preamp inverts polarity

Hey, I’m going to clunk your heads together. You do not (rpt not) obtain a vacuum by compressing air. You suck the air out of the container to produce a vacuum. A vacuum is measured by no. of air molecules per unit volume. A perfect vacuum would have no (as in zero) molecules at all. Obviously, a tremendous mass will compress air a lot but not (rpt not) completely, which is why air shock absorbers work even for very massive commercial jetliners. PV equals RT. The number of gas molecules in the container remains constant. Hardly a vacuum. In fact there are MORE molecules per unit volume when the air is compressed. A lot more.

geoff, read their posts again...they said “ decompress” air not compress. Either way, you’re correct about how vacuum is obtained. 
I’m still stuck on that single ended amp remark...😁

Kalali,

It’s pretty simple. The bias level in a single ended amp defines how negative a signal it can pull it down. If the source drives it below that bias level, the gain device goes more non-linear and eventually shuts off completely. It’s the electrical equivalent of trying to pull a vacuum in air. Push-pull amps don’t do that unless they’re deliberately mis-balanced. Instead, they transition to class AB and one half of the push-pull does all the work. That's why SET amps have such devoted followers. The way a triode distorts as it runs out of bias is very organic and similar to how air behaves.

@kalali

Just to be clear here, Push-Pull amps can be class A and also all triode. The 'air is single-ended' thing is a bit sketchy and also a bit of a red herring, since air has nothing to do with how amplifiers operate.

The transmission characteristics of air certainly don't resemble electrically balanced operation. And no doubt there's more to the character of a triode than how it distorts in a SET. Its undeniable that many fans of SETs like them because they distort in a way somewhat similar to how air does.