There's quite a lot to address here, so I may just comment on one issue at a time.
In terms of materials, all caps contain two layers of foil, which can be silver, copper, tin, or aluminum. That's the material which is electrically charged. The insulating material, which separates the foil layers, can be paper (treated with wax, oil, or other substances), plastic (such as Teflon, polystyrene, polypropylene, mylar, or polyester), something more exotic (silk), or some combination of the others. More expensive caps use individual, interleaved (or inter-wound) layers of conductors and insulators. Those are the "film-and foils." The less expensive ones spray or coat the conductive material onto the insulator. Those are called "metalized."
So, because they perform totally opposite functions, foil vs. plastic really isn't the correct vocabulary. What we're looking at is a wide mix of material combinations and fabrication techniques, some of which sound better than others when used in perfectionist audio components.
It's only been in the past 30 years that the concept of capacitors having distinct sound characteristics has come about. Traditionally, the best insulators, like Teflon and polypropylene, have been considered the best sounding, but they are also the most difficult and expensive to wind. Exotic combos, like the Jupiter or Duelunds are very recent developments.
What sets the Duelunds apart is their completely different approach, both in materials and fabrication. For the curious, there are papers by both Steen Duelund and Harvey Rosenberg (who was an early champion of Mr. Duelund's research) which can be found online. They go into much more detail about the theory behind his approach than one can easily summarize.
The other big variable is high voltage vs. passive circuit applications. Our experiments showed that the same cap used in high voltage electronics often sound very different when used in a passive speaker crossover (and vise versa). Also, putting them together in different combinations in the same circuit can greatly alter the end result. Throw in all of the other parts that make up a circuit (tubes, transistors, resistors, inductors, transformers, copper or silver wire, solder, and so on) and there's no end to it.
Basically, we found that certain caps (as well as other components) consistently sounded better when used in certain positions, combinations, or passive vs. active circuits. The Duelund caps and resistors are certainly at the top of the list, but others worked well, too. The issue of resonance in a capacitor is also worth considering and this may be a characteristic of how tightly a cap is wound, as well as the type and thickness of the insulating material. This may be why the "new" VCap TFTF sounds better than the original -- the Teflon insulation is substantially thicker.
For a long time we thought that foil-type inductor coils, such as those made by Alpha-Core, were the best to use. However, Dave Slagle pointed out to us that, abstractly speaking, these were built like capacitors (layers of foil wrapped with concentric layers of plastic insulation) and that we should stick to wire-wound inductor coils, like the Solen air-core. I actually did a direct comparison of Duelund's original wire-wound, iron-core, toroidal inductor with the Alpha-Core and preferred the Alpha-Core. This just shows that the ultimate proof is in the listening. Along those lines, my partner, Dr. Loesch, has historically preferred oil caps, but in our tests none of them really sounded as good as the film-and-foils.
My point is that there's really no "evil" material and that the reasons why we prefer one material or fabrication technique over another cannot always be applied as a general rule. To give you a more specific answer, as of this writing, I can't say that plastic, per se, has a sound that we can personally distinguish as plastic. Of course, we've never tried the Duelund caps or resistors in our electronics (the high voltage Duelunds weren't on the market when we were voicing our amp and preamp), so that view may change when we do.
In terms of materials, all caps contain two layers of foil, which can be silver, copper, tin, or aluminum. That's the material which is electrically charged. The insulating material, which separates the foil layers, can be paper (treated with wax, oil, or other substances), plastic (such as Teflon, polystyrene, polypropylene, mylar, or polyester), something more exotic (silk), or some combination of the others. More expensive caps use individual, interleaved (or inter-wound) layers of conductors and insulators. Those are the "film-and foils." The less expensive ones spray or coat the conductive material onto the insulator. Those are called "metalized."
So, because they perform totally opposite functions, foil vs. plastic really isn't the correct vocabulary. What we're looking at is a wide mix of material combinations and fabrication techniques, some of which sound better than others when used in perfectionist audio components.
It's only been in the past 30 years that the concept of capacitors having distinct sound characteristics has come about. Traditionally, the best insulators, like Teflon and polypropylene, have been considered the best sounding, but they are also the most difficult and expensive to wind. Exotic combos, like the Jupiter or Duelunds are very recent developments.
What sets the Duelunds apart is their completely different approach, both in materials and fabrication. For the curious, there are papers by both Steen Duelund and Harvey Rosenberg (who was an early champion of Mr. Duelund's research) which can be found online. They go into much more detail about the theory behind his approach than one can easily summarize.
The other big variable is high voltage vs. passive circuit applications. Our experiments showed that the same cap used in high voltage electronics often sound very different when used in a passive speaker crossover (and vise versa). Also, putting them together in different combinations in the same circuit can greatly alter the end result. Throw in all of the other parts that make up a circuit (tubes, transistors, resistors, inductors, transformers, copper or silver wire, solder, and so on) and there's no end to it.
Basically, we found that certain caps (as well as other components) consistently sounded better when used in certain positions, combinations, or passive vs. active circuits. The Duelund caps and resistors are certainly at the top of the list, but others worked well, too. The issue of resonance in a capacitor is also worth considering and this may be a characteristic of how tightly a cap is wound, as well as the type and thickness of the insulating material. This may be why the "new" VCap TFTF sounds better than the original -- the Teflon insulation is substantially thicker.
For a long time we thought that foil-type inductor coils, such as those made by Alpha-Core, were the best to use. However, Dave Slagle pointed out to us that, abstractly speaking, these were built like capacitors (layers of foil wrapped with concentric layers of plastic insulation) and that we should stick to wire-wound inductor coils, like the Solen air-core. I actually did a direct comparison of Duelund's original wire-wound, iron-core, toroidal inductor with the Alpha-Core and preferred the Alpha-Core. This just shows that the ultimate proof is in the listening. Along those lines, my partner, Dr. Loesch, has historically preferred oil caps, but in our tests none of them really sounded as good as the film-and-foils.
My point is that there's really no "evil" material and that the reasons why we prefer one material or fabrication technique over another cannot always be applied as a general rule. To give you a more specific answer, as of this writing, I can't say that plastic, per se, has a sound that we can personally distinguish as plastic. Of course, we've never tried the Duelund caps or resistors in our electronics (the high voltage Duelunds weren't on the market when we were voicing our amp and preamp), so that view may change when we do.