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How does one know when to double up (i.e. parallel) resistors?

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  • How does one know when to double up (i.e. parallel) resistors?

    I'm designing a small bookshelf for rear surrounds for my home theater.

    I'm utilizing Dayton Audio RS125-8 for the Woofer and Peerless BC25SC06-04 for the Tweeters

    The enclosure is 4.5Liters and port tuned to about 50hz (will likely change the tuning to a bit higher)

    Based on the above eclosure description, looks like the woofer will hit xmax at 18Watts.

    I've included a picture of my crossover schematics.

    When I use 18watt for the input signal, it looks like the series resistor for the tweeter circuit will exceed its 10watt rating (15.5watt @ 5Khz).

    Should I be concerned?

    Also, since power will be shared by the tweeter and the woofer, should I instead be using 36 watts as input (i.e - so it is split 18watts to the woofer and 18 watts to tweeter)?
    Click image for larger version

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  • #2
    Power density goes down by half with each octave increase in frequency. If the woofer takes a maximum of 18 watts then the power density above 5kHz probably wouldn't exceed 2 watts, and that's split between the resistors and the tweeter.
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #3
      I'm not familiar with VituixCad power dissipation chart but the graph looks a system chart, so it looks like 12v output and showing a nominal power into 4 or 8 ohms as a reference. There would be no other power adjustments if 12 volts equals 18 watts at your woofer. You may want to check the driver radial button to see what you get at the woofer.

      As far as the the wattage rating I derate the wattage by 50% for music, Kimmo might be doing something similar with the pink noise button, but the short answer would be no I would not be concerned.
      John H

      Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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      • #4
        Originally posted by billfitzmaurice View Post
        Power density goes down by half with each octave increase in frequency. If the woofer takes a maximum of 18 watts then the power density above 5kHz probably wouldn't exceed 2 watts, and that's split between the resistors and the tweeter.
        Bill - any chance you can make a second attempt at explaining the above - it went way over my head :-)

        John - You are correct. Per the VituixCAD manual, the "pink noise" button represents "effective RMS spectrum of music signal"

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        • #5
          One octave = doubling or halving of frequency. With typical source content if there's 100w of content at 100Hz there's 50w at 200Hz, 25w at 400Hz, 12.5 w at 800Hz and so forth. For a more in depth explanation google 'pink noise'.
          www.billfitzmaurice.com
          www.billfitzmaurice.info/forum

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          • #6
            Originally posted by billfitzmaurice View Post
            One octave = doubling or halving of frequency. With typical source content if there's 100w of content at 100Hz there's 50w at 200Hz, 25w at 400Hz, 12.5 w at 800Hz and so forth. For a more in depth explanation google 'pink noise'.
            Got it! However, I would imagine VituixCAD accounts for what describe above once I click the "pink noise" button. In the picture I've included above in Post #1, the default value of 3,000Hz was utilized. The manual states "Typical corner frequency of pink spectrum is about 2 kHz, but significant variation exists within music genres and recordings. Initial value is 3 kHz."

            If I drop the frequency to 2,000Hz, then the peak of 15.5 watts drops to 11.5 watts. - just slightly above the 10 watt rating of the resistor.

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            • #7
              I'm not familiar with your software, but your simulation would have to assume constant power density to get that result, and that's not the case. That's why crossover resistors larger than 20w are very rare. There's just no need for them, even in 500 watt pro-sound speakers.
              www.billfitzmaurice.com
              www.billfitzmaurice.info/forum

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              • #8
                Originally posted by dkalsi View Post
                When I use 18watt for the input signal, it looks like the series resistor for the tweeter circuit will exceed its 10watt rating (15.5watt @ 5Khz).

                Should I be concerned?
                Hopefully you test your crossovers before finalizing them, outside the enclosures, that way you can check the temperatures of the resistors. If they get hot, then yes, more or larger resistors should be used.

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                • #9
                  I've got a 3 way which is woofer excursion limited, and even with high treble content a series 10w resistor does not get warm. I am making an assumption here it is not inducing any distortion, but I can't hear any and will measure at higher power levels to see if distortion appears. I've never seen I don't think > 10w series resistors (or doubling of 10s) in the tweeter circuit in any DIY home speaker design - neither 3rd or 2nd way.

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                  • #10
                    Originally posted by AEIOU View Post

                    Hopefully you test your crossovers before finalizing them, outside the enclosures, that way you can check the temperatures of the resistors. If they get hot, then yes, more or larger resistors should be used.
                    AEIOU,

                    I did try this before posting the question. I left music playing for about 30 mins at moderate to loud volumes. When I touched the resistor, it was not warm at all. So I thought I was safe. The simulation software was indicating otherwise and so I wanted to get expert input here :-)

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