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 Forum index » DIY Hardware and Software
Electret humm...
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robynm



Joined: May 30, 2013
Posts: 10
Location: Melbourne

PostPosted: Wed Oct 30, 2013 6:08 am    Post subject: Electret humm...
Subject description: BZZZZZZZZZZZZZZZ
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So I just bought a Zoom H4N and I built myself a pair of earbud microphones to record my street wanderings. I bought some $2 earbuds, replaced the speakers with some cheap electret microphones from the electronics shop, and hooked them facing outwards. I just recorded my trip to the supermarket and it's awesome. But,

I'm getting hell of a lot of hum when I'm close to anything electric. I can even hear it from the automatic cash registers and the neon lights.

Now I've experimented a bit and if I hold the cable close to my computer I don't get hum. It seems to be only the capsule that catches the electric field.

If I grab the earbud (as in the plastic, doesn't even need to be the actual shell of the mike) in my hand the buzz gets way fainter.

If I touch the aluminium body of my computer while holding the earbud it goes down even more.

Because the issue seems to come from the actual capsule I doubt that a shielded cable will help. Some circuits involve cutting the trace that links one of the pins of the capsule to the shell and a moderately complex circuit.

I have only connected my two electrets to a stereo jack that goes in the back of the H4N. The recorder provides about 2.5V of phantom power through the tip (left signal) and the ring (right signal).

Anyone has a theory?

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JingleJoe



Joined: Nov 10, 2011
Posts: 878
Location: Lancashire, England
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PostPosted: Sat Nov 02, 2013 9:45 am    Post subject: Reply with quote  Mark this post and the followings unread

ground sheilding, that is what you need!

what's happening is simply inductance, strong electromagnetic fields are inducling current in your device, i assume it has sensitive receivers and high impedence circuitry? that would be congruent with what you have observed.

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robynm



Joined: May 30, 2013
Posts: 10
Location: Melbourne

PostPosted: Wed Nov 06, 2013 2:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Joe!

I tried to make a version 0.2 of the microphones the next day and I used a shielded cable this time (Ground all around, tip running through the middle).
The sensitivity to electric fields is way better but I still pick up some buzz if I'm really close (say less than 1m) to a 240V.

If I wanted to take this to the next step I would have to use triphase a la XLR right? Also, the capsule itself seems to be pickin up hum. Should I shield it within it's case with some sort of foil?

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