Audio Mods for the HT-37

[Steve - WB3HUZ]

Just posted on The AM Window Web Site, Audio Mods for the Hallicrafters HT-37 by Jeff, W2NBC.

http://www.amwindow.org/tech/htm/ht37mods.htm


Enjoy!

[WA1HZK]

Thanks
That's on my list of things to do.
Keith

[Opcom]

This is interesting.. The ELSIE program shows the original PI-type filter of 0.1uF-50mH-01.uF (assuming 500 ohms in and out) is flat up to 2KC, and sloped down fairly straight to:

-1dB @ 2.5KC
-3db @ 3.2KC
-6dB @ 3.8KC
-10dB @ 4.55KC
-20dB @ 6.82KC
phase is -180 at the -10dB point.

This is a pretty broad cutoff, and starts early in the voice range. Hallicrafters could have done a littler better, unknown about cost vs. space vs. philosophy, etc.
Would it be also useful to add more sections to a filter like this to bring the Fc out to 4.75KHz with a sharper decrease after that?

What I am asking about is certainly not a perfect filter, but it seems to make a better curve than the stock one, if it was desired to leave a filter in the transmitter..
0.082u-30mH-0.13uF-33mH-0.13uF-30mH-0.082uF

0db @ 3KC
-1dB @ 4KC
-0.5dB @ 4.4KC
-1dB @ 4.5KC
-3dB @ 4.65KC
-6dB @ 4.75KC
-10dB @ 5KC
-20 dB @ 5.4KC
phase is -180 deg at 2.4KC and -330 deg at 5.4KC

Does the phase delay with increasing frequency mess up the audio quality (speech), or is it "more better" as in the "all-pass filter" type of effect on the audio signal? I could not hear any difference with a breadboard and clip-lead setup, making a cassette recording and playing it back.