Why did we move to oil filled Mylar from oil impregnated paper?

When truth be told we have been experiencing increasing problems with the failure rate of our paper in oil capacitors over the past 3 to 4 years, and despite a major research program into the causes of this increasing break down rate we have really come up with a blank. We suspect that the cause has to do with subtle changes in the chemistry of the paper, the oil or some other key part of the caps, possibly due to new environmental requirements imposed on the production of the paper, but who knows? Anyway, when it became obvious that we were unable to fully establish and rectify the cause, we decided to try Mylar.


The initial experiments with oil impregnated Mylar were not sonically satisfactory, so we experimented with changing the construction to oil filled, the materials sealing the capacitor and the way we connect the foil ends to the lead out wires and voila! After some running in, the new Mylar/oil filled capacitors actually sound better in most respects than the previous generation of paper signal caps, the only real drawback is that they take nearly twice as long to run in and sound quite a bit worse than the old paper/oil type when they are new. So there you are, you don't get something for nothing!


As you can see from the above pictures, each of our signal capacitors is marked with a line on the right; this marks the start of the foil. It is our experience that the Audio Note (UK)™ capacitors perform best when the start of the foil is "facing" the incoming signal.

In all our Level One amplifiers, such as the OTO Phono Push Pull, we use the green ERO MKP metalized polyester capacitors, these cheap and cheerful industrial capacitors have been chosen through extensive listening tests against practically every commonly available polypropylene, polycarbonate, polystyrene and polyester capacitor on the market. They are far from perfect, but from a sonic standpoint they are purely detractive, meaning that they lose information rather than add it (which every polypropylene, Teflon etc. capacitor we have ever heard does to excess).

We used to use the Jensen made aluminium foil signal caps in our Level 2 and Level 3 products, and although we were never 100% happy with the sound of these, they were less flawed and thus far better than any of the available alternatives at the time.

When we stopped using Tobias Jensen in Denmark to make our branded custom paper in oil capacitors and started our own production of copper foil paper in oil (now mylar in oil), and silver/mylar signal capacitors, we decided against reinstating the aluminium foil type for reasons of sound and instead started exploring other materials, such as tin foil and various composite foils.


The resulting dry tin foil signal caps have now been on the market for a few years, and the move away from the copper paper in oil prompted us to introduce an oil filling in the tin foil caps as well, since this then makes the processes the same across the board for the tin and the copper foil signal caps, the oil filled tin caps do offer slightly improved sound compared to the dry versions.