Many audio circuits use PIO capacitors. What are the electrical Characteristics of PIO caps that make them special (i.e. justify the cost)? Which parts of an amplifier circuit benefit most from the use of PIO caps?
Oil submersion has been commonly used for high voltage applications such as power engineering, for capacitors as well as circuit breakers / relays up until the late 70s, and is still used today, albeit rarely, apparently.The benefits cited by the previous generation of engineers include:
- Suppression of arcing to a higher voltage than air
- Improved cooling of local heat spots due to circulation of the fluid
- Improved insulation between the contact electrodes, having a higher dielectric constant than air
- Elimination of bubbles within the paper or film, or between electrodes, thus avoiding sporadic arcing and variation in characteristics
- In specific high or low pressure deployments, oils were preferred due to being less compressible (almost not at all) compared to air. Compression or rarefaction would unpredictably change the distance between, and thus capacitance between, the electrodes.
Vintage audio amplifiers were vacuum-tube based, and often operated at high voltages, thus justifying the use of such high voltage friendly components. However, modern semiconductor based amplifiers work at (typically) much lower voltages, thus ought not have any strong engineering justification for continuing to use oil-filled capacitors. There's no accounting for the spiritual beliefs of audiophiles, though.
Footnote: Oil was later largely replaced in circuit breakers by SF6 gas, which is not flammable, hence eliminating the safety hazard of oil. Vacuum based circuit breakers are also now in use in high voltage deployments. These two options do not appear to be prevalent in capacitors, though.
As a professional engineer and amateur musician, I was intrigued by this question. All I could find on the subject can be summed-up from here ...
Many electronic experts will tell you that there is no scientific explanation of why paper in oil capacitors will give you better Tone when used in a guitar circuit. But it is a well known fact in the guitar community that paper in oil will be warmer, smoother and have more "Sparkle" than ceramic disc, mylar or polypropylene capacitors. The original Bumble Bees and Black Beauties were paper in oil and thought of by many to be the "Holy Grail" of Tone as far as capacitors go.
And from here ...
Paper in oil (or PIO) caps are just one variety of capacitor. Others are made out of mylar, ceramic, etc. PIO caps were used in the Les Pauls of the 50's and many appreciate their tonal qualities over more modern capacitors.
The effect of any capacitor on frequency response & transient response will depend on other impedances in the circuit in which they are fitted so a capacitor can have no 'sound' as such.
But we are straying into highly subjective (and some might say religious) territory here and fortunately, since I am not a guitarist I am quite unqualified to comment further ...
It's the same reason people will build a hi-fi amplifier on an 80 pound slab of granite, avoid op-amps in their designs, or use only 00 gauge pure gold speaker cables: they aren't proper engineers, and they are prone to taking a slice of truth, taking it to excess, and ignoring everything else.
When you read:
be warmer, smoother and have more "Sparkle"
thought of by many to be the "Holy Grail"
that really means:
some people think they are neat, for no particular reason at all.
If you are restoring - or recreating - a historically important amplifier, such as a 1940's Leak TL12 hi-fidelity amplifier or a Vox AC-30 guitar amplifier, it's important to use the correct parts or the closest modern alternatives you can.
At one time, the best and most reliable high voltage capacitors were paper-in-oil. So the original designers used them because there was no good alternative. (I find they tend to go leaky after 40 years or so)
This guy actually took measurements and put up some Lissajous, so at least we can see some data. I see a bunch of straight lines, but he says the paper/oil was "cleanest", but I see a bunch of straight lines (with some notable exceptions).
There might be some odd acoustic behavior as the foils of the capacitor will experience some force from the voltage across them, with different resulting effects for paper, mylar, etc.
Way back when, I had some oil filled glass body capacitors from a high voltage unit of an old oscilloscope. They were rated for 40 KV or some such. But when I put 60Hz ac across them (probably straight from the power line. It was a long time ago), they blew up. Go figure. I imagine they could not take the 60 Hz vibration.