Friday, November 27, 2009

Tone Test: Paper in Oil Caps

Gary Onofrio is right. He claims the paper in oil caps in his excellent Sonny Jr. harp amps contribute to their great tone. I cannot speak to his amps directly (I’ve played them but not had the chance to thoroughly review them), but I am a believer in his wisdom about the caps.

Coupling capacitors move the signal from one stage of the amp to another… In this case from the preamp section to the tone stack. Controversy rages on this topic among techie types. I’ve read very persuasive essays by highly qualified engineers who swear that it is impossible for coupling caps to affect tone in a way that can be heard by humans. I’ve also read tons of anecdotal evidence from musicians and audiophiles who swear that different caps have very distinctive tonal qualities. I decided to find out for myself.

A few days ago I tested several different coupling capacitors in my 5F2H harp amp. The method we devised was to compare the caps quickly, one after the other in rapid succession. The caps were soldered into the amp (it takes only a few seconds) and played with the same amp in the same position with the same control settings, the same mic and harp, and even the same licks. Each set of caps was in the amp for about 5 minutes.

I’ve gigged the 5F2H amp more than 50 times this year, so I am intimately familiar with its tone. Any change was easy to identify. It was up to me to take notes on each cap and decide which – if any – sounded better than the others.

We tried several different brands and types of capacitors, including all the well-known brands; paper, polyester, polypropylene, film, and paper in oil. We tested several different versions of the NOS Soviet military caps, including the K40, K42, and K72.

With most of the caps I could not hear any difference at all in the tone of my rig. However, there were four capacitors that did make a discernable change in the tone: The Mallory 150 poly film cap, the STK polypropylene cap, the Soviet K40Y-9 paper in oil, and the Soviet K72 Teflon cap.

The winner? The NOS Soviet K40Y-9 paper in oil capacitor. This cap was the clear winner in my mind, slightly broadening the tone and giving it a subtle yet pleasing vocal quality. I immediately liked it.

Next best was the Soviet K72. The tone was similar to the K40 but even more subtle.

The STK was notable for its airiness. I bet it sounds great in a guitar or guitar amp.

The Mallory 150 was slightly brighter than the others.

The Soviet K40Y-9 capacitors were not the most expensive caps we tested – not by a long ways. You can find bulk examples on eBay for as little as 20 cents. Good NOS versions range up to about $6.00. There are lots of sources for them in Eastern Europe and online.

Are these caps worth the trouble? Can you hear the difference in your amp? Good players – like Gary Onofrio – can hear it. It is part of that last 1 percent of tone we struggle to wring from our gear.


If you examine these
spectrum analyzer scans, you can see why the paper in oil caps appealed to me: The have more energy in the vocal range of sound, particularly in the 1000hz area. These tests were done in a guitar.


From an audiophile (scroll down):

Russian K40y Paper-In-Oil Capacitor
After the usual rocky burn-in ritual, this PIO cap settled into a confident, natural sounding device. There are some audiophiles who rank these PIO caps as the best of the Russian military caps, including the FT-3 and K72 Teflon caps. I may agree with this sentiment when it comes to utter naturalness and ease of presentation as well as the lack of a subtle "plastic" sound, which of course all plastic (film) caps have.


Don said...

Rick--thanks for the information and impressions re the caps. I'm liking the content of your blog--esp reviews of components such as speakers, caps, etc. Also your willingness to take a good look at such things as wattage claims, etc. I'd been obsessing a bit about what sort of coupling caps to use in my next project--a harp-specific 5f6a. I've used Mallory 150s before, but was wanting to try something different (and better), and was looking at the boutique pios ($$!!!) as well as Sozos, but just ordered some K40Y-9s on ebay--thanks for heads-up.

Rick Davis said...

Don, I think you'll like the K40s. I thought they sounded superior to everything else we tried, including more expensive pio caps; they were slightly warmer.

Let me know how your project 5F6A turns out.