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Old 19th Aug 2019, 6:00 pm   #1
Mikey405
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Default Spitfire AM Transmitter Hum (Buzz)

Hi all.

I wonder whether this has already come up or not and whether I'm asking a years-old question. Anyway, here goes...

I recently purchased a Spitfire MK2 AM transmitter from eBay but I'm having terrible problems with hum on reception. I say "hum" although most of the time it's actually a loud "buzz" (louder than the input). The problem occurs using all inputs I've tried - but I'm currently using a standalone (battery operated) MP3 player.

I've tried different and various PSUs but none seem to make much difference (even the bench variable PSU does the same).

I've tried earthing the device to various radiator or sink pipes, earthing to the "earth" pin of the mains and not earthing at all - and still the same. Not earthing at all means that the buzzing is so loud that you can barely hear the music at all.

I've tried using an isolating transformer, earthed and not earthed and still no (or very little) difference.

Even if I use 15V worth of batteries and a battery operated MP3 player and no earth, there is still a loud hum all the time the aerial is inside the building.

I've tried different frequencies from one end to the other, and used the ATU to "tune" the loading but barely any difference.

The only way I've managed to get the setup working without any hum or buzz is by running the whole thing outside - the modulator running with 15v worth of batteries, using a battery operated MP3 player and hanging the aerial in a tree. If I run a PSU to either the MP3 player or the Spitfire device from an outside socket then the hum is back with a vengeance.

Any help would be much appreciated.

Thanks everyone.

Kind regards.

From Mike.
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Old 19th Aug 2019, 6:07 pm   #2
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Default Re: Spitfire Am Transmitter Hum (Buzz)

This could be something in the house,a psu or router etc.I get a heck of a noise on AM near mine plus my Belkin anti surge socket puts rubbish out.

Do not think the Spitfire is to blame.
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Old 19th Aug 2019, 6:24 pm   #3
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Default Re: Spitfire Am Transmitter Hum (Buzz)

I'm thinking that this could be a form of "Modulation Hum" where something-mains-powered-in-the-house has diodes in it and the RF from your TX is passing through them as they're switching, so impressing 50/100Hz hum on the RF which is subsequently re-radiated by the wiring and picked-up by your radio.

In the past I had a 3-way 13A adaptor that had been gutted and the insides replaced with a triangle of 0.05uf capacitors (Live-to-Earth, Neutral-to-Earth, Live to Neutral) that I'd plug in to various outlets to see if the problem reduced - if it did I'd check anything plugged into nearby outlets. Alas in these days of RCDs the increased earth-leakage this caused often resulted in a loud CLICK! from the distribution-panel followed by radio-silence.
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Old 19th Aug 2019, 7:07 pm   #4
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Default Re: Spitfire Am Transmitter Hum (Buzz)

Hi Hamish and G6Tanuki.

Many thanks for the comments.

Re Hamish's comment - The loud "buzzing" is only there when the Spitfire is on - There is a little bit of noise from various electrical devices around the place between stations but not too much to speak of - and I can always find a "quiet" part of the spectrum. If I move the whole setup (including the aerial) outside then everything is quiet - but as soon as there is a sniff of anything to do with mains powering the Spitfire or the MP3 player then the buzz is back. Other local stations come in on the radios without any buzzing noises too.

The "Modulation Hum" thing sounds promising - Although I have tried turning off every electronic device in the building (including fridges, freezers, computers, the 19" rack and all its bits-and-bobs) - in fact everything except one AM radio and this device running on its batteries - but still the problem remains. In fact there is almost no discernible difference at all between everything "on" and everything "off".

One thing I did notice is that if I have a mains lead lying near to the Spitfire then it seems to pick up a lot of hum from that, so I wonder if shielding the device in a metal tin might be the answer.

So, I think I might be resigned to using the device in the garden with perhaps a small 12V lead-acid battery, the aerial in a tree, a 1200mm ground-spike, and a Bluetooth receiver, to which I can transmit music etc. It's just such a shame as my SSTRAN AMT transmitter works perfectly with virtually no hum whatsoever.
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Old 19th Aug 2019, 7:49 pm   #5
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Default Re: Spitfire Am Transmitter Hum (Buzz)

Hi all again.

I think a possibility presents itself - I can run a piece of coax from the aerial socket of the Spitfire to a little distance outside the building and then hang the aerial from a tree and it's not too hummy. The Spitfire still needs to be earthed near the device - and even then any mains leads coming anywhere near it introduce a lot of hum in the modulation. But at least I have a workaround which I am happier with then running everything outside.

