How can extend the cable distance from a condenser microphone to the ADC without loosing fidelity?

Question

I am new here so please forgive me for a vey basic question. I have a TASCAM iM2 in my possession. This is a stereo condenser microphone coupled to an ADC that connects to the the latest iPhone models via the dock connector.

http://tascam.com/product/im2/

I am in the process of dismantling it so that I can extend the physical distance between individual microphones. As you can see from the photo, there are 3 wires connected to each mic. I'm guessing that the black ones connected to the dock casing are ground, the yellow leads are probably a DC voltage to charge the plates and the red and white wires are probably for the actual audio signals.

I only want to extend the distance of each mic by 8 inches or so from the circuit board. I was planning on extending the wires using a cannibalised stereo audio extension cable (the ones with the 3-pin 3.5mm jack on each end). However, I want to extend the wires in a way that will not introduce much additional noise. One of the great things about this unit is that the ADC is so close to the microphones, so there is not much chance to pick up noise on the way. What would be the best way to shield this, or would the short distance I'm talking about not be a problem anyway?

Answer 1

I expect you are right about what the wires are. You should check with a voltmeter though to make sure. Ground should have the lowest voltage, power the highest, and the return audio somewhere in between although it may be almost the same voltage as the power but at a higher impedance. If you load the audio line with 10 kΩ or so, it should drop a bit whereas the power line should not.

By the way, unless you have specific documentation that says these are condenser microphones, I think they are more likely electret. However, that doeesn't matter for the purpose of extending the cables.

I would use stereo shielded cable to extend each microphone. Obviously the shield should be tied to ground. Since you're only going 8 inches, I don't expect any significant problems with that. Try to leave as little length as possible for the audio wire outside the shield.

Answer 2

Based on Olin's advice, I went ahead today and performed the hack. I also made the microphones removable by adding jack connections. I thought I'd briefly document my solution here incase anyone else googles how to hack a Tascam iM2, as this works really well.

Step 1: Wire 3.5 mm stereo female connectors to the unit's internals. I used the following pin layout:

    / \ 
   |   |   Audio signal
    \ / 
   +===+
   |   |   Bias voltage for microphone (+2.5V in the case of the iM2)
   |===|
   |   |
   |   |   Ground
   |   | 
 +=======+
 |       |

The yellow wires are the bias voltage, the black are ground, and the red and white wires are the audio signal as I originally presumed. I checked this with a multimeter. After this stage you will end up with something looking like this . . .

Because of the design of the iM2 chassis, I decided to make a barrel out of the two connectors using a combination of duck tape and heat shrink. This way, the chassis will hold the jacks in place firmly when reassembled . . .

Using a sharp knife, I cut the barrel of the original structure that held the mics in place and then wired up some shielded two core cable. You can then cable tie the cable to the remaining barrel that was left over after cutting the original structure.

Add stereo 3.5mm jacks to the other ends of the cables and then reassemble all the parts of the original chassis and viola!

Works great. Sounds great.