https://www.diyaudio.com/community/threads/square-law-amp.355067/


.ac dec 5 100 100meg

Trimmer 500 Ohm should be initially set to MINIMUM resistance.
Then adjust for IDLE current for 1 hexfet to be around 300mA. (340 Ohm in sim)

CCS: Q4 and Q8 should be temperature sensing - mounted to heatsink.

R2 and R4 (for 15V Zeners) should be:

2k2 for rails 35V
2k7 for rails 42V
3k3 for rails 50V


resistors 0.5W:   R15, R18
resistors 1W:     R5, R6, R7 (zener resistors)
resistors 2W:     R26

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My with corrections is here AUDIO_EW-WW_Hegglun_Square-Law-Rules_Sep-1995_errata.pdf - Google Drive
https://drive.google.com/open?id=0B3i4bQpWjZBtSmNHNFpKOGcwQlE

Most of my articles (apart from Linear Audio) are here IansArticles - Google Drive
(use List View for a compact list. Feel free to Downoad All for reading).

"Unusual" features are Common Source (CS) output stage with no source resistors for high idle current and
wider Class-A region. Why CS? Followers were used in original power amps when the FT of the power transistors
were abysmal.
Now with fast power transistors you can use CS. Also with followers using L or D MOSFET their high threshold
voltage reduces the output swing, but using CS this is overcome.
No need for extra rails to get the full output swing.

The original used laterals and did not need thermal coupling of Q1,Q6 to the heatsink.
But using DMOS will require thermal coupling of Q1,Q6 to the heatsink.
Also with DMOS don't forget zeners across gate-source (the Hitachi laterals have them built in).
Thses zeners can also provide current limiting for SOA, eg 6v2 for the n-ch and 6v8 for the p-ch).

I also ran R25,R26 from the zeners without CCSs. KISS.

Also I have since moved away from using capacitor C6 across the feedback resistor to
minimise the effect of RF from the speaker leads.

And for capacitive loads you do need an output inductor and damping resistor.

It did sound good. I built one for an audiophile friend, who compared it to a his
Williamson (UL version), and he like it.
His speakers were really old Midax? horns with 18 inch woofers in a huge cabinets -
I think they were 105dB/W/m and they sounded great on his Williamson. 

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Added LED's across the opamp feedback to limit the swing for faster recovery from clip
since the opamp only needs to output 0.5V to clipping the OPS.

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