At 9:09 AM -0700 7/6/08, Scott Fraser wrote:
><<Hi,
>I've been testing out various contact mics with my Sound Devices
>recorder & have come up against background noise (of course) on ones
>that are unbalanced. Does anyone have any tips for dealing with this -
>is there a way to deal with it ? & if not does anyone have suggestions
>for either pre-made contact mics that are balanced or for building
>some ?>>
>
>I've built many contact pickups over the years from inexpensive piezo
>buzzers from Radio Shack. It's a very simple matter of soldering a
>1/4" lead onto the piezo element, & it is an unbalanced connection. I
>use the instrument DI input on a Sound Devices USBPre to get into the
>laptop. Any sort of balancing device will get you into a suitable mic
>pre input. A passive transformer, such as the inline Shure or Sescom
>units will do, however there is usually an insertion loss with any
>transformer, a drop in volume which the mic pre must make up. An
>active DI will properly bridge the high impedance unbalanced output
>of a piezo element. Before doing any of that, though, I would
>recommend trying an XLR connector on one of your pickups, wired hot
>(tip) to pin 2, & shield (ground) to pins 1 & 3. You should also try
>it with no connection to pin 3 on the XLR, since, with some circuits,
>this will give a perfectly fine noise-free unbalanced to balanced
>connection, without the several db signal drop caused by tying pin 3
>to ground.
>
>Scott Fraser
Hi Scott--
Richard M, who has posted to this list and the phonography list on
numerous occasions, has a few pages about piezo discs illustrating
the wiring you are discussing and some buffer circuits.
http://www.megalithia.com/sounds/tech/piezo/ Rob D.
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