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Default issue with electret condenser mic

I have a couple of devices with electret condenser mics, one is an old Casio PDA, another is a 1/8" headphone adapter with a built-in microphone (for iPhone/iPad). Both devices have working mics, but recordings are noisy. The noise is slightly different in each case. With the PDA, recordings have a rumbling noise over them. With the iPhone adapter, recordings have a popping sound that appears every second or so. Does anyone recognize these symptoms and can they be fixed with a new electret mic element, or by some simple modification such as wiring a resistor or capacitor in? (I have a few electret mics taken out of old radios/etc, that may still be good.)
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Default issue with electret condenser mic

That's a fair question.

In the case of the Cassiopeia PDA (also happening in an old NEC "netbook" type PDA), the rumbling noise happens in isolated places where no air is moving and everything is quiet (ie in an automobile inside the garage [the engine was off, btw], in a walk-in closet w/no vents, etc.) The Casio & NEC are from the late 90s & the noise only began appearing in the past couple of years.

In the case of the iPhone adapter, I have 5 or 6 of these (ie for the car, living room stereo, radio in garage, for work, etc.) I am only getting the popping noise from 3 of the adapters I ordered separately from the first 4. The noise occurs in both my iPhone and iPad, only with the 3 adapters, and does not happen with the other 4 adapters, so I'm pretty sure the problem isn't in the power supply. (I paid less than $1 each & can generally use the ones with bad mics in places where I want to listen to music and don't need the microphone for recording.)

The cover of one adapter popped open and got lost so I cannibalized it to add a headphone jack to below iPhone 1/4" guitar input. That part is working great. However I added a DPDT switch to go between guitar input & condenser, and the condenser is noisy, hence my question.

Also I am generally curious about this since I have 3 devices with electret related noise issues, and it would be nice to fix.

PS here are the devices in case you are curious

----------------------------------------
The headphone adapter:

Headset Hands Free 3.5mm Adapter + Microphone for Apple iPod Touch iTouch 1 2 3 iPhone 2 3g 3gS

http://www.amazon.com/Headset-Adapte...482629&sr=8-16

http://www.amazon.com/Headset-Adapte...482655&sr=8-41

http://www.ebay.com/itm/Handsfree-ea...em19c9d0b7f c

http://www.ebay.com/itm/Handsfree-ea...em19c9da22b 5

----------------------------------------
The guitar input:

Peterson 3.5mm - 1/4in iPhone/iTouch Adapter Cable White

http://www.amazon.com/Peterson-3-5mm...2&sr=8-1-spell

----------------------------------------
The Windows CE devices:

NEC MobilePro 770
http://www.hpcfactor.com/hardware/de...tion.asp?d=129
http://www.necam.com/mobilesolutions...bilePro770.pdf

Casio Cassiopeia E-100 Palm-Size PC
http://www.amazon.com/Casio-Cassiope.../dp/B00000J0EN
http://pdadb.net/index.php?m=specs&i...ssiopeia_e-100
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Default issue with electret condenser mic

Anyone?
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Default issue with electret condenser mic

I'd go with Michael's suggestion. Put a 'scope on the B+ to the mics (in all
the devices), and see if there are any disturbances. It's certainly a
reasonable place to start.


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