Monday, December 29, 2014

Esoteric Shunt Capacitors - Part 2

If you were to develop a list of desired characteristics for high-end audio capacitors, what would that be? One example would be to look at the design parameters of well-known high-end capacitor manufacturers (Duelund, Mundorf, Hovland, etc.) and use them to model the theoretically best possible capacitor. However, such manufacturers hold this information close to their chests and are next to impossible to obtain. Mundorf uses the philosophy of two capacitors in series to lower the characteristic inductive reactance of a single capacitor. Teflon insulation is superior to Mylar and paper-in-oil is always a good option. All esoteric manufacturers offer some variation or combination of these designs without providing detailed electrical specifications so much is left up to your ear.

But there is another approach to creating such a theoretical capacitor and that is to see what other applications may fit your list of desired parameters. For example, looking outside of the audio realm and into the RF, Microwave, or higher frequency bands one may find an "off-the-shelf" capacitor that is designed precisely to your wish list, just at the wrong value or size or voltage. As the frequency increases upward from the audio band, the values of capacitors used in these higher-frequency bands decrease. So even though you may find a design that is theoretically perfect, it may just be too small to be of any use.

But therein is the rub. If you use these small capacitors in conjunction with a well-made large value capacitor (i.e., use it as a shunt capacitor), you may get the best of both worlds. What I considered to be an excellent design is the lowly glass dielectric RF capacitor, a design that has pretty much fallen from favor in conventional applications because of high manufacturing costs. Let's see what benefits using such capacitors could offer:

  • Low temperature coefficient: Glass capacitors have a very low temperature coefficient. Figures of just over 100ppm/C are often obtained for these capacitors.
  • No hysteresis: Some forms of capacitor exhibit hysteresis in their temperature characteristic. This is not the case for glass capacitors which follow the same temperature/capacitance when the temperature rises or falls.
  • Zero ageing rate: Many electronics components change their value with age as chemical reactions take place within the component. Glass capacitors do not exhibit this effect and retain their original value over long periods of time.
  • No piezo-electric noise: Some capacitors exhibit the piezo-electric effect to a small degree. This can result in effects such as microphonics on oscillations.
  • Extremely low loss / High Q: Glass capacitors are very low loss as there is virtually no dielectric loss. This enables very high Q circuits to be built using them provided the other components (e.g. inductors) are also extremely low-loss.
  • Large RF current capability: Some capacitors are not able to withstand large values of current. This is not the case for glass capacitors which are suitable for use in RF high power amplifiers, etc.
  • High operating temperature capability: Glass dielectric capacitors are able to operate at very high temperatures, some up to 200C without fear of damage or performance shortfall.
  • Manufactured to Military Specifications: Tighter tolerances, gold-plated leads, wider operating temperatures, and in general overall higher quality are available for military applications.

Does this list of design parameters sound like the same list you may have theoretically considered? I presume that many of the characteristics are indeed. The bad thing about such capacitors are that they are only available in the pico-farad range (10e-9 farads - or - 0.x, 0.0x, or 0.00x micro-farads); the good thing about capacitors is that you can place them in parallel to obtain larger values. So despite their tiny individual values, larger paralleled values are possible, just at a higher final cost.

I ordered a few of the largest value glass dielectric capacitors off of eBay I could find just to see how they would sound as tiny shunts across the tweeter and super tweeter signal-path crossover network capacitors. The results were pretty amazing but it also revealed other "hidden" issues I had built-into my design. (As mentioned in many other articles, your audio system is a chain whose strength is defined by the weakest link - here an assumption in crossover network design).

First, the sound is wondrous when shunting Mundorf Supreme capacitors. Although these capacitors sound marvelous by themselves, shunting them with these nano-Farad capacitors brought out a clarity that was otherwise veiled. Tiny brush whisks on snare drums came to life as did inner detailing of double-bass string plucking. 

One of my favorite albums for identifying acoustic assets or shortcomings is the superb 24/192 version of the "Wake Up Your Ears" sampler by Audiogon. Track 10 - You Haven't Done Nothin' - is a delightful jazz offering featuring double-bass, saxophone, drums, and vocals by Jen Chapin where this recording captures a significant amount of hall ambiance along with a very wide dynamic range. This allows you to listen deep into the music and appreciate the timbre and tonality of each instrument. The sax "pops" as the lower registers are artfully massaged into musicality blending with the hushed background grunts and whispers of the percussionist. Where these intonations were always there, they were thrust into the forefront with the aid of these tiny ultra-high quality capacitors.

Another thing I noticed was that not all capacitors responded in the same manner. For example, the Obbligato Gold series I use on the super tweeter sounded softer and less pronounced as if the ESR was high compared to the Vishay MKP it replaced. So these tiny capacitors are not the "magic bullet" to solve all of your audio dreams but I would give them a shot. Buy the largest values you can find and then parallel them to create at least 0.001uF in total capacitance. Voltages are typically 300V, far more than adequate for most crossover networks but your ears must be the final judge in tuning your network to your drivers and wires.

So there you have it. Glass Capacitors. Who'd a thought? Grab 'em while you can.

Related articles:
The Vishay 1837 Review and Modification

Bypass Capacitors
Mundorf Supreme Capacitor Review - Part 1
Mundorf Supreme Capacitor Review - Part 2
Capacitors: All Things are NOT Created Equal - Part 0
Capacitors: All Things are NOT Created Equal - Part 1
Capacitors: All Things are NOT Created Equal - Part 2
Capacitors: All Things are NOT Created Equal - Part 3

Yours for higher fidelity,
Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style, it targets one star that has never been considered before and builds a solid case for its candidacy.

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