RTA 12B Crossover Questions

Sitka
Sitka Posts: 74
in Vintage Speakers
So I just picked up a pair of RTA 12B's and plan to recap the crossovers. Each speaker has 9 caps, if you include the 2 mica caps. The schematic listed in our PDF library appears to match mine (circa 1982, and has a .75 amp fuse) I read in another thread that I can just delete those 750 pF mica caps. My question is this: There's a 55 mF cap with a 4.4 mf cap in parallel - can I combine the two values and go with a 60 mF cap?

Any of you who has additional suggestions, feel free to chime in! My experience has been the much simpler Monitor series - nice and straightforward.

Thanks!
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Comments

  • Schurkey
    Schurkey Posts: 2,104
    Sitka wrote: »
    I read in another thread that I can just delete those 750 pF mica caps.
    If you're using film caps instead of electrolytics, yes.

    Sitka wrote: »
    My question is this: There's a 55 mF cap with a 4.4 mf cap in parallel - can I combine the two values and go with a 60 mF cap?
    If you're using film caps instead of electrolytics, yes.
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  • F1nut
    F1nut Posts: 50,663
    edited May 2023
    Sitka wrote: »
    So I just picked up a pair of RTA 12B's and plan to recap the crossovers. Each speaker has 9 caps, if you include the 2 mica caps. The schematic listed in our PDF library appears to match mine (circa 1982, and has a .75 amp fuse) I read in another thread that I can just delete those 750 pF mica caps. My question is this: There's a 55 mF cap with a 4.4 mf cap in parallel - can I combine the two values and go with a 60 mF cap?

    Any of you who has additional suggestions, feel free to chime in! My experience has been the much simpler Monitor series - nice and straightforward.

    Thanks!

    That 55uF electrolytic is bypassed with a 4.4uF mylar, which is bypassed with the 750pF. There has to be reason Polk did it that specific way. By all means delete the 750pF when upgrading to film caps, but I would keep the rest of the circuit as designed.
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    President of Club Polk

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  • Sitka
    Sitka Posts: 74
    Thanks guys - I went with circuit as designed sans the mica caps. I'm going with all Clarity CSA, and even with that it was $500 for them caps. I looked at Sonicaps, but wow - sticker shock! Worth it I'm sure, but it seems like they tripled in price in one year.
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  • Schurkey
    Schurkey Posts: 2,104
    That circuit has three kinds of caps all piggybacked in that location.

    Cheap-junk electrolytic, bypassed by an ancient Mylar film cap that would respond faster and with less loss than the electrolytic, and the cherry-on-top is the silver mica that would respond faster than the old-style Mylar film cap.

    Replace all three with a single, modern, high-spec 60uF film cap and you'd be good-to-go. What you've done will work OK, too, but probably costs more and takes up more room on the circuit board.
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  • Sitka
    Sitka Posts: 74
    I'm sure your right, but I figured just stick to the schematic. The price difference was negligible.
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