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Old July 8th, 2011, 01:36 AM   #1
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power supply interference with radio broadcast

This thread is 'All things audio' meaning audio with video...but I thought someone here may be able to help with a radio issue I have...

I've been running a couple of low powered radio stations for about ten years here in NZ. I use a pre-recorded program. It has gone well...only I have a hiccup...

Until recently I used to run my program from a DVD in a DVD player but wanted to go solid state to avoid mechanical breadkdowns, so have been running for a while from a 16gb Ipod.

However when the ipod driving one of my stations is running on a usb power adapter plugged into mains supply, it causes a lot of interference on some short wave bands within the house I'm transmitting from. I haven't heard it has caused any issue for neighbours.

Pull the usb power supply to the ipod and the interference goes away. Plug it back it and there it is.

I need to do something quick as the ipod won't run for ever on batteries!

Does anyone have any ideas as to how I could power the ipod without getting the interference?
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Old July 8th, 2011, 02:21 AM   #2
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Re: power supply interference with radio broadcast

How long does it need to run for? One of these http://www.amazon.co.uk/Kensington-Rechargeable-Portable-Battery-Pack/dp/B000VG8GEA/ref=pd_sxp_f_pt/280-0170939-4422226 fully charged, plugged in to an iPod will keep it going for quite a while.

I bought one for my iPhone 3 because the battery life was dismal, and bought a second when I thought I'd lost it. The price has come down geatly, and the kit includes its own charger which might be cleaner on the RF interference front than the one you are using.

Or you could just try the Heath Robinson trick of covering the iPod, its charger and as much of the connecting leads as possible with aluminium foil and grounding it :-)
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Old July 8th, 2011, 04:59 AM   #3
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Re: power supply interference with radio broadcast

I often do theatre shows and have found that Mac laptop computers almost always give problems when running on mains, where as a windows based one doesn't... why I don't know.

My suggestions might start with the aluminum foil around the power supply, or use an isolation transformer on the audio connection... or try 240v - 240v (or 110 - 110v if applicable) isolation transformer on the mains side.
Is the mains power on the same circuit for both player unit and transmitter? If not this might minimise problems.
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Old July 8th, 2011, 06:58 AM   #4
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Re: power supply interference with radio broadcast

Quote:
Originally Posted by Colin McDonald View Post
How long does it need to run for? One of these Kensington Rechargeable Portable Battery Pack with USB: Amazon.co.uk: Computers & Accessories fully charged, plugged in to an iPod will keep it going for quite a while...
It needs to run 24 / 7 / 52...unattended...
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Old July 8th, 2011, 07:02 AM   #5
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Re: power supply interference with radio broadcast

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Originally Posted by Brian P. Reynolds View Post
I often do theatre shows and have found that Mac laptop computers almost always give problems when running on mains, where as a windows based one doesn't... why I don't know.

My suggestions might start with the aluminum foil around the power supply, or use an isolation transformer on the audio connection... or try 240v - 240v (or 110 - 110v if applicable) isolation transformer on the mains side.
Is the mains power on the same circuit for both player unit and transmitter? If not this might minimise problems.
Both transmitter and ipod are on the same circuit...

I have a UPS. Would that help? I haven't tried it yet.
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Old July 8th, 2011, 07:41 AM   #6
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Re: power supply interference with radio broadcast

Have you tried different mains-to-USB-power-adapters? There are lots and lots of them on the market, as well as USB-to-iPod cables from various manufacturers.

I'm not sure it would help, but a stereo isolation transformer like an Ebtech Hum Eliminator between the iPod and your audio system could be a good idea.
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Old July 8th, 2011, 09:14 AM   #7
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Re: power supply interference with radio broadcast

Not enough details/examples to narrow down the possible cause. It sounds like it might be related to the switch-mode power supply (SMPS) which operates at near-RF frequencies. Alas, the old-technology linear power supplies (with a heavy iron transformer operating at mains frequency, 50Hz) are all being replaced by more modern (and inexpensive) SMPS.

You could find an old-fashioned linear 5V wall-wart (they tend to be significantly heavier and larger) and wire it into the proper cable for your iPod.

Or you could try some ferrite devices that are used to filter out this kind of noise. You see some USB cables that have big lumps around one or both ends of the cable. And electronics vendors sell ferrite in various shapes that you can clip around a cable or thread a cable through. I have seen some extreme cases where a large doughnut-shape ferrite torroid had the offending cable looped through several times to fully attenuate the offending noise.

ferrite core - Google Search
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Old July 8th, 2011, 09:39 PM   #8
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Re: power supply interference with radio broadcast

I was going to say exactly the same thing Richard said.

Plug in the power adapter, without the iPhud connected to it, and see if the adapter itself is the source... I would bet it is. Then, yes, find an old linear regulated adapter and that should cause no problems.
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Old July 10th, 2011, 12:54 AM   #9
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Re: power supply interference with radio broadcast

Thanks all...

I've now tried magnets on the cable and wrapping the Usb power supply in many layers of aluminium foil but there was no change.

With the Ipod running on batteries - unconnected to the power supply but with the power supply plugged in and the power on, there is no interference. As soon as the ipod is connected to the power supply the interference starts...

