In yesterday’s post, we discussed the all-important input voltage gain stage at the heart of a power amplifier. The purpose of the input stage is to make the signal bigger. To increase the available pressure in preparation of converting that pressure into power.
I refer to the input stage as “all important” because this is where the main differences in sound quality take place. In the BHK amplifier series, for example, we use a vacuum tube to raise the pressure because it maintains musical timbre better than any other input stage I have ever heard. But that doesn’t mean there aren’t other ways to build such a stage: there are more than a few great sounding power amplifiers on the market that prove that. We have just found, over the last 45 years of designing, that a vacuum tube used as an input stage preserves and honors the music better than any other technique. Period.
Maintaining timbre is a term we use with abandon, but what does it actually mean? My good friend, the conductor, Lowell Graham, helps us to define it in words that assist me in explaining the challenge of input stage design.
“Timbre is the “quality” of sound (not good or bad), but how we recognize whether the sound is a clarinet, trumpet or vocalist playing/singing the same pitch. It is the absence or presence (strengths) of all the overtones above the fundamental or any pitch that is heard. Of course, this goes out at least four octaves and in reality, more. Ergo, if the bandwidth begins to drop at 20KHz, so does the ability to comprehend the full quality (timbre) of the notes, especially in the upper registers or tessitura.”
The point here is that maintaining bandwidth, and using every trick in the book to preserve musical timbre, happens in the input stage of a power amplifier. It is critical.
That said, once we’ve built up high enough pressure in the input gain stage it is time to convert that pressure into something useful to a loudspeaker. Power. This task takes the muscle of the output current stage, the meat of a power amplifier, where pressure is turned into power.
I want to spend at least one more day looking at the output stage without such a long, and necessary, preamble as we had today.