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Old 2nd October 2009, 05:45 AM   #1
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Default Mobile Phone Interference

A friend has dropped his Amplifon WL25 to me for a mod.
His problem is "dit dit dit" coming through the speakers when the mobile phone is in use.
Will a simple RF Filter on the amp input fix this or do I need to go through the entire amp and add grid stops etc.
Any experience or clues?
Cheers,
Ian
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Old 2nd October 2009, 07:02 AM   #2
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I've found that interference from cell phones can be fairly problematic,sometimes seemingly impossible to get rid of. Adding grid stoppers couldn't hurt. Small ferrite beads on the grid pins might help too.
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Old 2nd October 2009, 08:05 AM   #3
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A small capacitor, perhaps 100pf, between the grids of the low level stages and ground may resolve the problem. Concentrate on the low level stages. They are more likely to be the problem.
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Old 2nd October 2009, 08:19 AM   #4
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Hi Ginger - my understanding is that the cell-phone emits a very high power broad signal when "paging" a cell site. In my experience, it affects my car stereo, computer including screen, stereo whether the tuner is running or not.

My phone, if left sitting on an unpowered speaker, will couple the signal to the damn speaker!!!!

The only foolproof fix I have found is to turn the electronic leash off...
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Old 2nd October 2009, 08:52 AM   #5
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On more than a few occasions when I've been visiting a close friend who designs tube amps in Canberra I've heard the "dit dit dit" when an incoming call has been heading my way (before the phone has actually started running a ring tone).

May be a byproduct of hollow state that is hard to rectify as the bloke in question is pretty careful (bordering on obsessive) when it comes to making circuits tidy and immune to external crud.
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Old 2nd October 2009, 09:10 AM   #6
timpert is offline timpert  Netherlands
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Ginger,

Without designing a system from the start to be very well shielded against HF irradiation, you're practically out of luck if you wish to keep the high powered signal from a mobile phone out. Especially when using the amp with the tubes exposed, this will cause them to act as antennas. The low level stages are indeed most sensitive, but even the power stage is susceptible. This is not a problem you can solve with mods on the amp, you'll have to completely rebuild it with proper screening in mind, use balanced signaling only, and of course make the rest of the audio chain immune in the same way.

Or you can just keep the phone away from it.

If the amp is even a bit sensibly designed, grid stoppers should already be present, as well as a mains filter. Upgrading the filter could be worth the effort, because manufacturers often only install the bare minimum to pass CE qualification tests, in order to keep costs down. Also check if an input lowpass is present, if not, a simple RC with a corner frequency of about 150 kHz is advisable. Be careful with putting additional filtering in the amp circuit itself, especially within the feedback loop, because these introduce additional phase shifts and might render the amp unstable. The measures described here might decrease the intensity of the problem a bit, but won't eliminate it.
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Old 2nd October 2009, 10:35 AM   #7
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Don't forget loudspeaker cables acting as antenna.
Clipsable ferrite beads very near the amp may help.

The more unlinear the amp is, the more it's able to detect RF signals !

Yves.
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Old 2nd October 2009, 10:42 AM   #8
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Quote:
Originally Posted by Yvesm View Post
...
The more unlinear the amp is, the more it's able to detect RF signals !

Yves.

please explain.
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Old 2nd October 2009, 10:54 AM   #9
timpert is offline timpert  Netherlands
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A non-linear device (like a diode, transistor or tube - no device is totally linear) can act as an envelope detector for amplitude modulated HF signals. It does this by acting like a rectifier. A GSM signal has a strong on-off character, and its power is continuously adjusted to the actual needs. The resulting strong AM signals are readily detected by even a mildly non-linear device.
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Old 2nd October 2009, 04:10 PM   #10
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One possible option is to wrap the cellphone in tinfoil, and then ground that foil.

I'm going to put my former FCC tech hat on.

Be sure your patch cables are well shielded. I try to use shielded power cords too, but I don't have any proof that helps. Use a metal chassis if possible and then seal any gaps with copper tape. Ground the chassis. If the chassis is non-conductive, then you can spray an RF paint on the insides. Be sure your transformer enclosures are grounded.

Of course RF may get in through the speaker wire. Cellphones will operate at short wavelength so all sorts of things can act as an antenna, but with a cellphone in close proximity I'd guess what we hear are the harmonics from the signal generated that are in the audible spectra. Speaker wire could be a nice antenna for frequencies in the Mhz and Khz ranges if tuned to the right length.

Folks have already mentioned some of the tricks with ferrite beads and small capacitors.

Really the best thing is to just turn off the phone. Mine has a signal strong enough to crash my computer.
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