Topic: Cell phone interference: how can I shield against it / what's wrong with my rig?

[whatboutbob]

The good news is it would always stop if I turned it off and if it was on my far shelf in my cubicle (4' from the phone) it wouldn't interfere.  So it seems to be a GSA thing and has a very small radius of effect.

Hrmm...they must put the phones on 'roids down here (Australia) then. I stopped testing at ~30ft.  Admitedly the interference was faint at that stage, but still audible.

One thing I've been meaning to ask again, that I mentioned in my initial post...does anyone have any ideas why different mics in different channels would have different strengths of interference?  I'm really just making stuff up now, but the interference couldn't somehow be leaking from the ground on one mic to the audio on the other...and its stronger in one combination because the mic 'a' ground pin is closer to the mic 'b' audio pin than the mic 'b' ground pin is to the mic 'a' audio'.  Bah, I'm prolly confusing you all and I guess it doesn't really matter.

He's in Europe, so it will be GSM.  GSM phones tend to cause more problems with audio circuits if the high impedance points in the circuits are not bypassed with small caps like poorlyconditioned was mentioning.  He's right that a small capacitor would probably be a good choice.  You'll want to use low loss chip capacitors whose series resonant frequency is above the cellular band. (NPO or COG dielectrics have low loss at cellular frequencies.)  A typical 15 pF COG chip cap would have a series resonant frequency of around 2 GHz, so that would cover the 1900 MHz cellular band where your GSM phone probably works.

I'm pretty sure Australia uses dual-band GSM: 900 and 1800MHz...I think it jumps between the two in high traffic areas.  Would that change the caps required?

edit: Looks ilke it: http://www.accesscomms.com.au/mobnet.htm

Now I've just gotta go google 'NPO or COG dielectric capacitors' cos i've never heard of 'em and don't think the local Ratshack equivalent has 'em...unless I'm going nuts...

Yeah, I'm suspecting the battery box at this point too.

I thought the bbox and Church Audio bbox/pre could be ruled out cos I still get interference without them in the mix (AT853s > Y cable > iriver)?

[SparkE!]

Yeah, I'm suspecting the battery box at this point too.

I thought the bbox and Church Audio bbox/pre could be ruled out cos I still get interference without them in the mix (AT853s > Y cable > iriver)?

OK, I missed that.  I'm having a hard time keeping up with the signal paths you've tried.

If you are still getting the interference when just using the plug-in power of the iRiver, then it almost has to be the mics themselves.  With that in mind, I'd recommend getting some bypass caps and simply install them between tip and ground and between ring and ground on your mic connector (which I assume is a 1/8" stereo plug?).

You're probably not going to find the caps you need at the local Radio Shack.  NPO and COG refer to temperature characteristics of ceramic dielectric material, but your reason for wanting those dielectrics is not for the low temperature variation of dielectric constant.  Instead, it's because those materials tend to have low losses at rf frequencies, so the caps are still good caps at those frequencies.  They also can be made to have fairly high self-resonant frequencies.  Plastic film caps may be what you want to use for coupling caps at audio frequencies, but they aren't of much use at rf frequencies because their self-resonant frequencies are usually too low for their impedance to remain capacitive at rf frequencies.  At rf frequencies, most plastic film caps would appear to be inductive.

I don't know who to suggest for a source of caps in Australia, but I'd be looking for either Panasonic or MuRata caps in a 0402 package size and a COG or NPO dielectric and maybe 15 pF of capacitance.  That would work for shunting either 800 MHz GSM or 1800 MHz GSM signals.

[whatboutbob]

Chris from SP was good enough to reply:
=================================================================
> I hope you don't mind me asking this random question, but I was wondering if you guys have had any reports of problems with RF interference with the AT943/AT853 mics?

Not yet...

>Me rambling about the problem...

If you run the mics balanced as you are with the Denecke, you have very little chance of interference.

If you run the 853's balanced with true phantom adapters, etc or with this box:
 
http://www.soundprofessionals.com/cgi-bin/gold/item/AT8532
 
you will be balanced and should not have this problem. I think the 3 wire mod you have still runs the mic unbalanced (but I am not 100% sure of this).
 
Also, AT is aware of this issue in some extreme cases, even with balanced cable, and has a new version of the 853 available. It's more expensive and the benefits are only apparent if you run balanced with their power supply.

===========================================================

My battery box should be (assuming I haven't screwed anything up) and the Church Audio pre is balanced...right?  That's the whole point of the 3wbb, isn't it?

