If you have totally isolated the transmitter, then I have to wonder if something else in your home is picking up the RF and rectifying it. Most DC supplies, like "wall-warts" and those inside of small appliances do not have bypass capacitors across the diodes, as the ones in your SSTran do. This is one place where rectification can occur.
It appears that as you get further away from the antenna, the re-radiated signal picked up on your house wiring starts to dominate over the direct signal from the antenna. Then, you start hearing the hum.
BTW I found out that if you leave jumper S1 in due to the need to control hum from your audio source (as explained above), that will still allow a significant amount of RF to be imposed on the ac line through your audio gear. This of course pertains to normal operation, i.e. when you have something connected to the audio input. To eliminate this RF path, the easiest way I can think of would be to obtain a fairly large ferrite toroid of #77 material and wrap the audio cable around it for as many turns as will fit. This would serve as a common mode choke for the RF, but would have no effect on the audio signal.
WEAK-AM
Classical Music and More!
The other thing that bugs me is that I am about 7-8 miles from a 50kW AM transmitter and do not experience hum while tuned to this station. I don' think that the 100mW transmitter can generate more energy, as received by my house wiring, than what I am receiving from the flame thrower, so why don't I get hum when tuned to the flame thrower?
Here is what I have found so far:
- Used batteries to power transmitter and carried a small portable around to listen for hum.
- Unable to localize the source(s).
- Shut off my Main circuit breaker (then shut off each circuit breaker)
- The hum disappeared.
- Turned Main back on and started flipping switches, one at a time, until the hum cam back. I found that three switches resulted in a hum.
- Chose one switch to try to track source. I found an AC adapter used to charge my cell phone as the source of the noise in this circuit. Two circuits left.
- After a bit more investigative work sniffing around with my little AM portable I localized the next source to our refrigerator. Looks like I'm going to have to get an AC line conditioner for the fridge.
- The last circuit is a doozy. There are multiple dimmer switches on this circuit and it will take me some time to track down the culprit(s).
In any case, it's sure nice to hear what the actual transmission quality is like!
I don't think most people have the patience to try all of that! Luckily, most people don't seem to have serious problems with hum.
Despite the fact that your transmitter is low power, it can put a sizable amount of RF on the ac line in your home. In some cases, quite a bit more than the 50 kW station some distance away.
Today while driving to the store, I compared the signal from my SSTran in the living room to that from my Rangemaster out on the garage roof. Driving down my street, the SSTran put out a surprisingly strong signal all the way to the end of the block-- stronger than the Rangemaster in some spots-- but as soon as I turned the corner, it died out. All of the signal was coming from the power line! And this, despite the fact that I am not intentionally driving it with RF.
The Rangemaster, which is on a separate ground of its own and is mounted out on the garage, is well isolated from the ac line. It goes a lot further than my living room SSTran setup, but it does not benefit from the antenna provided by the power line that goes down my street.
Instead of putting line conditioners on all of your offending appliances, what about putting one on your transmitter? If you can do a really good job of isolating the transmitter from the line, it might not bother all of your susceptible appliances. Those little chargers can really cause problems!
WEAK-AM
Classical Music and More!
Interesting comments, WEAK_AM. However, since my transmitter was tested while running on batteries and totally isolated from ground and the AC line, I am thinking that it is all power coupling from the antenna into the wires in my house. That is why I am so dogged in my determination to figure this out.
Well, a little more investigative work and I found the final offending hum-producing device. The third, complicated circuit is also used to power some outside motion sensing lights in the front of the house. Fortunately, I have these on their own power switch so I can shut them off when listening to my transmitter. So, it still looks like the only device I will need to spend any money on is the fridge.
I hope this "investigation" helps others who are struggling with hum in their systems. The offenders can come from just about anything connected to the power line in your house. Also, I appreciate the patience of my wife through all of this. Obviously, she must be more patient than I to deal with powered down circuits and me walking all over the house with a noisemaker!
Hi all -
I just wanted to share some info on how I have solved the big HUM problems I had with the SSTran 3000. The things I did to get rid of the problem:
1. I use the trasmitter with an internet radio, so that I can hear "old sound" in my tube radios anywhere in the house. Now, most of the problem was not coming from the SSTran itself but from the power supply of the internet radio. When I welded 10nF capacitors in parallel with the diodes of the power supply most of the hum was gone.
2. Some additional switched power supplies throughout the house where also causing problem, even though they where not in the vinicity of the transmitter. So I got rid of them too. The power supplies of mobile phones are terrible, and if used close to the transmitter will cause a huge modulation hum. So, when I have to charge the phones I do it far away from the transmitter.
3. I operate towards the end of the scale, and I got better results by changing the antenna-coupling 180 micro-H inductor by another one with higher Q. Because I was operating without the 820 ohms resistor, I got a little bit of distortion on the signal after going for the new choke. I got good signal again by using a 2.2k-ohm resistor. With these changes I got better signal/noise, making any remaining hum almost imperceptible. After these changes, the level of signal in the antenna improved from 5-6 volts to 7.2 volts.
4. Another thing I did was to use a fixed telescopic antenna, fitted just oer the transmitter so that the size and position is always fixed and does not require frequent readjustment.
kind regards,
AI
AI--
Your observations are completely in line with what I have observed! You have done an excellent job of troubleshooting and correcting the problems.
If you have a good radio, you will really appreciate the sound quality that is available with the AMT3000 transmitter. I do!
My hum problem was largely dependent on how I oriented the receiver antenna. And, it seemed the farther away from the transmitter, the harder it was to "null" out the hum. I use the SSTran AMT3000 with an inexpensive MP3 player. This is one of those USB stick types that runs on a single AAA battery. I had the player "Jerry Rigged" to a switching 5V wall-wart using two forward biased series diodes as a 1.5 volt zener (with a dropping resistor of course). After reading the discussion here, I became suspicious of the wall-wart. I substituted an analog 5V power supply from an old computer project, and the hum totally disappeared! The MP3 audio was clean on the wall-wart, so I assume this was some kind of RF interaction on the AC power lines. Does anyone have a good explanation of how the hum is getting into the RF signal?
Bob D.
Bob,
Most AM hum problems are caused by indoor installations. The AC line becomes part of the ground system for the transmitter; hence a significant amount of the signal is superimposed on the AC line. When this RF energy enters a power supply, it will be mixed (or "detected") by the rectifier diodes unless they are carefully bypassed. The hum-modulated version will then be conducted along the AC line and picked up by nearby radios.
The best way to prevent this is to mount the transmitter outdoors away from the building, over its own completely independent RF ground system, and isolate any cables coming back inside. The amount of RF current induced into the AC wiring is typically too small to cause problems in this case.