Home › Forums › temp › Hum-free, Indoor LPAM transmission – even possible? › Re: It doesn’t know the words…
Total posts : 45366
It’s been a while since I looked at the SSTRAN schematic so I don’t remember the details of it 100%, but I do seem to recall it having isolation coils on the power lines and also a jumper to separate the audio grounds from the power grounds. In any case, I tried every conceivable combination thereof. I also tried unplugging the audio inputs. It is definitely an RF problem, not a ground loop in the audio chain per se.
I believe the only time I came close to getting the hum to diminish significantly was when I had the black wire supplied with the kit connected to the ground of the RF output laying at a diagonal across the livingroom floor and then had the white antenna wire up in the air somewhere. Needless to say this was not an acceptable solution.
Basically I think the RF ground is picking up hum from the power lines in the house. As I said, the hum would vary oddly at times depending on what loads (not noisy loads like dimmers or flourescent lights, but *any* loads) were switched on and off. One time I also took the transmitter out to the sunporch where the AM recieving antenna is located, and I had the ground wire attached to the ground rod that sticks up outside one of the windows there and the hum was still present.
I was thinking that perhaps if I could use some sort of electrically balanced antenna, then the ground would no longer be an issue – like some sort of transmitting loop or a coil-loaded dipole? Any other ideas? I will do an outdoor setup if absolutley necessary but I would rather not, due to cabling issues, lightning, weather, etc.
[quote=kk7cw]-But seriously folks, hum can be RF from the wire antenna being coupled back into the DC input to the transmitter. A greater value capacitor to ground on the DC input can be a real lifesaver. Also shielded wire and toroid cores are good to have around to decouple audio and DC to the transmitter from a variety of sources, including computer equipment. Also, remember ground the shield only on one end of the wire pair. Other wise you are guaranteed to have a ground loop.
And remember, if it won’t stop humming you may have to teach it the words.
Marshall Johnson, Sr.
Rhema Radio – The Word In Worship