powering electret capsules

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ambi_doug

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Joined
May 21, 2005
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26
ok, i realise this may be some orders of magnitude below the specs of most stuff that is posted/designed/discussed here, but please bear with me, i'm only just starting out...

anyways, for my ambisonic project, i've decided to get at least something working using very cheap electret capsules. i've ordered 5 different kinds (which are also all quite similar, but nevermind), and realise i still need some kind of interface between the capsules and my preamps.

the spec sheets for the capsules i'm getting state operating voltage of 1.5 - 2V.
i want to avoid battery power, imo that's a crap solution... especially as i'm lucky enough to have this thing:
SCV pre-amp set
which can provide 15V phantom power, or i can take +/-15V straight from it's module supply rails (+15/0/-15).

i also understand that the performance of electret capsules can be enhanced by using a higher supply voltage (ie, higher SPL capacity and better SNR). how far is it feasible to push the capsules before they stop working?

bah, sorry for all these questions in one go... and this isn't even half of what i still need to understand :(

any help would be much appreciated.
(btw, my goal for this part of my project isn't necessarilty audiophile precision, more just to get something thrown together to get my ambisonic rig basically working... the high quality stuff can come later - if it works).
 
I've run pretty much all my little capsules at 12 volts with no problem. I think somebody here had done a little research and figured out that the FET used in 99.9% of those capsules was good to at least 24 volts, or something like that.

All you really need to do is scale up the resistor(s) to keep the current about the same. So if you're running 12v instead of 2v, multiply the resistor value by 6.
 
scodiddly,

ok, that's good news as far as the operating voltage goes.

also good news insofar as adapting to a different supply voltage is a as simple as calculating a new value for one or two components.

the trouble is, i still don't know which circuit that pertains to.

so far, i've been looking here: http://www.epanorama.net/circuits/microphone_powering.html

and i've seen linkwitz's circuits for the modified panasonic capsules.
incidentally, the capsules i have on order are not panasonic (they are projects unlimited (www.pui.com), which by the looks of their drawings should support the same mods.
also, as far as the linkwitz mod goes, i'm still unclear as to how that would connect to a balanced pre input.
 
I've run ones just like them at 9V before now. The noise didn't really go down much: The mechanical energy is of course so much lower than a large or medium diaphragm, hence the signal-to-noise can almost certainly never be truly stunning, and since they were electret, the actual capsule electrical output didn't increase with increased voltage, unlike an externally-biased condensor.

None the less, the improvement is worthwhile.

I just decoded parts of Saturday's Soundfield recording of the Brass Band using VVmic. I made two versions of one tune: Stereo and 5.1 strip-outs. I burned both to CD, the surround one encoded using DTS. About 900 appreciative audience members... You don't really notice the 'surround' aspect until there's an applause reaction during a tune, say after a solo, or laughter after a comedic cue... The frontal image is pretty dammned good compared to stereo though!

Keith
 
ok, i did a few sums based on my basic knowledge....

the linkwitz circuit shows a 9V supply through the FET and a 10K resistor to ground...
here:
http://linkwitzlab.com/images/graphics/microph1.gif

am i right in thinking that the current flowing thought the FET would be 9V/10K = 0.9mA ?

therefore, with a 15V supply the resisitor would have to go up to 16K6 (or thereabouts) to make the same current?

what about the other resistor shown on his diagram?

also, am i right in thinking that the connections as shown would be
hot - top rail
cold - grounded
gnd
for balanced connection?

i'm getting there slowly i think...


(also keith, i'd be interested in hearing a couple of samples of your recordings, especially in b-format :wink: )
 

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