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Old 20th August 2008, 02:29 AM   #1
devzero is offline devzero  United States
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Default Yamaha HCA circuit.

Hello everyone.

I've been doing some research on an amp that I'll be receiving sometime next week, a Yamaha MX-1, which is a dual monoblock design and class a as well. However, it's not a conventional class a as I understand it, as Yamaha incorporated a circuit to which they refer as 'HCA', or 'Hyperbolic Conversion Amplification'.

Here is the bit on Yamaha's web site regarding this feature:

HCA (HYPERBOLIC CONVERSION AMPLIFICATION)
The Yamaha developed HCA circuit goes a step beyond the conventional Class A amplification generally used in a high performance amplifier. In a Class A amp, when output exceeds a limit determined by the idling current, operation degrades to Class AB, resulting in increased crossover and switching distortion. With the HCA there's less distortion and less heat generation.

Obviously, that doesn't explain very much. I've searched for descriptions of the circuit, and really haven't found anything that explains it. I've spent some time doing so, too, and mixed and matched terms, but to no avail. The service manual doesn't say anything and I've yet to find a schematic -- not that I'd be able to easily dissect it that way.

I'd appreciate it if someone could give me a brief explanation.

Thanks!

John
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Old 20th August 2008, 07:00 AM   #2
Mooly is offline Mooly  United Kingdom
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Never heard of it , but amplifier classes are precisely defined, and have been for decades.
It will almost certainly be a Pseudo type class A, perhaps using a sliding bias scheme or a floating type output stage, where the rails are always related to the output voltage and continually varying.
This kind of thing can be discussed for ever , Class A, the outputs pass current during the full 360 degrees of the input signal i.e. they are never cut off. If the bias is "varied" to "match" the expected output then it's not technically class A as we think of it, similarly if it uses variable rails to limit dissipation, again it's not true class A as we think of it. These tricks and techniques bring their own problems, that may only show under dynamic conditions.
There is nothing new out there, it's all been done many times before.
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Old 20th August 2008, 08:13 AM   #3
Bonsai is offline Bonsai  Taiwan
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This is marketing 'bollocks' (new word I learnt the other day) of the first degree.

Follow Mooly's advice and accept that its nothing new.

Enjoy the amp - Yamaha make some nice stuff.

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Old 20th August 2008, 05:21 PM   #4
devzero is offline devzero  United States
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Thanks for the replies, Mooly and Bonsai.

Oh, I agree -- it's definitely nothing new, as it's similar to what Krell has done, and another make whose name I cannot recall as of the moment.

Wireless World did an article on HCA back in the '80s sometime, and I was hoping to find a copy of that, although that's probably not going to happen. I'm not sure which amplifier they had, could have been the MX10000, but I have no idea. There were several amps they had at that time using it.

Ultimately I'm not hung up on it, and if the amp performs well, I'll be happy with it. I would like to try a true Class A sometime. It could always double as a stove too. It might be a 'winter only' amp.

John
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