I have a mobile headset setup which could be summed up like this:

phone→30cm cable→aluminum box→ 100cm cable → headset

The aluminum box is 50x50x20mm and contains one potentiometer and 2x 3.5mm female plugs to separate the mic and audio signals at will.

I want to place a ferrite bead to reduce EMI coming from the mobile phone.

It would be a lot more convenient for me to have the ferrite bead inside the aluminum box instead of outside...

Is there any reason important enough regarding sound quality or EMI removal to convince me not to place the ferrite bead inside the aluminum box (instead of outside- on the 30cm cable)?

Should the aluminum box be grounded in any case?


1 Answer 1


The ferrite choke is a CM impedance transformer, so CM noise coupling is high Z at RF and low current.

In order to attenuate , a simple RF cap shunt to gnd creates a large impedance divider for RF so yes a Gnd is necessary and any loops inside th box benefit from shielding at mobile frequencies, but if you gave a ground plane, and good RF decoupling, it is not necessary to have a grounded metal box.

CM chokes also come in SMT for board mount, ~100uH with high SRF, to make a Pi (LP) filter. A good search can visualize what I said.

  • \$\begingroup\$ Great thanks for that, I will look into it. Do you have an answer to my first question? \$\endgroup\$ May 28, 2017 at 9:05
  • \$\begingroup\$ ethernet baluns are always inside at both ends since bidirectional at point nearest entry to ptrevent further radiation inside box. same is true for applications needing CM chokes (Baluns), eg AC power and audio cables \$\endgroup\$ May 28, 2017 at 14:33
  • \$\begingroup\$ Regarding your comments, do you think it would be better to use a plastic case instead of an aluminum one? Is there a reason strong enough to stay with the grounded aluminum one? \$\endgroup\$ May 30, 2017 at 5:01
  • \$\begingroup\$ a grounded case offers better noise immunity, to a poor layout and design, but is not essential. USB headset amps are inline with tiny plastic cases. \$\endgroup\$ May 30, 2017 at 7:51

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