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

[Church-Audio]

Hi I heard about your problems my preamp, it is shielded with copper foil around the whole circuit board. Even though we use a plastic case. I would suspect that it might be the length of cable is acting as some kind of antenna for the cell phone aprox 860 MHz to 1.5 GHz 4 feet sounds just about right. Lifting the ground will not help (it will kill the bias voltage to the mic thus no sound. When you use your simple battery box with the mics do you get interference? (Make sure you crank the gain up on your iriver to be at the same level as my preamp) If your still having problems Send the mics and preamp back to me, I will look at them free of charge and ship them back to you if there is a problem with our preamp we will replace it or fix it. As for your mics if there is a problem there we will fix it for free as well.

 How long is and what type of cable are you using from my preamp to the A/D converter?

Chris Church

Your problem could be due to grounding of the shield to signal ground at both ends of the cable, thereby causing a ground loop.  You could try lifting the shield ground at the mic end of the cable.  If that's not possible, try lifting the shield from signal ground at the other end of the mic cable.  If that reduces the interference, then you know you've got a ground loop issue and we can continue troubleshooting from there.

I'd rather not do this at the mic end...I think I'd break something.

To 'lift the shield' at the mini-xlr end, do I just snip the shield in the mic cable at the mini-xlr and tape it up?

First thing I would try is taking mic cables completely out of the mix. Hook the mics directly into the pre and give it a shot.
Next put the cables back and hold the phone close to the mics or maybe borrow different mics from a friend.
Same thing for the pre.

The mics aren't detachable. They've got 4 ft of cable attached to 'em.  Don't know any other tapers in this country.

Seems like you should be able to move your phone up and down the line while someone is ringing you and the interference will get stronger as you get closer to the leak.

Moving the phone was my first thought...the problem is the interference isn't a consistant strength/length.  I can leave everything exactly as is, call 3 times, and get three different levels of intereference.  Maybe if the mic cables were longer than 4 ft...

[whatboutbob]

Hi I heard about your problems my preamp, it is shielded with copper foil around the whole circuit board. Even though we use a plastic case. I would suspect that it might be the length of cable is acting as some kind of antenna for the cell phone aprox 860 MHz to 1.5 GHz 4 feet sounds just about right. Lifting the ground will not help (it will kill the bias voltage to the mic thus no sound. When you use your simple battery box with the mics do you get interference? (Make sure you crank the gain up on your iriver to be at the same level as my preamp) If your still having problems Send the mics and preamp back to me, I will look at them free of charge and ship them back to you if there is a problem with our preamp we will replace it or fix it. As for your mics if there is a problem there we will fix it for free as well.

 How long is and what type of cable are you using from my preamp to the A/D converter?

Chris Church

Thanks Chris.  That's an extremely generous offer.

To be honest though, I don't think its your preamp that's at fault, so I would hate for you to feel obliged to look at this.  I get interference with 2 different sets of mics, my homemade battery box and through the PS2, along with your pre.  On the other hand, if you're still happy to look at it, then I'm not going to say no, because its frustrating me and my technical know-how is somewhat lacking.  Come to think of it though, if the issue is specific to Australia's cell phones, would you even be able to replicate/source the issue?

The cable I'm using from the pre to the AD is the two foot cable you supplied (unexpectedly).  It plugs into the AD20's XLR's via a one foot 'Y' cable.  Keep in mind though that I also get the interference without the AD20...just going direct to the iriver.

I don't think your cable length theory is the problem either.  I was getting interference using my old at853's w/ the original length of cable (25 feet?).  Just today I hacked them down to 6 feet and changed them to the 3 wire config. I grounded one at the mini-xlr end (I'm kinda simple and got on a roll and forgot I wasn't going to ground it) and didn't ground the other one.  They both get interference.

It kills me that the GSA omnis/battery box are the gear I have that don't get interference.

[rokpunk]

This cell phone interference is common. It happens every time I put my cell phone on top of any of my mixing consoles. It also happens if I mic up someone with a wireless lav mic and they have a cell phone/blackberry in their pocket. It also happens with wired mics. It happens when standing next to my self powered Meyer rig....it happens using cheapo "PA on a stick" systems. The only cure is to turn off your (and other peoples) phones. Shure and Audio Technica both know about the problem (been reported by me, plus lots of others, from what they tell me), neither has a solution as of yet.

Sheilding your head by wrapping it in an aluminum foil beenie is the only solution. It also protects you from evil alien life forms.

[Church-Audio]

I have had this problem with two way radios! I had keyed in my two way radio beside a console it muted the sound system! Very strange, it seems it overloads or causes some kind of oscillation. I have never had a problem with my cell phone on a console before or even near mics maybe it is certain types of phones or transmitter types. I guess this is why when you’re in a hospital they tell you to turn it off. Strange


Chris Church

This cell phone interference is common. It happens every time I put my cell phone on top of any of my mixing consoles. It also happens if I mic up someone with a wireless lav mic and they have a cell phone/blackberry in their pocket. It also happens with wired mics. It happens when standing next to my self powered Meyer rig....it happens using cheapo "PA on a stick" systems. The only cure is to turn off your (and other peoples) phones. Shure and Audio Technica both know about the problem (been reported by me, plus lots of others, from what they tell me), neither has a solution as of yet.

