Electret humm...

robynm · Joined: May 30, 2013 Posts: 10 Location: London

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?

JingleJoe · Posted: Sat Nov 02, 2013 9:45 am Post subject:

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.

robynm · Joined: May 30, 2013 Posts: 10 Location: London

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?