Hi, as promised. It took longer, because i experimented a lot with it to get
the best sound. I also tested a Macintosh microphone. 2 gif files can be
found on the ftp archive site to help (micro.arj).

(c) 1993 IdMEDIA & Francois Dion - any commercial use prohibited

    Here is a circuit to use a condenser microphone element with your Gravis
Ultrasound. It is a conventionnal way of using a condenser capsule in lots
of self powered condenser microphones.

    For this project i used 2 condenser capsules as the one you can find in
Radio Shack parts corner. I also use a 6 V battery because it was the
safest voltage, 9 beeing close to the limit. Some better capsules can take
12 to 15 volts. I dont have the part numbers handy, but i'll post them
when i have some time.
    Connect hi pole of capsule to the left or right ring of the stereo mini
jack thru a 2.2 uF (around that value. There seems to be a capacitor in
serie with the input of the microphone in on the GUS, but i cant say if it is
before a load or not, so in doubt, i put the 2.2 uF cap). Connect the shield
to the minijack tip. Connect the + pole of the battery to the shield. Connect
to the - pole of the battery a 2.2Ko (hey i like 2.2 8). It is red-red-red, not
red-red-wine 8) ) resistor. connect the other lead of the resistor to the hi
pole (hot wire connected to ring) thru a switch between the capsule and the
condensator. My god, is that it? Yup. Almost.
If you need a second microphone do the same thing except connect the hi pole of
the second microphone to the free ring (left or right depending on which you
did first). If you are not going to use a second microphone, connect the
left and right rings together, so that you have a signal on both channels.

    See fig1.gif for more detail (in micro.arj).

    You also probably tried a Macintosh microphone and were wondering why
it is not working. They are also condenser style, with about the same
circuit as in fig1, with the exception that there is no battery. Instead,
connected to the hi pole of the condenser capsule, there is a 2.2Kohm
resistor connected to the black wire. This wire is one of the 2 rings on
the minijack, thru which the macintosh supply the voltage to the microphone.
(It is not a stereo microphone as some could be lead to believe). So, to use
a macintosh microphone, cut the minijack off and use a small project box to
put your battery (a 9V is ok but i found that energizer A544 6 volts are
better suited and also smaller) and switch. Connect the white wire of the
Mac microphone to the left or right ring of the stereo mini jack (if you use
two microphones, you would connect the other to the other ring). Connect the
shield to the minijack tip. Connect the - pole of the battery to the
shield. Connect the + pole of the battery to the black wire thru a switch
(if you dont want to remove each time the battery). Another solution is to
replace the microphone in fig2 by a female mini jack so you dont have to cut
the wire.

    See fig2.gif for more detail.

    Last point: you probably have seen the PZM microphones at Radio Shack.
They can be used as-is, but they will work much better if you replace their
little power box and put the design of fig1.gif instead. You will get a much
better sound for the Gravis and for any other application. (The sound quality
falls in over 200$ microphones with a 40$ microphone).

    Also, if the use or misuse of these modifications cause any problems, i am
not responsible in any way. Use at your own risks. Also, be careful with the
soldering iron, ground it before playing with the capsule. Use a small
project box, dont mixup polarities (if you use an electrolytic cap). The capsule
as a hi and low pole, the low pole beeing connected to the exterior shielding.
Above all, if you have a gif viewer, do download the fig1 and fig2 gifs. BTW,
you can use a mylar 2.2 uF 50v instead of an electrolytic of the same kind.

Ciao,
--
Francois Dion
    '  _   _   _
 CISM (_) (_)  _) FM       Montreal , Canada       Email: CISM@ERE.UMontreal.CA
      (_)  / . _)             10000 Watts          Telephone no: (514) 343-7511
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