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91# WKJ
陈达财最近的我没看资料,我看08年的是Rd=150R,Rs=10R,
按照JC-2原作者本人John Curl的介绍:
Idss of input fets should be 25ma or more.
__________________14th April 2009, 12:11 AM
Parallel 2 V series Toshiba fets.
14th April 2009, 02:18 AM
输入级JFET的Idss要25mA或大于,达不到时用2对V档的东芝JFET管并联,
V档Idss:10~20mA
网友Aneat问
:less than 15mA why is not recommend for?
when Idss is smaller we can increase Rd & decrease Rs,the way whether not good?
John Curl昨天(2010/06/06)的回答:
Sorry that I did not get back to answer questions, I didn't see this thread for awhile.
Unfortunately, I cannot give additional info on the JC-2 attenuators, as I did not design them.
The original JC-2 design was done in 1973, or 37 years ago. At that time, all we had available to us were Siliconix j-fets, in general, and for special applications, Union Carbide, Motorola, or Crystalonics.
The JC-2 phono input stage used Siliconix J110 input devices that are high current switches, that usually have an Idss of over 20 ma. The input devices were used at about 15 ma ea for lowest possible input noise. When you change the Id of the input devices in order to use lower Idss types, then you MUST change the load resistors, and this LOWERS the open loop bandwidth. Still, the change is not too great, IF you use high Idss devices like the 2SK170 V series. Running at 1/2 the originally designed current is not too different, BUT it will be another design if you run less.
The output j-fets of the phono stage. Lowering the Idss on these devices will lower the slew rate significantly, if taken too far.
The same problem with the complementary j-fets in the input driver stage. It is designed with a very high open loop bandwidth, on purpose, and this will be lost IF the input stage load resistors are increased too much. Still the V series 2SK170 and 2SJ74 series will work pretty well, for the output of the phono stage and the input of the line driver amp, with resistor changes.
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