[Boatanchors] SSB Generation - Phasing VS Filter Opinion Sought
D C _Mac_ Macdonald
k2gkk at hotmail.com
Fri Jul 19 18:19:46 EDT 2019
The big problem with the phasing type SSB rigs is that the bandwidth no longer meet the current 3 kHz SSB standards. That is why the HT-37s sounded "good"
back then.
My 1st SSB rig was the W2EWL rig but I never knew how to align it to get rid of unwanted sideband. Only had a cheap VOM!
73 de Mac, K2GKK/5 in OKC, OK
Sent from my iPhone
On Jul 19, 2019, at 17:00, Robert Nickels <ranickel at comcast.net> wrote:
Having been a SWL and first licensed in 1965 I remember that well, because AM was still commonly heard and the HT-37 was the SSB transmitter that . at sounded most AM-like. If we look at the schematic (found on BAMA) the audio section is not unlike most AM transmitters of that era (before hammy mods to replace all the coupling caps with .1uF for the "broadcast quality sound"). I haven't swept one but I'd guess the audio bandpass will be pretty flat up to 3 kHz which will preserve the highs needed for good intelligibility. It's considered good practice in a phasing rig to include a lowpass filter before the phase shift network to prevent splatter and the HT-37 has a simple pi section filter using a pair of .1uF caps and a 50mH inductor. I put them in Elsie and the response is flat to 3kHz and down 6dB at 10khz:
https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fi.imgur.com%2F2TX3UIS.png&data=02%7C01%7C%7C89f64e6de9eb4be5e0ca08d70c948e05%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636991704480768980&sdata=31URx8aXeMbQ%2FxOhk4ZIouDFmTAa7IAjjpCVHlEgeM0%3D&reserved=0
This audio bandpass means that pretty much all speech-range frequencies that get to the mic jack will be reproduced and not clipped by the skirt of a filter. And since audio response is a function of the design, the "sound" of a phasing rig will not change much over time, whereas carrier oscillator crystals to drift and shift frequency with age, and either aren't properly aligned or end up drifting out of range of the alignment trimmers or the filter crystals themselves shift frequency which results in some filter rigs sounding bad (the "soup can" filters in the early Drake transceivers are notorious for this).
It would be easy to use an external audio processor to change the sound of a phasing rig, but I prefer to plug in a big honking chrome dynamic mic and let people remark on the audio quality.
73 Bob W9RAN
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