> This help you http://ideofunk.up.seesaa.net.....
Q2 has no gate resistor so its bias is uncertain.
I don't see why Q2 is needed at all; we have too much signal, why 2-stage amplifier?
>> compare to this one?
> distortion is often a problem
Yes, turn-down the guitar.
The mike input is probably scaled for 20mV-50mV maximum input, guitar runs up to 200mV even 500mV.
You need loss, impedance, and you need DC isolation because there is DC coming out of the iFone to power typical electret mikes.
The "this one
" plan isolates DC from the guitar, and has impedance to spare, but will overload either in the JFET or in the iFone.
Put a hi-Z pot in _front_ of that.
Pot may be 250K or 500K. Lower will suck the top off the guitar, and higher will add hiss.
I bet the pot wants to be Audio taper because we want about 10:1 loss in "normal" setting and with audio-taper that will be near "5". You may use linear but the "normal" setting will be near "1" and pretty twitchy.
It would be "better" to run the JFET with more supply voltage than what come out of a iFone, but that means a battery.