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Old 5th August 2012, 04:52 PM   #561
The Dastardly Dad of Three
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hi all,

Had time to fire up the iron again this weekend and wanted to run some tests in open loop (ie no NFB/degeneration) to see if i understood things correctly. Here are some observations, theory and then tests for some proof.

Setup:

Same schema as papas front end (just the tx+output stage). Supply rails +\- 24V. Output devices 2sk1058 hitachi latefts. Bias arrangement: 9V batteries with 5K pot to set voltage. The battery and pot bypassed by 220uF caps. Separate bias for top and bottom halves. Bias current set for 1.4A. Transformer was regular torodial power transformer. Stepup ratio approx 4x.

Results:

0.2V pk-pk in, 9.8V pk-pk out. Therefore gain = 49x. Given the tx gives us 4x, that means the fets are giving us approx 12.2X gain.

Theory:

Load R = 6.8R, therefore combined bottom n top half transconductance = 12.2/6.8 = 1.8 Siemens. Each fet therefore gives us 1.8/2 = 0.9 S transconductance. This seems to tie in with what i see from the hitachi datasheets.

Explanation:
Given the above, both top and bottom fets are acting in common source mode (despite the temptation to think about the top half as a source follower providing no voltage gain). If this hypthesis is true then both top and bottom halves provide equal amounts of gain. We can test this easily given the transformer coupling by just bypassing the top and bottom tx secondaries (ie in turn, each fet would see only the bias voltage, but no signal) individually and seeing the effect.

Results:

Both secondaries connected, trace 1. Output is 9.8V.
Top secondary bypassed, trace 2,. Output is 4.875V
Bottom secondary bypassed, trace 3. Output is 4.812V

Summary: So, the sum of the two halves individually = 9.7V vs 9.8V when theyre both connected. Seems close enough to suggest that the initial hypothesis that both halves act to provide equal amounts of gain (in common source) is correct.

Some other results/thoughts:

Warts n all open loop, the distortion performance seems pretty remarkable by comparison to a conventional complmenetary push-pull stage built. Here, (trace 4) you can see that theres zilch even harmonics and all were left with is 0.07% THD, almost all of it 3rd harmonic with very little higher order stuff.

The op stage needs some local feedback so as to reduce the miller effect on input capacitance which is very high. Having said that, the open loop freq response incl tx was quite usefully flat (trace 5). Seems to me that this would run quite respectably as-is w/o nfb if one could live with the HF rolloff and high o/p impedance. Addition of some feedback on this relatively linear stage will make for a pretty nice amp i think.....

Next steps while waiting (for the real F6):

a) Try the same local biasing arrangement i did last time but with lower impedance resistors (need to grab some WW low R pots tom)

b) Build the dual jfet input stage to see how that works....

I'm finding that part of the fun is in the journey .....
Attached Images
File Type: jpg trace1.jpg (125.7 KB, 515 views)
File Type: jpg trace2.jpg (123.9 KB, 507 views)
File Type: jpg trace3.jpg (122.4 KB, 510 views)
File Type: jpg Trace4-THD.jpg (126.2 KB, 494 views)
File Type: jpg Openloopfreqresponse.jpg (133.3 KB, 491 views)
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Old 5th August 2012, 04:57 PM   #562
The Dastardly Dad of Three
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Also while we wait, anyone on this list sells the semisouth R100s ? Thought id order them in ahead...I usually get stuff from zhoufang but he seems to be away these last few weeks .....
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Old 5th August 2012, 05:29 PM   #563
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Quote:
Originally Posted by kasey197 View Post
hi all,

Had time to fire up the iron again this weekend and wanted to run some tests in open loop (ie no NFB/degeneration) to see if i understood things correctly. Here are some observations, theory and then tests for some proof.

Setup:

Same schema as papas front end (just the tx+output stage). Supply rails +\- 24V. Output devices 2sk1058 hitachi latefts. Bias arrangement: 9V batteries with 5K pot to set voltage. The battery and pot bypassed by 220uF caps. Separate bias for top and bottom halves. Bias current set for 1.4A. Transformer was regular torodial power transformer. Stepup ratio approx 4x.

Results:

0.2V pk-pk in, 9.8V pk-pk out. Therefore gain = 49x. Given the tx gives us 4x, that means the fets are giving us approx 12.2X gain.

Theory:

Load R = 6.8R, therefore combined bottom n top half transconductance = 12.2/6.8 = 1.8 Siemens. Each fet therefore gives us 1.8/2 = 0.9 S transconductance. This seems to tie in with what i see from the hitachi datasheets.

Explanation:
Given the above, both top and bottom fets are acting in common source mode (despite the temptation to think about the top half as a source follower providing no voltage gain). If this hypthesis is true then both top and bottom halves provide equal amounts of gain. We can test this easily given the transformer coupling by just bypassing the top and bottom tx secondaries (ie in turn, each fet would see only the bias voltage, but no signal) individually and seeing the effect.

Results:

Both secondaries connected, trace 1. Output is 9.8V.
Top secondary bypassed, trace 2,. Output is 4.875V
Bottom secondary bypassed, trace 3. Output is 4.812V

Summary: So, the sum of the two halves individually = 9.7V vs 9.8V when theyre both connected. Seems close enough to suggest that the initial hypothesis that both halves act to provide equal amounts of gain (in common source) is correct.

