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Old 2nd October 2012, 11:15 PM   #1461
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I think you need to cascode the jfet input stage. And I think you need to go back to the + and - 30v ideal power supply rails. Ray
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Old 2nd October 2012, 11:39 PM   #1462
RNMarsh is offline RNMarsh  United States
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Scott -- Yes, it was thru 50K control contacts and all. The pot was about 10+ years old (Alps). No series R issues were found -- tested to 100K source. The C of those jFEts are very low. Used junk resistors. No special attempt to make it best possible - to sqeeze everything out of it. Different goals and appl. However, it too can be tuned up and I would start that, just for the heck of it, if I wasnt leaving for Asia in 2 days. But anyone pls sim away on it. I'll still have Internet some of the time. Not sure about Burma.

Circuit could use a little more OS drive and bias for the super-duper low thd results at 30 Ohms... as you alluded to. And, higher voltage devices and lower Cin output devices for +/-24v appls etc. for PRO use. Then it will be super-tuned.

However, I can't measure below what I got... probably mostly noise from thermal and power supply... though FFT showed the harmonics as such from the analyzer output. PS would also have to be upgraded even further to get that last drop of my blood from it. Something for a sim with perfect PS -who knows what that would produce for THD results. It'll be my first fall project when I return from my adventures abroad or others can play with it now. I do not want to start adding on to it, transistor wise. That is a whole other thing/direction - and probably not needed to get even lower THD (IMNSHO). But you DO have to match devices-select them- to get the lowest levels I get..... a down-side to using simpler symetrical circuits with moderate gnfb. Thx -RNM

Last edited by RNMarsh; 3rd October 2012 at 12:08 AM.
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Old 2nd October 2012, 11:45 PM   #1463
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Quote:
Originally Posted by grhughes View Post
I think you need to cascode the jfet input stage. And I think you need to go back to the + and - 30v ideal power supply rails. Ray
At 60v the cascodes become a mess since at that point they have to be either bipolar or something like a 2N7000.

OTOH I just ran it with cascodes and even a 100K pot is OK. We shouldn't go all the way back to one circuit for everything. I think a version with simple FET cascodes (gate tied down to bottom source) would be a good place to leave one (only 4 extra devices). We need to pick a Vds that keeps the input FET's linear.

Looks tight on the J111 .7V typ. but on the good side 5mA is the 0 TC point.
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Old 2nd October 2012, 11:47 PM   #1464
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Quote:
Originally Posted by RNMarsh View Post
Yes, it was thru 50K control contacts and all. The pot was about 10+ years old (Alps). No series R issues were found -- tested to 100K source. The C of those jFEts are very low. Used junk resistors. No special attempt to make it best possible - to sqeeze everything out of it. Different goals and appl. However, it too can be tuned up and I would start that, just for the heck of it, if I wasnt leaving for asia in 2 days. But anyone pls sim away on it. I'll still have Internet some of the time. Thx -RNM
I forgot you were doing only .25V or so input, the distortion goes as Vin squared.
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Old 2nd October 2012, 11:49 PM   #1465
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Don't the CCSs, J1 & J2 isolate Q6, 7, 8, 9, 11, 35 and hence Q15 from the signal path?

I can see a signal path through Q15 on some of the later 'symmetrical' versions but not on the simple () #1380 circuit.

Or are there some missing tracks on #1380

ricardo
The intent was to only change the input stage, yes the lines were JPEG'd out.
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Old 3rd October 2012, 12:10 AM   #1466
fas42 is offline fas42  Australia
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Hmmm, this should probably be looked at by somebody who understands the subtleties, tried 3 complementary pairs and got DC output levels of 1, 4.5 and -7 volts. My problem, LTspice, or something else ...?

Well, I've spent the last 10 minutes trying to upload a couple of files to attach -- Jeez, I'm sick of braindead software, glad to be out of that game ... try again later ...

Tried again, still not working ...

Hooray, tried a different browser, could be a coincidence, we're cooking ...

Frank
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Old 3rd October 2012, 01:54 AM   #1467
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Rather than answer questions piecemeal how about I put a device by device description in the articles section, this is a holiday weekend I might have some time.
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Old 3rd October 2012, 02:12 AM   #1468
EUVL is offline EUVL  Europe
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With real part J111 & J174s, I get about 5V Vds on the 2SK170/ 2SJ74s.


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Old 3rd October 2012, 02:40 AM   #1469
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by scott wurcer View Post
I forgot you were doing only .25V or so input, the distortion goes as Vin squared.
What is the affect of a lot of source degeneration on the jFETs re. Vin? Its pretty heavily degenerated. [Design for max 1v input.]

Last edited by RNMarsh; 3rd October 2012 at 02:45 AM.
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Old 3rd October 2012, 02:48 AM   #1470
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Originally Posted by scott wurcer View Post
The intent was to only change the input stage, yes the lines were JPEG'd out.
So excuse me if I'm stating the obvious ... but I take it #1397 completely discounts #1380.

#1380 was never Guru Wurcer's intention and is just as complex as #1397

#1397's only con wrt to #1380 is the need for Unobtainium FETs
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As far as I can see, there are only 3.5 gain stages; J7-9, Q1&35, Q14&58 and Q56&59. The 0.5 stage is cos the current gain of the last stage is 'sorta' set by R9&10 / R53/44.

It may be the 'alternative' signal path; Q15&16, Q10&13 to Q56&59 has higher gain but my head hurts trying to work this out and would appreciate education from the gurus.

Besides the compensation is in the 'main' path.
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It that is the case, IMHO, it should be possible to equal its performance with a much more conventional circuit ... not too dissimilar from 990. Allowing Iq 8mA, 5mA on the i/p and 25mA total can give a big boost to 20kHz performance of what is a small power amp.

Careful attention to compensation along the lines of Prof Cherry can improve both HF THD and also PSRR.

About the only thing which might be difficult is symmetrical slew rates but this can still approach zillion V/us
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The Unobtainium i/p FETS simplify providing a 'tail' current for the symmetrical cases but is there any other reason for using them apart from Golden Pinnae cred?
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So far, the only practical application mentioned is a headphone amp. On that basis, I think we'd be hard put to equal, let alone better the O2 headphone amp which prompted this article NwAvGuy: Op Amp Measurements

OK. It isn't zillion V/us ...
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Instead of +/- 30V capability, a far more useful requirement would be comfortable 10dBU (6.93Vpp) into 600R on a 9V PP3

For flexible use in EQ & filter circuits, no nasty TL07x or LM4562 latching.

For other stuff, I'm happy with AD797 .. even its high In.
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... in the interest of fun, exchanging ideas and hoping for pearls of wisdom from Guru Wurcer etc.
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