- "New Class A" bias module (see Technics folklore - synchro bias)
- "Super-A" bias module (see JVC folklore)
- "Non Switching" bias module (see Pioneer folklore)
The idea is to help hobbyists designing their own audio power amplifiers, making an effortless revival of the three non-switching class AB biaising methods. Hobbyists and designers would integrate the module of their choice onto their own main board.
Technics "New Class A" is relying on fast diodes, implementing a dynamic switch that is automatically chosing between the voltage that's coming from the driver (this is when the output device is pushed to drive the load), and between a fixed voltage that is preset and defining the minimum bias current (this is when the output device gets pulled out, with the load being driven by the other half).
Looks fine, apart from the inherent switching nature of the process. Technics engineers used to call it "synchro-bias", but the marketing rejected that name, and coined "New Class A" for very understandable reasons. I guess that nowadays ultrafast diodes can be used, specified with a less than 10ns switching time, an order of magnitude faster than the output devices.
JVC "Super-A" monitors the sum of Vbe of all output devices, plus the voltage drop that occurs on the 0.22 Ohm emitter resistance. This voltage gets then compared to a fixed threshold (like 2 Vbe in case of a Darlington), and when the total voltage gets beyond this threshold, plus a little something, a transistor fitted into the Vbe multiplier arrangement acts to gently increase the multiplication coefficient.
Looks weird, like a positive feedback ! The increase rate must remain well controlled.
Pioneer "Non Switching" monitors the sum of Vbe of all output devices, plus the voltage drop that occurs on the 0.22 Ohm emitter resistance. This voltage gets then compared to a fixed threshold (like 2 Vbe in case of a Darlington), and when the total voltage gets beyond this threshold, plus a little something, a transistor fitted into the Vbe multiplier arrangement acts to gently increase the multiplication coefficient.
In essence, this is exactly as JVC "Super-A". Pioneer scheme needs a supplementary current source, which would make transistor count higher than JVC "Super-A", but on the other hand Pioneer takes some shorcuts regarding the Vbe multipliers, saving transistors over there.
Sansui "Tanaka Non Switching" monitors the sum of Vbe of the last two power devices, plus the voltage drop that occurs on the 0.33 Ohm emitter resistance. This simple schematic seems to be effective as explained by Tanaka, the author of the design, in the AES publication he made in 1980.
Yamaha's Hyperbolic Conversion Amplification (HCA) Circuit
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