I wonder why this device is so sensitive to hum relative to the SSTRAN. Hmm.

Thanks all.

Kind regards.

From Mike.
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Old 19th Aug 2019, 8:10 pm   #6
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Default Re: Spitfire Am Transmitter Hum (Buzz)

Modulation hum is a phenomenon of mains operated receivers, and can be particularly noticeable on a local strong signal.
Hum modulated onto the carrier of your transmitter is not "modulation hum" in this context.

What you haven't told us, is whether or not you have tried a battery operated radio with your transmitter.

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Originally Posted by Mikey405 View Post
I wonder why this device is so sensitive to hum relative to the SSTRAN. Hmm.
One possible reason is that the Spitfire puts out a stronger signal than the SSTRAN. Modulation hum tends to exhibit itself on strong signals.
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Old 19th Aug 2019, 8:13 pm   #7
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Default Re: Spitfire Am Transmitter Hum (Buzz)

Hi Graham.

As much as I'm ashamed to say so - I don't have a single battery-operated AM radio. The few radios I do have are all mains operated.
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Old 19th Aug 2019, 8:23 pm   #8
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Default Re: Spitfire Am Transmitter Hum (Buzz)

This problem can arise when the transmitted signal is picked up by the house mains cables and then re-radiated with modulation from the mains. It can be very difficult to cure. Try to prevent the rf getting into the mains via the psu by all possible methods, incl ferrite, caps , earthing etc. The rf from the tx antenna is more difficult as the house wiring is all around ! Try putting the tx antenna in various places, in my home the landing gives the best results as the stairwell is free of mains. Use a modern battery powered radio to experiment with the tx side. Similarly, position the rx antenna as best you can. Good luck !

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Old 19th Aug 2019, 8:40 pm   #9
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Default Re: Spitfire Am Transmitter Hum (Buzz)

Hi Ken.

Thank you for the interesting advice. I will try a selection of those things tomorrow.

One thing I had considered is using a separate RF earth for the Spitfire by driving a separate 1200mm copper earth rod into the ground near the aerial and using coax from the Spitfire to both. (My radios are all currently connected to the same "earth" (water pipes) as the RF signal from the transmitter at the moment which may not be helping matters.) I think the RF earth on the Spitfire is the same as the signal earth so I'd still have to isolate the power and the audio signal.

Thanks Ken.

Kind regards.

From Mike.
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Old 19th Aug 2019, 8:46 pm   #10
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Default Re: Spitfire Am Transmitter Hum (Buzz)

This doesn’t help you as you’ve attached an earth but I had a similar problem and it was an earthing issue once I grounded the TXer it worked fine.

I assume when you say tried all inputs you’ve tried a different cable?

John
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Old 19th Aug 2019, 8:47 pm   #11
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Default Re: Spitfire AM Transmitter Hum (Buzz)

I don't have a solution, but I agree with others that these sorts of problems can be an absolute pig to sort out.
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Old 19th Aug 2019, 9:28 pm   #12
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Default Re: Spitfire AM Transmitter Hum (Buzz)

I had a very similar problem with an sstran transmitter which came on suddenly, presumably due to a fault with a neighbour's SMPS. It made it more or less unusable.

I greatly improved it by putting a class X capacitor inside a 13A plug across the live and neutral terminals. It might be worth trying that. Try a couple in various positions in the house. I also have similar interference on LW but the plugs do not help that for some reason.

I found the solution by chance when I switched on a lamp with a CFL bulb near the set. Here is a youtube video I made at the time https://www.youtube.com/watch?v=pAuHIr7xbn4
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Old 20th Aug 2019, 10:23 am   #13
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Default Re: Spitfire AM Transmitter Hum (Buzz)

Thank you for the replies John, Paul S. and Paul R.

Paul R. - Wow - That idea is excellent and it's made an enormous difference to the sets I'm using. I've made up a few plug tops with some suitable 0.1uF caps between Live and Neutral and positioned them around the building - and it's almost completely cured the hum - at least reduced it to a level which I'm quite happy with.

I think I might make sure I build in mains filter caps to radios which don't have them from now on.

Brilliant. Thanks Paul.

Kind regards.

