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5th element 11th February 2004 06:31 PM

Will this work for a WM61a?
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Here is a circuit I have drawn up, very basic, the 1pf cap represents the cap in the capsule, the opamp to the right is just there to represent the preamp.

Cheers Matt

Steven 11th February 2004 08:37 PM

2 Attachment(s)
Hi Matt,

No, this will not work. You've short circuited your input signal to the amp with the battery. A better way is to do it like in the picture below.
The external drain resistor of the FET in the capsule should not be too big, as the output impedance of the capsule is not so high (for most electrets it is about 10kOhm). High drain resistors cause some compression of the signal and more distortion. Typical values that can be used are 2k2 to 4k7.


5th element 11th February 2004 09:57 PM

Ah yes now that I look at it it is clear what you have said. I didnt really look at the diagram to see if it would or not work, I just copied it from somewhere else.

Although the circuit is faultly is does infact work to a certain degree it does successsfully charge the plates and retrieves a signal, but at a very low level which makes sense if the output is shorted.

I have now made the small adjustment from mine to yours and it works flawlessly, albeit with slightly different vaules. Measuring a SS d2905/97 now give the same as scans measurements. I had been using a dynamic mike, this was useless for measuring as its resonance is at 2khz, slap bang where u dont want it.
The only saving grace the dynamic has is its vocal performance, im quite a good singer and had it connected into a guitar amp which produced very nice results, its got very nice clarity for a 5 module.

Edit - Where are my manners!! I almost forgot to thank you!
so there we are cheers, the things working great!

Steven 12th February 2004 07:29 AM

You're welcome.
The WM61 is quite small and (therefore) a little noisy, but being omnidirectional it's frequency response is almost ruler flat from 20Hz to 16kHz, making it a nice measurement microphone. Not only for tweeters.
In case you need a spec sheet:


Centauri 12th February 2004 08:11 AM


Thanks for posting that spec sheet link. I have a WM60 and was not sure on its low end response.


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