How much current, depends totally on which JFET you use.
There is a value in datasheet MAX Current.
I usually set the current to be less than 50% of this MAX Current.
What value of R13/R14 also depends on which JFET transistor you use.
Voltage Gate-Source to set a specific current will be different
for different JFETs.
Also will be different for each individual JFET.
This is why JFETs often are 'matched'. This means you select two JFETs with similiar VoltGS value.
After you have chosen JFETs ( finding P-JFET is the tricky bit )
and set current with R13, R14 ( N-JFET and P-JFET have different qualities !)
you select R4/R7 to set a suitable current in Q1/Q2 stage.
There is a value in datasheet MAX Current.
I usually set the current to be less than 50% of this MAX Current.
What value of R13/R14 also depends on which JFET transistor you use.
Voltage Gate-Source to set a specific current will be different
for different JFETs.
Also will be different for each individual JFET.
This is why JFETs often are 'matched'. This means you select two JFETs with similiar VoltGS value.
After you have chosen JFETs ( finding P-JFET is the tricky bit )
and set current with R13, R14 ( N-JFET and P-JFET have different qualities !)
you select R4/R7 to set a suitable current in Q1/Q2 stage.
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