A friend of mine brought over a schematic of an Audio Research tube preamplifier yesterday. This is but one of several I have been studying as of late. One of the characteristics I find as an interesting pattern is that none I have studied are direct coupled; meaning at a minimum the output of the preamplifier are all capacitor coupled.
This is interesting because the vast majority of solid state designs are all sans capacitors in the signal path. Direct coupled. In fact, we work hard at removing these sonic changers from the musical path.
Capacitors, in general, soften the sound of music passing through them. I don't want to say blurred, because the best capacitors certainly do no such thing, but soften is the best I can come up. And soften isn't a very good word. Soften would indicate a reduction of transient information, a slowing of the signal. Modern tube preamplifiers are extremely fast and transient accurate. And cap coupled. I wish I had a better description of what they do. I don't.
One other pattern observation is their simplicity. The tube designs I've investigated are one, perhaps two devices long for the signal path. Most solid state designs have far more parts.
It is fascinating to me that tube preamps do all the wrong things as far as we solid state designers might think: add capacitors in the signal path and use crude, low tech amplification designs.
If it's not obvious to you, I am rethinking all that I know and take for granted in preamp design.
As Albert Einstein once said "If you can't explain it simply, you don't understand it well enough".
I'll keep you up to date as things become apparent.
