I have a class A amp, the F4 incidentally, and Nelson’s paper really works. You can design the crossover for your speakers using XSim based on Nelson’s paper. Yeah, there is power loss from the resistor but it is worth it. It sounds quite good.
Krglee, check out the F2 and F2J, which are simple current source amps.
https://www.firstwatt.com/f2j.html
Krglee, check out the F2 and F2J, which are simple current source amps.
https://www.firstwatt.com/f2j.html
https://hal.science/hal-02504314/document
But what NdFeB has is very high saturation flux. The very best magnet material is in fact old fashioned Alnico closely followed by Samarium Cobalt. Some way behind is the usual Ceramic Ferrite and way down the list is 'old' NdFeB.
Today, NdFeB has a huge advantage in $$$. But my $$$ no object units would have Alnico or SmCo magnets and NdFeB metalwork
Is it worth the extra $$$ today? Dunno as the distortions this would reduce are often inaudible and the ones which are audible can be reduced with conventional cheapo steel & ferrite systems.
There's a lot of good stuff in that paper. In da 90s, NdFeB started to be touted as the new wonder magnet material. Actually its awful for speakers cos its poor temperature characteristics. Dunno what modern formulations are like as they don't seem to show B-H performance with temperature like da old days.
But what NdFeB has is very high saturation flux. The very best magnet material is in fact old fashioned Alnico closely followed by Samarium Cobalt. Some way behind is the usual Ceramic Ferrite and way down the list is 'old' NdFeB.
Today, NdFeB has a huge advantage in $$$. But my $$$ no object units would have Alnico or SmCo magnets and NdFeB metalwork
Is it worth the extra $$$ today? Dunno as the distortions this would reduce are often inaudible and the ones which are audible can be reduced with conventional cheapo steel & ferrite systems.
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Seems like you didn’t do your homework. 
You can take out that 15 ohms and have 700 ohms instead. Read the F2 manual.
You can take out that 15 ohms and have 700 ohms instead. Read the F2 manual.
Thanks for this Jeff.
My thinking is that for 'pure' current drive (ie Ro >1k 20 - 20k Hz) most (all?) designs still require a Zobel which restricts Ro < 100R or so at 20kHz. The Vanderkooy arrangement suggests you can decouple the Zobels from the output. But you probably have to deal with power transformer capacitance. I'll have to get off my beach bum butt and see if I remember how to use LTspice 😱
Vanderkooy also gets around the main disadvantages of VFETs compared to lateral FETs. Alas, dis beach bum can't afford Linear Audio 🙁
I thought it was good for 100-200°C, so, probably best not to leave the speakers out in the sun.
If I understood it correctly, the main magnet has its otherwise open donut-shaped field compressed and linearised with a couple of bucking magnets. Getting a good, linear result would probably require an extensive mix of calculations, simulations and testing. As such, I found the paper a bit dry and lacking in eye-candy.
A wall of text just doesn't have the same explanatory power as a screenshot.
Besides that, getting rid of the iron would be a major benefit, as that eliminates a LOT of those bizarre distortion mechanisms that we've been discussing.
It looks like no one has done this.
“Current Source Amplifiers and Full-Range High-Efficiency Drivers” https://audioxpress.com/article/Current-Source-Amps-and-Sensitive-Full-Range-Drivers
So it's all about the single speakers.
That's what I'm suggesting too
https://www.diyaudio.com/community/threads/sound-of-the-bipolar-transistors.402327/post-7452463
https://www.diyaudio.com/community/threads/sound-of-the-bipolar-transistors.402327/post-7452463
There are several mechanisms.
The B-H curve changes with temperature well before 100C with NdFeB. Your 'magnetic load line' cuts the B-H curve which determines your operating flux; giving changes with temperature which are reversible with a well designed magnet system.
But if the changing B-H curve 'knee' crosses your load line, you get irreversible partial de-magnetisation.
This is NOT Curie Point when you lose nearly all magnetisation above a certain temperature.
SmCo & Alnico B-H curves move less with temperature and their shape is less likely to cross your load line
You may be thinking about magnet structures with small external field.
There are other ways of minimizing these THD mechanisms.. eg one that has been used from the 1960s is simply saturating the steel ie operating at > 1.4 Teslas for most steels.
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I've read of (and seen) people adding Neo magnets from HD drivers at the back of the rear steel plate of their drivers just for that (but they change other things too)
The subject of current drive /mixed mode drive has seen the light of day through many threads in this forum.
Here alone are 49 (from the 61 listed) with a simple search
https://www.diyaudio.com/community/search/1146749/?q=current+drive&c[title_only]=1&o=relevance
There are some 5-10 relevant threads more with slight different tittles, which do not show up in this search.
It's good to look back on those for valuable answers . 🙂
George