Remove 1170 KHz AM radio station from microphones

Interesting problem.

have you tried different types of mic?
A cheap moving coil mic may be better.

Del

Hi Del the cat,

Thanks for the reply. I am willing to try a moving coil microphone if it can be used with the same wiring and works outdoors. Do you know of a product that meets these requirements? My current microphones are:

"The Verifact Model E is an omni-directional, low impedance, electret condenser microphone with built-in preamp that is housed inside a single-gang weather resistant Bell Box. Design for outdoor applications."

In my opinion, you have given a good answer and this should help with fixing the problems seen.

Many radio amateurs have found that passing cables through ferrite rings can eliminate interference from RF sources. Many electronics and radio hobby stores carry these itmes as stock, and they are inexpensive therefore worthy of a first-stage experiment.

Hi PWSlack,

Thanks for mentioning the ferrite rings. We did use those at the beginning and it didn't help. I'll find more ferrite rings and test it again.

Best results will be obtained by inserting the ferrite rings as close to the amplifier or mixer input connection as possible.

These need to be in an impossible place, that is between the Mike and its pre-amp....not an easy place to get to usually.....

Most of the interference suppression ferrites are of the split clip-on type.

So its not a problem and certainly not impossible.

It really depends on the design as to how difficult they will be to place, some designs have no space left.

I tend to suggest that a suitable coil mike might be better.....testing on one link should be done - the longest cable run of course!

GA and my thoughts entirely.

Stagg MD 500 dynamic microphone £15

There are cheap dynamic mics for karaoke machines £10 or so... they give little tech info...but could be a good try.
The previous Digikey link turns out to be a tiny mic insert for hearing aid type applications...but it might be just what you need.

Del

I've three suggestions:

1. Try putting your microphone cables inside metal conduits. Make sure you have one continuous conduit connection and ground it at one end. Hopefully, this will prevent the radio signals from getting into your cable, which are acting like antennas.
2. As close to the microphones as you can, install a line amplifier. Your mics are already pre-amplified and connecting them to a line amplifier might give you a better signal-to-noise ratio.
3. You mentioned that the mics are housed inside a Bell box. I don't know what that is but if it's not metallic, try spraying the inside with aluminum paint so that it can act like a shield. Also, keep the stripped-off wires of your cables as short as possible.

As far as filtering goes, I'd use a simple passive lowpass filter. You only need to pass audio, and the frequency you need to filter is 1.17MHz. Use high quality resistors and capacitors and your problem should go away.

your problem should go away

Not if the mic itself or the preamp built into the mic (Electrets usually have their own built in preamp) has already pulled the audio component off the interferrence?...any semiconductor junction could have that effect.... You can't filter audio off audio with a low pass filter.
Or have I mentioned this alread..
BTW How the heck do you shuffle those damn frogs

Del

50 kWatts only 5 miles away!!!!!!!

My Gawd you're going to have fun filtering that out

At that frequency you have a wavelength of about 900 feet so your feed wires are going to be excellent receiving aerials, quarter, half and full (dipole) wavelength!!!

I would be surprised if you weren't in danger of getting electrocuted from the open end of the sheild wire!!!

I think with that sort of energy you will need metal conduit earthed several feet apart to enclose the cable runs and even then you are putting the cables into a PC which is not the best screened instrument in the world...

I bet if you unplugged the microphone inputs from the PC you would still pick up this AM station

John.

It was a guy who could listen to an AM station with the sound coming from inside him, most precisely, from his mouth. The filling of his tooth was demodulating the am signal and his tooth together with his buccal cavity were amplifying the sound.

People living very close to AM stations can light bulbs from the radiated signal.

Your microphones capture the signal, your FET amplifier demodulates it, and overlays on the audio signal coming from the mike. So, besides the precautions you should take with a long antenna (your mike cable) I think that you should get rid of the RF signal on the input of your FET amplifier. A capacitor that will be low impedance for 1170KHz and a high impedance for the 5-6 KHz of audio (are they PA mikes of HiFi?) should do the trick. Perhaps a combination RC RC is even better. Those who suggested coil mikes are absolutely right, they present a very heavy load for RF (that means almost no interference) and the matching transformer make an almost immune low frequency system.

