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Hi all,
A few months ago I put together an SSTRAN transmitter to use as a ‘personal radio station’ since I have a lot of old radios from the 1920’s to the late 1940’s and I enjoy listening to other source material on them (old radio broadcast recordings, etc.)
I got the transmitter working with the simple wire antenna included with the kit but there was always a hum, particularly predominant on quiet passages. To add to this, it would vary with changes in electrical loads throughout the house. As some things were turned on (even simple incandescent lights, no dimmers or other noise sources), the hum would get lower or higher – there was no obvious pattern to what might happen.
Hi all,
A few months ago I put together an SSTRAN transmitter to use as a ‘personal radio station’ since I have a lot of old radios from the 1920’s to the late 1940’s and I enjoy listening to other source material on them (old radio broadcast recordings, etc.)
I got the transmitter working with the simple wire antenna included with the kit but there was always a hum, particularly predominant on quiet passages. To add to this, it would vary with changes in electrical loads throughout the house. As some things were turned on (even simple incandescent lights, no dimmers or other noise sources), the hum would get lower or higher – there was no obvious pattern to what might happen.
I tried all possible combinations of internal grounding schemes, changing the ground jumpers in the kit, no ground, AC outlet ground, even tried a short ground rod outside a window, nothing seemed to help. I am at the point where I just gave up trying to get the signal quiet.
The next step is to set up a proper tuned outdoor antenna but before I invest the time and money in this, I would like to know if anyone has an idea as to a way to set up an indoor AM station for personal use only (max 60 foot radius of transmission) that would be totally hum-free. If you have any suggestions I would be most appreciative.
I would rather not have to put the transmitter outdoors and run the power and audio out there as would be required for part 15 operation. If there were a simple way to do this with some sort of counterpoise, dipole, or loop antenna I would be glad to know. By the way I am in a small one-story house with a full basement and the stereo system I am using as an audio source is approximately in the center of the house.
thanks in advance,
Dave
Total posts : 45366Is the hum still there when you remove the audio input to the xmtr?
Total posts : 45366Hello Dave,
I have a SSTRAN AMT-3000 and it is perfect, so don’t give up….yours will be also.
The usual cause for hum is what is called a ground loop. This translates to having more than one ground in the system. I am a bit surprised that you have a hum problem with only a wire antenna. There is the possibility that something is wrong with the power supply to your transmitter, or something is not right with the wiring on your board, but there are simpler things to try first before tearing into your unit. Try what was suggested about unplugging the audio and report back here.
Also, start with all front panel controls set to counter clockwise positions. Is the signal clean?
In my experience, if the “compression” control is set too high, you will get hum.
I think your problem is that the front panel controls are not set right. Try it with the “modulation” and “compression” pots set CCW and work only with the “gain” control. Let us know what you find.
I also have some antique radios and it is possible that your signal is just overloading the receivers. Their AGCs are not the best. Does your transmitter signal sound OK on a modern receiver?
If you try these tests and post what you find, I and others here can probably help you.
Neil
Total posts : 45366I also have a SSTRAN unit. Currently, my antenna is about 7ft of wire, stuck to the wall with push-pins. Not the best, but I can actually receive a good signal over 100ft away, using the radio in the truck. One of these days I’m going to buy another SSTRAN to use with an outside antenna.
As for hum, I had a problem with that, also. In my case, I had three culprits. The first was a UPS in my computer room. In reality, it wasn’t the UPS, it was the power cord. I changed the power cord and the hum disappeared. The old cord was thinner, so I suspect that either it didn’t have a ground wire, or the wire gauge was just too small. This was inducing hum into the transmitter, as the SSTRAN is situated about 5ft above the UPS.
The second case was with the old tube radio in the living room. This had a noticeable hum. Again, it was caused by a UPS. The radio wasn’t plugged into this UPS, but my TiVo was. I tried switching power cords on the UPS, but no luck. I finally switched the UPS with another brand to silence the hum.
The third case wasn’t a UPS, but a surge strip. After repairing another old tube radio to use in the computer room, I found it had a bad hum. I had just put all new caps in the radio, so I suspected it wasn’t the radio. When I plugged the radio directly into the wall outlet, instead of the surge strip, the hum stopped. I was surprised about this, as the surge strip was a Tripp-Lite (although an old one).
What this proves is that you need to suspect almost anything as being the source of your hum. As for the SSTRAN, I have never had a hum problem directly related to it.
You may also want to get one of those cheap AC line testers (Radio Shack, Home Depot, etc.) to make sure the outlets in your house are wired correctly. Even if the house is fairly new, it wouldn’t hurt to check. Besides, those testers are cheap enough.
One other thing to look at is the orientation of your antenna. If it is close to an AC line that is in the wall and especially if the antenna runs parallel to it, that will induce hum into the transmitter. I originally had that problem and moved the SSTRAN across the room.
As for ground loops, simply unplug the audio cable from the SSTRAN to see if the hum goes away. If that is the problem, Radio Shack used to carry isolation transformers. Jameco and other hobby electronics places also carry them. You’d need to solder on a jack and plug to use it as a pass-through for the PC sound card output.
Total posts : 45366-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
http://www.rhemaradio.org
Total posts : 45366It’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.
thanks again,
Dave
[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
http://www.rhemaradio.org
[/quote]
Total posts : 45366[quote=polaris34]I am at the point where I just gave up trying to get the signal quiet. … The next step is to set up a proper tuned outdoor antenna but before I invest the time and money in this, I would like to know if anyone has an idea as to a way to set up an indoor AM station for personal use only (max 60 foot radius of transmission) that would be totally hum-free. If you have any suggestions I would be most appreciative.[/quote] Dave – You might try lashing together a battery pack with enough power capability to operate your tx with no connection whatsoever to anything else except its short, wire antenna and the tx ground terminal connected to your separate ground stake (not the “AC ground”). If with no audio input connected you can’t produce a hum-free carrier that way, then probably you never will get hum-free performance in any other configuration, including with an outdoor antenna. But then you can confine your troubleshooting to the tx to find the source of the hum.
//
Total posts : 45366If you have a ground loop, normally you will hear a smooth 60 cycle or so hum. If you are coupling RF back into the rig, it usually sound like a buzz. Try this: put your hand near the counterpoise (ground wire), then around the antenna and notice if there is a detectable difference in the sound. Your audio ground (unbalanced) and your DC ground should (must be) tied together. Otherwise the RF will couple back into the modulator.
Second, get rid of the wall wart power supply, period. They possess NO filtering. And finally, use shielded wire on all wire pairs (audio, DC) into the transmitter, grounding only the source end of the shield.
And finally, there is little question in my mind that impedance match between the transmitter and the antenna is not correct. In your make of transmitter, that is why the manafacturer uses the swamping resistor in the output. And BTW, a balanced antenna won’t solve your problem; only make it worse.
Marshall Johnson, Sr.
Rhema Radio – The Word In Worship
http://www.rhemaradio.org
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