removing condenser mic from a PDA and modifying circuit for line-in ?

[Mad Scientist Jr]

I have an old PDA that I would like to modify (if possible) by taking
out the built in condenser mic and replacing it with a 1/8" or 1/4"
jack that I can plug another device into. Mainly I want to plug a
standard stereo headphone output (from iPod/cellphone/walkman/etc)
although it would be cool to be able to connect a line-level out from
a mixer, or directly plug a guitar in. The latter ones would probably
require a different circuit so for simplicity I'll just stick to the
first scenario (headphones out). Assuming I successfully can de-solder
and remove the condenser mic from my PDA and have two leads (which
went to the mic) free to solder my new input to, what kind of circuit
do I need to place between the new jack and the leads, so the input
works? The PDA must have some kind of preamp built in to drive the
condenser mic, so I am thinking maybe some kind of resistor or volume
control to "step down" the signal strength before it goes into the
PDA's preamp? I suspect the audio fidelity won't be the greatest, but
as long as it's passable I would like to try. Any info would be most
appreciated... thanks.

 

[jwvm]

I have an old PDA that I would like to modify (if possible) by taking
out the built in condenser mic and replacing it with a 1/8" or 1/4"
jack that I can plug another device into. Mainly I want to plug a
standard stereo headphone output (from iPod/cellphone/walkman/etc)
although it would be cool to be able to connect a line-level out from
a mixer, or directly plug a guitar in. The latter ones would probably
require a different circuit so for simplicity I'll just stick to the
first scenario (headphones out). Assuming I successfully can de-solder
and remove the condenser mic from my PDA and have two leads (which
went to the mic) free to solder my new input to, what kind of circuit
do I need to place between the new jack and the leads, so the input
works? The PDA must have some kind of preamp built in to drive the
condenser mic, so I am thinking maybe some kind of resistor or volume
control to "step down" the signal strength before it goes into the
PDA's preamp? I suspect the audio fidelity won't be the greatest, but
as long as it's passable I would like to try. Any info would be most
appreciated... thanks.

Lots of luck! The audio input circuitry is almost surely only intended
for dictation. To save memory, the audio is very likely heavily
compressed so artifacts will really destroy any semblance of fidelity.
It also has very aggressive AGC that is great for dictation and
horrible for music. There are probably five other issues that will
further degrade music quality. BTW, the condenser microphone might be
the highest fidelity component in the PDA. How does music sound that
has been recorded with the microphone? It probably won't sound any
better using a hacked line input. You will get much, much, much better
results (and multiple channels) using some like a Zoom.

 

[Richard Crowley]

"Mad Scientist Jr" wrote ...

I have an old PDA that I would like to modify (if possible) by taking
out the built in condenser mic and replacing it with a 1/8" or 1/4"
jack that I can plug another device into.

The PDA must be *really old* to have enough space for a 1/4"
jack! OTOH, the smaller 1/8" jacks might fit in the space
occupied by the mic capsule. Note, however that most 1/8"
jacks are configured to plug into the *side*, so you may have
a mechanical puzzle on your hands if the mic capsule isn't at
the edge of the enclosure.

Mainly I want to plug a
standard stereo headphone output (from iPod/cellphone/walkman/etc)
although it would be cool to be able to connect a line-level out from
a mixer,

Headphone level and *consumer* line-level are effectively
the same thing. Pro line-level is considerably higher and
would require additional attenuation.

or directly plug a guitar in.

The output from a passive, high-impedance guitar pickup
may be too low (and the impedance too high) for the kind
of circuit typically used for built-in electret condenser mic
(ECM) capsules. OTOH, a single-transistor (FET) "preamp"
circuit could likely make it work. And it could be powered
from the same voltage already provided to run the FET in
the ECM. But the output from guitars with built-in buffer
amps (requiring a battery) would be closer to the level that
came out of the original ECM.

The latter ones would probably
require a different circuit so for simplicity I'll just stick to the
first scenario (headphones out). Assuming I successfully can de-solder
and remove the condenser mic from my PDA and have two leads (which
went to the mic) free to solder my new input to,

Probably. But some ECM used three wires: ground, signal, and power.

what kind of circuit
do I need to place between the new jack and the leads, so the input
works? The PDA must have some kind of preamp built in to drive the
condenser mic, so I am thinking maybe some kind of resistor or volume
control to "step down" the signal strength before it goes into the
PDA's preamp?

Yes, you will almost certainly need to attenuate the signal so
it won't overload the existing input circuit. This can be as simple
as two very small resistors (an "L-pad").

I suspect the audio fidelity won't be the greatest, but
as long as it's passable I would like to try. Any info would be most
appreciated... thanks.

That is a whole separate discussion and depends a lot on exactly
what old PDA you are talking about (undisclosed). But as others
have mentioned, likely none of them have any significant music-
quality performance.

 

[whit3rd]

I have an old PDA that I would like to modify (if possible) by taking
out the built in condenser mic and replacing it with a 1/8" or 1/4"
jack that I can plug another device into.

Why not just glue a bit of Velcro next to the microphone? A suitable
speaker-on-a-wire can be stuck on whenever you want. It can
be as simple as half a dead iPod earbud, or you can experiment with
matching transformers and/or amplifiers.