I am going to try a computer PS directly powering a usb plug and see if that makes a difference.
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Old July 10th, 2011, 01:30 AM   #10
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Re: power supply interference with radio broadcast

Aluminum foil is a joke for this kind of problem.
Nobody suggested "magnets" because that won't work either.
Since you have not tried ferrite inductors, we don't know if that is the solution or not.
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Old July 10th, 2011, 02:18 AM   #11
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Re: power supply interference with radio broadcast

'Aluminum foil is a joke for this kind of problem.
Nobody suggested "magnets" because that won't work either.
Since you have not tried ferrite inductors, we don't know if that is the solution or not. '

Opp's...not magnets - ferrite inductors from a reputable electronics store...
Re Aluminium foil...I'm just trying what some have suggested. It certainly didn't do anything...and neither did the ferrite inductors...
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Old July 10th, 2011, 02:39 AM   #12
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Re: power supply interference with radio broadcast

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Originally Posted by Colin McDonald View Post
Or you could just try the Heath Robinson trick of covering the iPod, its charger and as much of the connecting leads as possible with aluminium foil and grounding it :-)
Yes, I suggested that but the :-) at the end is the giveaway - it was a joke, but not entirely irrelevant. The earthed/grounded foil would form a "Faraday cage" - but only if it enclosed the entire system, which it wouldn't because of the mains and audio wiring sticking out. The theory of the Faraday Cage does work when correctly designed as the foil screening fitted inside many electronic iappliances will testify.

Sorry if I have misled anyone in this discussion. Surely it can't be that hard to find an RF clean power supply?
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Old July 10th, 2011, 09:26 AM   #13
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Re: power supply interference with radio broadcast

You say you're going to try a computer power supply, wired to a USB connector.

Be aware that almost all computer power supplies (with the exception of a few 286-era supplies that I can recall) are switching supplies. They, too, will most likely generate RF interference to some degree. True, they may be a bit cleaner than the no-name "wall wart" that caused your original problem, but they won't be 100% clean.

If you are really interested in an RF-free solution, you really need to get a supply with a linear regulator inside, nothing with high-frequency switched mode regulation (as was suggested earlier).
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Old July 10th, 2011, 01:52 PM   #14
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Re: power supply interference with radio broadcast

'If you are really interested in an RF-free solution, you really need to get a supply with a linear regulator inside, nothing with high-frequency switched mode regulation (as was suggested earlier).'

Thanks Greg. I certainly want to beat this. Do you have any suggestions as to a power supply that you think may be suitable?
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Old July 10th, 2011, 03:02 PM   #15
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Re: power supply interference with radio broadcast

Hi Renton,

As an amateur radio operator, and a sometimes FM-broadcast DXer, I am certainly aware of RF noise issues. Even the dreaded "CFL" or Compact Fluorescent Lamp puts out a lot of RF noise! The world is getting noisier and noisier.

I'm sorry to tell you that I can't suggest a specific make or model supply, especially one that is easily available to you.

But here's the deal. When we say a "linear regulator" what we mean is this. You start with a mains step-down transformer running at your mains frequency (50 or 60 Hz as may be), and step down the voltage to something in the right ballpark... perhaps around 6 or 7 volts. You rectify the output, and filter that... the result will be around 9 or 10 volts DC. Then you run that through an inefficient old-time regulator with a zener diode and some transistors to get your desired 5 volts. Wasteful of power... runs rather warm.

The newer "switching" supplies directly rectify the mains voltage and filter it (here in the US, 125VAC gives you roughly 175VDC). Then you switch that high-voltage DC on and off at a very high frequency. The resulting square wave goes through the step-down transformer and is rectified and filtered again to produce the desired 5 volts. More efficient... usually runs cooler. Higher frequency means you need less iron in the transformer, and less capacitance in the filter caps. That means smaller size and lower price... that's why switchers are popular.

The RF noise problem is caused by the high frequency square waves which can put out harmonics all over the RF spectrum. Very hard to control unless the entire circuit is inside a grounded metal case, the wiring goes through a ferrite choke immediately upon leaving said case, etc. etc.

Switched mode supplies need much less iron in the transformer. So when shopping for a new "wall wart," heft it in your hand. If it feels as light as your old, noisy one, it is probably another "switcher." If it's bigger and heavier, that probably means the transformer has a lot more iron (to operate at mains frequency) so it is not a switcher.

Another thing... switched mode supplies typically will accept a very wide range of input mains voltages, perhaps even 100 - 240 VAC. Linear regulators will probably be a narrower range, perhaps 110 - 125 VAC or something similar. Switched mode supplies most likely accept 50-60 Hz mains input. Some linear supplies are also dual-frequency, but some are single-frequency. If you find a supply that is narrow voltage range and only one frequency, it's almost surely a linear regulator.

If you're buying from a really legitimate, and somewhat commercial vendor, their product description might even specifically tell you whether or not it's a switcher. If you're buying from the neighborhood battery shop or music store, they may not know what they're selling.

I also note that switchers are becoming more and more common; older linear regulated supplies are becoming "old hat."

AND REMEMBER: There are also unregulated supplies out there. Those will probably blow up your iThing. So be 100% certain the supply you buy is regulated!

Of course you could build one yourself, if you're a tinkerer. ;)

Good luck, and let me know how you make out.
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