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Need explain of this jfet schematic please?

Hello . i need help with this jfet shcematic .How to caculation thi gain of the jfet , and how to reduce the gain ??
Thanks in advance!
sorry for my bad English!

Picture of Need explain of this jfet schematic please?
Something is missing from your circuit diagram: a source of DC current for the JFET. Compare yours to the diagram in this instructable by Pyrohaz:

https://www.instructables.com/id/Building-JFET-Preamplifiers-for-musical-instrument/

The only other thing I know about JFETs is that diode junction at the gate always has to be reverse biased, and I think that means the gate always has to be at a voltage more negative than both the drain and the source.
login721 (author)  Jack A Lopez4 years ago
This is the DIY irig for iphone, it's dont need DC source because the ipod will provide it .The main aim of this circuit is make hiZ input for guitar and lowZ output for ipod mic input.It also reduce the electric guitar signal (~400mv) to 30-50mv for ipod.Thats all i know.
The problem is i want to reduce the signal a little bit by reduce the gain of jfet to avoid clipping when i use humbucker pickup.
Thanks!