I had one once too (yamaha HCA amplifier). I agree about it. What's always funny to me is that when one manufacturer actually develops something new and improved which really does perform better than anything else, other manufacturers are quick to dismiss it.
Why? because they don't have it and can't make money by licensing it from the true inventor of the technology who did all of the R&D in the first place (in this case Yamaha).
I actually thought that HCA sounded at least as good as a pure class A amp I had once. Maybe slightly better. This was decades ago though and is only just a vague memory now...
cheers!
Well that stands to reason, as HCA was real class A. Not that it would matter, they perform admirably.
The Titanium series MX-2000, and MX-10000 used advanced approaches for variable level fulltime Class A to maximum power. Bedini also did variable bias but it was rather simpler and more of a slow descending bias circuit. These yamahas are not push-pull like Pass' Threshold stepped bias amps, nor Quad's current dumping, as upon closer inspection to the schematics we realize Yamaha's approach was with totally different.
Each amp employs two rows of Class A amps who's output was inverted to each other but with equal phase bias. One signal would then be inverted on one side, summed to the opposite, and the bias and distortions were cancelled, not unlike balanced interconnects.
The input signal was referenced by the error correction amp, and used to further eliminate transient and harmonic distortion that may remain on its output.
The MX2000 and MX10000 will maintain a flat response into any load due to this servo, and was voltage biased in contrast to the current bias topology of other amps and this bias was modulated with the audio signal. I'm not sure about the MX1000, I know its Class A but to full power or not, I can't say having never owned one, but at its price what more could be expected? Word is, they sound super.
And, with class A, this was one of the big considerations. Temperature coefficients would change the any class A amps specs wildly if the temp changed, again in the yamaha's its all servo controlled to correct this.
Intriguing technology hardly seen anywhere, really.
All of it had to be faster than the audio signal, which we know was fast and the amps stated bandwidth was 200kHz, but much higher. In the end the HCA allowed a small amp (well, they're actually pretty large) to provide bounties of power per channel into 1 Ohm without compounding constant bias as it reduced the duty cycle.
So what is it? Its two single ended class a amps on each side, with nulled distortion and music controlled bias. Nothing short of brilliant.
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