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Old 11th November 2010, 12:33 AM   #1
teleman is offline teleman  Norway
diyAudio Member
Join Date: Jul 2008

I have an AKG C411L condenser contact mic,but without the power pack.So now I've built this simple buffer to power & boost the signal as well.

When I connected the C411L to it,although the mic is working,the circuit is "motorboating .The buffer works otherwise just fine with other back electret mics like Panasonic WM61 etc.I have no idea why this mic is behaving this way! Buying a power pack is out of the question,as I think it is unreasonably expensive!

The buffer is running on +/-12v dc , proper psu & the load resistor feeding the mic is 15k

As you will notice,in the original wiring, both pins 2 & 3 are tied to the mic "hot"! wire.

This is my modified wiring
Pin 1 screen
Pin 2 blue to buffer input
Pin 3 red to the 15k supply resistor.

Can anyone please tell me what I've done wrong here?
Attached Images
File Type: jpg AKG C411L.jpg (9.1 KB, 87 views)
File Type: jpg MIC PREAMP.JPG (154.7 KB, 86 views)
Attached Files
File Type: pdf C4114055d1f49698a.pdf (115.8 KB, 28 views)

Last edited by teleman; 11th November 2010 at 12:41 AM.
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Old 13th November 2010, 09:27 PM   #2
mjf is offline mjf  Austria
diyAudio Member
Join Date: Jan 2009
Location: Austria
i am no mic expert.but i saw a few older versions of akg headsets.the input configuration was:
instead of 15k there was 8,2k..........perhaps you will use 10k - similar to the attached schematic.
4,7k left out,none.
8,2k behind the input cap..........here 10k,not far away.
i guess the motorboating will not stop,but perhaps try it out.
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Old 13th November 2010, 10:39 PM   #3
teleman is offline teleman  Norway
diyAudio Member
Join Date: Jul 2008
Hi mjf,
Good to hear from you again! I will of course try your suggestion in the morning & will report back.What puzzles me is that the recommended input impedance value for this C411 mic is > 1k ohm!

So do you think making the buffer to an inverting input instead will be any good?

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