in the 'narrow' mode
> with active and passive band-pass and band-width filters (homebrewed, of
> course)
> -Geoff/W5OMR

Actually, the narrow filter cuts off 3400~, with an extremely sharp cutoff. 
Since that's an audio, not rf filter, the total bandwidth would be 6800~ 
including both sidebands.  I use it only when the band is so congested that 
I can't get good reception using the 8 kHz mechanical filter in the 
receiver.  When I can  receive well with the 8 kHz filter, I switch to 5000~ 
cutoff, which is more gradual, with complete attenuation somewhere around 
7500~.  I could run with no filter in line at all, but the undistorted 
frequency response of all my transmitters is at least 40-11,000~.

Wider bandwidth is no problem, if you use it sensibly.  We use variable 
selectivity with our receivers, and adjust according to band condition, so 
why not do the same with our transmitters?  We don't keep the receiver in 
the narrow selectivity position, which severely restricts audio quality, 
when QRM is not present.  So why should we limit ourselves to 6000~ 
transmitting bandwidth when the band is lightly occupied?

My passive low pass audio filter modules themselves are not homebrew, but 
surplus items, potted in cases like small transformers.  One is rated at 
10,000 ohms in/out and the other is 50,000 ohms in/out.  The low-pass filter 
unit is homebrew and rack mounted, built upon the remains of a late 30's 
Gates broadcast studio audio line amplifier.  I drive the filter modules 
with a cathode follower, and terminate them into a resistor at the rated 
load impedance.  That way, I get the same signal level at the output, 
regardless of which filter is in line, and each filter is properly 
terminated.  The filter module is followed by a triode connected 6J7 output 
stage working into a UTC LS series (broadcast quality) plate-to-line output 
transformer. I use a simple double-pole switch to change filters.

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