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In Reply to: Re: The color of my parachute! posted by Kevin on May 06, 2002 at 22:28:59:
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
Follow Ups:
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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