The circuit with the NJFET will work with with the MIC input only, because it has a drain/biasing resistor built in. For the line input that resistor will have to be added. A quick simulation shows, that such a circuit built with J201 NJFET (or MMBFJ201 - SOT23 equivalent) with the 10k input Z of the SGTL5000 will deliver about 10.5dB gain (~x3.5). With typical output (10-20mVpp, let's assume 15mV) level from a piezo pickup it will result in about 50mVpp signal coming into the Line Input. And an inverted phase, which can be easily flipped back in software.
Luckily, it can be amplified further using the SGTL5000 ADC gain stage, up to 22.5dB. With so much gain it might be noisy, though.
Try it with the Mic input first, it's only 2 components:
Through-hole JFETs in TO92 package are getting obsolete, not many component vendors have them. They were very popular in DIY stompbox projects and some shops selling parts and kits might still have them in stock. Otherwise, lots of SMT equivalents are available and cheaper.
I still think that testing and resolving the issues with power decoupling would be a better idea. Piezo acoustic guitar pickups are kind of special (impedance, output level) and require a dedicated preamp to get the best signal quality. LR Baggs is well known and respected in that area.
Not sure if the Teensy-Only solution will deliver the same quality. Might be good enough, though.
Another effective way of decoupling the analog power rail from the digital or noisy DC-DC converters is the capacitance multiplier. It's been very often used in the Boss/Roland digital pedals:
It's only one transistor, resistor and a few capacitors. It should be placed in series with the piezo preamp 9V power.
Luckily, it can be amplified further using the SGTL5000 ADC gain stage, up to 22.5dB. With so much gain it might be noisy, though.
Try it with the Mic input first, it's only 2 components:
Through-hole JFETs in TO92 package are getting obsolete, not many component vendors have them. They were very popular in DIY stompbox projects and some shops selling parts and kits might still have them in stock. Otherwise, lots of SMT equivalents are available and cheaper.
I still think that testing and resolving the issues with power decoupling would be a better idea. Piezo acoustic guitar pickups are kind of special (impedance, output level) and require a dedicated preamp to get the best signal quality. LR Baggs is well known and respected in that area.
Not sure if the Teensy-Only solution will deliver the same quality. Might be good enough, though.
Another effective way of decoupling the analog power rail from the digital or noisy DC-DC converters is the capacitance multiplier. It's been very often used in the Boss/Roland digital pedals:
It's only one transistor, resistor and a few capacitors. It should be placed in series with the piezo preamp 9V power.
