Hum problem with SSTran AMT3000

Here are some suggestions which you might try.

First, disconnect the audio source. If the hum goes away then you have a ground loop. Try disconnecting the "good eight foot ground" and see if this helps. It is a good idea to have a single point ground, most likely at the audio source (computer, etc.). The jumpers you removed isolate the power feed and the audio ground at RF but not at audio frequencies, so though this might reduce hum (it did here) it is not a cure for a ground loop. Also check on the positioning of the audio cables. Make sure they are not near power wiring or transformers. This caused hum here. Make sure your antenna is in the clear also.

If removing the audio connectors does not help, then it s possible that the hum is coming from something other than the transmitter. Is there hum when you listen to other stations? If so, then check for light dimmers or other things which can cause hum.

Try increasing the audio signal level from your source and turning down the transmitter "gain" control. Mine hums a lot if this control is turned up too far.

Finally, it is possible that there is a problem with assembly or a bad component. Check escpecially the rectifier diode directions, voltage regulator installation, and the filter capacitor on the SSTRAN board.

Without knowing your installation details it is hard to pin this down any more than with my suggestions above. It is a real treat when the hum is gone and I am sure you can make this happen. Just takes some detective work. I was able, by doing the above, especially adjusting the "gain" control and single point grounding to get mine so quiet that I often forget to turn off my battery portable when the audio program ends because there is no noise at all to remind me it is on.

Neil

I have established that it is not the audio source. I can induce a different kind of hum when I turn up the gain or compression too high, but get relative quiet when operated at proper levels. As a precaution I have already installed a RS ground loop eliminator in the audio line. My antenna wires go through a plastic cable pass-through tube in the wall to the outside. The white (hot) wire then goes vertical up the side of the house made of painted cinder block. Once outside the black (ground) wire is clamped to an 8 foot commercial grade ground stake. In the process of passing through the wall the antenna wires do pass within a 1/2 foot of a wall AC outlet in which the transmitter power supply is plugged in.

I don't have any problems listening to other AM stations in the house. Just mine. I have noticed that the hum is much worst on some frequencies than others. It was unlistenable at 1360, for example. I normally run it at 1620 and the hum there is still present, just not quite as loud. Could the RF be getting into my house wiring somehow and causing this problem? If I take a battery powered radio into the yard it is pretty clean. I can still move the radio orientation around and get some hum, but not like in the house. Same story in my car. The biggest problem is with the radios in my house that are plugged in. As a matter of fact when listening on a plugged in radio it seems that the further from the transmitter that you get, the worse the hum is. The exception is very close to the transmitter. I have the RS version of the SuperRadio plugged into the same outlet as the transmitter power supply and have no problem. That radio is about 4 feet from the SSTran. 30 feet away in my living room on a Sangean HD receiver every AM station is clean except mine. 50 feet away in the master bedroom the buzz is the worst on both the clock radio and my old RS Long Range AM portable that they made in the early 1980s. Even a battery powered radio will hum in the house if it is not positioned into the perfect orientation.

The fact that I can get a reasonably clean signal on a battery powered radio that is properly oriented leads me to believe that the transmitter is not suffering from a bad component. The whole situation reeks of RF somehow getting into the house wiring or the plumbing and with the over the air signal strength weakening with distance from the transmitter, the hum takes over. But since I am not an engineer, I have no idea what to check or look at to prove this theory or how to combat it.

I have been fighting this battle for weeks and am just about ready to give up. Again, any help from the engineers and other smart people on this board would be most appreciated.

Deano

My antenna wires go through a plastic cable pass-through tube in the wall to the outside. The white (hot) wire then goes vertical up the side of the house made of painted cinder block. Once outside the black (ground) wire is clamped to an 8 foot commercial grade ground stake.

What exactly are you using for an antenna? If you are just using a length of wire without a loading coil to compensate for its reactance, then it will radiate very little.

It may be that signal is so weak that it cannot overcome the AM noise present in many homes and neighborhoods these days from light dimmers and other sources. In which case the fix is to improve the impedance match between the transmit antenna and the transmitter.
//

The antenna that came with the SSTran kit consists of two pieces of wire soldered to an RCA connector. The white wire goes to the center pin and the black wire is the ground. The SSTran has a built-in PI network. By adjusting dip switches and a ceramic pot allows you peak the voltage to best "match" the antenna and transmitter. Without a workbench of equipment, I just tuned up at night. When the incoming skywave chatter vanished to silence I assumed that I was peaking the antenna as best as I could. Would a loading coil improve the match?

