restorer-john
Grand Contributor
It's more complicated than that, an ordinary class A/B amplifier biased heavily in class B would be unlikely to have stellar THD performance without a healthy margin over the theoretical limit on slew rate. The internal slew rate in this case is larger than the output slew rate at the zero crossing. I suspect (not really, I know) the error correction schemes in the Benchmark are necessary to compensate for this. If one simply does not have quiescent power as a design parameter there are other paths to sub-ppm distortion amplifiers. No criticism implied or intended, just that if you allow for 50W quiescent power in a 100W amp it is easier to achieve linearity before any feedback or error correction.
Scott, the post above simply asked whether the AHB-2 rated amplifier slew rate of 16V/uS was "sluggish".
Clearly, the 16V/uS quoted is surely an outer loop (overall) worst case number for the AHB-2 amplifier as a whole, wouldn't you say? Wouldn't you consider 16V/uS overall (line in to speaker out) sufficient at large outputs to reproduce content in the audible bandwidth without slew-induced distortion rearing its head?
Other feedforward/'non-nfb'/direct distortion cancelling power amplifiers from the past specified huge numbers for slew rate and not just internal. My Denon POA-1500 is +/-400V/uSec from main in to speaker out, a S/N of 123dB and a FR of 1Hz-300KHz (+0/-3dB @ 1W) THD of <0.002% and IMD <0.0015% (60Hz/7KHz 4:1 at full power). That needed 150MHz fT transistors in the front end/driver and 35MHz output stages and it was made in 1983!
It uses a most interesting design and I wonder how it differs or is similar to other systems like the Benchmark? Perhaps you can comment on the topology and any similarities, this amplifier has always intrigued me (I have few of them) and apparently caused more than a few techs and designers to scratch their heads back in the day.
Schematic is attached, VAS is virtually a complete amplifier on its own. There are a few small schematic drawing errors in the power stage- ignore them, they don't translate to the actual amplifier.
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