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Old 23rd June 2013, 11:39 PM   #4691
Zen Mod is offline Zen Mod  Serbia
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and then there goes simplicity outa window.........

even if idea is interesting , per se .......

remember that all 3 (4) windings in F6 are the same , so you're having same AC on disposal ( to feed output gates ) , having it through or without xformer , when input buffer is used
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Last edited by Zen Mod; 23rd June 2013 at 11:43 PM.
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Old 24th June 2013, 12:52 AM   #4692
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Originally Posted by Zen Mod View Post
and then there goes simplicity outa window.........

even if idea is interesting , per se .......

remember that all 3 (4) windings in F6 are the same , so you're having same AC on disposal ( to feed output gates ) , having it through or without xformer , when input buffer is used
Thanks Zen Mod. Please show a clarifying schematic as your time allows. One may also consider a quasi P-channel complement of R100A [or other JFET] by compounding it with a leading small signal bjt; for example. Mr. Pass used quasi PNPs in older work you showed in a post of Papa's Pub's. It is less symmetric; but the input capacitance presented to the secondary windings is like that in diyF6.

Last edited by Antoinel; 24th June 2013 at 01:03 AM.
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Old 24th June 2013, 01:26 AM   #4693
lhquam is offline lhquam  United States
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And there you have it. Probably the next issue you might want to think about
is the increased capacitance of the gain devices operated at 3 volts.

I went with the FQA28N15 and FQA36P15 primarily for their high transconductance. Given that Vdsmax and power dissipation requirements are lower for the drive FETs, I could probably find a N/P FET pair with better characteristics. How about sending me some LU1014s and their P-channel complement?

Next task: run the sweeps without the cascode FETs, and with the cascode FETs at a higher voltage.
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Old 24th June 2013, 02:23 AM   #4694
lhquam is offline lhquam  United States
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Here is a comparison of my F6CC with and without cascoding. The left plots are with cascoding and Vds=3V on the drive FETs. The right plots are without cascoding. In both cases adjustments of P3 and P4 were made to minimize H2 as 1kHz, 1watt.
What is really interesting is the difference at 10 watts, and how flat the THD curve is for the cascoded circuit.
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File Type: jpg F6p5-build1-fsweeps-1.jpg (151.4 KB, 199 views)
File Type: jpg F6p5-no-cascode-build1-fsweeps-1.jpg (156.8 KB, 198 views)

Last edited by lhquam; 24th June 2013 at 02:47 AM.
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Old 24th June 2013, 03:33 AM   #4695
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These are nice comparisons - thank you for posting them.
If you do try the cascoded circ at some point, there are a lot of new and many not-so-new N channel low (and flat) Ciss Fets that you can use in the outputs. You might find that you like the results quite a lot - even without cascoding.
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Old 24th June 2013, 03:54 AM   #4696
lhquam is offline lhquam  United States
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Originally Posted by kasey197 View Post
These are nice comparisons - thank you for posting them.
If you do try the cascoded circ at some point, there are a lot of new and many not-so-new N channel low (and flat) Ciss Fets that you can use in the outputs. You might find that you like the results quite a lot - even without cascoding.
Are there any N/P complements with those characteristics?
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Old 24th June 2013, 05:07 AM   #4697
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Wow, I guess I've been kind of snoozing, and preoccupied with life changes, and haven't been on Pass Labs forum for awhile and just saw this thread. Great thread and lots of cool ideas and circuits shared here. Very nice amp and article Nelson! Thanks for sharing again ... and to everyone else too.
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Old 24th June 2013, 05:07 AM   #4698
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I think you know the 1530/201 pairs and they may be worth a try. I used the n channel only and found it pretty decent. These might do better in your design compared to some direct coupled designs due to their low Vth. The p channel 201s, I haven't tried apart from just measuring them but many say that they are good.

Last edited by kasey197; 24th June 2013 at 05:12 AM.
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Old 24th June 2013, 05:31 AM   #4699
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here are the Ciss charts from the datasheets - in both cases the P channel parts have higher capacitances compared to their n channel pairs but the toshiba complements look closer on paper. They appear flatter also.
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File Type: jpg fairchilds.JPG (38.2 KB, 168 views)
File Type: jpg tosh-fets.JPG (31.5 KB, 166 views)
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Old 24th June 2013, 05:49 AM   #4700
PKI is offline PKI  United States
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Fired up F6 couple of hours ago. Such a lovely amp! :-) Thank you Nelson for the design and Buzz and Tea for parts :-)!
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