Three Parallel Film Caps in RTA-12B??

I am presently rebuilding the crossover for an RTA-12B. The consensus on this forum seems to be to remove the 750pF bypass caps shown in the tweeter schematic below. Common practice on this forum also seems to be to replace the 4.4uF and 55uF with good quality film caps, e.g. Sonicap or ClarityCap. As well as other caps in the crossover. 55uF is hard to find and fit in the speaker. They are often split into 2 - 27uF. They would also be in parallel with the 4.4uF in the tweeter circuit. Why have 3 film caps in parallel? Is it wise to use just 2 film caps, e.g. 2 - 30uF? This would probably be less expensive.

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Comments

  • delkaldelkal Posts: 673
    What am I missing? With capacitors you add the values of everything in parallel. Why can't you use one 59.4 cap?

    Yes I know you probably cant find a 59.4 cap.......but how about a 60?
  • The whole concept of bypass capacitors is very nebulous and a mystery to me. I know that shortly after I purchased my 7B's I was sold a kit of 4 silver mica bypass capacitors (don't remember how many pF's) that was supposed to improve the SQ and I installed them in parallel with the capacitors in the crossovers as instructed but I can't honestly say that I ever noticed any improvement. That's not to say that there wasn't, just that I couldn't detect it lol. I think the whole thing with this hobby is you can make lots of somewhat imperceptible incremental improvements that as a whole are perceptible, if that makes sense.

    I think it's interesting that Polk themselves were apparently believers though and that counts for something with me.
    George / NJ

    Polk 7B main speakers (1979, orig owner)
    Martin Logan Dynamo sub
    Crown D150 amp
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14 gauge, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
  • Discussion on bypass caps:

    https://www.audiocircle.com/index.php?topic=154260.0

    "Smaller value caps are faster in output than larger value ones and therefore their information reaches the speaker drivers sooner than that of the larger value capacitors. They may indeed perhaps extend the frequency response of the unit they're used in (or just add "air") but at the cost of smearing the timing. People are more or less sensitive to timing issues so, as usual, it becomes a personal preference."

    "Danny Richie stated something similar on his board. He liked using bypass capacitors, on larger values, as he felt it improved the discharge rate of the cap. I believe Danny felt that it improved the transient response of the speaker; the perceived 'speed' of the speaker."


    George / NJ

    Polk 7B main speakers (1979, orig owner)
    Martin Logan Dynamo sub
    Crown D150 amp
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14 gauge, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
  • from http://www.humblehomemadehifi.com/Cap.html

    "Cornell Dubilier 940C / 0,01uF / 3000VDC bypass - 10% tolerance

    Technical specifications (according to manufacturer): "Type 940 round, axial leaded film capacitors have polypropylene film and dual metallized electrodes for both self healing properties and high peak current carrying capability. This series features low ESR characteristics, excellent high frequency and high voltage capabilities; high dV-dt for snubber applications; capacitance range 0,01 to 4,7µF; capacitance tolerance ±10%; rated voltage 600 to 3000 VDC (275 to 500 VAC, 60 Hz); operating temperature range with ripple 55ºC to 105ºC / full rated voltage at 85ºC - derated linearly to 50% rated at 105ºC; insulation resistance more than 100.000 MO x µF; test voltage between terminals @ 25ºC 160% rated DC voltage for 60s; test voltage between terminals & case @ 25ºC 3kVac @ 50/60 Hz for 60s; life test 2000 h @ 85ºC, 125% rated DC voltage; life expectancy 60.000 h @ rated VDC 70ºC; case material UL510 polyester tape wrap; resin material UL94V-0 epoxy fill; terminal material tin plated copper."

    Sound: Using the 3000VDC Cornell Dubilier 940C as a bypass capacitor in the value 0,01uF really is the "icing on the cake" when you are looking for a low cost bypass capacitor that gell's everything into one coherent sound. Best bang for the buck! Over the years I have used these in many different combinations and everytime they just added more to the overall sound. More coherency and more ease. Especially when you have been "cooking" to find your favourite mix of capacitors and only need to cement the different characters together. Adding the 940C creates an overall more mature sounding speaker. And no, they are not better than the Duelund or VCap bypass capacitors that excel's in naturalness and richness of tone and in transparancy and air. The Duelund and VCap are in another league, not only in sound quality but also in price ;-) Compared to the Jupiter Copper Foil Paper & Wax used as a bypass, the Jupitor is has richer overtones and has a tonally slightly warmer balance. The Vishay MKP1837 used to be my favourite low cost bypass capacitor but now it is the Cornell Dubilier 940C / 0,01uF / 3000VDC. It surpasses the Vishay with quite some margin :-)

    Verdict: use them everywhere possible!"
    George / NJ

    Polk 7B main speakers (1979, orig owner)
    Martin Logan Dynamo sub
    Crown D150 amp
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14 gauge, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
  • F1nutF1nut Posts: 44,481
    edited February 7
    but at the cost of smearing the timing.

    I call it artifacts and it's not natural. In fact, the effect becomes annoying as hell. Bypassing film/foil caps should to be avoided, period.

    That said, there is some benefit to bypassing electrolytic caps due to their inferior nature.
    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."


    President of Club Polk

  • TrapJawTrapJaw Posts: 12
    Replying to @delkal, one 60uF is probably too big for the space and there is an advantage to paralleling caps: cuts the resistance and inductance in half. This reminds me of something else I read somewhere that at high frequencies, lead inductance arguably becomes a concern with large caps (not sure that I recall that totally correct). This may be contradictory with the first statement which is a fact. The lead inductance thing, I’m not so sure.
  • delkaldelkal Posts: 673
    That is a big cap! Do you have room to mount it sideways or even over the other caps to shorten the leads?
  • CH46ECH46E Posts: 1,960
    edited February 11
    Im pretty sure it would interfere with the screen top if you tried to put it on top of the other caps.

    I used to hook up wires to increase the leads to I could relocate it there. I knownits not ideal but I had to do what I had to do to make it all fit. It really was the only way I could see to get it in there. I had not idea how big it was going to be when I ordered it. But these speakers sound so good now!
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