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Realizing that hum apparently remains a problem for some users when listening on ac line powered radios in their homes, I resolved to continue looking into this a little further.
First, I turned down the input gain of the SSTRAN all the way and listened very carefully on my best portable battery powered radios. No hum was audible. Then I went around the house and listened, with no audio input, to a variety of ac line powered sets. One or two exhibited some hum (not a major amount). I picked the worst one (in another room) and cranked up the volume.
I tried disconnecting the audio input cables from my stereo system, which is currently feeding the SSTRAN. This made no change in the hum level. Next, I disconnected the ac transformer that ships with the SSTRAN and substituted a few different “wall-wart” dc power supplies, including a beefy IBM ThinkPad laptop supply. This also made no difference. I tried moving the SSTRAN antenna around– no change. I tried grounding the transmitter to various ground points in my living room where it is presently situated. Grounding it to the chassis of my hi-fi equipment did not affect the hum at all. The only thing that changed the hum level was connecting it to the outlet ground directly. This reduced the hum somewhat, but did not completely eliminate it.
Remembering my ac isolation transformer upstairs, sometimes used with my Knight Wireless Broadcaster to eliminate the shock hazard and allow the chassis to be safely grounded, I got that and tried plugging the stock SSTRAN ac transformer into it. The thinking there was that the isolation transformer would serve as a pretty effective RF choke (it certainly reduces line-conducted noise). This did not change the hum level. Then I tried grounding the case of the isolation transformer to the ac line and that didn’t do anything either.
Along the way, I happened to notice two other interesting things. Flipping the light switch on and off in the room where the ac radio was operating changed the hum level slightly. When the light is on, the hum level drops. The other thing I noticed was that when I pull the SSTRAN power supply connector out of the back of the transmitter, the hum drops immediately, before the carrier dies. This means one of two things: there is still a residual amount of hum coming from the power supply that for some reason is just not audible on a portable radio, or that the hum disappears when the transmitter is completely isolated from the ac line.
Based on these empirical experiments, I tend to think that powering the transmitter from batteries would largely eliminate hum on any type of receiver in the home. Of course, for most people this would not be practical, but it is a start. My next experiment will be to retrieve my lab dc power supply from work and see if there is any hum when the transmitter is operating from that. If there is no hum, that would indicate to me that I could look at modifying the SSTRAN power supply section to further improve hum rejection.
Hope these experiments are interesting to the readers. They are not yet finished; as they say, “please stay tuned”!
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