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I am trying to connect the speaker out jack from a PC Sound Blaster card to my cell phone, a Nokia e72, using the headset 3.5mm connector. I have already converted stereo to mono and routed one of them to the correct ground and mic pin in the 3.5mm connector, but I get a lot of noise.

What kind of device/product could I use in between the PC and the phone in order to reduce the noise.

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Try a better power supply and make sure your computer is properly grounded.

I think beyond that I'd start breaking out the highpass filters for constant noise, lowpass filters for ground loops, and maybe cut my power back if I'm getting clipping.

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Do these same cables produce any noise in any other application? As in, is the issue with the cables you are using?

Also, using the PC volume control, have you muted all extraneous sources? (leaving the CD player unmuted can produce quite a bit of noise in low volume situations).

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The main issue here is usually that the microphone connector on your handset is used to work with much much lower signal levels. In best case it could help to reduce the volume on the soundcard output (line-out) to a much lower level by lowering the output volume to a minimal level. However also the impedance is different and might require some adaptation. The circuit needed to connect a line-out to an e72 might also depend on the type of mic supported by the phone. I found some basic information here here.

What puzzles me a bit is why you want to do such a thing. The e72 seems to be able to play digital audio files directly (transferred via memory card, bluetooth or cable I guess). So it might be much easier to record on your PC and then convert to any suitable digital format for transfer. There should be some software included with the device already. For audio manipulation I can recommend using Audacity.

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