3, Interfaces, MK-121 capacitor types

Read more on the Acoustat 3 in our home audio section

Dave Pitt2004-02-23 22:21

I am refurbishing a pair of model three Acoustats, putting in the ”C” upgrade, and am somewhat puzzled by the .01uf/6kv cap discussion in Electrostatic Loudspeaker pages.

The untouched MK 121 - 2 ( not Medallion upgraded) interfaces have the original .01uf caps with black epoxy filled ends with an unmarked brown paper wrapper - one end of each capacitor has an 1/8 inch metal barrel extruding before the lead attachment - they all measure OK for value and leakage current - manufacturer and dielectric type unknown.
These are larger diameter caps than the Sprague 430p

(Note: the Absolute Sound Medallion review refers to large black .01 caps as polypropylene - could these non-Medallion ones be as well? )

The Ronald Hicks cap discussion says to use 01/10kv Sprague 430p series caps ( as that was what Acoustat-Rockford used in the medallion upgrade) - I have obtained some Sprague 430p caps, but from what I can find on the Internet it seems that the Sprague 430 series are polyester.

I would keep using the original black caps if they are a better dielectric than the Sprague polyesters ? If neither the original or the Sprague are polypropylene I will try to get the Electronic Film .01/6kv polypropylene capacitors - if they are still available). Thanks for any input you may have. Best Regards.


Andy Szabo2004-02-23 22:21

The MK-121 interface, in all of its variations, was manufactured for nearly twenty years. In the early days, the audible benefits of polypropylene were not widely known, so Acoustat used oil/paper and polyester (a.k.a. Mylar) dielectric capacitors for the high-voltage couplers. In the later years of production, polypropylene was used exclusively. Many different brands were used, so it is often difficult to identify what type of capacitors you have. This problem is further compounded by the fact that many of these capacitors were custom made, and were often marked, if at all, with non-standard part numbers. Many of those capacitor manufacturers are now out-of-business.

I remember the particular capacitors you describe. I do not know what dielectric they are, but I have a strong suspicion that they are not polypropylene. I do know for sure that these caps were replaced with polypropylene when the full Medallion Upgrade was done at the Acoustat factory. The 0.01-uF 6kV capacitors were replaced in almost all of the interfaces sent in for the Medallion Upgrade - unless they were very recent units already using polypropylene.

If you are not sure that your replacement capacitors are polypropylene, then you might as well leave the originals intact. The audible benefit of upgrading these capacitors is subtle at best, so unless you can use the best dielectric type (i.e. polypropylene) then it is usually not worthwhile to change them unless one is defective.

I will also recommend against trying to create the required 6000-volt rating by connecting lower-voltage caps in series, as some people have done. This is less reliable from both a mechanical and electrical standpoint. If you can\'t find a capacitor with the proper electrical ratings, in a case-style that will readily fit the board, then it is better to leave the originals intact.

If you find a good source for these capacitors that is not already listed on this site, please share the information!

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