Re: input buffer for mic impedance matching

["IronMan Mike Curtis" <ironman@xxxxxxxxxxxxxxxxx>]

From: "bcdee" <bcdee@xxxxxxxxxxx>

> I havn't built this one, but I built the "MikeMojo" by IronmanMike that
> works well for me.  This one is not quite as elaborate as that one in that
> it only uses one 9V battery.  I may try this one to compare.

I looked at the other circuit.  It provides about 1 to 1.5 megs input
impedance, due to the input balancing resistors.  The Astatic MC151 is
spec'd as "equivalent to a 390 pF capacitor.  While it will improve the
sound of a crystal mic into the Dano, it's input impedance is still not as
high as it should be for the fullest sound.  At 1.5 meg, the -3 dB high pass
is around 270 Hz.  At 1 meg, it's closer to 400 Hz.  If the Dano is 100k,
the high pass is a very trebly ~2700 Hz!

The Mike Mojo has a true infinite input impedance.  If your mic has no VC,
the -3 dB frequency is zero.  If your mic has a volume control, the high
pass point is predetermined by the value of the pot.  If it's a 5 meg pot,
the high pass is around 80 Hz (which is fairly hi-fi).  If it's a 1 meg, the
high pass is a midrangey 400 Hz - still an improvement over the "raw"
DanEcho, but not as good as it could (should) be.  Using a 1 meg pot with
the other circuit, hi pass will be around 700-800 Hz.


BTW some LIVE tunes by The IronMan Curtis BonTemps Blues Band:
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http://www.billhouse.com/songs/YouAreSoBeautiful.mp3
http://www.billhouse.com/songs/ironman/PrettyWoman.mp3
http://www.billhouse.com/songs/StormyMonday.mp3
(and - hint hint - we're looking to tour)

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- -IronMan Mike Curtis http://www.ironmancurtis.com *Southland Blues Magazine
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