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In Reply to: The color of my parachute! posted by j.l.guillebeau@att.net on May 06, 2002 at 10:42:37:
James,
Wow, I have to say I'm pretty much in agreement this time. One thing about university-taught engineers in the U.S. is that, no, if an engineer has the passion for what he/she is designing, then there are no limits. I believe that we have some of the finest engineers and finest minds on the planet in the U.S. Unfortunately, unless those bright engineers can dump a limitless amount of money into their own hobby or their own business, they are controlled by the business world. It impresses me to see how much you know about audio electronics being self-taught -- my hat's off to you. But, from an engineer's perspective, please understand that it is EXTREMELY frustrating to work for a company doing design work. It is not that engineers don't think outside the box; they do. If engineers were turned loose to compete against each other, then companies would start making products that simply can't be beat, because product development would be driven by engineers instead of business majors. Either the companies would unfortunately go under because nobody can afford the high quality products, or the companies' marketers would suddenly have to step into overdrive to keep their company competitive. It's that employers, like you said, are trying to churn out mass-produced, mediocre-at-best quality products. I know how the American buying public is. They don't want the best in one or two categories, they want to have everything, and get as much for as little as possible. I think this sort of makes America the great place that it is. If I had to buy all Ping to get into golf, buy all Theta and Classe to get into power amps, and buy all Ferarri to get into cars, then I'd never have it all. But, give me some Wilson clubs, a Sony receiver, and a Chevy Cavalier and suddenly I'm the American that has it all, even though I might not have the best. Does that sound right? Audio is an area where I personally put more emphasis into quality however, yet perhaps not as much emphasis as others, including yourself. If I could have the opportunity to listen to some of your designs, I would probably save up and stand in line to purchase an amplifier from you. You heart is certainly in the right place with your audio likes.Now, about operating in pure class A... ouch, pure class A limits the amount of output power you can have naturally. I know that Krell uses some gigantic tubes to make a respectable amount of power from pure or quasi-class A designs. But, is it reasonable to expect 300W per channel from a class A design? About the topic of negative feedback, please share, what are your dislikes with using negative feedback? My BGW amp sounds more open and musical than my Crown amp most likely for the reason that it uses very little negative feedback. I'm interested in hearing your thoughts on neg. feedback. Regarding the input drive stages you mentioned, most pro audio products feature a discrete class A drive stage. Many even use opto-couplers to control the gain knobs so that the pots don't get into the signal path. I would bet that most home products (beyond the run-of-the-mill receivers) use a discrete drive stage. Some pro amps have power factor correction features, but those are typically on the multi-kilo watt models. All I have considered in my personal designs for power supplies is: use a huge, well-wound, clean power transformer, beyond the rectifier use a generous amount of capacitance. Please share thoughts you have along these lines.
Follow Ups:
Kevin: Push pull topologies are just wrong! Some weak musical signals that are not strong enough to push or to pull get cancelled, erased, eviscerated by push pull topologies. This is because if the signal expresses itself as common mode disturbances then it falls by the wayside. Diff amps belong in calculators and power supplys not music amplifiers. The tail current in a diff amp is loaded and there is no way around it except not to use it. There are useful alternatives but I may want to consider it proprietary. Summing the output signal and feeding it back to the - input multiplies odd distortion products exponentially. Odd distortion products are mostly responsible I believe for most of the solid state grunge or glare we hear that is fatigueing. Feedback will multiply these out to the 99th. Feedback is a pandora's box if not laboriously and infinitely controlled. It must be something akin to giving an elephant an enema. One doesn't have enough means to control it and invaribly in most every case is gets out of hand. It changes every single parameter in the amp. I would go for lead compensation of each stage as opposed to overall feedback. And when we slow the transistor down with lead compensation, then we invaribly also multiply odd distortion products. They are generally applied to make an otherwise unstable transistor work or behave. If we had better transistors that would work without the lead compensation this would be un-necessary. The semiconductor industry has turned away from the design of disreet transistors for the audio industry in preference for digital processors. One of the best transistors I use was a fairly early device 2N2219A and 2N2905A but I have to go through barrels of them to find two that have the same characteristics that will behave properly. And then they are getting increasingly hard to find. And output devices in TO-3 or TO26 cases are abysimally slow in turn on and turn off times. The higher the power the slower the device. The output stage in the amplifer is the weakest link in the chain. No one yet has perfected controlling a output stage where the collector is turned toward the output (the best current transfer) as opposed to the emitter follower topology. Crossover notch distortion and dead zone cannot be cleaned up and is extremely audible in a class AB amp. The only away around it is to bias the output stage Class A. You better get out your water-cooled or refregerated heat sinks and get the transformer out of mothballs and re-invent the transformer coupled Class A output stage because it's coming. We could conceivable get 50% efficiency. The well designed low turns ratio transformer is still the best way to interface a reactive load like a loudspeaker to an amplifier. In every solid state amp virtually on the market, the speaker is looking into an unprotected output stage that the only thing seperating the speaker and enough stored energy in capacitance to lift a twenty pound dog six feet into the air is two tiny emitter to collector junctions. It's just not safe for my $3000 speakers to be abused so unfairly. Cordially, James
I will always agree that class A is the most pure design. I really like the way you look at everything that determines what happens in the signal path. Thanks for the info. on neg. feedback... I've heard a lot of mixed reactions on that topic. I know tube lovers have to their advantage the way tubes tend not to amplify odd order harmonic distortion... making higher THD levels in those amps acceptable. But like you said, until new class A designs take hold, we'll have to bias large class A designs and cool them with ventilation systems designed to chill a walk-in freezer. Plus, don't forget and leave the amp turned on when you're not listening to it or you're electricity bill just doubled :) Hmm, class A topologies, including power supply loss approaching 50% efficiency? That's great! It's been a long time since I've studied some simple transformer coupled output designs. I remember reading some bad effects of doing that, and of course, some good effects. I cannot remember what those bad effects are. But, I wouldn't trust my expensive speakers to my own designs. That's why I like buying what I can afford from reputable vendors, so if their amp fries my speakers, then they have to pony up the cash to replace them.
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