Ok so basically I have a small FM transmitter that has a small electet microphone that I want to replace with a simple RCA jack so I can plug my mp3 player in directly to it and use that instead of having to put up with the wonky microphone. It feeds back and is just generally displeasing. Now the thing is that the microphone connects to two terminals and the RCA jack I have has one solder tab. Do I just simply connect that one tab to both terminals? Heres a pictorial diagram to help you out.
Ok so basically I have a small FM transmitter that has a small electet microphone that I want to replace with a simple RCA jack so I can plug my mp3 player in directly to it and use that instead of having to put up with the wonky microphone. It feeds back and is just generally displeasing. Now the thing is that the microphone connects to two terminals and the RCA jack I have has one solder tab. Do I just simply connect that one tab to both terminals? Heres a pictorial diagram to help you out.
So do I simply solder both of the terminals to the RCA jack or do I need an RCA jack specifically with two terminals?
Sorry for such a boneheaded question its just that I've never worked with audio and jacks and such.
Here's some schematics: http://www.epanorama.net/circuits/line_to_mic.html
Use the "20 dB PAD for line to electret microphone input" one but maybe add a trim-pot between your output device and this schematic. This is what I did to use one of those little "FM Transmitter Bug" kits as a cheap mp3 transmitter.
It's hard to tell from your drawing if you are trying to solder an RCA jack or a plug. In either case, there has to be two different connections, one to the isolated center of the jack/plug and one to the outer ring. Usually, each of these has its own soldier tab but sometimes the tab for the outer ring is missing (or it's supposed to be panel-mount in which case your supposed to use a little washer-type-of-thing with a soldier tab on it). I hope I made this clearer; it's hard to describe these little parts. Anyway, soldier your 'positive' audio wire to the center part of the RCA jack and soldier the ground audio wire to the outer metal ring somehow and you'll be fine.
I did something similar with a little low power transmitter board, and I used this circuit to put it all together:
The resistors load the stereo output of the MP3 Player or PC Sound Card. Mine was used for a sound card line out at 600 Ohms, you can use lower Ohm values for headphone jacks.
(thanks to MRAM for pointing out my error below - this red text show the previous WRONG INFO: I suggest the power blocking diode (any low voltage (6-25v) unit from the junk box should work)
CORRECTED INFO - a power blocking capacitor is suggested by a more skilled contributor, as follows
With the low value of input resistance, a larger electrolytic
would be in order. The input resistance along with the capacitor forms a high-pass filter which if the cap is too small would limit lower frequencies.
As a passive 1st order RC filter, the cutoff frequency would be calculated using the capacitor value and the input resistance of the transmitter audio input as
f=.159/RC
Solving for C you get
C=.159/RF
where C is capacitance, R is input resistance and F is the cutoff
frequency.
In your case, about 4.7 microfarads should take the frequency down to around 50 Hz with an input resistance of 680 ohms. Of course the actual audio circuit of the transmitter would affect this calculation.
Make sure polarity is observed unless you have a non-polarized cap.
(because electret mic modules often receive a low voltage DC input to power the microphone, and you don't want that wandering up the line into your audio device. Older units had 3 leads - audio, power and ground - but newer units take power and audio in the (+) terminal and separate the two internally.
And Tomi's circuit linked above, repeated for reference:
Experimental broadcasting for a better tomorrow!
I would think you'd want a blocking capacitor in place of the diode.
As shown the diode would only conduct positive audio peaks that exceed the "phantom power" DC supplied by the microphone jack. This circuit would act as a biased rectifier.