Square Law Class A Amps

[Onvinyl]

Hi,
in the latest edition of the fabulous Linear Audio magazine we have a comprehensive article about a so-called Square-Law Class-A amp written by Ian Hegglun.

I find this design quite remarkable and appealing and I'm musing over building it.
Are there inmates who build this thing? How did it compare soundwise to your stuff?
I freely admit that I didn't really get how the arrangement of the floating supply would ensure the amplifiers gain.

I would love a discussion about that design here.

I did a search and to my surprise I found no thread concerning this amp, should it exist we'd probably don't need another one...

Rüdiger

[kipman725]

I have read that artical and to be honnest wasn't impressed at all perfomance is very poor! and heat production still a lot.

[ArthurDK]

Hi Rüdiger, I just got my copy of Linear Audio vol 1 last week, and I too think this concept sounds very interesting.

I am though in the middle of building a Pass F5, and have some of the items to build a F4 also, so I will probably not be ready to participate on building a square-law Class A amp soon.

Did you get any experience, or did you give up the project.

Regard
Arthur.

[kenpeter]

Schottky Diodes have square law curves, and much lower voltage drop than other
such device (MOSFET diode for example) you could throw in series with a totem to
sense and shape conversion of linear voltage to complimentary square law currents.

A recurring theme in most of my schematics, search my recent threads. I have not
seen what Hegglund has done for comparison, can you point to any drawing of his
that isn't hiding behind a subscription fee?

[Bigun]

Originally Posted by kenpeter

I have not
seen what Hegglund has done for comparison, can you point to any drawing of his

http://perso.orange.fr/francis.audio2/AmpHegglun.doc

[kenpeter]

Voltage across Schottky stack deliberately insufficient to bias
all four diodes ON at the same time. Yet plenty high enough
to turn on two, and hard... Or all four jammed halfway-on,
riding the curve. Square law curve, if those Schottkys are
sized right...

R7 provides some always-on - do-nothing-reserve current.
Even if a Schottky might turn off, transistors still conduct
this small amount.

[KSTR]

Undegenerated complementary (or circlotron) lateral MOSFET outputs are already pretty much D2S to start from, plus they have the desireable feature that the transfer characteristic tends to flatten out to linear at higher currents. Exactly what is required to drive a "square-law" amp into class-B and maintaining transconductance. N-ch tend to have higher gm and lower capacitance, so a gate divider works out nicely as gate stopper and attenuator to get the transfer statically and dynamically complementary. That's IMHO why lateral outputs have something to them, raising interest as can be seen on this forum, lately.

Kenpeter's Allison O/S looks promising, too, when getting all 4 Schottkys a bit conducting at idle, otherwise the voltage transfer has a dead zone a global feedback would have to deal with.

- Klaus

[padamiecki]

Originally Posted by kipman725

I have read that artical and to be honnest wasn't impressed at all perfomance is very poor! and heat production still a lot.

yes, lots of the heat
but poor? no hf harmonics...

[padamiecki]

Originally Posted by kenpeter

Voltage across Schottky stack deliberately insufficient to bias
all four diodes ON at the same time. Yet plenty high enough
to turn on two, and hard... Or all four jammed halfway-on,
riding the curve. Square law curve, if those Schottkys are
sized right...

R7 provides some always-on - do-nothing-reserve current.
Even if a Schottky might turn off, transistors still conduct
this small amount.

Hi Kenpeter
I found simulating your design with LTSpice, that above input voltage 10Vpp is completely unstable, oscilating like mad, could you send asc file for this circuit, I will compare it with mine to find my eventualy mistakes... thanks in advance!

[kenpeter]

I discovered the folder where this old .asc file had been hiding.
Also tried 10V @ 1KHz and saw no oscillations, thats not really
proof of stability. Any crazy oscillation wouldn't surprise me...