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On Fri, 21 Sep 2012 15:52:17 -0400 |
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Michael Mol <mikemol@×××××.com> wrote: |
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> On Fri, Sep 21, 2012 at 3:45 PM, Alan McKinnon |
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> <alan.mckinnon@×××××.com> wrote: |
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> > On Fri, 21 Sep 2012 15:25:49 -0400 |
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> > Michael Mol <mikemol@×××××.com> wrote: |
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> > |
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> >> Guys, this fell into politics, one of those categories of things |
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> >> you don't discuss in polite company. Not that I'm accusing anyone |
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> >> of being polite, |
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> > |
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> > Phew, glad to hear that last bit. You had me worried for a second, |
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> > what with my reputation to uphold and all |
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> > |
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> > |
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> >> Now, could we go back to discussing software, packages, electronics |
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> >> and amplifiers? I found that portion of the thread utterly |
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> >> fascinating... |
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> > |
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> > That's a good idea. Would you like to hear about Hitachi Class H |
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> > amps? |
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> > |
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> > I'm forever fascinated that I seem to be the only person that ever |
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> > heard of them. Most techies know A, AB and B. Some know Class C but |
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> > I get blank looks everywhere I mention Class H... |
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> |
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> I would indeed. And a primer (or reasonable reference for someone with |
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> just a technician's amateur radio license) on class C. :) |
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> |
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IIRC this was back in the late 70s or early 80s. Someone at Hitachi |
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figured that amps (like code) spent 90% of their time doing 10% of the |
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effort. If you had a 100W amp, it wasn't trying to drive 100W into the |
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speakers all the time - only when the input signal was large enough. |
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And yet, the power source for the output stages was permanently running |
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at 70V or so (that's what it takes to get 100W into speaker coils back |
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then). A transistor isn't a perfect isolator when biased off, so some |
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of that voltage gets dropped somewhere (across the output transistors) |
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and the result is a lot of wastage. |
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wikipedia has a quite good summary of the usual classes - A, B, AB, C & |
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D: |
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http://en.wikipedia.org/wiki/Electronic_amplifier#Class_C |
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But, Class H. A Hitachi engineer had a brilliant idea: |
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Run the damn thing class A all the time (for the audio quality) but at |
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around 24V. Heat generated is minimal. The power supply had a fancy |
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voltage tripler circuit and when the input warranted it, the supply |
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voltage would (very rapidly) switch over to the full 70V and the amp |
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would deliver the full rated output. There was fancy circuitry in place |
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to avoid distortion at the switch on point of course, but that is a bit |
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OT. |
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An interesting take on the problem. Mechanical engineers do this all |
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the time with engines - turbos only kick in when you need the power |
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boost they provide, the rest of the time the motor is in regular mode. |
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I've promised myself for years since my apprentice days that I would |
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one day built a valve amp from a kit. There's something about the warm |
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glow from the tubes on a winter night that is appealing :-) I'd better |
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hurry up and get on with it, I read that decent quality valves are |
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becoming scarce and are generally only available from (what used to be) |
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the USSR. |
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-- |
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Alan McKinnon |
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alan.mckinnon@×××××.com |
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