Mic mod for audio testing (change FET?) - diyAudio
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Old 21st January 2004, 12:30 AM   #1
effect is offline effect  Australia
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Default Mic mod for audio testing (change FET?)

Ive searched these forums and found a mention of changing the FET in a condenser mic such as the wm-60ay, but havent found any info on it.

Can anyone tell me how much difference to the noise changing the FET would make, if at all?
and how hard was it to do?
In the same thread someone mentioned using an opamp onstead of a FET, I cant remember all of the ECM theory I learnt but I guess that would work since the ECM is permanently charged?

I have taken apart ECMs before and seen some with a welded fet gate lead, and others with just the pressure of the ECM case holding the gate lead on. I guess the hard part might be to put the ECM back together and have not damaged anything.

Any feedback, and experience welcome.

Effect
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Old 21st January 2004, 04:17 AM   #2
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Here is the link from Linkwitz's home page.
http://www.linkwitzlab.com/sys_test2.htm

After reading instructions on this, I am not sure if the WM 60AY and the WM 61 have the same modifications procedure. I don't know if you feel the same way.

There are people on this board who have done this modification, so if you have any questions, I am sure they will post here. If not, I can contact somebody on this board who I know for a fact has done this modification.
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Old 21st January 2004, 05:16 AM   #3
effect is offline effect  Australia
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kelticwizard

I know about the Linkwitz mod I have already used it succesfully, on a couple of different model of ECM, this increases the max SPL that the ECM will handle without distortion.
However what Im talking about is bypassing the internal FET altogether and using external circuitry, and wondering if there are any noise benefits from this (eg. by using less noisy FET)
I can see there would be other possible benefits like being able to further increase the max SPL (assuming the diaphragm itself has the headroom for this and not go into non-linearity).

So I would like to hear from anyone who has tried any type of mod that involved bypassing or swaping of the internal fet, successful or not.
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Old 23rd January 2004, 06:00 AM   #4
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Default ^bump

thats interesting as i want to do this too-i have RS analog meter,and want to upgrade at a later date-and dont know if the 50$ behringer mic will be any better...

Cheers!
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Old 23rd January 2004, 07:21 AM   #5
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Hi,

There are numerous threads around here concerning modding the Panasonic. Replacing the internal fet does not make much sense regarding noise. Note that the capacitance of the diaphragm is very small. All external circuitry directly to the diaphragm ads so much extra capacitance that you loose a lot of sensitivity. For lower noise you need simply a bigger diaphragm and/or higher bias voltage for the diaphragm. Since it is electret you can do nothing about build in bias voltage.

BTW the Behringer ECM8000 contains the Panasonic capsule.

Cheers
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Old 23rd January 2004, 09:53 AM   #6
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I don't really know, but I would have thought using an external fet could actually increase noise, unless you keep the leads very short (which may not be practical if you are using for speaker testing, due to not wanting any bulky stuff near the mic due to refections). I'm speculating that the signal level from the mic would be a lot lower, and therefore more suceptable to noise........

I've done the linkwitz mod (haven't tried the one with the 9V battery though, which looked kinda interesting, but it would probably overload the preamp I'm going to make.)

Tony.
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Old 23rd January 2004, 01:54 PM   #7
Wizard of Kelts
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Default Re: ^bump

Quote:
Originally posted by mikee12345
thats interesting as i want to do this too-i have RS analog meter,and want to upgrade at a later date-and dont know if the 50$ behringer mic will be any better...

Cheers!
For the modifications to the Radio Shack analog meter, go here:
http://www.gti.net/wallin/rsmeter.htm

Just click on the catalog number of the meter, and the instructions for that catalog number will appear.

I believe Eric Wallin, the author, is also the fellow behind Speaker Workshop, because Speaker Workshop makes mention of the Wallin Jig.

PS: The page also contains a detailed description of the Frequency Response Curve, so that you can make accurate measurements even without modifying anything. However, the modifications look easy, cheap and fun enough that I am sure you will want to do them eventually.
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Old 24th January 2004, 07:39 AM   #8
effect is offline effect  Australia
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Pjotr said:
Quote:
BTW the Behringer ECM8000 contains the Panasonic capsule.
Well thats good to know.
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