From Mike.
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Old 20th Aug 2019, 10:37 am   #14
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Default Re: Spitfire Am Transmitter Hum (Buzz)

Quote:
One possible reason is that the Spitfire puts out a stronger signal than the SSTRAN. Modulation hum tends to exhibit itself on strong signals.
Very interesting to read that. I’d wondered if that was the case. Any idea how much stronger? I’d imagine they’re both FCC part 15 compliant, so 100mw, but possibly more of it getting to the antenna
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Old 20th Aug 2019, 11:51 am   #15
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Default Re: Spitfire AM Transmitter Hum (Buzz)

Not sure if you did this too but also try a cap in the Spitfires mains plug ? Glad you are getting there !

Ken
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Old 20th Aug 2019, 11:53 am   #16
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Default Re: Spitfire AM Transmitter Hum (Buzz)

When I first built and installed my SSTRAN it worked perfectly but the levels of hum and noise just got worse making it totally unusable.
I also find the same levels of hum and interference on the stronger broadcast stations such as BBC R5L and BBC R Wales.
I thinks its a kind of modulation hum from cheap and nasty SWPS's, cheap and nasty LED and CFL lamps and ageing poorly designed Plasma TV's all radiating back along the mains wiring affecting all or many surrounding houses. It a problem that has been allowed to get totally out of hand.

The interference levels on mains operated radios is very bad, not quite so bad on portable radios running on batteries. I find that the levels of noise and interference can be reduced to not so annoying levels by moving a portable radio running on batteries to different parts of the room.

Going back to the pantry transmitter problem I hope to start experimenting with the earth, it looks like a direct earth connection to the pipe work or mains earth is a bad idea. Using a counterpoise earth, leaving the earth wire of the Spitfire or SSTRAN just trailing across the room not connected, say around the skirting board, this slightly reduces there range but does seem reduce the hum and buzz interference by a useful amount.
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Old 20th Aug 2019, 12:51 pm   #17
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Default Re: Spitfire Am Transmitter Hum (Buzz)

Quote:
Originally Posted by rambo1152 View Post
Modulation hum is a phenomenon of mains operated receivers, ...
But it could also arise during transmission.

If the Spitfire has a dodgy earth, so that the RF earth path is partly through a diode which is also handling 50Hz, then the radiated field will be different according to whether that diode is conducting or non-conducting. So the emission will be modulated by 50Hz, even though the Spitfire itself is pushing out a steady CW.

Similarly, if the Spitfire aerial is a bit dodgy, so that there is capacitive coupling to some wiring with a diode in it, switching at 50Hz, then again the radiated field will be modulated at 50Hz.

However, reading the posts, you seem to have eliminated these.

As the hum is really strong, can you 'scope the Spitfire output to see if the generated signal is modulated? That should give a clue. At least with a severe fault, it should be that much easier to find!
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Old 20th Aug 2019, 7:36 pm   #18
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Default Re: Spitfire AM Transmitter Hum (Buzz)

Hi.

I've also found hum problems when experimenting with small AM transmitters and must try out the suppression cap trick in the mains plug for a number of locations throughout the bungalow.

I guess for safety reasons it would be a good idea to fit say a 1A fuse in the plug in the event the cap goes short circuit. Those polypropylene caps rated at 1000VDC seem a better bet than the normal class X types provided they will fit in the plug body.

I was also wondering if a bleed resistor of say 1M Ohm across the cap is needed to prevent getting a belt after removing the suppression plug and touching the live/neutral pins. Those blue Vishay metal glaze resistors (VR37 series for example) are often used in this application.

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Old 20th Aug 2019, 9:11 pm   #19
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Default Re: Spitfire AM Transmitter Hum (Buzz)

Quote:
Originally Posted by Philips210 View Post
Hi.

I was also wondering if a bleed resistor of say 1M Ohm across the cap is needed to prevent getting a belt after removing the suppression plug and touching the live/neutral pins. Those blue Vishay metal glaze resistors (VR37 series for example) are often used in this application.

Regards,
Symon
That is a very good point. I forgot to mention to be careful of the plug pins if you unplug it. They can give you a bit of a nip.
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Old 21st Aug 2019, 1:28 am   #20
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Default Re: Spitfire Am Transmitter Hum (Buzz)

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Originally Posted by G6Tanuki View Post
I'm thinking that this could be a form of "Modulation Hum".
Yes I have experienced that. Looking for sources of hum and using batteries to supply the modulator didn't remove the hum. Researching on the web, I came across modulation hum and it seemed a very good explanation. All the wiring in a house is carrying a high amplitude 50Hz ac waveform. It also acts as an aerial, receiving and re-radiating other transmissions. These get modulated with the mains frequency.

Of course, you shouldn't really be radiating anything with your modulator. Keep it all screened and you won't have a problem.
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