If you have to have Black Gates, Michael Percy sells them and has explanations of the types. But Loon is correct--I'd never put a 'lytic into a speaker, and SCRs (Axon, Solen, etc.) 'propylenes are great, bypassed with something better, for the high-value locations and are highly affordable. Percy also sells LOTS of other high-quality film caps. http://www.percyaudio.com/Catalog.pdf
For higher-quality caps for use in series with the signal, I prefer North Creek's Zen, Harmony, and Crescendo 'propylenes, with the Harmony metalized also highly affordable. http://www.northcreekmusic.com/CrossoverComponents.html
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