The Plastic Disc At The Capsule End Of Mic PCBs...

I've noticed that most if not all mics have some sort of non conductive disc/ cylinder at the 'Top' end of the pcb, between it and the capsule. In some instances this material is obviously a form of shock mounting for the capsule, but in some others the material is very solid and clearly not to protect from vibration.
Is there a reason for this, or is it just cheaper and lighter than an equivalent metal disc? :?

chef

> most if not all mics

Condenser mikes....

> have some sort of non conductive disc

....are extremely high impedance. The leakage in untreated PCB material would cause hiss and random rumble. Quartz and glass have been used as insulators, today I'm sure Teflon is the logical first choice.

In DIY, the mike will work with good clean dry PCB material (glass-fiber much better than brown paper-phenolic), but after you get it working pretty good you should upgrade all insulation in the capsule-Gate path to get the leakage noise down as low as possible.

Cheers PRR! I actually thought I had typed "condenser" but clearly I didn't... That'll teach me for not previewing...
Thanks again!

chef

Sometime the plastic mount is shaped to "reflect" the sound the sound at an angle or a curve so as not to promote refections from the top of the grill to a flat surface.

After capsule design the grill and cavitiy it forms is the next hardest thing to get "right". I think the electronics are the easy part.

Nice one Gus!!! But one problem at a time...!
Would a chunk of PTFE do the right thing...?

chef

For the Hi-Z problem, yes.
I use perspex on mine - cheap and seems to do the job. I've also bought some teflon/PTFE tubing (from RS) to use instead of heatshrink in future.
Stewart

PTFE should be fine like PRR posted. You can also use plastic teflon tubing to pass thru a metal piece.

FWIW the solid state DIY microphone of mine at Zebras site has a machined nylon capsule mount/top plate and it seems to be fine.

I the USA you can find plastic cutting boards.
I believe they are high density polyethelyne(sp).
I cut disks with a cutter and then machine them in my lathe. IIRC good coax uses foamed polyethelyne(sp)

Gus - Nylon absorbs moisture from the air. I think that increases its conductivity. In NJ, you will soon know if it matters.

Cutting boards seem to be made of several different plastics.

> good coax uses foamed polyethelyne

Yes, but coax works at 50-100 ohm impedance. In low-level work the resistivity is not an issue. (In big radio work, even trivial leakage is big heat in the cable, but they go to ceramic beads at some point.)

However, my Heathkit VTVM uses rather ordinary coax for a lead and I've never had reason to suspect that the coax leakage is at all significant compared to the 10Meg input impedance. And that poor thing has been living in a humid climate for decades.