Newsgroups: rec.audio.tech
Subject: Re: Crappy microphone cabling problem
References: <[email protected]>
Glenn Booth  writes:

> Hi,
> 
> This should be easy, but it's driving me nuts.
> 
> for various reasons, I need to record some stuff using a really nasty,
> cheap condenser microphone that is fitted with a 3.5mm connector, but
> I want to find a way to feed it into a balanced XLR microphone input,
> as that's all I have.


Here are two plans that could be of some use to you 
(use fixed width font like courier to view the images)


General powering circuit for computer microphones designed 
originally to work with Sound Blaster and similar souncards:

   Soundcard
   Microphone
 
        / \
       |   |--------------------------------- Audio signal out
        \ /
       +===+      ____
       |   |-----|____|--------+
       |===|    2.2 kohm       | +
       |   |              Battery (3--9V)
       |   |                   | -
       |   |-------------------+------------- Ground
     +=======+
     |       |


Multimedia microphones to normal microphone input:

                                                        Soundcar microphone
    Normal mic in                                         Microphone
                           10 uF
        / \                 ||+                              / \
       |   |----------------||------------------------------|   |
        \ /                 ||                               \ /
       +===+                                                +===+
       |   |         +5V ----------------Resistor-----------|   | 3.5mm plug
       |   |         power               2.2 kohm           |===| to microphone
       |   |          |                                     |   |
       |   |----------+------ ground -----------------------|   |
       |   |                                                |   |
     +=======+                                            +=======+
     |       |                                            |       |


Both circuits are my designs from 
http://www.epanorama.net/circuits/microphone_powering.html#soundcard
 
> The microphone was supplied to me as part of the materials for an
> acoustics course. It has the type of connector found on stereo
> headphones - 3.5mm, with three (Tip, ring, shield) sections, but it's
> definitely a mono unit. A quick check with a continuity tester shows
> that tip and ring are connected together. 

This is typical to most PC electret microphones connected to PC 
soundcards (but you can't count on it on all microphones).

> I have to use this
> microphone in order to get measurements that will give the answers the
> course wants, so using another mike is not acceptable.
> So, I need to build an adapter cable to feed a Soundcraft Spirit
> mixer. Anyone care to suggest how I should wire it?

The circuits are described above and here is modification for 
this for XLR connections:

   Soundcard
   Microphone
 
        / \
       |   |--------------------------------- XLR 2
        \ /
       +===+      ____
       |   |-----|____|--------+
       |===|    2.2 kohm       | +
       |   |              Battery (3--9V) +-- XLR 3
       |   |                   | -        |
       |   |-------------------+----------+-- XLR 1
     +=======+
     |       |


Do not use mixer "phantom power" when using this circuit, 
because turnign "phantom power" can damage the microphone.

> I could chop off the end of the mike cable and just fit an XLR, but I
> have to return this microphone, so the alternative is to wire a cable
> from a female 3.5mm jack to a male XLR, and put it between mike and
> mixer. Sounds easy, but my first effort was a failure.

THat is the best idea to build the converter.

> The Soundcraft is wired pin 2 hot, pin 3 cold, pin 1 screen.
> I wired tip to pin 2, ignored the ring (it's shorted to tip) and wired
> screen to pin 3. No go. What did I do wrong?

The microphone need operating power to work. 
When connected to PC the soundcard supplies that power. 
But the mixer does not supply suitable power to microphone 
(the possibilities are no power and "phantom power" that 
is too much for this microphone type).

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/