Electret microphone phantom powering idea

This article can be seen as continuation to my Powering microphones document and  PC microphone phantom powering improvements blog posting.

Jon Blackstone said in comment:

Tom -

I’ve got a solution for this that’s very simple, and is working for me.

I connect the ECM capsule (cheesy computer mic) directly to pins 2 and 3, and just pull pin 3 towards ground (pin 1) with a 47K resistor to create a 6V differential across the ECM.

(2)---------------+
                 MIC
(3)----+----------+
       47K
(1)----+
(both mic wires insulated from any metal casing

This avoids attenuating the signal as the 3-resistor scheme does. And I may be mistaken, but I think it’s even roughly “impedance balanced”, as the ECM floats much the way a transformer in a dynamic mic would (the 47K additional loading on one side being negligible).

Am I missing something?

And my comments to that was:
That stripping down of spaces is a feature of this blog system commenting.

I still understood the drawing anyway.

And I think that your idea is a really good idea indeed.
I have to test it.

I tested the circuit and got it working… So the idea works in practice. Here is prototype I used for testing:

This circuit can be viewed as kind of modification of this circuit from Powering microphones document:

                     _____
   HOT (2) ---------|_____|------+
                    47 kohm      |+
                                MIC
  COLD (3) ----+              CAPSULE
               |                 |-
GROUND (1) ----+-----------------+

But the suggested new circuit (fshown below) improves this design by getting rid of signal attenuation caused by 47 kohm resistor and the signal from mic is fed to XLR input in balanced format.

(2)---------------+
                 MIC
(3)----+----------+
       47K
(1)----+
(both mic wires insulated from any metal casing

There is still ther downside is that this circuit loads the phantom power in very unbalanced way which can disturb some older mixers. Other downside of this simple design is very high output impedance (logn cables atenuate signal high frequencies very much). Also one doenside od this new design is that both sides of elecret capsule are at quite high potential (easily over +40V DC with 48V phantom power). So this does not suit for PC microphones that expect to be grounded. But for discrete small electret capsules this works – make sure that mic case does not make accidental contact to anything (ground or people, 40V can cause some electrical shock that can be nasty). You have been warned.

I think thait improvemrnt idea somehow imperives the original deisgn, but it is not optimized and right in any way, but coudl be good enough in many cases. If you want to read how to do the adaptation correctly my Powering microphones document docment and Electret Microphones – Powering & Uses web page for details.

 

 

More material:

http://vogelchr.blogspot.fi/2016/04/superlux-hmc660-headset-and-ps-418-s.html

https://electronics.stackexchange.com/questions/257819/phantom-power-supply-for-electret-mic

http://education.lenardaudio.com/en/10_mics_4.html

http://sound.whsites.net/articles/mic-electret.htm

http://www.dutchforce.com/~eforum/index.php?s=1be5092ffbc368eda9287be7ada9d427&showtopic=3450

Phantom power adapter circuits:

http://www.audioimprov.com/AudioImprov/Mics/Entries/2014/12/18_Another_Chinese_mic_Circuit.html
https://electronics.stackexchange.com/questions/214861/humbucker-style-microphone-interference-cancelling
https://www.gearslutz.com/board/geekslutz-forum/764918-what-should-follow-electret-mic-capsule.html
http://www.circuitdiagramworld.com/test_circuit_diagram/Recording_and_Measurement_Microphones__OPA2134__1051.html
http://sound.whsites.net/project93.htm

 

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