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

[Mad Scientist Jr[_2_]]

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]

On Aug 28, 1:33*pm, 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. 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.

[Mike Rivers]

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.

> 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

You will probably need an attenuator and possibly a blocking capacitor.
You'll have to experiment to determine the values however. No predicting
them, and probably nobody reading this has ever tried to implement such
a crackpot idea. <g>

Just for starters, use a 2-circuit (TRS/"stereo") jack with the tip and
ring tied together, maybe through 100 ohm resistors so as not to feed
one channel's output into the other channel's output directly. You'll
probably be plugging a stereo device into it and the mic input is almost
certainly mono, so you'll need to combine them. The circuitry part isn't
difficult. The hard part will be the mechanics.




--
If you e-mail me and it bounces, use your secret decoder ring and reach
me here:
double-m-eleven-double-zero at yahoo -- I'm really Mike Rivers
)

[Kevin T]

Dear Mad
Just spend the $150 foe a Zoom H2 and be done with it :)

On Aug 28, 3:04*pm, Mike Rivers > wrote:
> 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.
> > 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
>
> You will probably need an attenuator and possibly a blocking capacitor.
> You'll have to experiment to determine the values however. No predicting
> them, and probably nobody reading this has ever tried to implement such
> a crackpot idea. <g>
>
> Just for starters, use a 2-circuit (TRS/"stereo") jack with the tip and
> ring tied together, maybe through 100 ohm resistors so as not to feed
> one channel's output into the other channel's output directly. You'll
> probably be plugging a stereo device into it and the mic input is almost
> certainly mono, so you'll need to combine them. The circuitry part isn't
> difficult. The hard part will be the mechanics.
>
> --
> If you e-mail me and it bounces, use your secret decoder ring and reach
> me here:
> double-m-eleven-double-zero at yahoo -- I'm really Mike Rivers
> )

[Mad Scientist Jr[_2_]]

For my purposes (copying voice memos from analog to the PDA) the audio
quality of the PDA is actually fine.

BTW I have used various PDAs to record guitar lick ideas, a lot of
them at 11KHz 16-bit mono, and it's not super crisp sounding but it is
decent for a lofi document.

On Aug 28, 3:04*pm, jwvm > wrote:
> On Aug 28, 1:33*pm, 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. 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]

On Aug 28, 10:33*am, 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.

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.

[Bob Masta[_2_]]

On Thu, 28 Aug 2008 10:33:24 -0700 (PDT), 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. 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.

Fidelity will probably be poor by hi-fi standards, but fine for guitar. Mic
inputs typically include low-pass filtering to remove frequencies above 8k or 9k
or so (at least on laptops I've measured), but that's well above the guitar
range. The mic capsule has a single-FET "preamp" internally, powered either by
a separate connection (3-connection capsule) or through a dropping resistor
(2-connection capsule). The case of the capsule is usually one of the
connections, and may go straight to the circuit ground. Or there may be a wire
to a pad on the capsule, with a very fine trace from the pad to the case. That
would be typical if the capsule is not mounted directly on a circuit board of
the PDA.

If you have the 3-connection style you can ignore the one going to the supply
voltage and feed your signal to the other 2. If you have the 2-connection style
with a dropping resistor, you will need to find the hot end of the resistor and
disconnect it before using the 2 leads.

Yes, you will probably need an attenuator, which can be a simple 2-resistor
divider.

Best regards,






Bob Masta

DAQARTA v4.00
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
FREE Signal Generator
Science with your sound card!