Author Topic: electret mic bias  (Read 9237 times)

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tempus

electret mic bias
« on: November 04, 2012, 01:41:09 PM »
Hey all;

Maybe a bit off topic, but does anyone know the best way to bias an electret mic? Most specify a voltage range of 1.5 - 10 volts or so. In terms of distortion/headroom, is it best to bias it at the midway point (sort of like virtual ground), or is it better to get the max voltage? I know there are condenser mics that perform better at the actual 48v they're spec'd for, for instance.

This isn't the actual mic I'm using, but it's this kind of thing: http://ca.mouser.com/ProductDetail/PUI-Audio/AOM-5244L-R/?qs=sGAEpiMZZMsYQxPYEnGVuL%252bd%252bkvNNR2zoEwMwUhh6%252b0%3d

Thanks

Quackzed

Re: electret mic bias
« Reply #1 on: November 04, 2012, 04:53:20 PM »

2.2k resistor 10uf cap... worked well for me at 9v. btw theyre nice room mics ,very sensitive. and no, you don't want to bias em at middle vref, just max voltage via a 2.2k or so resistor. although i have seen other possibly better active circuits to power them...
http://www.scotthelmke.com/microphones.html
or just search...
alice mic
« Last Edit: November 04, 2012, 05:02:35 PM by Quackzed »
nothing says forever like a solid block of liquid nails!!!

Minion

Re: electret mic bias
« Reply #2 on: November 04, 2012, 06:52:36 PM »
Here is a Electret mic curcuit that will run off Phantom power from a standard mic preamp .....


Go to bed with itchy Bum , wake up with stinky finger !!

PRR

Re: electret mic bias
« Reply #3 on: November 04, 2012, 08:02:28 PM »
The 2-pin mikes, as long as they get a couple volts, more makes no difference. Resistor value has only slight effect on output level.

BTW: "good" 48V mikes work fine down to 12V supply 9V inside. The 48V-only mikes are either very special or have taken shortcuts based on the fact that most mike inputs do give near 48V.

All of these are optimized for phone and answering-machine duty. For serious work you'd want a 3-pin (backplate, source, drain) connection. The market for these is so very small that they are hard to find. There are tricks to mod the 2-pin mikes for higher sound levels.

Jazznoise

Re: electret mic bias
« Reply #4 on: November 04, 2012, 08:05:38 PM »
Hey, PRR, why is the impedance of that capsule so low?

I thought most Electret capsules needed to see a load near enough to 1G to avoid undue loading?
Expressway To Yr Null

Crontox102098

Re: electret mic bias
« Reply #5 on: November 04, 2012, 08:28:28 PM »
Recommend that you use a lower voltage to 9V, 48 Shorten lot its lifetime, I recommend this scheme, or you need to look in google simplest one based on a 2N3904 transistor having high gain.

I'm Carlos.

I speak spanish, just in case you do not understand what I say.

tempus

Re: electret mic bias
« Reply #6 on: November 04, 2012, 09:06:08 PM »
Wow - thanks for all the replies guys - got way more info than I needed here. Re-reading my original post I can see that I didn't ask my question as clearly as I could have. I already know how to bias one of these (have been working with them for years) and I don't need a mic pre (have been building those for years too), I just always wondered how much voltage would be optimum for this type of mic, which has been answered.

What I need to do is build a biasing circuit that will give me the required voltage no matter what it's plugged into, i.e., if I plug it into a board with an actual 48v phantom power supply, I want it to get, say, 5v (just picking something in the safe operating range here). The same if I plug it into some piece of gear that has less than 48v for its phantom (and there are things that do).

The obvious solution is a voltage regulator, but the max I/O voltage differential is usually 35-40v, so if I do plug into a 48v (or slightly more if things aren't quite in spec), I'll be pushing my luck on the regulator. I suppose another option is to use a zener diode to make sure I get the right voltage... I'm open to other suggestions though if any one has any ideas.

Thanks

Minion

Re: electret mic bias
« Reply #7 on: November 04, 2012, 09:50:31 PM »
The schematic I posted will work at pretty much any voltage down to 9v up to 48v phantom , it uses a 2.4v zenner diode to regulate the voltage so the capsule only gets 2.4v no matter the input (within reason).......
The schematic for a Naiant MSH-1 (now discontinued) which used a panasonic electret capsule .......
http://www.naiant.com/naiant/microphones.html
Go to bed with itchy Bum , wake up with stinky finger !!

Minion

Re: electret mic bias
« Reply #8 on: November 04, 2012, 09:54:48 PM »
PS: 48V phantom power can only supply about 14ma (because it is fed through two 6.8k resistors which limits the current) so you shouldn"t fry a regulator or zenner ....... 
Go to bed with itchy Bum , wake up with stinky finger !!

PRR

Re: electret mic bias
« Reply #9 on: November 04, 2012, 10:17:42 PM »
> most Electret capsules needed to see a load near enough to 1G

There's three levels.

The naked capsule needs load >>1Meg for decent bass.

The $2 capsule-units such as Tempus linked-to (or in many electronic gizmos) *have* an FET buffer inside, and can face loads under 10K.

The complete mike with XLR and everything is more complicated.

Tempus needs to interface the $2 capsule to various XLR+Phantom inputs. The plan posted by Minion is an example. Phantom from 10V to 80V is all regulated to 2.4V. (Over 80V, the resistors may smoke.) However it "floats" the capsule shell off of ground. You can't mount the capsule with metal (you want rubber anyway), and you may need a grounded screen around the capsule to ward-off hum.