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