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Old 14th March 2013, 05:45 PM   #241
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Originally Posted by wrenchone View Post
Then it's not an SRPP. I will continue with this project as an an SRPP so that I can suss out the implications. Juma has already done the gyrator thing and tacked it on to another one of my threads. We'll see about output impedance when I measure it.
Suit yourself. I wasn't attempting to hijack your thread, only to a suggest a solution to the problem of metastable quiescent drain voltage that you had raised.
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Old 14th March 2013, 06:58 PM   #242
mlloyd1 is online now mlloyd1  United States
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ken:
i think what you're describing is what was done for the phono preamp used in the Hafler 915 jfet preamp, page 26.
yes?
they add complimentary symmetry and use the unobtanium jfets, of course.

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[QUOTE=Ken Newton;3410399]Perhaps, the most reliable means to address this problem is to apply fixed-biasing to the upper JFET, turning it in to a gyrator. You could create a bias reference voltage by dividing the supply in half with a string of two very high value resistors. Tap the midpoint of this string for biasing the upper JFET's gate. Then, A.C. couple the upper JFET gate to the bottom of it's source resistor, at the lower JFET's drain). You can use a small value capacitor because of the high impedance terms involved.
...
QUOTE]
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Old 14th March 2013, 08:44 PM   #243
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Originally Posted by mlloyd1 View Post
ken:
i think what you're describing is what was done for the phono preamp used in the Hafler 915 jfet preamp, page 26.
yes?
they add complimentary symmetry and use the unobtanium jfets, of course.

mlloyd1
Hi, mlloyd1:

Since you ask (no hijacking is intended, Wrenchone ), yes, the front half of that Hafler circuit is it. A fixed bias gyrator allows us to have our cake and eat it too. We obtain both a high A.C. impedance - giving us maximum gain, and a low D.C. impedance - giving us a stable and predictable D.C. bias voltage across the FETs. I know an good idea worth adopting when I see one.

I would not worry too much about having complementary symmetry, along with sourcing the scarce P-channel JFETs that would require. I would build this circuit single ended utilizing only N-channel parts. I would cascode those FETs, and also implement the indirect feedback technique on the upper FET, per the below linked paper from A.N.T. Audio. My best sim of just such a circuit predicts a gain of around 46dB. It will produce a 1kHz 2VRMS output signal with a sim'd 2H at around -75 to -80dB, and 3H at nearly -120dB. All higher harmonic are beneath -140dB. All without resort to loop feedback, only low value (10 ohm) source degeneration resistors and the indirect feedback technique cited above. Okay, unplanned hijacking over.

http://www.ant-audio.co.uk/Theory/Ap...e_Feedback.pdf
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Last edited by Ken Newton; 14th March 2013 at 09:00 PM.
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Old 15th March 2013, 04:57 AM   #244
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To put it bluntly, I'm not in love with gyrators butting up against triode equivalents. A current source works better against a "triode equivalent" than a voltage source (at least in my book). Anyway, I want to investigate what can be derived from a simple SRPP before I make a leap (if I ever do) to some other alternative. It's my time (I don't have a lot of it, for various reasons, none of which I have to justify to anyone here), my resources (ditto).

At any rate, I've already figured out how to compensate an SRPP (the position for the capacitor was not where I initially thought it would be, and I got a nice, overdamped response with 15 pF of compensation)). I suspect quite strongly that most hot-doggers here have let fly with SRPP designs here, SRPP there, without any sort of time domain/frequency domain investigation of stability. To be brutally honest, I never did a time domain test of a simple SRPP stage until the results I've posted here. Others might benefit from such a simple investigation...

Maybe a simple unquestioned SRPP topology works for vacuum-state triodes that have all their intrinsic parameters built-in ( maybe not, who knows?), but it appears to be a different situation for triodes synthesized from enhancement mode devices (with feedback) that have their own agenda regarding parasitics. At any rate, unless someone tells me differently, it appears that I might be forging some limited new ground. You are totally free to build actual circuits and post results. People who will actually do this (a mere schematic doesn't count) appear to be rather thin on the ground.
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Old 15th March 2013, 12:52 PM   #245
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Originally Posted by wrenchone View Post
To put it bluntly, I'm not in love with gyrators butting up against triode equivalents. A current source works better against a "triode equivalent" than a voltage source (at least in my book). Anyway, I want to investigate what can be derived from a simple SRPP before I make a leap (if I ever do) to some other alternative. It's my time (I don't have a lot of it, for various reasons, none of which I have to justify to anyone here), my resources (ditto).
To also put it bluntly, I'm a bit mystified by your defensiveness. Who here has suggested that you must like or dislike one topology or another? Do you really feel it's necessary to declare to us that you do not have to justify the use of your time or resources? Really?? Sure, do or don't do whatever you wish (do I really need to say that?). Yet, can no one else submit directly pertinent thoughts and suggestions intended only to be helpful? Should you personally not find such thoughts and suggestions useful or interesting, so be it. However, there are others reading this thread who might. You are reacting almost as though this thread were your private blog. We are here as a knowledge community, each of us teaching and learning as we interact, aren't we?
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Old 15th March 2013, 04:49 PM   #246
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Take a time out Gentlemen. Wrenchone stated twice that he was continuing research on the SRPP circuit as a purely acedemic exercise. Even if it is for personal edification there are people following this thread with interest. This is not a thread started as a group design project unless I missed something somewhere, and I have been flollowing this thread since it's inception many years ago. The written word, even at it's best, does not convey nuances that personal interaction does. Trying to ferret out what emotions a person wants to evoke is an exercise in futility.
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Old 15th March 2013, 07:56 PM   #247
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Take a time out Gentlemen. Wrenchone stated twice that he was continuing research on the SRPP circuit as a purely acedemic exercise. Even if it is for personal edification there are people following this thread with interest. This is not a thread started as a group design project unless I missed something somewhere, and I have been flollowing this thread since it's inception many years ago. The written word, even at it's best, does not convey nuances that personal interaction does. Trying to ferret out what emotions a person wants to evoke is an exercise in futility.
Forgive me, but I don't believe a referee was required to either interpret, mediate, or call a timeout for us. Nothing ugly was taking place, nor about to take place. We were simply negotiating through an understanding of personal perspective, upon which we could more effectively communicate. From my perspective, it was as simple as that.
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Old 15th March 2013, 08:33 PM   #248
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My sincere apologies. Play ball. Sorry, it's March Madness time.
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Old 15th March 2013, 09:38 PM   #249
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No worries, all in good fun.
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Old 27th March 2013, 11:20 PM   #250
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Any progress being made? Hope the lapse in postings is just due to time constraints and nothing else.
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