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Speaker crossover power calculation question — Parallax Forums

Speaker crossover power calculation question

Don MDon M Posts: 1,653
edited 2012-09-04 05:05 in General Discussion
I'm wondering if someone can help me out with the math for figuring out how much power (wattage) C1 can handle for its component rating of 100v. I have someone who has asked me to look at their PA speakers that keep destroying this capacitor. The first time they also burnt the tweeter coils.

I was using Ohm's law and figuring C1's voltage rating divided by the sum of R1 & R3 (10.5 ohm) which gives 9.52 amps times 100 volts should equal 950 watts. The input is paralleled with the woofer/input which is 8 ohms and the tweeter measures approx 8 ohms.

Crossover.png


I'm wondering if maybe there is some high frequency distorted output coming from the amp that may be destroying this that you maybe can't hear. I think he mentioned the amp was rated at 400 watts. I'm going to take my scope over and look at the output and see if I can see anything unusual.

Thanks.
Don
Crossover.png
800 x 471 - 15K

Comments

  • Don MDon M Posts: 1,653
    edited 2012-09-03 06:19
    Actually looking at the schematic again if I figured the tweeter resistance in series with R2 and that combo in parallel with R3 then the new resistance figure is approx 6 ohms into 100 volts which makes the wattage even higher?
  • Mark_TMark_T Posts: 1,981
    edited 2012-09-03 15:59
    Normally capacitors are rated for ripple-current rather than power handling. I can't see how you've applied ohms law to a capacitor BTW - impedance depends on frequency and you haven't mentioned the frequency range (clearly its something like 5kHz to 20kHz, but you haven't mentioned a figure.

    Typically for high power use you'll have to use a low-loss capacitor, something like a high-grade plastic-film (polyester, polystyrene, ptfe) - and large enough that it can take the current/power (ripple-current ratings are usually frequency-dependent since they are accounting for both the resistive losses and power-factor losses.

    Are we sure the cap is not being exposed to voltages that are too high BTW?

    [edit - is this for a piezo tweeter or a moving-coil tweeter?]
  • Don MDon M Posts: 1,653
    edited 2012-09-03 18:21
    Moving coil tweeter. I don't know the crossover frequencies as I would imagine that the values of these components would determine that. I connected the crossover to my digital frequency generator and connected the scope to the input and output. I could see the slow rise of response starting at around 400 Hz. I didn't plot it to determine anything further than that. I could see that by the time I got to 4K the output amplitude was getting fairly close to the input.

    After I re-installed the crossover back into the cabinet I connected my scope to the crossover while he had the PA in operation. I was looking for any DC voltage present as well as any high frequency AC wave that maybe the system was producing that maybe couldn't be heard but would damage the components. Neither was present. What it is most likely is the fact that the band they were using this PA with was a loud screaming hard rock band and the speakers were just over driven to the point of destruction.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-09-04 05:05
    You might try this site - Elliot Sound Products - for a good answer. He has written quite a bit of good material that is available for free.

    sound.westhost.com/
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