SS 120R085 Depletion Mode Jfet - Page 19 - diyAudio
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Old 25th May 2012, 10:48 PM   #181
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You mean in line with cap to top fet.
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Old 25th May 2012, 11:20 PM   #182
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something in line with this : L'Amp: A simple SIT Amp

I'm not saying all values are adequate for your cause , but what's important is principle

I don't have depletions , so I can't help more , except with dry datasheet based conclusions
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Old 25th May 2012, 11:48 PM   #183
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Thanks. Downloaded that as well as another one of his schematics. Gonna build it even if its wrong. Always learn more from mistakes anyway. Hardheaded like that.
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Old 26th May 2012, 12:03 AM   #184
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Quote:
Originally Posted by buzzforb View Post
........ Hardheaded .........
welcome to the club

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Old 26th May 2012, 01:05 AM   #185
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Buzzforb,
Your pic was supposed to look like smth attached. SRPP.
In this case top tr do not need to be of depl type, ench SJEP will be
equally good.
But remember, that this circuit with SemiSouthes will have high output impedance =current drive. And you will not achieve muF with this vertical positioning of one on top of the other - only if you "disassemble" the pair.
Those SS will require volt FB to have sufficiently low output imp.
Smth along the lines of post by Knutn -also attached.
Attached Images
File Type: jpg Knutn SRPP.JPG (26.7 KB, 179 views)
File Type: jpg untitled.JPG (27.5 KB, 165 views)
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Old 26th May 2012, 11:10 AM   #186
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Thanks Steven. Why does the top resistor need to be .9 vs 1R? Isn't this the bias setting resistor for the top fet? If i take the signal from above this resistor, do I have a Mu Follower? I must admit that I do not understand the "disassemble" part, but that is not shocking. I guess feedback and output impedance are a reason Nelson uses the LTP input in his FW amps, although i thought that Zout would be reasonable. I am going to measure curves for the R085 and start doing some actual math today. May also consider using FQA up top as it has higher Yfs than R085 and that should help with Zout, correct?

One more thing. The reason I used a voltage divider on the R085, is that Nelson had stated in other threads that stability of the bias of the upper fet was problematic, even with depletion devices. This was not a direct comment from him, but extrapolated from what was stated about smaller Jfet cousins, like sk170.
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Old 26th May 2012, 11:23 AM   #187
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as I wrote few times - try to find (buy , steal , borrow ) book called How to gain gain_A Reference Book on Triodes in Audio Pre-Amps

interesting reading , back to the roots and up to the sky

goooogl is your friend

then you'll learn difference between mu, srpp etc.

for output impedance of mu follower , ratio of these two resistor in totem is dominant - over xconductance , of used part
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Old 26th May 2012, 11:27 AM   #188
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Will do. Been reading up on SRPP and Mu follower, but most just touch on differences without much explanation.
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Old 26th May 2012, 11:30 AM   #189
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output signal point

witty Pa made bazztardo in some of his (known ) amps - modulation (of upper element) from one point , output signal takeover from other point
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Old 26th May 2012, 11:36 AM   #190
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It seems i have much to read and do this weekend. Beats typing
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