It may not be putting out much smoke but something is radiating because with a battery powered radio you can listen to the signal anywhere in the house loud and clear and for several hundred feet up the street before, like passing through a curtain, the signal just fades into the noise of the band. A very weak signal can be identified (but not listen to) for about a 1/4 of a mile.

Deano

Deano,

I have an SSTRAN, and it works quite well for me! It is hard to guess at exactly what might be causing your problem, but let me suggest a few more things to try.

Operating a transmitter with an indoor antenna is always challenging! One possibility would be to mount the transmitter outside and ground it to a ground stake, like some users do who want to use the unit for micro-broadcasting with an external loading coil. You don't need the loading coil unless you are looking to make the unit cover a larger distance. You should pick up a small multimeter to help you tune the output. These meters can often be found for under $10 at hardware stores and home centers. It's worth having one.

Putting the transmitter outside may not be your idea of a convenient setup, so here are some more thoughts:

* Move the antenna as far away as you can from other appliances, wiring, and metal objects.
* Try connecting the ground wire to a different ground. Try leaving it disconnected and just short the jumper inside the unit to tie it back to the audio system ground.
* Try powering the transmitter from batteries and see if the hum goes away (yes, it will run on DC).
* Try moving the transmitter to another part of the house.

My final suggestion may require a little detective work. Many small appliances use "wall wart" or internal power supplies that can generate hum by rectifying RF present on the AC line. Don't assume that something is "off" because the power switch is "off"! Unplug it! This includes cordless phones, games, audio equipment, radios, and TV sets. You can try unplugging these one at a time (or all at once) and see if this makes any difference in the hum level. Don't ignore the ones on the other end of the house (or even out in the garage, including the door opener), because AM has a long wavelength and the problem could be occurring a long way from where you are experiencing it, relatively speaking.

Finally, the ac-powered radio you are listening to can be causing the problem, because its own power supply might be rectifying the RF on the AC line and causing hum. The two best ways to remedy this would either be to move the transmitter outside the house and isolate its power supply from the AC line using an ac line filter, or alternatively to install a line filter on the radio. Keep in mind that ANY radio or other electronic device plugged in the wall could rectify the signal and cause hum, potentially EVEN IF it's "turned off"!

Please let us know how things turn out!

WEAK-AM
Classical Music and More!

LOL,I had hell with hum here as well. Step1,power the SStrans from a battery. It needs 18 volts and works fine with 24 volts. If clean,replace your wall wart.
This was the cause of my major hum!@*#%$~!
I use TWO 12v DC(not AC) wall warts in series and mine never sounded better! Step2.If it still hums,go to your fuse or circuit breaker panel and turn them off one at a time to see if you can isolate something in the house. Check the ground continity of all your outlets in the house,you can use a cheapie checker like the ones found in hardware stores. I found several bad grounds on outlets. Check to see that the grounds from your service panel,cable,gas,water and phone are in tact and not broken,blown off or corroded. Also check the ground leads on any telephone poles in the area. I found many bad ones this way. Step3. Look for a bad flouescent light,tube type and the new screw in replcements can cause grief. Step4. Make sure your audio/power wiring does not run past 120VAC power wiring,near transformer type power supplies or computer CRT's or TV sets which can induce hum. Biggest hum producer is placing a power supply on the desk,the flux lines of force extend outward even through the enclosure for up to a foot! If that doesnt cure it,there is always Harey-Carey.....
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm

Deano,

In my setup, I have the SSTRAN plugged into a multiple outlet adapter that has a built-in ac line noise filter in it. This filter is designed to reduce line-conducted noise coming into a hi-fi system, and it also has a surge protector built in.

The effect of this on the SSTRAN is that it probably significantly attenuates any RF energy coming from the SSTRAN power supply that might otherwise be conducted back onto the ac line. Thus, it is probably is somewhat the equivalent of powering the transmitter from batteries because that would isolate the transmitter from the line also, which is actually the reason I suggested you try that.

