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Old 10th June 2012, 02:46 PM   #11
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For the A-T, try adjusting the bottom 82 Ohm resistor higher (or the ratio of bottom to top 82R). This needs to be slightly higher to supply the drive signal for the Mosfet (for finite gm). Broskie's SRPP+ writeup gives the design rules, but since the Mosfet gm varies with current like tubes, easier to just tweek the R ratio till the odd Harmonics minimize. At some R the odd harmonics should largely null out since it will be closest to P-P, but it will still have the odd harmonics from the tube, since A-T preserves them.

SRPP+ (tube up top instead) should be able to largely null out the odd harmonics, with matched gm top and bottom, like normal P-P. (will need a much bigger 82R/82R ratio for the tube drive though)

-------

The 3rd circuit, the Mosfet follower, should be doing the best performance, since it nearly completely unloads the tube, which becomes just the driver stage for it. (as long as it has a high gm Mosfet for the follower)

The A-T reduces the loading on the tube by 2x (its just a 2x impedance converter circuit), and the gyrator is stuck at 1x. So I would expect the A-T to have lower harmonics in general versus the gyrator. (Assumes that the Mosfet gate capacitance does not get in the way though. That STP8NM60 is an awfully big Mosfet. Have a spice model for an FQP1N60 ?)
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Old 10th June 2012, 03:24 PM   #12
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STP8NM60 Cin 440 pF, Cout 100 pF, Crev 10 pF

FQP1N60 Cin 150 pF, Cout 25 pF, Crev 4 pF

IXTU01N100 Cin 54 pF, Cout 8 pF, Crev 2 pF
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Old 10th June 2012, 05:58 PM   #13
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Been there, done that. Best results with lower 164ohm and upper 0.

A little surprising to me actually.
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Old 10th June 2012, 08:48 PM   #14
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What's the OPT turns ratio? I.e. what's the OPT primary impedance. I don't see it anywhere.

The AT circuit should produce the same spectrum as the gyrator only if the AT is loaded by 1/2 the OPT primary impedance vs. the gyrator, and the resistor ratio tweaked a little to compensate for the finite gfs of the MOSFET. This would make the load line the same as the gyrator case.

I don't know much about the big MOSFETS except that they're barely turning on at 100 mA and may not be in a good part of their gfs curve for this. I use a 2N60. The DN2540 is pretty linear in the 10-100 mA range.

How much cancellation is going on between the stages?

Are you planning to post the actual results and measurement conditions?

Thanks,

Michael

Last edited by Michael Koster; 10th June 2012 at 09:12 PM.
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Old 10th June 2012, 09:36 PM   #15
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Have only tried AT for line amp use.

The idea of an AT parafeed follower poweramp is new to me. Has anyone made it work?

Donīt mind the "big" MOSFETs, donīt think they are the issue here. Using a depletion FET would of course make things practically easier.
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Old 11th June 2012, 06:47 AM   #16
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Hey Don and Michael,

We know AT with PP works after me building the 6B4G/6P3S SEPTOR and you having built other prototypes with MOSFETs as the AT.

The SEPTOR have also been tried with MOSFET instead of 6P3S and showed even better results.

So, always high Gm devices at the AT side in the PP designs. The lower the Gm the higher the distortion. This probably goes for the SE follower being mu or AT.
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Old 11th June 2012, 02:00 PM   #17
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Hey Lars,

Now I'm not sure exactly what you tested and what the results are.

Did you test with both tubes and MOSFETS as the top devices? And with tubes you had some harmonic cancellation? But the MOSFET "works"?

I think I'm more confused than ever. Can you present the results with their circuits?

Thanks!

Michael
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Old 11th June 2012, 03:39 PM   #18
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Hey Michael,

?? now its my turn to be confused.

My last post didnīt concern the parafeed. It was about the poweramps built a few years ago in PP configuration, not follower. You did some prototype with triode+DN2540 and I did the SEPTOR in another thread.

Only the last sentence was about the follower.

I am trying to start a theoretical discussion here, nothing else.
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Old 13th June 2012, 01:43 PM   #19
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Just remember we had all this covered before:

Spud-Assist: Totem-Pole Current-Mirror PP Hybrid

Still using the highest possible GFs/Gm device on top, seem to minimize odd order harmonics.
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Old 13th June 2012, 04:48 PM   #20
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Quote:
Originally Posted by revintage View Post
Corrected the errors and indications are AT will have the highest odd order harmonics. About the hybrid sims that where correct, AT seems still marginally better.
I guess I don't understand this statement and wondered if you were going to show the plots to illustrate.
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