[shinew]

does anyone know how to build an amplifier shield for cell phones? I sometimes use my amp with an iphone for portable listening, the buzzing sounds is really bothering me(i know i can turn on the airplane mode, but i would like to still get phone calls).

With cell phones everywhere these days, I'm sure this issue has bothered people before with other phones. I have tried to use the cell phone shielding pouch and put the amp inside of it, it doens't help.

I know this can be done though because the built-in amp inside of the iphone has no interference at all when they phone is on. Just not sure how it's done.

any idea? thanks.

 

[joneeboi]

Let's see if I can apply my knowledge from my electromagnetism class. If you wrap something in a conductive material, electric fields don't penetrate the conductor. My prof demonstrated this by wrapping a radio in alumin(i)um foil, which cut off its reception; it wouldn't work when wrapping his microphone in aluminum foil. Wrapping the radio in alumin(i)um cuts off the reception because the charges balance each other out throughout the conductor, so a positive electric field on one side pushes the positive charges to the other side of the conductor, and the negative electric field on the opposite side would attract the positive charges. This doesn't work when the source is inside the conductor because all of the positive electric field will be either inside or outside the metal and the negative electric field will be either outside or inside the metal, respectively. Cell phones use radio waves, or electromagnetic waves, which are covered by this principle.

The short answer is that you could wrap your amp, and not your phone, in some sort of conductor, meaning you should use a metal case. Some great examples are the AMB Labs Mini^3 and HeadAmp Pico, which are cased in metal and connect their grounds to the case. I can still hear that funny buzzing if I leave my phone right next to my Mini^3, but that's only when they're hugging each other. Give them a bit of distance and they should be fine.

 

[shinew]

I'm using Corda 2Move, the case is solid aluminum and the ground is connected to the case, but still hear the buzzing sound when the phone is close to it.
Does anyone know how to completely eliminate the buzzing?

 

[Uncle Erik]

A metal case isn't enough to stop RF - it can still get in. RF will also be picked up by patch cables and the wires to your headphones/IEMs. Cell phones are constantly calling out to cell towers and hopping frequencies. They're really radio tranceivers, not telephones. They only transmit about 0.25W, but that's enough to be picked up by nearby audio gear.

Killing the RF interference can be tricky. You'll have to use shielded cables for everything (including IEMs) and probably putting the amp into a different shielded enclosure.

 

[shinew]

Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif Killing the RF interference can be tricky. You'll have to use shielded cables for everything (including IEMs) and probably putting the amp into a different shielded enclosure.

yes it certainly seems to be tricky. However, it shouldn't require the cable & IEM to be shielded though, because I have used several headphones(including the er4s i'm using with 2move) to listen to music through my iphone, none has had the interference issue. So somehow the headamp inside the iphone has found a way to shield itself from the interference. I've seen some guts-out pictures of the iphone, but I didn't see anything shielding the amp(granted I didn't even know where the amp was

....)

 

[ericj]

I'm not a shielding expert by any stretch, but, I know enough about RF to know that the difference between a shield and an antenna can be maddeningly subtle.

The last time i successfully defeated an RFI issue, it was a wall-powered mint-tin amp. The external power supply was a center-tapped transformer and the center tap was brought though as the signal ground.

I tried a lot of stuff, but what ended up being successful was isolating all the connectors from the enclosure, installing small ferrite beads on the inside of the mini-din power connector's metal barrel on each power lead *and the ground lead, and connecting ground to the casing and thus the shield on the power cable on the inside of the enclosure.

After that, all the noise i had been picking up was gone. even after i took off the beads on the audio input and output - so the source was apparently the power cable.

But i presume you're using a battery powered amp, so you're on your own.

 

[Uncle Erik]

Quote:

Originally Posted by shinew /img/forum/go_quote.gif yes it certainly seems to be tricky. However, it shouldn't require the cable & IEM to be shielded though, because I have used several headphones(including the er4s i'm using with 2move) to listen to music through my iphone, none has had the interference issue. So somehow the headamp inside the iphone has found a way to shield itself from the interference. I've seen some guts-out pictures of the iphone, but I didn't see anything shielding the amp(granted I didn't even know where the amp was ....)

I haven't had any interference issues directly out of my iPhone, either, but I haven't tried it with a portable amp yet. I believe the antenna is on the bottom reverse, under the black plastic shroud. My guess is that the amplifier circuit is up near the headphone jack. I'd think Apple spent some time engineering interference out of it.

Still, anything that's unshielded or not shielded enough is a potential antenna. You could experiment with tinfoil by wrapping it around cords, etc. until you find where the RF is getting in and then deal with it directly. RF can be a real gremlin.

 

[blackinches]

Find yourself a Faraday cage.

 

[dgbiker1]

Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif I haven't had any interference issues directly out of my iPhone, either, but I haven't tried it with a portable amp yet. I believe the antenna is on the bottom reverse, under the black plastic shroud. My guess is that the amplifier circuit is up near the headphone jack. I'd think Apple spent some time engineering interference out of it. Still, anything that's unshielded or not shielded enough is a potential antenna. You could experiment with tinfoil by wrapping it around cords, etc. until you find where the RF is getting in and then deal with it directly. RF can be a real gremlin.

I'm wondering if the iphone amp & phones don't pick up the noise because they're parallel to the antenna whereas parts of the separate amp are perpendicular (assuming the amp and phone are held together).

Just brainstorming, but if the amp has a metallic case it might be worth a shot to put some kind of insulating barrier between the amp and tin foil if straight foil on the amp doesn't help.

 

[shinew]

just to be clear, i'm using the LOD on iphone, not the headphone output, but that doesn't make any difference anyway...

I'm pretty sure it's not the cable, I have tried disconnectting the cable from the iphone to the amp, same level of interference. However, when I wrap the amp inside of the cell phone signal blocking pouch(inside material of the pouch is made/painted completely with conducting material), the intereference decreases but it's still there. I don't have any tin foil at home at the moment but I can get some tomorrow.

I have a feeling that if I keep wrapping around the amp with layers & layers of tin foil, eventually the intereference will be gone, although I can't help but wonder if there is a "smarter" way to do this...

 

[joneeboi]

Quote:

Originally Posted by dgbiker1 /img/forum/go_quote.gif I'm wondering if the iphone amp & phones don't pick up the noise because they're parallel to the antenna whereas parts of the separate amp are perpendicular (assuming the amp and phone are held together).

In my experience, direction has a lot to do with it as well. Putting my phone right on top of the amp causes the sounds to get through, but putting them side by side doesn't.

 

[Nebby]

It's a sign you should go balanced