CJ

Effect of lower R-drain in J-Fet amp?
« on: March 08, 2020, 04:04:02 PM »
working on Marantz tuners, 2270, 2275, notice changes in R-d in some units for the first fet in the front end,

, have seen, 100 ohms, 47 ohms and 10 ohms being used,

was wondering why they do this, is it simply using different resistors to bias the right I-d ?

what happens to noise, gain and bandwidth with different values of R-d?

thanks!
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


ruffrecords

Re: Effect of lower R-drain in J-Fet amp?
« Reply #1 on: March 08, 2020, 04:54:55 PM »
It is usually just to set the quiescent current. Value io resistor does not affect noise because it is bypassed. Like tubes, shot noise in FETs is inversely proportional to gm so higher gm gives better noise. Generally gm is high at higher drain currents. Setting quiescent current roughly sets gm.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

CJ

Re: Effect of lower R-drain in J-Fet amp?
« Reply #2 on: March 08, 2020, 05:38:28 PM »
Thanks Ian!  :D :D
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


 

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