Updated 10 May 2018
After two and a half years and thousands of Phantom Piezo Preamps sold, we finally ran out of 2SK596S-C JFETs (which have been discontinued for some time by On-Semi). Going forward, the best option is the Fairchild PF5102 (now also an On-Semi product). We have slightly redesigned the Phantom Piezo Preamp PCB to allow for the use of the ‘5102 (which has a different pinout to the ‘596) and also we have incorporated a pair of zener diodes across the input to protect the circuit (and your loudspeakers) from large voltage spikes (which will be generated by a piezo transducer if you give it a significant physical shock). As a result, the redesigned PCB is 1mm longer than the old one.
Here is the revised schematic:
Note that R10 sets the bias current in Q1 and Q2. You should aim for about 2mA current flowing in R10. If you use Q3 with a random Idss, you will need to adjust R10 so that the voltage across it divided by its resistance is about 2mA. In the photograph above, R10 is the resistor right above where it says “V2”.
Note that a small number of modules have been shipped with a higher value for R10 (4k7). We have identified that some preamps may have slightly elevated distortion and somewhat lower gain than the nominal 12dB as a result of the value of R10 being a little high for the Idss value of Q3 installed. Thanks to Luc for bringing this to my attention. In almost all cases, this effect will be inaudible and those of you affected will actually benefit from increased headroom. However, if you want to restore the missing gain, please reduce the value of R10. We suggest that you do not try and replace R10 unless you are expert at soldering and we recommend that you solder a 220 Ohm resistor across R10 (i.e. in parallel) on the top side or the underside of the PCB. All preamps in future will ship with the 220 Ohm resistor for R10.
Luc also mentioned that the schematic is inverting as it is shown. This is true. If you want the preamp to be non-inverting, you need to connect pin 3 HOT and pin 2 COLD at the input.
Finally, I ordered some 2SK170 JFETs from China. We will see if we can confirm whether they are real Toshiba ‘170s or fakes and, if they prove to be good, we will be building prototypes with the 170’s to see if we can further improve the noise and distortion figures. Watch this space.
The new preamps are available in the shop here and a version with the XLR not soldered is also available for those who want to hard-wire the output.