Thursday 5 December 2013

Contact microphone pre-amp

Collin's Lab: DIY Contact Microphone

It’s amazing what a little disk can do … when it’s layered with piezoelectric crystals. Piezo disks are impressively sensitive to vibration and can easily be adapted to work as a contact microphones. The trick is the preamp – a basic circuit used to match the piezo’s signal to levels compatible with modern audio gear inputs. The resulting piezo/preamp combo can be used for electrifying an acoustic guitar or simply exploring the lesser-heard world of small sound around us.

Or try the original...

(J201 Transistor is not as easy to obtain but, if you can manage to source it, is definitely worth it for its superior sound.) 

I've been performing experiments with guitar preamps and on-board electronics since, oh, the Nixon administration or thereabouts.  Sometime around 1990 I designed this preamp circuit, and have been using it mostly unchanged ever since.  In 1992 I posted a schematic of the preamp to one of the Usenet groups and the circuit became somewhat popular.
The goals of the preamp are:

  • Sounds great.  Of course.
  • Discrete FET (Field Effect Transistor) design.  Discrete because I don't like the sound of opamps, and FETs because the devices operate in a manner somewhat analogous to vacuum tubes.
  • Runs off a 9v battery.  In practice a decaying 9V battery, possibly as low as 8.0 volts.
  • Very high (3.0 Mohm) input impedance.  1.0 Mohm is the minimal acceptable input impedance for a guitar device.
  • Medium (6.0 Kohm) output impedance.  There's no need for an especially low output impedance as we're only using the preamp to counter the effects of cable capacitance and loading.
  • 3 dB gain.  You don't need much gain, and given the guitar signal level and the 9v supply voltage, there isn't room for much gain.
  • Low noise.
  • Graceful overload.  Overload happens; the circuit should clip gracefully and recover gracefully.
The preamp can also be phantom powered and mounted in a phone plug.  See my article "An FET Preamp Cable".

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