Sheilding your head by wrapping it in an aluminum foil beenie is the only solution. It also protects you from evil alien life forms.

[Church-Audio]

So it's safe to say it’s not the mics or your preamp. Have you tried a different recorder? If not give that a shot try using the line input on your computer if you do not have another recorder.

Chris Church

Hi I heard about your problems my preamp, it is shielded with copper foil around the whole circuit board. Even though we use a plastic case. I would suspect that it might be the length of cable is acting as some kind of antenna for the cell phone aprox 860 MHz to 1.5 GHz 4 feet sounds just about right. Lifting the ground will not help (it will kill the bias voltage to the mic thus no sound. When you use your simple battery box with the mics do you get interference? (Make sure you crank the gain up on your iriver to be at the same level as my preamp) If your still having problems Send the mics and preamp back to me, I will look at them free of charge and ship them back to you if there is a problem with our preamp we will replace it or fix it. As for your mics if there is a problem there we will fix it for free as well.

 How long is and what type of cable are you using from my preamp to the A/D converter?

Chris Church

Thanks Chris.  That's an extremely generous offer.

To be honest though, I don't think its your preamp that's at fault, so I would hate for you to feel obliged to look at this.  I get interference with 2 different sets of mics, my homemade battery box and through the PS2, along with your pre.  On the other hand, if you're still happy to look at it, then I'm not going to say no, because its frustrating me and my technical know-how is somewhat lacking.  Come to think of it though, if the issue is specific to Australia's cell phones, would you even be able to replicate/source the issue?

The cable I'm using from the pre to the AD is the two foot cable you supplied (unexpectedly).  It plugs into the AD20's XLR's via a one foot 'Y' cable.  Keep in mind though that I also get the interference without the AD20...just going direct to the iriver.

I don't think your cable length theory is the problem either.  I was getting interference using my old at853's w/ the original length of cable (25 feet?).  Just today I hacked them down to 6 feet and changed them to the 3 wire config. I grounded one at the mini-xlr end (I'm kinda simple and got on a roll and forgot I wasn't going to ground it) and didn't ground the other one.  They both get interference.

It kills me that the GSA omnis/battery box are the gear I have that don't get interference.

[it-goes-to-eleven]

Come to think of it.. I used to have a stereo that would once in a great while blast some trucker's CB at extreme volume as he drove past.  I'm pretty sure it was an illegal high output CB..  The signal bypassed the volume control and would just BLAST even when nothing was playing or even when you were playing very low volume.. It would sure get your attention.  Now the interesting part.. It was a Creek integrated amp. Really great little amp BUT I now see the significance of the wood cover.


Did you ever test with the cables out of the pot but the mics and the bbox/pre/iriver in?

[SparkE!]

 I would suspect that it might be the length of cable is acting as some kind of antenna for the cell phone aprox 860 MHz to 1.5 GHz 4 feet sounds just about right.

Anything longer than about 1/4 wavelength can inadvertently become an effective antenna, so any cable longer than about 3 1/2" can be the problem.

 Lifting the ground will not help (it will kill the bias voltage to the mic thus no sound.)

So these are not shielded cables? I was suggesting to lift one end of the shield or the other, but if the only ground is power ground, then you obviously can't do that.

So it's safe to say it’s not the mics or your preamp.

I get interference with 2 different sets of mics, my homemade battery box and through the PS2, along with your pre.

The cable I'm using from the pre to the AD is the two foot cable you supplied (unexpectedly).  It plugs into the AD20's XLR's via a one foot 'Y' cable.  Keep in mind though that I also get the interference without the AD20...just going direct to the iriver.

I don't think your cable length theory is the problem either.  I was getting interference using my old at853's w/ the original length of cable (25 feet?).  Just today I hacked them down to 6 feet and changed them to the 3 wire config. I grounded one at the mini-xlr end (I'm kinda simple and got on a roll and forgot I wasn't going to ground it) and didn't ground the other one.  They both get interference.

It kills me that the GSA omnis/battery box are the gear I have that don't get interference.

So why do you think that it can't be the mics or the preamp?  Aren't they always in the circuit when there is a problem?

[Church-Audio]

I dont think its the preamp or the mics because he has tryed a different battery box and mics and still has the same problem.
I don't know if he has tryed just going direct in without the d/a converter or if he has tryed a different recording device. I would think that because the preamps and mics have been switched that the problem would be elsewhere. IMO.


Chris Church

 I would suspect that it might be the length of cable is acting as some kind of antenna for the cell phone aprox 860 MHz to 1.5 GHz 4 feet sounds just about right.

Anything longer than about 1/4 wavelength can inadvertently become an effective antenna, so any cable longer than about 3 1/2" can be the problem.

