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Crossovers: Passive Parts Selection Guide

dnewma04

dnewma04

The Healer
Capacitor Considerations/Recommendations

Content care of Bob Stout of www.snippets.org , DIYspeaker mailing list (formerly the Basslist), and the Loudspeaker Design Selection Guide (LDSG). Most of this content is just cut and pasted from Appendix E of the LDSG, this is really a great source of info, albeit a bit dated as more and more new drivers appear on the DIY scene. In any case, here are some things to consider about various passive components when you start your new speaker rebuild or some scratch build. I have copied some of the content here, but have also included links to each of the sections with the source of this data. I have also taken the liberty of removing some of the more advanced properties but they can be found through the links if you are compelled to learn more about each component.
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Capacitor Definition

A capacitor is a component consisting of two metal plates separated by an insulator (dielectric). The value of the capacitor is derived by the area of the plates, the thickness of the dielectric, and the dielectric constant (K) which is the ability of the dielectric to store electrons compared to air which has a K value of ~1.0001. The higher the value of K, the smaller the physical size of the capacitor.

Capacitor Types

  • Bi-polar (Non-Polarized Electrolytic (NPE))
  • The Polys (Polypropylene, Polyester (mylar), Polystyrene)
  • Impregnated caps (Oil or wax impregnated types like Paper in oil caps (PIO))
  • Film and Foil

The most common type you will run across in your speaker rebuilding efforts will be bipolar, also referred to non polarized electrolytic (NPE) capacitors. These are especially common in typical moderately priced speakers. NPE caps aren't good in critical locations like the signal path. Replace with a Polyproylene capacitor at the minimum. If you are on a budget, NPEs are effective for especially large positions on low pass sections where Polypro caps become excessively expensive. Be sure to bypass with a higher quality capacitor if in the signal path regardless of the application.

Here is an example of an NPE that might be typical of something that you’d find in a loudspeaker.

NPELarge.jpg


Polypropylene capacitor examples care of Erse, Dayton, and Solen:

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027-200m.jpg


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Capacitor Specifications

Definitions of each of the following terms can be found at the link within the title of this section. But these are the important properties of a capacitor:

  • DIELECTRIC COEFFICIENT (K): The dielectric coefficient (or constant) determines the size of the capacitor for a given set of values.
  • ESR: Equivalent Series Resistance is the value of a resistor in the capacitor's model to account for its deviation from a perfect 90 degree phase shift. Inversely proportional to the caps Q (quality factor).
  • Q: The Q is a frequency dependent value where a higher Q, especially a consistently high value across the frequency band is important.
  • DISSIPATION FACTOR: Reciprocal of Q. Obviously a low value is beneficial.
  • DIELECTRIC STRENGTH: Also known as breakdown voltage, the strength of the dielectric is expressed in the voltage required to pass through a specified unit thickness of the dielectric.
  • DIELECTRIC ABSORPTION: DA is part of the capacitor model which represents the tendency of the dielectric to retain a charge. Low values of DA are beneficial in audio applications.
  • TEMPERATURE COEFFICIENT: Defined as the change in capacitance per unit change in temperature. Relative unimportant in speaker building/rebuilding if using higher grade caps than electrolytics. If you are using NPE (not recommended), TC can become more important.
  • INDUCTANCE: At audio frequencies, the inductance of a capacitor is usually not significant.
  • MICROPHONICS: Not generally an issue in the kind of capacitors you’d select for loudspeakers.


More capacitor related sites that go into even more detail about the specs above including some moderate level math.
Picking Capacitors

Learn Electronics - Capacitors

Dielectric Comparison Chart (PDF)