Indel,

You raise a very good point with the guy whose lose filling rectifies/demodulates the signal.

First thing for our friend to do, assuming all of the system is in metal already, is to check every ground, i.e., disconnect every ground, polish them absolutely clean, and then reconnect making sure the connection point is also clean. The entire system should be checked for any loose high resistance connections that could act as demodulators.

As has been said before the entire system should be encased in metal tubing with a soldered bond at every joint.

As to the mikes I don't see why they should be a problem. I have worked in studios right down the street from more than one 50KW station, with any number of condenser mikes and never had a problem.

Only time I remember a problem was in a theatre with a relatively low power tower not a hundred or so feet away. The communication system picked that up and they were never able to get rid of it.

As to the PC checking it free of any of the feeds is a good idea. If alone it is picking up signal it will need to be enclosed in a Faraday cage.

The other thing I would look at is ground loop. Assuming a good solid ground make sure that it is singular, that you do not have multiple grounds.

Funny thing is here in the "big city" Atlanta, I live within a mile or two of two or three 50KW transmitters. Never a murmur. I can look out my window on the sixth floor and see two or three towers. There are in fact major broadcast transmitters all over this city and doing video and audio near many of them has never been a problem.

Check the neighborhood. See if the problem is endemic. If it is, and you have carefully checked all your own gear, you might want to talk to the FCC technical guys. They see this sort of stuff all the time.

But again, do check your equipment and make sure you have no "loose tooth" connections that might be rectifying the signal.

j.

Try putting a pi filter at the input to the amplifier, as once the RF gets into it there will be rectification effects in the amplifier chain. That is, a capacitor in parallel with the microphone line, then series inductors (signal and ground lines), then another capacitor at the amplifier input. Try 0.1uF capacitors and inductors of around 10uH. Ideally, instead of individual inductors you could use an IF transformer of the type used in transistor radios, this has two windings, one for the signal line and one for the ground to pass through. You should be able to tune this kind of inductor to get rid of that station's carrier. The IF transformers used in AM radios are designed to trap 455kHz and already have the capacitors across the windings, unless you remove them and replace them with smaller ones probably you will not be able to tune it to 1170kHz. But perhaps if you get one that is designed for the FM band (10.7MHz) and add extra capacitance in parallel with the windings, you can tune it down to 1170kHz.

I am sure the pi filter will reduce the noise a lot even if you can't tune it to resonate at the carrier frequency. You may even find something ready made at your local electronics store, this is a common problem for amateur radio operators who often have to install this type of filter on TV's and audio equipment to avoid interference. Experiment!

That's a twisted 22 AWG coming back I take it?

Use a bunch of ferrite beads (the more the better) to block the 1170 KHz AM, then use 0.01 uF disc ceramic capaictor to short any exta RF to ground in the following way:

Those beads sould be put in series with the wires going to the mics (at the mics proper), AND also at the entrance to your preamp. Use a bunch of ferrite beads (the more the better on each line) to block the 1170 KHz AM entering the box at the far end from the mic.

Finally just inside and outside the metal box housing of yout preamp/amp short each line to ground along with with the shield with a 0.01 uF disc ceramic capaictor (i.e., short them to the metal box at the entrance). Or use feethrough caps at that box along with series ferrite beads inside and outside of the noxe -- right where the wire ho through).

Rather than beads you could put three raps around some W material toroids. "W" material is a high permeability material used for frequency attenuation from 100 KHz to 1 MHz in EMI/RFI filters. https://www.amidoncorp.com/items/22

Cheers,

-- Rob

What effect does reducing the transmitter power have?

What you are suffering from is called "Audio Rectification". This was a very common problem years ago. Any amplifier could develop the problem. Most commonly Church Organs, Mortuary Sound Systems, Courtroom microphones, etc. would suddenly pick up strong radio signals. Usually from a police car, fire engine, or overpowered CB Radio. It was often loud, inappropriate, and really obnoxious. We used to solve the problem by making minor modifications inside the amplifier. Basically to force it to act like an amplifier and quit thinking itself to be a radio receiver.

