The Sweet SpotAuthor: Nelson Pass, Pass LabsThis article will concern itself with finding the sweet spot for each gain device in audio amplifiers.
Every audiophile understands the concept of the sweet spot, that happy balance of conditions that elicits the best possible sound. It can be a particular listening spot, the best positioning of loudspeakers, or the fortuitous combination of components which complement each other perfectly. It should be no surprise that the desire for the best performance takes the search for the sweet spot into the interiors of the components themselves.
This article will concern itself with finding the sweet spot for each gain device in audio amplifiers. It is a commonly held belief in audio that the best amplifiers are composed of one or more active gain stages, each made as intrinsically linear as possible before negative feedback is applied to further improve performance.
Taken to the extreme, linear stage = good / negative feedback = bad. The latter half of that statement is the subject of controversy, but there is no argument about linear stages being good.
Achieving intrinsically low distortion in active devices (tubes or transistors) is usually not easy. We can use good parts, see to it that they have adequate voltage and current available, and use them in topologies that play to their strengths. These are the basics, and this is what you generally see when you examine schematics of popular amplifiers.
However each individual device has a sweet spot where its performance is the best, often dramatically better, and finding that sweet spot is a powerful technique for maximizing the performance.
I am writing about this because over the years of communicating with DIYers and many professionals I have discovered that this is a poorly understood concept, and I have seen many circuits which have failed to take advantage of it.
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