[Church-Audio]

I think the solution Chris mentioned would be very expensive, you would need two converters $300 + a new preamp $$$$ I would try the caps first. If that does not work send back the preamp and your battery box and I will mod them so this does not happen again. I have sold 100's of preamps to the UK never had a problem, but I can see you are so I want to help get that stupid metal pot off your head

All mini three wire mic inputs are unbalanced. they have three conductors they are GROUND-SIGNAL-BIAS ( BIAS JUST A FANCEY WORD FOR POWER) what Chris at SP is talking about is a balanced converter that is three pin to XLR I do not personally think it will do anything more then a few well placed bypass caps. As a matter of fact balanced is + signal - and ground. The advantage is less chance of inducted interference for long cable runs. (Not an issue here) and more common mode rejection (a different issue here) but Like I said before I think this problem can be fixed with a simple mod (have you tried a different recorder?)

if you still have problems send me your battery box and my preamp and I will fix the problem for FREE with out spending $300 on converters there are tons of guys on here that run unbalanced inputs and never have a problem I think there has to be a simple solution that will not cost an arm and a leg.


Chris Church

Chris from SP was good enough to reply:
=================================================================
> I hope you don't mind me asking this random question, but I was wondering if you guys have had any reports of problems with RF interference with the AT943/AT853 mics?

Not yet...

>Me rambling about the problem...

If you run the mics balanced as you are with the Denecke, you have very little chance of interference.

If you run the 853's balanced with true phantom adapters, etc or with this box:
 
http://www.soundprofessionals.com/cgi-bin/gold/item/AT8532
 
you will be balanced and should not have this problem. I think the 3 wire mod you have still runs the mic unbalanced (but I am not 100% sure of this).
 
Also, AT is aware of this issue in some extreme cases, even with balanced cable, and has a new version of the 853 available. It's more expensive and the benefits are only apparent if you run balanced with their power supply.

===========================================================

My battery box should be (assuming I haven't screwed anything up) and the Church Audio pre is balanced...right?  That's the whole point of the 3wbb, isn't it?

[poorlyconditioned]

Chris from SP was good enough to reply:
=================================================================
> I hope you don't mind me asking this random question, but I was wondering if you guys have had any reports of problems with RF interference with the AT943/AT853 mics?

Not yet...

>Me rambling about the problem...

If you run the mics balanced as you are with the Denecke, you have very little chance of interference.

If you run the 853's balanced with true phantom adapters, etc or with this box:
 
http://www.soundprofessionals.com/cgi-bin/gold/item/AT8532
 
you will be balanced and should not have this problem. I think the 3 wire mod you have still runs the mic unbalanced (but I am not 100% sure of this).
 
Also, AT is aware of this issue in some extreme cases, even with balanced cable, and has a new version of the 853 available. It's more expensive and the benefits are only apparent if you run balanced with their power supply.

===========================================================

My battery box should be (assuming I haven't screwed anything up) and the Church Audio pre is balanced...right?  That's the whole point of the 3wbb, isn't it?

No battery boxes are balanced!

Balanced is what regular XLR cables are.

  Richard

[whatboutbob]

Right...my bad then. Clearly I have no idea what I'm on about.

Picked up some 15pF NPO dielectric caps today...will see if they make any difference tonight.

[Church-Audio]

Its that dam pot its not making you think straight

Chris Church

Right...my bad then. Clearly I have no idea what I'm on about.

Picked up some 15pF NPO dielectric caps today...will see if they make any difference tonight.

[whatboutbob]

Either that or aliens have fought through my shield...

[SparkE!]

Right...my bad then. Clearly I have no idea what I'm on about.

Picked up some 15pF NPO dielectric caps today...will see if they make any difference tonight.

What case style is used for the caps?  Are they leaded or are they chip caps?  If they are leaded, then you'll probably need a large electrical value, more like 22 pF or 27 pF in order to overcome the inductance of the leads.  Nonetheless, the 15 pF caps will still probably help if the rf interference is related to the mic cables.

On the other hand, if the interference is unchanged, then you're probably looking at needing to bypass the mic element itself.  There are back electret mic elements that come with 10 pF or 15 pF bypass capacitors installed inside the capsule specifically to avoid the problem of cellphone or radio interference.  One of the most common uses for these bypassed electret elements is the mic in cellphones, but I also remember a few years back that there was something in the news about hearing aids that were being driven crazy by cellphones.  There were lots of recalls on hearing aids when GSM was being deployed in Europe.  Apparently the cellphone interference would be played loudly into the ears of the person wearing the hearing aid.  Yikes!