I don't think there is anything wrong with your ac adapter from SSTRAN. If there was, you'd hear hum no matter what you did, not just on some radios and in some locations. That is strictly an ac transformer; there are no diodes in it. The diodes are on the transmitter board.

WEAK-AM
Classical Music and More!

It is NOT the same as a battery. A battery is DC and is isolated form your mains! The mere fact that you are sending AC along the same path,maybe even the same cable,to the SStran can conduct its own AC or any conducted AC into the transmitter or even the radiated signal! A ground loop is also a big hum recieving antenna! Believe me when I say feeding it with pure DC is the ideal way to go. Try this as an experimental way to find stray hum fields,probe around with a DYNAMIC mic connected to an amp and sniff around your cables,wall warts,power supplies,ect. you will be suprised what you find! I have no trace of hum,even with the studio silent and crank up the volume,I cant find any hum at all. It took a lot of rearranging and cleanup to get it clean,knowing where to look...priceless!
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm

But mine works fine with the supplied ac adapter, and I didn't need to resort to using a dc power supply to eliminate hum. In my setup, hum is avoided by preventing RF from feeding back into the mains through the power supply by plugging the transmitter into a line filter.

I am very persnickety about hum, because my programming is classical music. I use very little compression and any hum or noise would be clearly audible. I monitor on a GE Superadio III in wideband mode. The sound quality is very good.

WEAK-AM
Classical Music and More!

Im glad it works for you,the supplied wall wart that came with my SStran and the replacement he sent both died witin a few days of plugging them in and the hum was bad. There was a marked improvement when I switched it to 24VDC,better range,sound and no hum at all. I,too have been very picky about how my station sounds. I do not use the SStrans compression at all,its done in the studio chain. My station sounds as good or better than the local flamethrowers,IMHO,lol...Its just interesting how well it cured the hum problems.
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm

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"!

WEAK-AM
Classical Music and More!

Taking the advice from people on this forum, I went to the breaker panel and shut all the circuits off except the one powering the SSTran. Even with ALL other breakers off I had a hum issue. The only thing energized in the room was the transmitter. So, somehow the SSTran is getting RF into the AC lines and causing issues. Like WEAK-AM, if I unplug the SSTran, for that brief moment before the voltage falls off the transmitter is clean and hum-free. As I brought each breaker back on-line I did notice an increase in the hum level each time another fluorescent screw-in bulb was energized. It was also weird that on some circuits when the breaker was reset, the hum came up considerably. But if reset again would yield different results. I have not managed to figure out the variable in each circuit that was causing it to act that way.

So the experiment did yield some results. I know that the SSTran is somehow interacting with the AC wiring even when all circuits other than the one powering the transmitter are turned off. Fluorescent screw-in bulbs do add to the hum level and some circuits would produce varying results each time the breaker was thrown and reset.

Any more ideas? Your help is most appreciated!

Deano
Sun Country 99.5
www.suncountry995.com

I recommend you try to reduce the RF being fed back into the power line as much as possible. Remove jumpers S2 and S3 inside the SSTran, which will insert two chokes in the line to the power supply. Do not remove S1. Ground the transmitter to the best ground you have available; either a ground rod or your cold water pipe.

You can also try powering the transmitter from batteries, which will totally isolate it from the ac line. You will need to provide 20 Vdc or more at 50 mA. If this does not remove your hum problem, it is in the radio's power supply.

One more thing-- this may seem rather obvious, but if the radio has an external loop antenna, try moving it around to see if the hum level decreases in another position. If the radio only has an internal antenna, try rotating it or moving it to another spot to see if that affects the hum. It could be picking up hum from some other nearby appliance.

Nothing I tried completely got rid of the hum while listening on my new Cambridge SoundWorks 735i-- until I picked up the AM loop antenna and tried repositioning it. Sure enough, the small amount of remaining hum pretty much disappeared.

Moral of the story-- don't overlook the obvious!

Also see my blog for more info.

WEAK-AM
Classical Music and More!

I am also having a similar 120Hz hum problem. However, I have my matched AM antenna inside. I notice that the hum recedes and is nearly eliminated when the radio is near the antenna and gets worse farther away. I have used a battery to power the transmitter, with the audio input disconnected, to see what happens with a totally isolated system. I still get the hum. I have not yet attempted to shut down circuits, so I will see if I can isolate the problem to a particular circuit.