I am not familiar with the newer types of amplifiers, so I won't waste your time explaining what we used to do. Just google "Audio Rectification" and follow the links. You are sure to find some great up-to-date info on how to solve the problem.

Best of Luck

That's exactly what it is. Audio Rectfication.

The RF has bypassed what little filtering there is present (jumped around it because signal is so strong), has encountered a non-linerarity (like a diode junction), and you are listening to the envelope of the RF signal (that's audio) on top of your intended microphone signal. It is a pretty common problem.

[See my previous response.]

Thus the 0.01 disc ceramic capacitors required, the torroids or large beads required, the sheilded box (metal at your end) required, AND a shield going around the mic canister itself (but not touching anything conductive on the mic proper).

That last item, the tubular shaped sheild around the mic (the mic has built in JFET source follower I would assume) should be attached to the sheild of your twisted pair comnig back to you.

That shield should be insulated from any 'real' ground all along the way. The only exception being at you preamp box (or amplifier box, if you want to call it that) where it can be grounded not only to the box itself - but via the boxes sfatety grounding, to real earth ground.

Regards,

--- Rob

G'day folks,

There has been some good advice here but there are a couple of things I would add.

First off is the shielding. I know it stipulates the use of shielded cable but how have you earthed the shielding properly. Shielding electronics from RF interference can be a very complex problems and its really easy to mess it up. Here's a couple of thing to look at.

• Ground Loops: Ground loops are where you have the shielding connected to ground in two or more places or have a loop in the shielding. The shielding must and I really mean must be connected to ground at a single point and only a single point then radiate out in a branching pattern from there. If you don't do it that way you end up with a loop of wire that will pick up any old electromagnetic disturbance and transfer it to the signal cables. So, to check if you have a ground loop disconnect the ground from the main amplifier and test the resistance to ground of the shielding. If it isn't open circuit then you have a second earthing or grounding point that needs to be removed. Use your usual troubleshooting techniques to find any secondary grounds and remove them.
• Twisted Pairs: Check to see if the cables you are using have the signal cables twisted or not. By twisting the cables in a balanced pair cable, as your should be using, will counter much of the interference as the twisting will mean that both conductors have pretty much the same interference which will be rejected by the amplifiers common noise rejection ratio.

As Del the cat alluded to in post #11 and has been thrown around by others is the filter you have tried to use. With an AM or amplitude modulated signal as is with KCQB you have a 1.17 MHz carrier that has its amplitude or signal strength modulated by the audio signal it is carrying. If you filter out the carrier, as you are doing with your filter, you end up with the audio that it is carrying and as that's in the same bandwidth that your microphones are producing so I'm not surprised it won't work. What you are doing is basically what happens in an AM radio receiver so you may actually be making it worse.

Finally as Electroman stated in post #14 a 50 kW signal is very powerful and is going to be difficult to filter out, so it's even more critical that you do everything correctly.

In summary, first off is to start looking for a problem in the shielding and if that doesn't fix it try using a shielded two core cable that has a tighter twisting and multiple shielding layers.

Everyone is talking about shielding and filtering the outputs of the microphones. I think that the problem here is the signal may already be in the audio band at this point. I think that the preamps in the electret microphones are the problem. Being that they are most likely high impedance FET amps, they are picking up the signal,at their inputs, demodulating it, then inserting it on the lines in the audio spectrum. You should probably concentrate on the preamps themselves. Maybe take the mics apart, decouple the inputs of the preamps with RF bypass caps (0.01uf maybe?) then shield the hell out of the mics. Or better yet, do away with the high impedance high gain amps and mics altogether. Use low impedance moving coil mics, as already suggested, and use low to medium gain, bipolar preamps. I would still shield them and use decoupling caps on the inputs and outputs of the amps. Regardless, 50KW is a buttload of power to deal with at that distance. So, you've definately got your work cutout for you.