[whatboutbob]

What case style is used for the caps?  Are they leaded or are they chip caps?

Errrrrrr....they look like clay....?

Ok...just got done testing with the 15pF caps across pins 1&2 and 1&3.  There's still a lot of interference, however it seems to be marginally lower than w/o the caps, though it is difficult to measure precisely due to the varying strength of the interference at any one time.

Should I try a different capacitance (sp?) cap?

[SparkE!]

What case style is used for the caps?  Are they leaded or are they chip caps?

Errrrrrr....they look like clay....?

Ok...just got done testing with the 15pF caps across pins 1&2 and 1&3.  There's still a lot of interference, however it seems to be marginally lower than w/o the caps, though it is difficult to measure precisely due to the varying strength of the interference at any one time.

Should I try a different capacitance (sp?) cap?

By case style, I meant do they have leads or are they chip caps with metalization for connection directly to the body of the caps?  If you are connecting by wire leads, then the ideal capacitance will be slightly more in order to overcome the lead inductance.

Regardless, if there was only a small difference, then you probably will not be able to make much difference by putting caps between the signal lines and ground, even with larger caps.  In that case the problem has almost got to be with the mic elements themselves.  You'd have to place the caps inside the mic elements in order to fix the problem.  It sounds like that's what AT is planning to do with their new design, so cellphone interference may just be a given with your current mic elements, which would be a bummer for you.  I'm pretty much out of ideas here unless you can talk AT into trading your old mics for the new ones.

[Church-Audio]

or send them to me

What case style is used for the caps?  Are they leaded or are they chip caps?

Errrrrrr....they look like clay....?

Ok...just got done testing with the 15pF caps across pins 1&2 and 1&3.  There's still a lot of interference, however it seems to be marginally lower than w/o the caps, though it is difficult to measure precisely due to the varying strength of the interference at any one time.

Should I try a different capacitance (sp?) cap?

By case style, I meant do they have leads or are they chip caps with metalization for connection directly to the body of the caps?  If you are connecting by wire leads, then the ideal capacitance will be slightly more in order to overcome the lead inductance.

Regardless, if there was only a small difference, then you probably will not be able to make much difference by putting caps between the signal lines and ground, even with larger caps.  In that case the problem has almost got to be with the mic elements themselves.  You'd have to place the caps inside the mic elements in order to fix the problem.  It sounds like that's what AT is planning to do with their new design, so cellphone interference may just be a given with your current mic elements, which would be a bummer for you.  I'm pretty much out of ideas here unless you can talk AT into trading your old mics for the new ones.

[whatboutbob]

Right, I've found my partner's phone emits more regular interference.  So using that I've run it up and down the length of the rig, and can confirm, that the problem is definitely the mic caps.

Thanks very much to all who have contributed.  I give up.  Chris, I'm going to take you up on your generous offer.  Wil email you shortly.

[Church-Audio]

Hey I just thought of another solution that "MIGHT" work create what is called a faraday shield by going to a hobby store and pickup some fine brass screen and make little mic pockets surround the mics in this fine brass screen I bet that will solve the problem.


If you don't want to try that it’s cool, but I think it might work. You can sew the screen togeather or use solder at the edges create a windscreen out of the stuff it should work, the problem is the front of the capsule is letting the inducted signal in. With the fine brass screen it will allow the sound to come in with out any effect, but will or may block the bad cell phone stuff from hapnin. Also it will allow you to take the pot off your head another added bonus!

Right, I've found my partner's phone emits more regular interference.  So using that I've run it up and down the length of the rig, and can confirm, that the problem is definitely the mic caps.

Thanks very much to all who have contributed.  I give up.  Chris, I'm going to take you up on your generous offer.  Wil email you shortly.

[whatboutbob]

Hey I just thought of another solution that "MIGHT" work create what is called a faraday shield by going to a hobby store and pickup some fine brass screen and make little mic pockets surround the mics in this fine brass screen I bet that will solve the problem.

I was initially looking at faraday cages when I thought the problem was with the iriver.  Didn't think about using it on the mics. I wonder if I can do this without screwing the sound?

Would I need to ground the cage somehow?  Otherwise wouldn't it act like an antenna?

Edit: hrmm...I wonder where I can source the mesh.  The local hobby shops don't seem to have it.

I may need to order from these guys: https://www.twpinc.com/twp/jsp/product.jsp?type=11# (some interesting info about RFI there too).

edit2: gah...just noticed the fine print...minimum order is $75. :-/