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Preamp for Piezo Pickup

12/29/1999 5:50 PM
Preamp for Piezo Pickup
A while back, we were discussing the idea of modifying a typical equalizer pedal for use as a suitable preamp for a guitar having a piezo saddle pickup. Most guitar amps built to accomodate acoustic bridge pickups have a very high input impedance, which is required to match the typical piezoelectric crystal element. Regular EQ pedals have an input impedance similar to that of a tube amp for use with a magnetic pickup.  
I wanted to use a common Boss GE-7 pedal for a piezo preamp. It has an input impedance around 500kohms, one tenth of that which would be a good match for a bare piezo. Unfortunately, the input shunt resistor is part of the IC, so it wasn't possible to simply replace a 470k resistor with a 4.7 megohm unit. R.G., Gus, and others suggested adding an FET buffer to achieve the desired high input impedance.  
Well I finally got around to doing the surgery on a GE-7 pedal. The buffer is a 10kohm source follower, using a 2N5457 with a 4.7meg gate resistor and a .02uf gate isolation cap. My prototype circuit was built on a perf board, about half the size of a postage stamp. There's just enough room to sandwich it between the input jack body and the top of the case.  
Performance was checked using a Parker Fly. The buffer is active as long as the cables are plugged into the pedal. Footswitch activates the preamp circuitry only. So, in order make a comparison, the guitar cord had to be connected straight into the amp or through the pedal. There was a noticeable increase in bass response using the modified pedal. The overall guitar sound seemed slightly "more alive" (sorry for the lackluster subjective description- -I could never get a job writing gear reviews for a magazine) when the pedal was in line, whether activated or not.  
The nice thing about this mod is that it provides a cheap impedance-matching preamp, complete with a 7-band EQ to tailor the guitar's voice, (and provide an output boost, if desired) within easy reach of the guitar player. As with many pedals, it has a relatively low output impedance which allows use of a generic cable or extra long cable to get from the pedal to the amp without degrading the frequency response.  
Acoustic preamp pedals are now offered specifically designed for use with high impedance piezo pickups, some with nice enhancements like chorus and stereo outputs. But they will set you back 3 to 4 times or more what this common equalizer pedal will. Many players have a spare EQ pedal just lying around anyway. It could be converted into a useful piezo preamp. (I haven't tried any brand other than Boss, but DOD and others make a similar pedal. Tha same mod should be feasible.) After the impedance buffer is installed, the pedal can be still used with a standard magnetic pickup, just like before.  
A tiny pcb for this buffer would be nice. But it's only a five component circuit, so perfboard is fine.  

Mark Hammer Bright idea. Nice work.... -- 12/30/1999 5:11 AM
friday I have had excellent results record... -- 1/2/2000 9:54 AM
friday I stand corrected, the Super Hard O... -- 1/2/2000 7:12 PM
JoeArthur I'm sorry... I don't understand som... -- 1/3/2000 5:39 PM