Note: One should avoid using JFETs with low IDS – especially in buffering circuits.
For example, J201 is a commonly used FET - probably because it was once used in
the famous “Till” guitar preamplifier and “FET Preamp Cable”, both designed by
Donald Tillman. However, the gate cutoff voltage VGS (OFF) of a J201 is about the
lowest amongst all depletion mode JFETs and with moderate source resistor values
this device can’t even handle input signals that are greater than few hundred
millivolts peak-to-peak. This FET is a horrible choice for buffers and basically for
common source circuits as well: In an equal circuit, a higher current FET, like J309,
can handle input voltages higher than 1 VPP and even offer slightly greater gain.
Although popular, J201 is really not that marvelous device. Note that Donald Tillman
originally substituted a higher current model with a J201 solely because of improved
noise performance. Let this be a lesson to you: Always base your component selection
principles on circuit theory – not to a fact that a particular component was used in
some famous circuit! It might have worked there – likely it will not work as well in
now, i know that the j201 has been in widespread use in diy stompboxes for quite a few years now(mostly due to ROG and DIYS), but this paragraph does make me wonder if there are better devices out there for our purposes- especially since i recently found a stash of j309s that i picked up in a sample order ages ago. i've tried using the 309s in buffers and mu-amps and have had great results so far. considering that there are a number of FETs out there, does anyone have any experience with anything besides the usual j201,2n5457, 2n3819(?) fare? and what characteristics do you look for in a JFET?
BTW- i just have to say again that this is a great book. even if you aren't building a solid state amp, the preamp section has lots of good stuff for the intermediate stompbox builder. thanks again, teemu! great job!!!