various mic questions...

[michaelq]

what exactly is FET impedance conversion? i'm trying to build my own condenser mic using an old tapeop article found at http://www.prosoundweb.com/recording/tapeop/buildmic/buildmic_16_1.shtml ...one problem is that the mic cartridge suggested is now obsolete. i called digikey and they gave me "similar" options, but with one the frequency was from 100-10khz, and the other 100-15khz. the 100-10 khz seems to be cutting too much out of the high end, but apparently that one does have an internal FET impedance converter. (not sure about the 100-15khz but on digikey the # is 359-1011-nd ) as stated earlier in this post i do not know what FET impedance conversion is so i dont know how to make an external one if needed. if anyone can explain a. FET impedance conversion or b. have any suggestions for omnidirectional mic cartridges with ranges preferrably close to 20khz (or even places to look other than digikey) it'd be appreciated. thanks in advance. -michael q

 

[darnold]

100Hz-10KHz is weak for both condenser and dynamic mics. 100Hz-15Khz would work ok with a dynamic mic but not really on a condensor mic. Well i guess thats not all the way true, that has no sub frequencies at all so it would only work good for things that dont need sub bass frequencies.

I believe the FET Impedence conversion is put in after the diaphragm. FET i believe just means non tube. After the signal transfers through the diaphragm, it is a very high impedance (or low, cant remember) so it needs to be converted to a usable impedence to the preamp. Im sure there is more involved but this might help answer your question. Im sure other people here can answer your question better because i dont know all the details.

Danny

 

[crazydoc]

It just means that it has an integral FET incorporated in the capsule - you don't need an external FET in the circuit. I think most all the electret capsules have the integral FET.

Here's Digikey's manufacturer's page for that capsule (and others.)
http://dkc3.digikey.com/PDF/T042/1116.pdf

I think I'd get the 359-021 as it has leads on it, and a slightly better sensitivity.

Here's one of Panasonic's with the full frequency response:
http://rocky.digikey.com/WebLib/Panasonic/Web data/WM-61A,B.pdf
It's Digikey part #P9925-ND, and they have about 20,000 in stock at $1.83 for 1.
It's not as sensitive as the spec'd part, but a slightly higher S/N ratio.

 

[crazydoc]

mq
Here are some more links to sites about making electret mics:

Linkwitz modification (you'll hear about this if you delve into this very deeply)
http://www.linkwitzlab.com/images/graphics/microph1.gif
https://homerecording.com/bbs/showthread.php?t=111051
https://homerecording.com/bbs/showthread.php?t=36674
http://www.linkwitzlab.com/sys_test.htm

Here's a mic builders newsgroup which is heavily into building electret mics:
http://groups.yahoo.com/group/micbuilders/

This site has links for parts, and has capsule mods and some nifty DIY mic circuits and configurations:
http://mp3forkidz.com/mic/

Have fun!

 

[cornjchob2]

what exactly is FET impedance conversion?

They're using transistors instead of a transformer to match impedance. FET = field effect transistor. It's a powered mic as well, I'd imagine?

 

[crazydoc]

They're using transistors instead of a transformer to match impedance. FET = field effect transistor. It's a powered mic as well, I'd imagine?

Actually the FET is used instead of a vacuum tube to match the high impedance of the condenser. Most condenser mics use one or the other, though CAD has some that use op amps.
And yes, all condenser mics need power, whether derived from phantom power, battery or dedicated power supply.

 

[cornjchob2]

Actually the FET is used instead of a vacuum tube to match the high impedance of the condenser. Most condenser mics use one or the other, though CAD has some that use op amps.

I could've sworn I've heard "transformless output" a bunch of times from various condensor makers--if there were a transformer to match impedance, it would be at the output as opposed to directly after the capsule? Are there any other methods besides tube and fets?

 

[crazydoc]

...it would be at the output as opposed to directly after the capsule?...

Yes. The capsule has to be connected to the electronics of the mic's internal amplifier through some mechanism to couple the extremely high impedance (low current) of the capsule to the lower impedance of the rest of the circuitry.

The output of the mic's internal amp then has to be made compatable with the input of the next stage of the signal chain, generally a preamp, and also converted to a balanced signal. This can be done either with a suitable transformer, or by an electronic circuit - "transformerless", or by a combination of both. See the circuit in this thread:
http://www.homerecording.com/bbs/showthread.php?t=129008

This has nothing to do with the input from the capsule being FET or tube coupled.

Edit: although as far as I know, every tube mic has a transformer coupled output as it is so much simpler electronically to do it this way because of the voltages and impedances involved. It's probably acoustically superior also, avoiding the capacitors that would be involved otherwise.