Why have capacitors improved so much over the years?

The best are still vacuum dielectric or air dielectric. They’ve been around forever, but they come in small values at high cost. I use these exclusively in my phono / pre.

Film and foil are next best; styrene is pretty neutral, teflon a bit bright, polypropylene a bit dull. Some examples are ’very’ rather than ’a bit’.

Metallized are constructed by depositing metal onto the dielectric. Quality appears to depend on the thickness of the metal and the junction of the metal to the wire leads. They can be almost as good as film and foil, or almost as bad as electrolytic. Mica and teflon are the best.

Ceramics tend to be inferior to metallized, the quality seeming to depend on the dielectric constant of the ceramic, lower being better.

I don’t have strong opinions on paper in oil, but I really don’t like leaks, so I avoid them.

Good luck!  YMMV

Barjohn, that's the engineer's answer from 1980.

The book that changed all that was Horowitz and Hill, 'The Art of Electronics', which had a table of capacitors and their actual characteristics. With real information about the physical effect known as 'Dielectric Absorption', or DA, applied to everyday electronic components, the stage was set for a genuine improvement in sound quality.

There are a few topologies which sound good and don't much profit from better components, but the best ones can be much improved thereby. In my experience, anyway.

Build yourself a breakout box in which you can change an important cap from one technology to another with the flick of a switch. Get a book on psychology experiments to learn how to do a single-blind experiment, and recruit your lady as a test subject. I think you'll get quite a surprise.

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.

Since you seem to be interested, one of the best books you can buy is an edition of Art of Electronics by Horowitz and Hill. First edition 1980, third edition just a few years ago.

Redwood, it’s just a matter of getting used to them. Capacitance is an effect of two conductors separated by an insulator. The bigger the plates (conductors), the more capacitance. The closer together, the bigger the capacitance.

Accordingly, the capacitors are made so that every other plate is connected together, and the shaft turns the plates so that they become close together and alternating. This increases capacitance to the maximum value, while turning past that place decreases the capacitance towards the minimum.

With the help of a meter you’ll figure it out in a few minutes. Nothing exotic here, the theory’s been known for 200 years.

Yep, all that heat in Class A is murder on those electrolytics. But who wants to rule out Class A, even for power amps?

It’s a good thing to keep the hot spots (rectifiers, output transistors) away from the electrolytics and noisy power transformers away from the listening chair. I solve the problem by putting a spacious power supply in another room, with reactances well away from the heat, but not everyone is lucky enough to have the space.