Guidelines copied verbatim from the article at LDSG

Following are my personal guidelines which others have found to be useful:
  1. Film and foil is superior to metallized film, if you can tolerate the size. By superior, I mean primarily, lower noise. Capacitor noise is generally caused by flaws in the dielectric or poor terminations. The reason film and foil is "better" than metallized foil is one of metallized film's special "features". You will hear vendors speak of metallized foil as self-clearing. All this means is that if there's a pinhole in the film, a temporary arc will vaporize the metal around the pinhole. This is great for long term viability, but bad for noise since each time this happens, it adds noise. Also, most don't tell you that this only happens when the leakage current through the pinhole is adequate to heat the metallization to the point of vaporization. This is primarily a factor in polypropylene, since of all the popular high-stability dielectrics, it's the softest and therefore most likely to: a) have pinholes as received from the film vendor, and/or b) develop pinholes during winding. Finally, although the theory is that this only needs to happen once per pinhole, after which the metallization around the hole is blown away, it often doesn't work that way in practice. In the real world, all the metallization around the pinhole won't necessarily be cleared by the first (or second or third or…) peak.
  2. Especially with polypropylene, the best caps used two layers of film. Doing so virtually guaranteed that the inevitable pin holes wouldn't line up. Single film and foil polypropylene is, unlike Teflon and polystyrene, only somewhat better than metallized film. Note: Most manufacturers consider whether or not they use two layers of film to be a trade secret, so you'll almost never see it advertised. However, it's often a safe bet that film and foil capacitors with excellent reputations are quite likely to be using them. At one time, AudioCap Theta's were noted as using two layers of film, but you won't find that factoid in any current official information. One notable exception is Intertechnik, whose Audyn-Cap Plus premium caps are advertised as "double wound".
  3. Although significantly heavier, tin foil is generally preferable to aluminum foil, although the degree of improvement is debatable. Presumably, this is due to fact that tin is softer, resulting in less residual stress and reduced susceptibility to microphonics.
  4. Stacked foil caps are generally better than wound caps since the material isn't stressed during assembly.
  5. Impregnated paper caps (the paper is typically kraft paper and the impregnant is usually either oil or wax) can be quite good for audio, although for a number of non-technical reasons, they're less convenient and economical to use than plastic film types. Offering extremely low leakage/high resistance (but at the expense of high dissipation factor and often high inductance), they're still one of the best choices for high voltage work, but that hardly applies to audio. Of the choice of impregnants, wax offers significantly improved mechanical damping, but only at low temperatures - at higher temperatures, the wax melts and any advantage is lost.
    There are very few sources of impregnated paper caps for the simple reason that most manufacturers simply don't want to mess with them. Whether they offer audible improvements in crossover designs is a debate I really don't care to get into - akin to the tube/valve vs. solid state or LP vs. CD debates. As with the tube/valve vs. solid state debate, there are measurable differences and no shortage of explanations on both sides why people might prefer one over the other.
  6. Polyester (Mylar®) capacitors should be avoided for all critical applications! If your budget's tight, try using them in trap or Zobel circuits before putting them in the signal path. With poorer electrical characteristics across the board than polypropylene or polystyrene, Mylar® is the entry level for film capacitor dielectrics. For non-critical systems where cost is a major issue, they're at least better than NP electrolytics.
  7. Polystyrene is a dielectric which is superior to just about all others. It's electrical properties improve on polypropylene and even Teflon®. Some premium crossover capacitor lines are made with polypropylene, but do they offer any audible improvements? As with many other issues, the improvement of polystyrene over polypropylene has been hotly debated. Many of polystyrene's electrical advantages (e.g. lower temperature coefficient) are irrelevant for crossover work where temperatures are relatively stable and drivers have looser tolerances which drift much more with temperature than crossover components. What's not arguable is that polystyrene has a much lower dielectric constant (K) than other dielectrics, so polystyrene capacitors are correspondingly larger for the same values.
  8. Avoid oval caps! They're made by winding them round, then flattening them in a clamp while the epoxy sets. This leaves a lot of residual tension which can lead to value creep and microphonics.
  9. Everyone buys their raw film from the same suppliers. Any vendor claiming to use, e.g., "premium" polypropylene film is, at best, exaggerating. Note that this applies generally to *virgin* film. You can also buy recycled PP film which is generally used for non-critical applications such as packaging and wrapping, or where only the mechanical properties are important. Although I don't know of any capacitor vendors who exclusively use recycled PP film for capacitors, it would be naive to assume that none do. Certainly, the type and class of capacitors I was discussing (MIL spec, low-noise, high reliability) all use virgin PP film.
    "Raw" polypropylene typically comes in pellets, either from a chemical company (usually virgin) or a recycler. Further clouding the issue, pelletized PP may contain a mixture of virgin and recycled material. The pellets are then fabricated into film by a mill, which may, or may not, be related to the raw material supplier. Finally, the mills are the folks who actually supply the film to the capacitor vendors. In the case of metallized films, yet another intermediate vendor is usually involved.
  10. The biggest difference in vendors is QA of the winding operation (for wound vs. stacked foil caps, obviously). Constant light tension is required. This is especially critical with polypropylene since it has such little mechanical strength and stability (i.e. it stretches, tears, develops pinholes, and permanently deforms easily!)
  11. The attachment of the metallization/foil to the leads is critical. This has to be a relatively low temperature operation or the film will be compromised. Poor attachment increases ESR and can introduce noise caused by rectification and/or thermoelectric effects.
  12. If the rest of the structure is sound, the lead material is immaterial. Tinned OFHC is as good as you'll ever need.
  13. Popular audio voodoo is that there's some magic in which lead is attached to which foil. These are all non-polarized parts! Since there are such wide tolerances in the component materials, capacitors are wound to a target value, then unwound as required to achieve the exact target value and tolerance. Until then, which foil is which is undefined.
    A couple of final notes are in order…
    Some well-regarded brands do sell exactly the sort of flooby dust that the preceding discussion attempts to debunk. As we all know by now, there's very little correlation between audio quality and marketing hype. I'd never rule out the possibility of a capacitor vendor marketing flooby dust, while still delivering top quality parts. The trap and the fallacy is in thinking that the parts' quality derives from the marketing claims, rather than simply sound engineering and manufacture.
    DIY'ers spend ridiculous amounts of money for voodoo caps, when they could buy better caps, in most cases, from a wider variety of vendors if they only knew what to order. "Audiophile" caps are priced around the same as MIL spec caps, yet often have less to back up their claims of superiority.
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Recommendations of various crossover components/vendors. AudioKarma sponsors are enlarged and italicized, please support them if you can.


To be added as time allows and pending information collecting:
  • Erse
  • Jantzen
  • Jensen
  • Jupiter
  • Mundorf
  • Obligato
  • whatever Samra pimps as the greatest thing ever this week.
  • Solen
  • Sonicap
  • V-caps
  • Xicon
 
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L pad burn out

replacing burnt up L- Pads (violet AT-50 L * ohm Frazer Mark 5) what`s good or better
are pin outs the same for L pads ? No markings on these.... Thanks Vern
 
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