N-Channel JFET analog switch clarifications

[redbagy]

I've read a few other questions on here which helped me understand analog FET switches better but I still have a couple of questions/clarifications which I'd like to understand better.

Consider a supply of 9 VDC with an AC guitar signal. With reference to the below circuit:

• Is R37 necessary? I know that it is there for biasing, but I did an LT Spice simulation without it and the circuit worked the same. I understood that R38 is necessary to have a fixed DC voltage, else the NMOS would conduct on the negative portion of the AC wave.
  
• What is the purpose of D4 at the gate? I think that the RC combo adds some filtering, but I'm still unsure about D4 in this case.
• Can someone recommend an N-Channel MOSFET Switch schematic as well? I'm curious to understand better the difference to using a JFET vs a MOSFET as an analog switch.

[iainpunk]

1.) yes, in the perfect world of SPICE, it can be omitted, but IRL, it prevents POP sounds when switching
2.) this diode prevents any DC current from passing through the gate of the Jfet. look at it as if its a safety feature.
3.) you'd need 2 mosfets to block any signal due to the intrinsic body diodes they have, if only 1 is used, youll get some half wave leakage if the signal is large enough.

also, the drive voltages are different for mosfets to work well

cheers

[duck_arse]

use the instawires seeker to locate "The Technology of the Tube Screamer" at geofex, in which RG tells us the diodes leakage allows the gates to float to a happly, clunkless place. etc.

I don't see any NMOS in your diagram.