[sdiy] preamps for piezos (slightly OT?)

[Donald Tillman]

> On Feb 14, 2016, at 10:20 AM, Busby Bergson <busby.bergson at gmail.com> wrote:
> 
> i'm currently experimenting with contact mics. they sound pretty good going into a guitar amplifier, directly - but if i want to send them through my other circuitry, it seems that they require a specialized preamplifier, to compensate for their impedance. 

"Compensate" isn't the best word, but...

Piezoelectric pickups have a very high impedance that's mostly capacitive.  That means that if they are loaded down, you'll lose the lows first.  Compare that to an inductive electromagnetic guitar pickup which loses the highs when loaded down.

So you'll want a preamplifier with an input impedance of 1 Mohm or higher, and the higher the input impedance the better the low end response.  It's a good place for an FET.  You probably don't need much voltage gain, maybe 10dB worth, but that varies wildly with the specific application.  So no specialized preamp is necessary, just a very high input impedance.  (Guitar amps generally have an input impedance of 1.0 Mohm.)

There's also the other extreme... with a sufficiently high load impedance a piezo response can go down to almost DC.  Which means that for an instrument like a guitar, the string plucks can induce DC thumps, and it might be appropriate to filter those out for your application.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com



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