[Harp-L] Re: I-mic (Harp-L)

[Isaac Ullah <iiullah@xxxxxxxxx>]

Hey, I've made a bunch of low-z dynamic mic's, all
basically from those plans (for the I-mic). I can just
tell you what I've learned from my experience building
these things, but your expereince may be differnt from
mine. It sounds to me that you are trying to use your
I-mic in the same manner you would use a bullet mic.
ie., you are plugging it straight into the input of a
guitar amp. There are some major issues with doing so.
1) Most guitar amps have a fairly high input
impedance, usually about 1 megaOhm. 2) This is because
the magnetic pickups on electric guitars have pretty
high output impedance (signal strength), usually
around a few hundred kiloOhms, and probably pushing
close to half a volt. 3) Bullet mics with crystal or
ceramic (or controlled magnetic) elements also have
pretty high output impedances (similar to electric
guitars, in the kilo-Ohm range). 4) Dynamic
microphones have low output impedances, however,
(usually on the order of 100 to 300 Ohms) and only
push a few hunderd milivolts, which means they send a
much much weaker signal to the guitar amp than the
other types of signal sources I mentioned (guitars and
bullet mics). 
   So, the moral is that if you are using a guitar
amp, it expects to see a signal of about the same
strength of an electric guitar's output signal. You
can use a low-Z dynamic mic, but the final result of
the amplification will be much quieter because there
is less signal to amplify in the first place. 
   You can overcome this in one of two ways. You can
use a low-z to high-z transformer, or you can use an
additional preamp or booster between the mic and the
amp. Personally, I go for the preamp/booster route,
because I like to build preamps/boosters. I think they
give you a lot more control over your tone, and that
they allow you to dynamically change the tone of your
mic without having to switch mics... If you are
interested in building your own boosters, they are
pretty easy to do. I would suggest starting simple
with a single transitor (FET type transistor) circut
like the fetzer-valve (from www.runoffgroove.com) or
the AMZ mini-booster (from the AMZ website), or
another similar one (google search "FET preamp", you
will find a lot of circuits). Personally, I have had
fantastic success with the circuits found at
runoffgroove, and have found a LARGE group of folks on
the net who also enjoy making them. This means you
will more easily find help out there if  you go with
one of those designs, not to mention that you will
find ready made templates and notes on how to build
them. 
   If you already have a soldering iron and know how
to solder (which i expect you do, since you've built
the mic), you can build a fetzer-valve with parts from
radioshack (they carry the MPF-102 FET transistor you
will need) for under $5 (not counting the enclosure,
which you cna get creative with). You could also build
a "noisycricket" or "little gem" amp with parts from
radioshack for about $12, and this will give you all
that you need...

Cheers,

Isaac