capsule polarity?

[hitchhiker]

I've been comparing different mic circuits and I see that some have the diaphram to ground and others have the backing plate to ground.

My question is .... Are capsules bi-polar?...in other words, can they be put
in either type of circuit?

 

[bradb]

Maybe the capsules you are coming across are interacting with the amplification circuits in a different way.

Maybe the question could also be phrased like, is a condenser mic capsule more like an electrolytic or a standard ceramic capacitor.

??

 

[SSLtech]

I don't get Brad's question at all...

Anyhow... Grounding the diaphragm will attract less dust, which is a consideration. Other than that, the polarity DOESN'T change, since no matter whichever side is grounded, the capacitance still increases with an increase in air pressure.

'Polarity' as such isn't affected by which plate is grounded, however, some capsules/circuits cannot do multi-pattern without grounding one plate or the other, see Jakob's notes on the subject.

Wait... I think I get Brad's question. -But no. It's usually to do with common backplates etc. If backplates aren't a problem, then it's prefereable to ground the diaphragm for dust-attraction reasons.

keith

 

[hitchhiker]

thanks Keith,

I'll look for Jakob's notes.

Lance

 

[SSLtech]

Here's a cut-n-paste:

We want to keep the front electrode of the capsule at ground potential - 0V - at all times, both to act as a shield for incoming electrical disturbance and to avoid electrostatically attracting too much dust from the environment. So to keep a voltage charge across the capsule, we bias the center electrode by the means of two 470K resistors dividing our 160V supply voltage in half - resulting in +80V.

Now we have -80 volts at the front electrode, referred to the center electrode. If we now bias the back electrode with the same (-80V ref. Center = 0V polarization voltage), a positive sound pressure on the back capsule will have the same voltage-potential effect on the center as when applied on the front capsule. This sensitivity pattern is then OMNI directional.

If we polarize the back electrode at +80 Volts, no voltage difference will exist between this and the center electrode, already offset at +80V. This in effect mutes the back capsule, resulting in the CARDIOD directionality.

At last, if we polarize the back capsule at +160 Volts, we'll have a charge of +80 Volts relative to the center electrode. Now a positive sound pressure applied to the back electrode will produce an 180 degrees out-of-phase signal compared to the front capsule. When applying a sound pressure from the side of the microphone, so both capsules sees the same sound pressure, the signals coming from the two capsules will be in opposite phases, effectively canceling each other. This is the FIGURE-OF-EIGHT directionality.

That's using a common-backplate (3 terminals). If you have 4 terminals (two seperate backplates), you can do it differently and only need one voltage instead of two...

Keith

 

[hitchhiker]

thanks again Keith,

That's exactly what I needed to learn.

cheers,
Lance

 

[Gus]

Keith wrote

"Grounding the diaphragm will attract less dust"

Keith this post is meant as a discussion starter

I have read that as well and I have two questions about it. I don't think it matters.

I would like to seem some posts from others on this, I think it is something that at first read makes sense enought that one does not question it.

1) Does 40V to 60V realy make any difference in the real world?

2) What charge is on the dust in the air on average?

I think if this was real effect microphones would be attacting dust like the front of a CRT like a TV or PC monitor.

Plus the Grill and body of a condenser microphone should be grounded and the dust would have pass the grill.

 

[SSLtech]

It's real! -I know 48V/80V/160V doesn't seem like much but:

Pull out modules from a V-series or an SSL E or G, and you can observe the traces on the motherboard for the 15V/18V DC power rails have attracted a fur coat, where the ground rails don't.

A vocal mic is also in a stream of contaminated air, where console modules only have a very tiny amount of air getting to them. After cleaning a number of mic capsules, I wonder how some survive! :wink:

It doesn't matter that the grill is grounded, this isn't electrostatic interference as such, dust particles are attracted to charged surfaces, no matter how many un-charged screens they've made their way past.

Keith