Quick question on protective high pass cap... - AVS Forum
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post #1 of 7 Old 10-07-2013, 07:31 PM - Thread Starter
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For a protective capacitor placed in series with a compression driver and using this formula to size the cap:

(µfd) = 1,000,000 / (2pi x Impedance x Crossover Frequency)

what do you enter for the Impedance, when you have this:



Target is to highpass around 500hz.

Is 6 ohms, 7 ohms, or something else used?

Thanks in advance.

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post #2 of 7 Old 10-07-2013, 08:57 PM
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Well if its just for protective purposes in an active design or something, I'd use 6ohm. Whether you use 6 or 7 won't matter all that much for that purpose.

But you've identified one of the biggest reasons you can't just use that simple formula for XO design. You should use a program like sound easy or PCD or something that will calculate it every 1/24th octave. Then it'll give you the true response. For instance. Using that formula you'll get something like 25uF using 6 or 7 ohms. But a 25uF cap will cause some attenuation above 500hz as well. I'm sure you already knew that, but worth saying.
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post #3 of 7 Old 10-07-2013, 09:23 PM
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Quote:
Originally Posted by LTD02 View Post

Is 6 ohms, 7 ohms?
It doesn't matter, as a turn on transient protection cap is typically sized for a corner frequency one octave down from the actual crossover frequency.

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post #4 of 7 Old 10-07-2013, 09:34 PM - Thread Starter
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I got that far. :-) So, let's say the crossover target is 1khz and the target for the protective cap is 500hz. What size cap do you use given the impedance of the compression driver shown?

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post #5 of 7 Old 10-07-2013, 09:46 PM - Thread Starter
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"You should use a program like sound easy or PCD or something that will calculate it every 1/24th octave."

Ah! OK. It just "clicked". Thank you Tux!

So the program will go through and calculate the attenuation that will occur at each frequency (or small group) given the value of the cap and the impedance at that particular point based on the ever shifting actual crossover point that will produce.

In this case, since the impedance *rises* above the target point of 500hz and since having a little lower effective point for the protection is better than getting too high, we are good because a higher impedance will produce a "lower" crossover point and therefore less attenuation in the region where the impedance rises than would occur with the theoretically ideal 500hz crossover point. Yeah I know. Clear as mud. :-)

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post #6 of 7 Old 10-07-2013, 10:23 PM
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You got it.
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post #7 of 7 Old 10-08-2013, 08:35 AM - Thread Starter
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yeah...thanks again. two thumbs up. :-) when you said that it recalculates every 1/24th octave or so, the intuition clicked for what was going on.

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