Most that I have seen are transformer powered. Not that it couldn't be done with switchers but if the transformer solution is only one cent cheaper that's how it'll be done.
Ah, I guess that's what I meant to ask. So, how are the speakers driven nowadays? Are they step-down DC-DC converters? (DC coming from a filtered bridge rectifier, connected to the mains?)
Many mfgs are pretty "creative" when it comes to power data. I have seen
200W amps that had a transformer no larger than that in grandpa's tube set, 50-70W.
With the rising oil prices that might play an increasing role. It's also size. Nowadays the macho thing isn't all the rage anymore. We bought a new set and my wife made it very clear that it should not be as large as the old Kenwood stack. It's amazing, the new stereo is barely larger than a shoebox but with the speakers that came with it the thing sounds almost as good as the old one. Plus it was a whole lot cheaper than the old one had been.
Well, you can reduce line voltage to something more reasonable (depending on your power spec) with an iron transformer or a switcher, and you can drive the speaker through a linear amplifier similar to say, this:
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This circuit can be class A, B or C. The only difference is the power dissipated at idle (heatsinks, eh?), and a small "tit" of crossover distortion in class C. Just add a small filter choke on the output to clean that up. Efficiency runs from maybe 45% class A (sine wave output) to oh,
75% I think.
Or, you can use a reasonably fast PWM circuit and drive the speaker that way. Class D with stiff MOSFETs nets you 90-99% efficiency. Linearity is questionable; with a fast, stable circuit you can at least wrap it with feedback and correct it back more or less.
Tim
--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Whyso? Bipolar transistors turn on pretty quickly, you can bias it within say -0.3V. A fast op-amp front end can correct that in a hundred nanoseconds. After the transient, one transistor is conducting and it's linear as any other amp is, with 60dBV NFB.
Tim
--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Distortion at the zero crossing point is highly unpleasant. I'll defy you to find an output stage that responds adequately in a hundred nanoseconds too.
I have *never* seen a commercial Class C amp for audio.
Strange. I know a fellow who built his own 1kW amp and he says it's a few bits of a volt (just as I described) into class C. He says it's good at full power out to something like 300kHz, so damned if it isn't fast enough.
OBTW, any regular output stage using reasonable transistors (fT > 20MHz or so, not wimpy-ass 2955/3055 or TIPxx stuff) will certainly respond in 100ns. I'd be more concerned about the diff amp responding in under 300ns... which can certainly be done, especially with somewhat faster transistors than generic (maybe 200-500MHz fT) and careful design.
Tim
-- Deep Fryer: a very philosophical monk. Website:
Must have repaired several hundred units back in the 60's. All ran in the A-B region. Just biased to conduct slightly at zero signal to avoid the dreaded 'crossover notch'. The last place you want distortion, is near zero.
I don't think I was ever impressed by their home amplifiers weight, allthough it usually goes by price. Switching, probably not. Most of these cheap amp have overdriven transformers, so they skimp.
Many current designs use the brute force transformer-rectifier-filter approach. However, new regulations are proposed that would require all consumer products to meet a minimum power factor requirement. This requirement would eliminate the old approach in favor of switchers with power factor correction. Also, class D amplifiers require tightly regulated supplies for the output stages, which make switchers more attractive, since linear regulators are notoriously inefficient.
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