I have had hum problems with my FM signal before but these were resolved by moving the plug transformers ("wall warts") away from the transmitter but I recently encountered hum on my AM signal where repositioning the offending wall wart did not help.
First a bit of background information. My center of operation is in the basement (my cave) and includes the AM transmitting antenna mounted on the ceiling. Unfortunately, our city has experienced many auto break-ins and since I park my vehicle in the driveway it is vulnerable. So I found this nifty passive infared wireless motion detector which alerts me if there is motion in the driveway. It works well with no false alarms but we have family and a border coming and going day and night so the chime sounds frequently. I decided though I was in a cave I was not in a submarine and don't have to be blind so I installed a video camera to watch the driveway. This is really neat since I have a small video security monitor which I acquired at an auction for a few bucks years ago and I can leave it on all the time so when the motion detector chimes I can see who is in the drive.
To make this work I ran two lines, one for video and one for power, about 80 feet through the house to the outside. The power is sourced by a 12VDC 1A switcher in a wall wart. The video coax is grounded at the monitor so I figured that all would be well. When activated, the camera system produces a very strong hum in my AM signal audible even with the receiver outside and away from all electrical stuff. Repositioning the monitor and wall wart has no effect, even if they are connected to another power circuit.
I wasted a lot of time because I assumed that since the video coax was grounded at the monitor and the power negative and coax were connected together at the camera that all was proper but this assumption was wrong. The coax ground and the power negative are isolated at the camera according to an ohmmeter check. The solution was to ground the power supply negative to the monitor ground and the hum is completely gone. This camera system was apparently designed to have a floating DC power circuit perhaps to avoid a ground loop but I am surprised about the isolation at the camera since the wall wart is not grounded..
I know from my experience and that of other posters that hum can come from wall warts but usually it affects FM rather than AM so this was new territory for me. Maybe by sharing this experience I can help others who have a similar problem.
Neil
Just a guess, but the hum came from the camera in the first place. The video refresh rate on cameras like your likely using is around 60 Hertz. It sounds similar to AC hum, however if you look at the hum on an o-scope you will see something that looks like a saw tooth pattern. The isolated P/S in the camera is to eliminate the possibility of ground loop hum in the video. TV video signals are AM by their very nature. In TV, the picture is vestigial sideband (AM) and the audio is FM that are combined at the output of the transmitter and delivered to the antenna. Your setup has only the video circuits.
Ground loops usually show up as bars of video distortion that scan vertically across the screen of the video monitor. If no ground loop, that still doesn't mean there is not a link through the ground or neutral of the power supply; commonly known in TV as "common mode noise".
And that is very likely the gremlin you found. Great little article.
I've noticed hums appear on some radios but not others, when listening to micro-1670 here at the house. Plugging a power strip into the wall NOT TURNED ON added to the hum on a nearby radio. Turning on the bedroom overhead CFL lights puts hum on certain other radios. At different times of the day there are other hums or no hums.
God help us all.
Marshall,
I think you are on track with your analysis but I did more experimenting while tackling this which I did not report. I disconnected the power lead at the camera to kill the video and the hum was still there unchanged. Thus, I eliminated the baseband video as the cause.
As you stated they probably floated the power leads to prevent hum bars from a ground loop but I was surprised (and fooled) that they didn't connect the power circuit to the video coax common at one end which would not produce a loop. I don't like floating power or signal circuits and usually do a single point ground so I am happy with my solution.
Carl, I tried three different receivers at various locations and found the hum was not being induced into the receivers but was combining with the transmitter signal.
My best theory is that the DC line from the wall wart was acting as an antenna and was coupling the RF into the wall wart where nonlinearities were modulating the signal which was then re-radiated by the camera power line. What was surprising is the strength of this signal which could be heard almost as far away as the transmitter signal. Grounding the camera power lead apparently shorted the induced RF to ground.
Sometimes things get difficult but it is good to try to understand what is happening.
Neil
In my experience, hum is most often (not always) caused by a potential difference between AC grounds.
First, your wall wart IS grounded through the neutral. That's how they work, and it's also a reason why non-compliant unfiltered ones raise so much havoc with RF interference. And also why sometimes you can get rid of hum by simply flipping the plug.
