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Old 7th June 2010, 10:56 AM   #291
Aneat is offline Aneat  China
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Quote:
Originally Posted by john curl View Post
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.
thank you very much
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Old 9th June 2010, 04:29 PM   #292
gk7 is offline gk7
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Thatīs very valuable information ideed, thank you very much. So I will use matched 2SK170V
for the input stage and adjust their drain resistors accordingly.
For the output stage the sad news is that more and more parts are unobtainable,
even Borbely Audio no longer has 2SK170V matched to 2SJ74V (all 2SK170V they
have left are 11.5-14 mA and all 2SJ74V 15-20 mA). Seems I have bought some
of the last matched quads when I built the line stage.
Maybe using BL grade and increase the number of pairs from 2 to 4 could be
an option. Or which other FETs for the output stage would be usable ?
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Old 9th June 2010, 06:52 PM   #293
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Doubling up is a reasonable solution for output devices, but resistance scaling is with a single pair of devices is better for input, because of the problem of higher nonlinear input capacitance.
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Old 10th June 2010, 06:59 AM   #294
Aneat is offline Aneat  China
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Quote:
Originally Posted by gk7 View Post
Thatīs very valuable information ideed, thank you very much. So I will use matched 2SK170V
for the input stage and adjust their drain resistors accordingly.
For the output stage the sad news is that more and more parts are unobtainable,
even Borbely Audio no longer has 2SK170V matched to 2SJ74V (all 2SK170V they
have left are 11.5-14 mA and all 2SJ74V 15-20 mA). Seems I have bought some
of the last matched quads when I built the line stage.
Maybe using BL grade and increase the number of pairs from 2 to 4 could be
an option. Or which other FETs for the output stage would be usable ?
2N5434 Idss=30ma Vp=-1~-4v Ciss=30p Crss=15p
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Old 10th June 2010, 06:36 PM   #295
HKC is offline HKC  Hong Kong
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Quote:
Originally Posted by john curl View Post
Doubling up is a reasonable solution for output devices, but resistance scaling is with a single pair of devices is better for input, because of the problem of higher nonlinear input capacitance.
Hi John

Could the input capacitance be minimized if each of the parallelled JFets connected with a source resistor?
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Old 10th June 2010, 06:55 PM   #296
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No.
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Old 11th June 2010, 02:33 PM   #297
HKC is offline HKC  Hong Kong
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Hi John

Even for input stage of the line amp section?
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Old 11th June 2010, 04:13 PM   #298
gk7 is offline gk7
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If you want to reduce the input capacitance you could cascode them. In general I think you should not have problems with capitance modulation as long your source impedance is below 50 kOhms or so.
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Old 12th June 2010, 12:39 AM   #299
HKC is offline HKC  Hong Kong
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Quote:
Originally Posted by gk7 View Post
If you want to reduce the input capacitance you could cascode them. In general I think you should not have problems with capitance modulation as long your source impedance is below 50 kOhms or so.
Hi gk7

Thank you for the inputs, cascode will change the structure of the circuit and that is not what I want to do. So I will leave it as the original as you said as long as the source impedance is below 50K Ohm.
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Old 12th June 2010, 02:13 AM   #300
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Please don't parallel input devices of the line amp section. Worst place possible.
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