Pok RTA 12B Refurb

I bought my Polk 12b over 35 years ago!. They were relegated to my office when I got my home theater setup about 20 years ago. It took me a while to realize how crappy they started to sound and the inevitable damage to the top hats made them a sorry sight to see. Fortunately I had the inspiration from the many contributors to this forum and made them sound new again.

The project included
1) recapping the top crossover and the LFM crossover with polypropylene Audio Caps
2) Sonic Barrier damping material behind each 6.5" speaker. (similar to Black Hole 5)
3) Sound dampening sheets on the speaker frames
4) Rebuilt the top hats (the old plastic supports had broken, make new ones out of wood)
5) re upholstered the top hats (thanks to my lovely wife and some speaker fabric)
6) New speaker terminals (The old ones had disintegrated)
7) Rewire internals with high quality speaker wire (O2 free thick gauge)

All told I spent around $150 bucks from Parts Express and a few quiet evenings with the soldering iron and wood working tools. No major hiccups but getting the LFM's back in place was challenging since the new caps were much bigger than the old ones.

The project was a huge success. Even my wife, never an audiophile, was impressed. My new old polks sound as good as I could remember. I hooked up my Project Turtable to it and replayed some of my remaining vinyl.

My next step will be to replace the bottom stand (the old ones rotted out) and replace the faux wood grain on the cabinets.

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Again thanks to folks here like Michael8it, FesYboy and the many others I have read here.

Comments

  • CH46ECH46E Posts: 1,029
    Nice man. Congrats. Is that pic of the crossover a finished one? I only ask cause it looks like you are missing 2 caps? Or did you relocate them?
  • FaustinFaustin Posts: 879
    Nice job. I does appear that some caps are missing. Here is a pic of mine.okukvp5kycvd.jpg
  • jmurjmur Posts: 2
    I'm thinking he's not missing any capacitors. I think the only thing that looks unusual is the big cap at the top. I'm guessing that big one is either a 60uF, or it's a 56uF and there's a 4.4uF and a 750pF hiding behind it or under the board.

    I have some RTA-12B and have a couple questions.

    1. Is there a sonic reason for the 4.4uF in parallel with the 55uF?

    In the original, there is a 55uF in parallel with a 4.4uF and 750pF. I know everyone says to get rid of the 750pF because the quality of the capacitors we're using today is better and they are no longer needed. The total capacitance is 59.4uF.

    It seems to me that people put two 27 in parallel for the 55 and then add the 4.4. Why not just get one 60uF Dayton or two 30uF Sonicaps? The cost isn't much different but there isn't a lot of room.


    2. What is the difference in sound between using Dayton vs Sonicap for the capacitors in parallel with the MW drivers? I hear this discussed a lot but have not found exactly what the trade off is in how they sound.

    If I do these speakers I would use Sonicap to replace the caps in series with the tweeters, but it would save about $250 to do all the low pass with Dayton or $200 with Solen. I have read people saying that they can't hear the difference with their old ears, but what is the difference? Mo better bass, or is the sound slightly smoother?

    Thanks,
    Jim
  • F1nutF1nut Posts: 41,971
    The Dayton caps are exactly the same as the Bennic XPP. Same sound, same color, just a different name.

    "Sound: The Bennic XPP has a slightly more open top-end than the Bennic FPP. The FPP is extremely neutral making the XPP sound a tad "toppy" but till overall reasonably neutral. Personally I prefer the FPP to the XPP for souding less "cheap". Although the XPP seems more detailed than most standard quality MKP's it lacks coherency and overview making complex programme material, such as large big band music, choirs or symphony orchestra's sound confused and muddled. The Bennic FPP performs better in this respect. The XPP give me a headache after a while of having to listen to them. Even a cheap Solen PB-MKP-FC is more pleasent to listen to, the Solen is more spatious and more neutral. Switch from the Bennix XPP to a Clarity Cap CSA and everything falls back into place in regards to the neutrality and coherency of the musical image. Compare an XPP to the Wima MKP-10 and the Wima is more neutral, maybe not as detailed as the XPP, but definitely more coherent. The Bennic XPP is one to skip if possible."
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


  • ddogddog Posts: 194
    Congratulations on your rebuild/referring of you 12b's.
    I bet they sound excellent now...

    Enjoy them.
  • Tony MTony M Posts: 8,144
    What kind of tweeters do you have in yours?
    Most people just listen to music and watch movies. I EXPERIENCE them.
  • SchurkeySchurkey Posts: 1,875
    jmur wrote: »
    1. Is there a sonic reason for the 4.4uF in parallel with the 55uF?
    In the original, there is a 55uF in parallel with a 4.4uF and 750pF. I know everyone says to get rid of the 750pF because the quality of the capacitors we're using today is better and they are no longer needed. The total capacitance is 59.4uF.
    First Guess: A 4.4uF Mylar (film) cap bypassed with a 750pF, and paralleled with a 55uf electrolytic sounds better than a single 60uf electrolytic. A single 60uf modern film cap might be a different story.
    jmur wrote: »
    It seems to me that people put two 27 in parallel for the 55 and then add the 4.4. Why not just get one 60uF Dayton or two 30uF Sonicaps? The cost isn't much different but there isn't a lot of room.
    That'd work for me--if I could fit it to the boards.

  • CH46ECH46E Posts: 1,029
    I went with this for the 55 uf.

    https://www.partsconnexion.com/MUNDORF-76413.html

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