Fast Amplifier

Cornfed pretty much summed it up. The three terms that summarize a "fast" amp to me would be rise time, slew rate and ultra wide bandwidth. You can't have really wide bandwidth without having pretty solid measurements on the other two categories though, so it is kind of redundant. If the device can't due at least 100+ Khz with excellent linearity, you will have "side effects" that are audible and measurable within the normally accepted "audio range" of 20 Hz to 20 Khz. This can be seen as rounded edges of square waves, ripples or sags on the tops of the square waves, ringing on the trailing edge of the square wave, etc... Most of these things would be quite visible on a good quality scope with a test frequency of 10 Khz or so if the amp was "too slow". Sean >

I think that most everybody has hit upon certain design aspects that are all quite valid. Macm brought up some simple yet good observations. Gmkowal also mentioned an interesting viewpoint that has proponents on both sides of the fence. This stance differs with the designer of one of my preamps. His feelings were that the only reason that a pre / power amp has problems with RF is that they are TOO slow to respond to them. He was designing audio circuits that slewed at over 350 v/ Usec and had rise times in the area of .01 Usec back in 1974. Then there is the fact that ANY filter causes phase shifts, impedance bumps, increased noise and circuitry, etc.. to deal with. My thoughts are to make the unit as fast and simple as possible, the theoretical "straight wire with gain" school of thought. Using that school of thought, Trelja's observations that smaller ( and therefore simpler ) amps sound better would be logical. In most cases, i would say that this is PROBABLY true. I think that it boils down to the fact that the larger amps have more circuitry and output devices. This requires a tighter tolerance for all parts involved with much greater care necessary in matching active components. This takes a great amount of time and is therefore probably neglected in most production based situations. The result is good sound with high power but not quite as good as what could be done if EVERYTHING were extremely tightly matched. While this could be said of smaller amps also, there is simply more parts in a bigger amp and more to go wrong in terms of parts tolerances, making the effects more noticeable. Another factor to all of this as mentioned above is speaker efficiency. Modern technology has raised the efficiency of speakers to the point that less power is now required to get the job done. To follow suite, designers have therefore put more attention into making very good sounding small amps. Once that is achieved, they simply expound on that curcuit to increase power output while PROBABLY not paying quite as much attention to parts tolerances. That is the very reason why some of the small to meduim amps sound slightly sharper and more detailed than the bigger amps with more parts and circuitry. Sean >

Your findings are only limited by the equipment that you've used and have seen in production. There were prototype amps made back in the early 80's that slewed at 1000 V / uS and had rise times as fast as those mentioned for the preamp in my previous post. Unfortunately, those prototypes were never put into production as the designer changed avocations and went into designing and manufacturing equipment for nuclear power plants. Sean
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