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Preamp for Piezo Pickup

12/29/1999 5:50 PM
Doc	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. Doc

12/29/1999 5:50 PM

Doc

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.

Doc

12/30/1999 5:11 AM
Mark Hammer	Bright idea. Nice work.

1/2/2000 9:54 AM
friday	I have had excellent results recording acoustics w/piezo pickups using the "Super Hard On" from ZVex. This is a high impedance JFET circuit. This pedal is excellent for piezo pu's straight in to the board. OK, it's not a fancy smancy tube pre, but warm and sparkly to my ear. I have also used the outputs to run 2 tube guitar amps and been very impressed with the results. It occured to me that it would be cool to have the Preamp and your GE-7 as seperate pedals thus avoiding "surgery" . But, speaking of surgery, I've thought of modding the SHO with a third output. This would give me a straight track to tape as well as the stereo "vibe" tracks from the amps.

1/2/2000 9:54 AM

friday

I have had excellent results recording acoustics w/piezo pickups using the "Super Hard On" from ZVex. This is a high impedance JFET circuit. This pedal is excellent for piezo pu's straight in to the board. OK, it's not a fancy smancy tube pre, but warm and sparkly to my ear. I have also used the outputs to run 2 tube guitar amps and been very impressed with the results.
It occured to me that it would be cool to have the Preamp and your GE-7 as seperate pedals thus avoiding "surgery" . But, speaking of surgery, I've thought of modding the SHO with a third output. This would give me a straight track to tape as well as the stereo "vibe" tracks from the amps.

1/2/2000 1:56 PM
sunday	>>>>the "Super Hard On" from ZVex. This is a high impedance JFET circuit. the SHO is a mosfet circuit. the Rogue GEQ-5 is a 7-band equalizer stompbox that is on sale at Musicians Friend for $19.95! Sounds like a good item to use for mods.

1/3/2000 5:19 PM
Doc	Yeah, I noticed those pedals in their latest catalog. If anyone is placing an order with them anyway, it can't hurt to buy one or more of these cheap (Plastic?) pedals along with your regular order (to save on shipping costs). They could turn out to be good enough pedals to actually be useful, or they could serve as a nice box for any home brew project of your choice.

1/2/2000 6:08 PM
Gus	Doc. and myself have talked about this mod. IMHO the high Z input is how this petal should be built High Z is not bad for mag pickups IMHO

1/2/2000 7:12 PM
friday	I stand corrected, the Super Hard On is a Mosfet circuit. The input impedance is 5 Meg or so. It really is worth checking out as an acoustic pre.

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