Next, Marshall is right that since the camera creates and AM signal even wired, it can have properties which can cause hum ... and so can florescent lights. Both are pulsing at AC rates. E.g., TV signals are 30 fps dual interlaced ... takes a set of two to create the screen, coming from 60Hz. If both things are happening, I can see broadcasts becoming unlistenable.
In any building, you can have outlets which have disparity. In the real world, there is no such thing as a perfect ground. One circuit may be leaking a different amount of very low voltage to ground than another. Thus the chassis ground of, say, your transmitter, may be getting a potential across it from the ground of another device plugged into an outlet on a different circuit. If you connect the audio of one to the audio of the other, you can get some hum.
Even your body can generate enough difference to create hum.
On sound stages (anywhere, any kind), this can cause serious problems. Gear plugged into one wall of the building with audio connections to gear plugged into another wall can easily develop hum in the audio signal. Also, I've noticed the condition usually worsens when you try to connect consumer-based gear to professional gear. The best solution is to have a dedicated ground for all your audio gear, and let everything else be. Shy of that, try plugging everything you can into the best AC power supply you can get, which can cleanup, stabilize, and protect.
Even in my studio, I can sometimes hear hum and have to spend time tracing it down, and most of my gear is powered from a Monster Power Unit, heavily filtered, with very fast-reacting (nanosecond range) resettable internal breakers.
On my boat, where the TX is located, when I found hum from a portable radio in close proximity, I backed through the system. First, I tried plugging the TX and computer (which is the audio source) into a different circuit, but that circuit had corrosion and I had to pull it out. Next I pulled the power strip and plugged the units directly into the two-plug outlet. which killed the hum.
At that point, I was blaming the power strip, but even tho' tired and frustrated because the power strip is needed for other devices, I kept going. I plugged it back in and started pulling devices one at a time until the hum quit.
Turned out to be a cheapo DVD player. I'm guessing its internal power supply is creating the hum, because it's there even with the unit shut off ... like a cheap wall wart (poor or virtually non-existant filtering).
Other than that, my first Talking House TX had hum anywhere I tried it, even with its own dedicated PS. The only thing which finally got rid of it was a big heavy 18vdc HP Printer Power Supply brick. No more hum 😉 Other similar ones I've tried with my latest TH unit. including afore-mentioned dedicated one, all produce some hum ... so I'm on the hunt to get another HP Printer PS, exact same model. It's probably at least 10 years old, kinda hard to find, no joy yet.
I watch movies on that DVD player late night, and overnight I'm just running NOAA Weather from a dedicated scanner and the hum isn't noticeable. But sometimes I forget to unplug it, and on songs with quiet passages I can hear it ... so I have to traipse on down the dock, climb aboard and unplug the stupid thing.
I'm just speaking from experience here ... HTH ...
First, your wall wart IS grounded through the neutral.
No, it is not. This would create a dangerous condition since the blades are both the same and it can be plugged in with either connected to the hot. If there was a connection to the neutral to the supply output then this would be at line voltage when it is plugged in reversed.
Even polarized ones are not grounded through the neutral since if, as does happen, the AC outlet is wired reverse polarity the circuit ground would be at 120 Volts.
The wall warts with the third round plug pin may or may not have their output grounded to the power ground. I have seen both situations.
As you commented, reversing the plug can affect hum but this would be due to an unbalance to the blades at high frequencies.
Neil
don't most wall warts have the same blade size?
I know for nearly all of mine it doesn't matter which way you plug em in.
Most but not all of mine are not polarized. The ones that are polarized are always a pain as they're always turned the wrong way to fit on the power strip.
Only my Talking House and home alarm supplies are three pronged. I'm not counting things like printer, laptop and monitor inline supplies which are either three pronged or two blade polarized.
This thread seems to have piqued interest in wall warts which is fine since it gives an opportunity to pass along some information I have accumulated on the topic. This applies only to the simple warts and not to those which have switching supplies or regulators contained within.
I have taken apart many warts and here's what can be found. Usually there is a small transformer with the primary winding connected to the blades. Some have fuse links which look like a short length of solder wire and this may be in the primary or secondary circuit. This fusing serves both as an over current and over temperature safety. Once the fuse melts the wart is "dead" though a working transformer could be salvaged. The simple AC warts are just a transformer (maybe fusing) with the secondary connected directly to the output lead.
Some of the "DC" types include diodes which may be half wave or full wave and may or may not include a filter capacitor. Rarely they may contain a regulator IC such as a 78XX series device.
