The simple one with exactly the same J-FET works fine. I put it in a small aluminium box with a rechargeble battery (ready-to-use or similar low discharge type). WOrks like a charm. The critical part is the J-FET. Even the resistors around the J-FET are dependent on the type. I did simulations with Spice to find out the J-FET ist indeed critical.
For the EUB with the two sensors I used the J-FET J310, that comes close to the 2N-types. I once tried a BC245C but this type will loose quite some volume.
For the EUB I have only very, very little room for the electronics, used a printed solder dot board (about two by five centimeters only), two of the simple J-FET buffers, one for each piezo, then mixed them by a simple potentiometer (not a good solution but acceptable) then put a passive tone control in and then one more J-FET buffer to get a higher output current. I also used a rechargeble battery and a connector to load it from outside (by a good -DeltaV charger).
My problem is, that the piezos I use have a weak signal and I have found no useful way to enhance the signal by 6 to 12 dB yet. Maybe I need to make other piezo sensors with a higher signal.
Personally I'm not too happy with pickups that need 10 MOhms.
Put a cable into the impedance buffer input, connect the buffer output to your amp and connect nothing to the other end of the buffer input cable.
Switch the amp on an knock at the cable, try different cables, also some thinner ones. You might hear the knocking at the cable out of your amp. THis is due to microphonics of the cable. The reason is a rather soft dielectric between inner cable and shielding.
The lower the input impedance of the buffer/amp, the less microphonic influence would be transmitted by the cable. So there is a drawback of high impedance inputs.
Therefor some of the buffer designs and preamps have a switchable input impedance (i.e. 1MOhms, 4,7 MOhms, 10 MOhms).
If you study more of FDecks buffer schematics you will find one for phantom power.
