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Strange feedback on an FM transmitter

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SlicRic

New Member
Hi,
I have just recently modified a small FM transmitter which I have built to hold an external microphone, the system does work, but when I hold the external mic away from the transmitter, sometimes, the reciever starts emitting a high frequency feedback like sound, but if I touch the mic, or even bring my hand near, or hold it up to the transmitter it goes away, the internal mick(Half in the baox I.E) does not have any problem, presumably because it is near the transmitter circuit, also sometims holding the mick away from the transmitter a little farther stops it, and both mics are on different frequencies when I try t tune them on a reciever, they evn vary when I move the microphone. Neither mics have any shielding, can anyone help me with this?

Thanks, SlicRic
 

audioguru

Well-Known Member
Most Helpful Member
A microphone has a very low level signal and must be connected wirth shielded audio cable.
The wires from my electret mic to my FM transmitter circuit board are only 5mm long so i did not use shielded audio cable to connect them.
My Sound level Indicator project has a shielded audio cable that is 2cm long and it does not pickup interference.

Maybe the high frequency is acoustical feedback howling from the radio? Then keep the transmitter away from the radio or turn down the volume on the radio.
 

SlicRic

New Member
Thanks Audioguru, so when the mic is closer to the transmitter there is no need for shielding? because it say cancels out the feedback? Also why does touching it stop the problem as well?
Thanks again, im quite new to the audio side of radio, im still learning!
SlicRic
 

audioguru

Well-Known Member
Most Helpful Member
I suspect the mic on your transmitter hears the sound from the radio and the sound goes around and around in acoustic feedback howling.
Touching the mic changes the frequency of the transmitter so the feedback stops.
 

Sceadwian

Banned
If there's an AGC stage on the mic pre-amp the 'tap' of the microphone will drop the gain dramatically and temporarily reduce the gain bellow the feedback threshold. I've noticed that on many small mic amp circuits that have a high gain preamps, like baby monitors.
 
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