Hi all,
I am modifying a digital recorder board that uses an electret mic for recording the sound into an ISD 2560 chip. I would like remove the mic and use a 1/8" mono jack so I can input .wav files directly from my computer. The board runs on a 9 volt battery and the mic has 9VDC across it. Is there a way to eliminate or otherwise negate this voltage so my sound card isn't damaged? Perhaps an appropriate cap in series? All advice appreciated.

 

Yes you can use a capacitor values between 0.1uF to 10uF without problem. If you use an electrolytic capacitor, be sure that the positive connection of the capacitor is connected to +9V side.

Another problem is the rather "large" output signal of your sound card compares with the small signal from the MIC. This has the possibilities of overloading your chip. If you cannot control the amplitude of the .wav file to avoid distortion, then you will need to add a variable resistor volume control between the sound card and the chip's input.

 

Thanks very much. I've used this solution in the past for line voltages of ~4VDC but didn't know if it would work with 9VDC. I've used 100uF electrolytics - perhaps overkill?