Some people swear by paper-in-oil (PIO) capacitors for electric guitar tone circuits. They say all else being equal (same capacitance, similar tolerance — voltage is not relevant in guitar circuits), a PIO capacitor will sound different, meaning better. Words like “warmer”, “mellower”, “livelier” are being employed to describe the difference. The theoretical basis for this would be the dissipation factor, which obviously varies with capacitor “architecture”.
Not surprisingly, PIO capacitors are significantly more expensive. Some of the models and brands being tossed around in these discussions are the famous Bumble Bee, Angela/Jensen, Vitamin Q, and the cheaper, but well regarded Russian K40Y9.
Since I had to replace my factory stock .047µF tone capacitors because they are way too dark for a humbucker guitar, I decided to go for paper-in-oil, see if there’s any gain. I opted for the cheaper Russian K40Y9, of which I also got the .047µF, just for the purpose of this test, before moving on to a .022µF.
Are paper-in-oil capacitors different/better sounding than factory stock polyester film capacitors, all else being equal?
The awful stock wiring was converted to 50’s wiring. The volume potentiometer are stock 500K. The bridge tone pot is a Dimarzio 500K, push-pull. The neck tone pot is the stock 500K. All pots are audio taper.
The recording was performed direct, via the Hi-Z input of an Edirol UA-25EX interface, input gain knob at 12 o’clock. No amp sims, no effects applied. During all recording, the volume pots were kept at 10. The difference in volume between samples is due to the 50’s wiring, and it is the same between both sets (polyester and PIO).
The stock capacitors are polyester 2A473J:
The new capacitors are Russian paper-in-oil (PIO) K40Y9:
The important thing is to remember that the setup was identical in both situations, the only variable was the capacitor.
The following video is a thorough A/B test of polyester vs. PIO for various tone knob positions, first for the neck pickup, then for the bridge. I chose to roll off the tone in increments of 2, which appeared to be enough for the purpose of the comparison. The video is about 9 minutes long.
The polyester capacitor samples were recorded with the cap soldered in place. The PIO samples were done with the cap connected via test leads.
I played a simple G-A minor-F-G sequence, and a basic A minor Pentatonic lick, flatpicking.
I suggest actually not watching the video initially, but instead just listening to it in order to avoid confirmation bias (you’re convinced PIOs should sound better, and the samples will sound better to you) by reading the captions, try to see if you get any “a-ha” moments.
My conclusion obviously applies only to the Russian K40Y9 PIO capacitor, as tested against the polyester film 2A473J.
They offer identical results, for all practical intents and purposes.
I have no direct experience with $15+ capacitors, and I think I’ll never have, judging by the test I’ve just done. I really doubt that, since there’s no practical difference in this test, there will be a significant one using a Vitamin Q, for instance.
I can definitely fool myself into hearing just a tad more harshness for the polyester in the bridge (where the Dimarzio pot is), but I have to try really, really hard, and it’s so minute it makes no practical difference whatsoever.
Some samples may seem to sound very slightly different, but not consistently, and not between corresponding samples (polyester vs PIO). That is attributable to my playing dynamic and getting in and out of tempo occasionally.
This guy’s results were one of the factors that determined me to try PIOs. However, looking back at them, I’d say his very audible differences (with the tone pot on 10!) is due to switching from modern to 50’s wiring, and upgrading to CTS pots, not to the capacitors. Instead of keeping all else equal, and just test with different caps, he compares two completely different guitars from the point of view of electronics! That test is therefore very misleading.
Perhaps there would have been a significant difference between really cheap, high-tolerance ceramic caps and PIO, but that difference would have been the same between said ceramics and the polyester ones.
As you can imagine by now, my opinion is that there is no real, usable difference between the two types of capacitor in the guitar tone circuit, all else being equal. None whatsoever.
Unless you have really crappy stock capacitors in your guitar, for significant improvements in tone you should probably look elsewhere first.