issue with electret condenser mic

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.)

What makes you think it's the Electrets? A rumbling noise can be an air current across the microphone. The popping could be a decoupling problem in the power supply.

That's a fair question.=20

In the case of the Cassiopeia PDA (also happening in an old NEC "netbook" t= ype PDA), the rumbling noise happens in isolated places where no air is mov= ing and everything is quiet (ie in an automobile inside the garage [the eng= ine was off, btw], in a walk-in closet w/no vents, etc.) The Casio & NEC ar= e from the late 90s & the noise only began appearing in the past couple of = years.=20

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 th= e ones with bad mics in places where I want to listen to music and don't ne= ed the microphone for recording.)=20

The cover of one adapter popped open and got lost so I cannibalized it to a= dd 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 & condense= r, and the condenser is noisy, hence my question.=20

Also I am generally curious about this since I have 3 devices with electret= related noise issues, and it would be nice to fix.=20

PS here are the devices in case you are curious=20

---------------------------------------- The headphone adapter:=20

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

---------------------------------------- The guitar input:=20

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

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

NEC MobilePro 770

Casio Cassiopeia E-100 Palm-Size PC

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.