Why have capacitors improved so much over the years?

It’s market driven. Who cared about capacitors 20 or 30 years ago. People like us who tweek and upgrade electronics have found that capacitors make a difference in sound quality. Like changing tubes and interconnects.

Engineers who designed professional gear 40 and 50 years ago cared. Back then I knew some then 'old timers' who were aware. Can't comment on earlier, but no doubt some were aware.

I had a tube 'guru' tweak my Citation II with better resistors, caps & wire. Same cct & old tubes, but OMG.

jfd11, hmmm, you have to tell us the name or it's like it never happened. 

Redwood, I bought most of mine from Surplus Sales of Nebraska, when I didn't make my own.

At the Surplus Sales website, click on 'capacitors' in the left column, then on 'air gap variable', then go to the 'quad' section. There are some with 4 sections, each of which maxes at 460pf, for about 1.9nF or 0.0019 uF when paralleled. With judicious design, one can create an RIAA network with values in that range.

Warning: they're big and expensive. But it's the best sound available.

@ieales - thanks for the link, cool history.

@terry9 thats a wild website… I have no idea what I’m even looking at with those air caps!