Some other results/thoughts:

Warts n all open loop, the distortion performance seems pretty remarkable by comparison to a conventional complmenetary push-pull stage built. Here, (trace 4) you can see that theres zilch even harmonics and all were left with is 0.07% THD, almost all of it 3rd harmonic with very little higher order stuff.

The op stage needs some local feedback so as to reduce the miller effect on input capacitance which is very high. Having said that, the open loop freq response incl tx was quite usefully flat (trace 5). Seems to me that this would run quite respectably as-is w/o nfb if one could live with the HF rolloff and high o/p impedance. Addition of some feedback on this relatively linear stage will make for a pretty nice amp i think.....

Next steps while waiting (for the real F6):

a) Try the same local biasing arrangement i did last time but with lower impedance resistors (need to grab some WW low R pots tom)

b) Build the dual jfet input stage to see how that works....

I'm finding that part of the fun is in the journey .....
Great work kasey. You've separated [and delivered] fact from fiction.
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Old 5th August 2012, 05:43 PM   #564
lhquam is offline lhquam  United States
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Quote:
Originally Posted by kasey197 View Post
hi all,

Had time to fire up the iron again this weekend and wanted to run some tests in open loop (ie no NFB/degeneration) to see if i understood things correctly. Here are some observations, theory and then tests for some proof.
...
I'm finding that part of the fun is in the journey .....
Excellent. How are you driving the transformer? The suggested Jensen transformer is JT-123-FLPCH which has 4 windings with equal numbers of turns. Thus it must be run either 1:1 or 2:1. I suspect that it should be run 1:1 with the 2 primary windings paralleled, which will reduce the gain by a factor of 4 vs. your transformer configuration. That will not leave a lot for negative feedback. It also suggests that the degeneration resistors should be kept low in order to keep the transconductance as high as possible, but having the MOSFETS be thermally stable.
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Old 5th August 2012, 06:03 PM   #565
The Dastardly Dad of Three
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Quote:
Originally Posted by lhquam View Post
Excellent. How are you driving the transformer? The suggested Jensen transformer is JT-123-FLPCH which has 4 windings with equal numbers of turns. Thus it must be run either 1:1 or 2:1. I suspect that it should be run 1:1 with the 2 primary windings paralleled, which will reduce the gain by a factor of 4 vs. your transformer configuration. That will not leave a lot for negative feedback. It also suggests that the degeneration resistors should be kept low in order to keep the transconductance as high as possible, but having the MOSFETS be thermally stable.
You have a fair point with the fets im using since they have low transconductance.... The r100s have more than 10 times as much transconductance so there should be PLENTY of gain end to end even with 2:1 step down tx (!) or the 1:1 ...

Re your first question, the test setup is driven direct by the hp 35665a signal analyzer which has a very low impedance..... Real world use is going to need the fet buffers ala f6 schematic. That's on the to-do list (eventually) after I try the local biasing n fb network
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Old 5th August 2012, 07:01 PM   #566
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Quote:
Originally Posted by kasey197 View Post
Also while we wait, anyone on this list sells the semisouth R100s ? Thought id order them in ahead...I usually get stuff from zhoufang but he seems to be away these last few weeks .....
I sell the JFETs for less than Newark does. PM for details.
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Old 5th August 2012, 07:14 PM   #567
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TeaBag is excellent to deal with. Much more prompt than I.
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Old 5th August 2012, 07:25 PM   #568
generg is offline generg  Germany
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or me.....

thank you kasey197!
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Old 5th August 2012, 07:54 PM   #569
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Old 5th August 2012, 10:02 PM   #570
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by kasey197 View Post
........

Setup:

Same schema as papas front end (just the tx+output stage). Supply rails +\- 24V. Output devices 2sk1058 hitachi latefts. Bias arrangement: 9V batteries with 5K pot to set voltage. The battery and pot bypassed by 220uF caps. Separate bias for top and bottom halves. Bias current set for 1.4A. Transformer was regular torodial power transformer. Stepup ratio approx 4x.

....
had some C in series with secondaries , to save them of DC ?


Quote:
Originally Posted by kasey197 View Post
...... Seems close enough to suggest that the initial hypothesis that both halves act to provide equal amounts of gain (in common source) is correct.
......
yup

as I wrote - quasicomplementary is in function same as complementary ; with even greater symmetry between windings (which we can expect from signal xformer , comparing to crude mains one ) and , say , if we can use really dynamically matched (even best in situ , observing THD spectra ) outputs , we can expect even greater amount of suppression of even harmonics ; say that with some feedback that will be less critical than in case of OL .

Quote:
Originally Posted by kasey197 View Post
.....
a) Try the same local biasing arrangement i did last time but with lower impedance resistors (need to grab some WW low R pots tom)

......
having more than , say , 3-5mA through bias net is hardly of any benefit ; remember that you need to decouple gate from bias net with at least 10K , to avoid unnecessary loading of secondaries

then secondary's action will give enough AC mA for proper feed of gates

that's what counts , not brute force in bias net

anyway - did you observe near clipping action of tried OS ?
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