 Lifting the ground will not help (it will kill the bias voltage to the mic thus no sound.)

So these are not shielded cables? I was suggesting to lift one end of the shield or the other, but if the only ground is power ground, then you obviously can't do that.

So it's safe to say it’s not the mics or your preamp.

I get interference with 2 different sets of mics, my homemade battery box and through the PS2, along with your pre.

The cable I'm using from the pre to the AD is the two foot cable you supplied (unexpectedly).  It plugs into the AD20's XLR's via a one foot 'Y' cable.  Keep in mind though that I also get the interference without the AD20...just going direct to the iriver.

I don't think your cable length theory is the problem either.  I was getting interference using my old at853's w/ the original length of cable (25 feet?).  Just today I hacked them down to 6 feet and changed them to the 3 wire config. I grounded one at the mini-xlr end (I'm kinda simple and got on a roll and forgot I wasn't going to ground it) and didn't ground the other one.  They both get interference.

It kills me that the GSA omnis/battery box are the gear I have that don't get interference.

So why do you think that it can't be the mics or the preamp?  Aren't they always in the circuit when there is a problem?

[SparkE!]

I dont think its the preamp or the mics because he has tryed a different battery box and mics and still has the same problem.
I don't know if he has tryed just going direct in without the d/a converter or if he has tryed a different recording device. I would think that because the preamps and mics have been switched that the problem would be elsewhere. IMO.

The way I understand, the preamp is always in the circuit.  Is that not the case?

[Church-Audio]

He has removed my preamp from the chain and he still gets the interference.

I dont think its the preamp or the mics because he has tryed a different battery box and mics and still has the same problem.
I don't know if he has tryed just going direct in without the d/a converter or if he has tryed a different recording device. I would think that because the preamps and mics have been switched that the problem would be elsewhere. IMO.

The way I understand, the preamp is always in the circuit.  Is that not the case?

[whatboutbob]

The way I understand, the preamp is always in the circuit.  Is that not the case?

There's always something between the mics and the iriver...whether it be the preamp, my homemade bbox or the AT8505s/PS2. They all get interference.  The PS2 less than the other two.

Can the at853's in the 3-wire config be powered by the iriver plugin power?  If so, could someone please tell me how to wire a 1/8 plug to 2 mini-xlr(m).  That way I can test with just the mics and iriver.

Unfortunately I don't have a different recorder.  Oh, I suppose my partner's got an iriver H10 I could use.  Only records to mp3...but it may be better than nothing for testing purposes.

I thought it was safe to say that it wasn't the iriver because I don't get the interference from the GSA mics (w/ ~5 foot cables)?

So these are not shielded cables? I was suggesting to lift one end of the shield or the other, but if the only ground is power ground, then you obviously can't do that.

Sorry...I don't know (showing my ignorance here but i thought shield/ground were interchangeable)...there's two yellow wires, two red wires and the shield/ground 'thing' wrapped around 'em.

Did you ever test with the cables out of the pot but the mics and the bbox/pre/iriver in?

I did, but unfortunately the slightly open pot-lid makes these tests inconclusive.  I had interference with everything, apart from when the whole kit was in the pot (ie the only time the lid was fully closed).

Maybe I should just wear the pot to gigs? ;-)

[Microbe]

Hi,

If you've got a soundcard on your pc with line in you can always try that as a recording device.  If you've got the right software you can turn the gain up and test the mics and boxes.  I don't know how it would work out, but in theory you should be able to do it.

[SparkE!]

Can the at853's in the 3-wire config be powered by the iriver plugin power?  If so, could someone please tell me how to wire a 1/8 plug to 2 mini-xlr(m).  That way I can test with just the mics and iriver.

Ahhh, so you're talking about poorlyconditiond's style of battery box, like this?



When you said 3-wire, I thought you were talking about 3 wires per channel, balanced, like is used in XLR mic cables. So, you're running unshielded or perhaps your ground is your shield.

...there's two yellow wires, two red wires and the shield/ground 'thing' wrapped around 'em.

Now wait a minute.  I thought we were talking about 3 wires.  There are 5?  Do you have a schematic of how this stuff is connected?  Maybe you do have separate signal ground and shield paths.

[whatboutbob]

Hehe...sorry for the confusion.

The battery box I made is very similar to Richard's, but is actually from schematics drawn up by Graeme Cogger:
http://taperssection.com/index.php?topic=57512.0

The AT cables have two red wires, two yellow and the ground.  In the 3 wire config the red wires go to pin 3, yellow to pin 2, and the ground to pin1.

Does that clear anything up?

[SparkE!]

+T for notchin up the g33k factor with a chalkboard schematic!!
love it!

Well, poorlyconditioned deserves that +T because it was his drawing.  I think he said that it was the chalkboard in the physics(?) lab where he works.  Here's where he keeps all of the info:

http://www.cs.uwaterloo.ca/~mannr/AT853/

But hell yeah! I got a good chuckle out of the great mix of modern technology with erasable manual media.