Unusual audio power amp configuration

Ian0

Joined Aug 7, 2020
1,933
Interestingly, the topology in Jony130's attached file, 88_1235904466..jpg , was used for many years by QSC in their commercial amplifiers. The key advantage was that the collectors of the output transistors were all at "ground" potential, and could therefore be used without mica (or other) insultation on a common heatsink - which was also grounded. Substantially better heat conduction without the insulators...
They are lateral MOSFETs and the case is the source (unlike normal MOSFETs where the case is Drain).
It can also be done with the conventional circuit, but the heatsink is connected to the output. It means you can't connect the heatsink to the case, but generally, you can't anyway because it would exceed the standards for case temperature.
 

Ian0

Joined Aug 7, 2020
1,933
The "conventional" circuit can be morphed into the "unusual" one as shown:
From the top:
1. Usual amplifier circuit (biassing not shown)
2. Using a op-amp as drive. Obviously, this would need an op amp capable of ridiculously high power supply voltages. (Some do exist, but have you seen the price?)
3. Instead of the full supply, make a 15-0-15 supply, bootstapped to the output. This will almost work, but the input common-mode voltage of the op-amp will be exceeded at large output voltages
4. Same as drawing 3 (just noticed, but I'm not going to draw it again)
5. Move the ground connection, and swap the input polarity to remove the phase inversion. Now it looks an awful lot like circuit in the original post.
 

Attachments

Veracohr

Joined Jan 3, 2011
750
Looks similar to this circuit from Douglas Self’s “Audio Power Amplifier Design”. He calls it “Super-Class A” and says it was introduced by Technics in 1978.

F3B0504D-3941-4ED6-B489-B7C4B40B95C2.jpeg
 

Ian0

Joined Aug 7, 2020
1,933
Looks similar to this circuit from Douglas Self’s “Audio Power Amplifier Design”. He calls it “Super-Class A” and says it was introduced by Technics in 1978.

View attachment 220609
Superficially, but the Self/Technics circuit is two amplifiers - a class B driving a class A, the Peavey/QSC is quite a lot simpler.
As drawn, the Self/Technics circuit also runs out of input common mode range!
 

Jony130

Joined Feb 17, 2009
5,230
If we are talking about unusual amplifiers topology with floating supply. For a couple of years now the Yamaha uses this topology in A-S series (from A-S1000 to A-S3200) power amplifiers.

As-1000.png
 
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