[ltp] Thinkpad R40 mic input and phantom power

[Richard Neill]

Ben Pearre wrote:
> Thanks!  In case anyone else is interested: it turns out that there's
> 4.77 volts across the pins (at least, on my R40; YMMV etc).  That
> counts as a "bias voltage", not as "phantom power" which is usually
> somewhere in the 9V--56V range.
> 
> The mic in question (Audio Technica AT822) is a condenser, but is
> self-powered with an AA battery and doesn't want phantom power.

That's interesting. I didn't really appreciate the difference between
bias voltage (which will power a small electret+FET) and phantom power
(which will power a pre-amplifier).

Anyway, if you want to block it, it depends how sensitive the MIC is to
the bias voltage ever being applied. If it can cope briefly, just put a
a capacitor in series. If it cannot, then use a transformer:


 M  ---}  ||  {------- Think
 I     }  ||  {        Pad
 C  ---}  ||  {------- Input

You'd need some care selecting the transformer - it mustn't short out
the bias voltage (so the secondary needs a high enough DC resistance) -
and it must cope with a small DC current.

Richard


> 
> On 2005-12-20  4:42, Richard Neill wrote:
> 
>>
>>Christopher Sawtell wrote:
>>
>>>On Tue, 20 Dec 2005 15:44, Ben Pearre wrote:
>>>
>>>
>>>>I know, I should call IBM or Intel, but I like you guys better :)
>>>>
>>>>Any idea whether the R40's soundcard supplies "phantom power" to the
>>>>microphone jack?
>>>
>>It's certainly true for the A22p, since I use an electret microphone
>>with it. But if you are worried, put a capacitor of perhaps 0.1uf in
>>series with your Mic: this will block the phantom power. That said,
>>iirc, the only mics that don't use phantom power are
>>
>>i)crystal piezo mics (very cheap/indestructable)
>>
>>ii)moving coil mics - which shouldn't care if you pass a small current
>>through them.
>>
>>
>>Richard
>>-- 
>>The linux-thinkpad mailing list home page is at:
>>http://mailman.linux-thinkpad.org/mailman/listinfo/linux-thinkpad
> 
> 
> 
> --
> Ben Pearre     http://koryukanboulder.com/ben      PGP: CFDA6CDA
>          Free music at http://hebb.mit.edu/FreeMusic
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-- 
rn214@hermes.cam.ac.uk  **  http://www.richardneill.org
Richard Neill, Trinity College, Cambridge, CB21TQ, U.K.