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Re: plug-in power mic

Subject: Re: plug-in power mic
From: "Klas Strandberg" klasstrandberg
Date: Fri Jul 9, 2010 2:35 pm ((PDT))
Sorry to say, the question does not have a straight answer.

A fet is a fet. A IC is a fet with a electronic circuit in the same
body. How the FET IC works depends on this internal electronics.

But the gain of a fet and a fet IC is determined by the drain load.
The drain load, in turn, depends on the driving voltage. The more
voltage, the lesser load (higher resistor value),  the more juice you get.

So far you can say that you get more output with a higher voltage.

But if the input impedance is, say, 2,2 kOhms, (like LS-xx and Zoom
H4n) there is not much to gain by having a high drain resistor (low
load) with a higher voltage, as the higher gain that you get  will
get lost by the low input impedance.

To maximize the output of a FET, the entire chain has to be right:
Take a Telinga PRO7W handle and a Twin Science microphone, for example.
The mic capsule itself faces the fet IC, which has a very high input
impedance which means a very low load. The fet faces nom 7,2v which
means that the drain resistor can be as high as 23k, which means a
low drain load. This mic output, in turn, faces another very high fet
input in the handle, which means another low load. The output
impedance of the PRO7W handle is less than 200 Ohms, which means that
a 2,2k input impedance of a LS-xx or a Zoom recorder is high enough.
In such a case the entire chain is correct, and you can say that "the
higher the voltage, the higher gain."

Sony recorders don't use a drain resistor, but a active device which
I have never understood. The principle is really smart, though, as
you get a much higher input impedance than 2,2k, which means a lower
load, especially if you only have 1,5v to deal with.

Even though the battery voltage is only 1,5v, the PIP input impedance
of a typical HiMd is around 10kOhms, isn't it??
Which.... means that a HiMD input is a low load on the mic's fet
drain, which is one of the reasons why HiMD's are so low noise.

You are right about voltage and max. SPL. However, if you drive a fet
with 9v and a 27k resistor, and put this output via a condensor into
a 2,2 kOhm impedance, then you get some -20 db attenuation by the low
impedance and the SPL is gone anyway. You need a impedance follower
in between, with a high input impedance and a low output impedance.

You are right that the limited SPL is of little importance to the
naturerecordist.
And if you do "the chain" right and get maximum output from a
microphone, you also amplify the self noise.
For a naturerecordist, I would say that - in practice - maximizing
the output from a microphone is only required when you have a noisy
recorder input which must be overrun by this high output / low impedance.

I hope I wrote it all correctly.

Klas.


At 04:20 2010-07-08, you wrote:
> > So guys, you're saying that mic sensitivity will vary with PIP
> > voltage? Is this limited to the unnamed capsules Mike has tested, or
> > is this a general rule of thumb that will apply to other PIP mics
> > also, like WL-183s?
>
>I've been thinking that the sensitivity of PIP mics is constant with
>bias voltage but that the maximum SPL before distortion
>varies--which is why concert tapers use 9V supplies, but nature
>recordists don't have to worry at 1V or even less.
>
>But I may be wrong. Or there's a difference between electret mics
>with FETs and those with ICs?
>
>Can the mic builders clarify this?
>
>-Dan
>
>------------------------------------
>
>"While a picture is worth a thousand words, a
>sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause
>Yahoo! Groups Links
>
>
>
Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
email: 
website: www.telinga.com









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