Most of the warts I have were designed to operate with a specific device and were included with the device. A common characteristic is that these have a high secondary resistance so the transformer size can be kept small. The voltage spec. on the case is the voltage output at the rated load current and this voltage will be substantially higher at a lower current. It is not unusual to measure 13 volts or so output from a 9 volt wart under no or light load. Conversely, a higher current output will give a lower voltage. Bottom line is if you use one other than for its intended purpose it is a good idea to check the voltage under load.
If you have a dead wart you might be interested in taking it apart just to see what's inside. This involves breaking the case so the wart will be trash afterward. I wouldn't recommend fixing one since safety may be compromised.
Neil
Neil:
Your contribution to wart intelligence is appreciated and will be printed for the power supply folder. You have described more than I previously imagined about the inner-doings of the plastic blocks.
Here's yet a more question, pardon my broken English, but the wires that come out of the warts are so thin they can either be ripped lose if caught by a passing foot, or maybe don't supply the full voltage or current because thin = not low impedance.
Maybe you should write a paper on how to upgrade these various kinds of wart, i.e., building in the proper grounding or improved features.
I was using a wall wart for an application
it was not designed for (although the
voltage and current were not critical.)
I had chopped the connector off of the end
of the cord. And I was connecting the bare
copper positive
and negative leads to the thing I was
powering (don't remember what it was.)
I accidentally shorted the leads together
(with the wall wart operating) and of
course it went dead because of
that internal current/thermal fuse.
And this is a good thing. I would rather have that
happen than have something bang or melt or
make a stinky smell or oooze all over the place.
Or all of the above. Also, I'm really glad that
this wall wart HAD a fuse.
Best Wishes
Bruce, MICRO1690/1700
Transmitter power supplies are extremely important to the quality of the transmitted signal on both AM and FM stations. A rule of thumb is, "The signal from the transmitter can be no cleaner that the DC power supply energizing it."
Dirty power, dirty signal. And not just the audio, the RF signal as well. RF bypassing and decoupling is very important to the integrity of the output audio and on-air received signal.
I don't use wall warts at all for any of the equipment in my radio station; 12 or 24 volt regulated and filtered DC power supplies with the lowest ripple rate at full load I can afford. Even the 48 volt phantom supply for the condenser microphones is regulated and ultra-filtered.
If you have to use wall warts, you can build external "ripple" filters with inductors and capacitors to clean up the DC output to the equipment.
The number one gremlin in commercial radio stations when trying to conduct successful engineering performance tests on the transmitter audio chain is power supplies. The better grade of power supply the better the radio signal.
I occasionally use wall warts for skeet shooting. They make a nice cloud of smoke when you them with bird shot. Pull!
Just the other day I confidently told a friend that the wire with a white stripe coming from a wal-wart is the positive DC wire.
But after saying it I lost confidence and started doubting myself. In electrical code the black wire is the "live" side and white is the neutral. That always gets me confused, compared to other color codes.
Question: which polarity is indicated by the white stripe in a wal-wart cable?
Best to dig out the meter when in doubt.
There appears to be no universal color standard for secondary power wiring. Yesterday, I wired a 12 Volt cigarette lighter female connector to a battery and found that the white stripe designated the positive lead. Sometimes, when both wires are black, the negative will have a molded rib or ribs but I have found this not to be reliable. Also the small coax connectors for power vary with the positive being either the pin connection or the shell. Follow MRAM 1500's advice and use the meter to check it.
Marshall raised a good point with his post above regarding the use of quality regulated supplies. Many times this is not necessary but other times it is a good idea. AM transmitters and audio amplifiers, for examples, require a "stiff" supply so the voltage doesn't change at an audio rate with the modulation which would cause distortion. Transmitters with internal filtering and regulation such as the SSTRAN work with the supplied warts which provide sufficient voltage to maintain regulation but others might not. This is something to check if audio distortion appears.
This check can be done by connecting a scope or voltmeter to the circuit positive rail and looking for changes with modulation.
In some of my ham radio escapades I have found regulated supplies which fail regulation in the presence of strong RF fields. This should not happen at part 15 power levels but it is possible.
Don't be discouraged by the possibilities raised in this thread. Just be aware that they may happen and it sometimes takes some technical detective work to track them down.
Neil