Maybe take the mics apart...

Surely, you've seen an electret microphone, right? Kinda hard to take them apart since they're usually built as one, compact unit.

As you can see, one of the terminals is actually connected to the shielding so the device itself is completely shielded. The only things not shielded will be the cabling, at least the part before the wires go into the shielded portion of the cable, and the input to the amplifier.

It is, therefore, understandable that people would suggest shielding and filtering the outputs instead of the mics themselves.

Greetings.

The Bell System used to have a filters for specific radio frequencies that they put on the telephone line to filter out the extras.

If I remember right the radio interference also comes in through intrinsic components. Maybe a shield around any components in the individual mics. Does your amp etc have shields around them that are grounded. Make sure the shield has enough air as to allow cooling.

Trying to eliminate radio frequencies from any system, telephone, audio, paging can be a real pain.

Had one in the radio station's Merling Telephone system. I put the controller in a clean metal garbage can and put the lid on to isolate where the signal was getting in. Building a screen around the controller totally eliminated the problem and the transmitter was in a tower just above the building. You could try something similar with your amp etc to isolate the unwanted input.

Good Hunting.

1) Use wireless microphones.

2) Accidently hit the transmitter tower with your car. At 50 mph.

3) Get the signal to the PC with fiber optics (SPDIF)

It has been my experience that 80% of the audio rectification problems are inside the amplifier itself. First I would see if the volume control affected the interfering signal. If not, then the rectification is happening in the stages following the volume control. If the interfering signal increases and decreases with the volume control, then the signal is getting in before the volume control. If this is the case, I would disconnect all input devices and see if the problem goes away. If it does, then reconnect them one at a time till you find the one causing the problem. If the interference exists with all inputs disabled then the RF signal is coming in through the power line or the speaker wires. Having solved dozens of these problems, it is my experience that most of the time the signal came in through the power cord. Power cords make great antennas. Speaker wires were often to blame. I never once found a problem with a microphone or other input device. (Trading out the main amplifier always eliminated the problem). Though a higher quality amp would always solve the problem, it was not always feasible due to cost considerations. Usually replacing a bypass capacitor in the offending amplifier stage would suffice, otherwise placing a ferrite bead on the base lead of the transistor would almost always remove the problem. (Any PN Junction can act as an RF Detector). All of my experience with this problem has been several years ago, so what worked then may not work with current technology. Best of luck, I hope this helps.

Lots of good stuff so far. I see that someone has given Stevem and off topic, but taking the light hearted stuff aside, why not fiber optic?

The OP indicated that for the shortest lead, the problem has gone away after some additional actions, but the longer leads are troublesome. Thus we know the microphones and short leads can give an adequate response and the base unit is able to cope with that signal without significant interference.

Converting to fiber optic will absolutely de-couple the electrical signals and thus remove the interference source, or alternatively instal a digital transmitter at the microphone amplifier.

A differential amplifier on the front end may help, the induced voltages from the radio station output will be in common mode and therefore should easily be removed. Winding several loops of each conductor in opposite phase through a ferrite torroid will help attenuate any induced currents.

Hi All,

I got all (8) of the microphones to work.

It turns out the microphone boxes were made of metal and there was a screw to connect the metal box to the microphone cable signal ground. This helped filter out the radio station.

I also added ferrite clips to the microphone cable inputs in the server room.

Lastly, the audio recording computer in the server room had a software setting that I set to no amplification given that the microphones already had built in amplifiers.

Thanks for all your helpful comments.

It's good to hear what happened in the end. The number of people that ask for help and never bother to come back and post what transpired in the end would astound you.

So well done for both solving the problem and getting back to us with the outcome.

You could try a low pass filter off an LC type (or a high pass across the conductors) at the microphone end, it's possible the RF envelope is being demodulated through an output pn junction at that end. The microphone I'm assuming from the twin conductors is balanced in which case the amplifier input would be differential and therefore interference induced on the mic line arrives in phase and is filtered (in theory). Just a thought.