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Old 3rd July 2006, 02:18 PM   #1
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Question Class A/H amplifier

Class A/H
In 1988, Stan Gould, a co-founder of BSS Audio (cover picture, upper), then based in London, took class H still further. In BSS’s model EPC-780, shunt-mode commutation involves progressive (‘non-switched’) drive transfer between three supply rails, instead of hard switching. As if this wasn’t enough, the output stage is configured as an asymmetric bridge (Figure 4.55) [60], with one half operating in class A. The output of the low voltage class A amplifier is connected to the ground (0v); the bridge’s HT1,2,3 rails and their common centre-tap midpoint are all floating. This most innovative circuit may be viewed as a hybrid of Super classA, NSB, and class H, and bridging! Efficiency is 70% at full power. At average program levels, up to around 20w (13v rms) the signal experiences class A operation with a relatively
low dissipation. In these conditions, the amplifier is operating as a half-bridge on +/ –27v rails (HT1). Since 1989, the topology has seen use in thousands of concerts worldwide, as the EPC amplifiers are part of the Turbosound’s Flashlight and Floodlight
PA systems.
From "DUNCAN, Ben (1996): High Performance Audio Power Amplifiers for Music Performance and Reproduction. Oxford, UK: Newnes. An imprint of Butterworth-Heinemann Ltd. ISBN: 0-7506-2629, p.p. 142-143."

If I replace the right part to the simple stage with one-level PSU, we can see that the scheme similar to A+. What do think about this scheme ?
I want to use TDA7293 or LM3886 as the right part of this scheme and the JLH amplifier as the left part . What the lower voltage should I utilize to the JLH stage for the power-down?
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Old 5th July 2006, 01:06 AM   #2
djk is offline djk
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Hmm, I wouldn't pick the JLH for the class A amp portion. It's not that it's not a good amp, but not being DC coupled I just wouldn't use it. The feedback loop and DC centering might be tough.

If I really wanted to do it right, I would order a copy of the Yamaha B-2X service manual. It is what you described as wanting to build. The block diagram is in the owner's manual:

http://www2.yamaha.co.jp/manual/pdf/.../SepA/B-2x.pdf

IIRC, the class A amplifier outputs run on ±6V or so with a pair of 100,000µF filter caps. The class AB amplifier on ±63V or so with 22,000µF filter caps. The front-end that driver the class A output stage runs off the same voltage as the Class AB amplifier.

There is another two-part article on this type of bridge amp in Wireless World in the 1980~81 time frame. It would be worthwhile to check the library for it for reference purposes. It shows the left half of the bridge being a high quality amplifier, controling a lesser quality right half that is class AB (essentially what Yamaha is doing). It only uses a single supply voltage (what you want to do), and the input is referred to the negative supply rail(like the JLH). Two designs are presented, a 20W one, and a 200W one (100W RMS).
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