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Old 8th October 2015, 09:21 PM   #1
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Default Bob Cordell complementary input stage with N channel Jfets

since the scarcity of p channel how about using N channel Jfets in complimentary input stage?
well its stated in cordell book Im sure we will not have serious matching issues but atleast we can get jfet input amp and not in uni polar style. How will be the THD at 10k and 20k?
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Old 9th October 2015, 05:05 PM   #2
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no replies?
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Old 10th October 2015, 12:08 AM   #3
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No way to do a fully symmetric input stage. The best compromise is
the sansui way , just N-channel Jfets. (below). Single ended driving
a complimentary second stage/VAS.

I'm not going to develop that posted "kypton V" further , use it as you wish.
Member thimios was quite impressed by it's SQ , BTW .

OS
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File Type: jpg KVcomp.jpg (162.3 KB, 1004 views)
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Old 10th October 2015, 03:37 AM   #4
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OStripper.

LTspice simulation of your Krypton schematic generated an output FFT that has higher THD on Odd_Harmonics than the THD level of the preceeding even order harmonic. I had expected the single JFET Diff stage to lower the progressive odd harmonics.

The Symetri topology with both complementary diff inputs and complementary diff VAS circuits has a similar FFT harmonic profile and is more robust to clipping. Naturally, you will need to send a "Dear Santa" letter to Nelson Pass to get PJFETs before Christmas.
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File Type: jpg Krypton JFET_BJT.jpg (92.2 KB, 968 views)
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Old 10th October 2015, 04:22 AM   #5
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Quote:
Originally Posted by LineSource View Post
OStripper.

LTspice simulation of your Krypton schematic generated an output FFT that has higher THD on Odd_Harmonics than the THD level of the preceeding even order harmonic. I had expected the single JFET Diff stage to lower the progressive odd harmonics.

The Symetri topology with both complementary diff inputs and complementary diff VAS circuits has a similar FFT harmonic profile and is more robust to clipping. Naturally, you will need to send a "Dear Santa" letter to Nelson Pass to get PJFETs before Christmas.
The FFT not looking good. I think its better to go complimentary jfet input than using one.
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Old 10th October 2015, 04:49 AM   #6
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Even with that horrible looking fft , you are still 95+db down.
That is 30ppm , BTW.

My 1k sound card FFT shows a more realistic harmonic plot.
edit line ... that "spray" of odd harmonics does not show in real life - model generated.
get rid of the input cap , input filter cap ... disconnect the servo ... it goes away.

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Last edited by ostripper; 10th October 2015 at 04:52 AM.
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Old 10th October 2015, 04:54 AM   #7
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Quote:
Originally Posted by rhythmsandy View Post
The FFT not looking good. I think its better to go complimentary jfet input than using one.
Full complimentary cancels even order .
A single ended cancels all.

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Old 10th October 2015, 05:56 AM   #8
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For comparison. No input cap, no DC cap, no servo.

Kypton FFT of JFET input and BJT VAS
Symetri FFT of JFET input and BJT VAS
Attached Images
File Type: jpg Krypton JFET_BJT.jpg (102.3 KB, 844 views)
File Type: jpg Symetri JFET BJET.jpg (100.2 KB, 804 views)
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Old 10th October 2015, 06:24 AM   #9
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are they bad ?

are you using cordell/keentoken models ?

Slew master must be a distortion master ... right ?

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Old 10th October 2015, 08:04 AM   #10
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Quote:
Originally Posted by LineSource View Post
For comparison. No input cap, no DC cap, no servo.

Kypton FFT of JFET input and BJT VAS
Symetri FFT of JFET input and BJT VAS
Yes the symetri is better since they both even and odd are kind of closer.

But my question is about compared to a circuit like this does the Kypton THD better?

http://m.eet.com/media/1196048/F11large.jpg
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