Minimum frequency to put protection capacitor in active systems?

[DingDing]

In an active system the crossover points often change. Can the protection cap be chosen independent of the crossover frequency to allow for maximum flexibility and at the same time protect the driver and be sonically negligible?

Ie, is there a minimum frequency for every driver which can be chosen to yield maximum flexibility, minimum interference with the XO point and offer good protection?
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For example, I'm using 2446 and 2404.

1) Per JBL minimum recommended XO is 12dB/oct @ 500 Hz for 2446. A protection cap is only a 1st order high pass filter, hence 6dB/oct. At which frequency can this be placed to minimize risk of damage and at the same time minimize the audible effects of the lowest potential XO?

An octave below XO would be at 250 Hz, but is a 6dB/oct HPF sufficient protection against an amp blowing up, user errors in setting crossovers etc?

2) Same for 2404, which is 12dB/Oct @ 3kHz minimum per JBL. What would be the minimum frequency at which to set the cap to offer good protection and be sonically negligible?

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Yes, I know this information has been discussed a million times over, but I've yet to see a clear answer to this question. People often say "put the protection cap an octave below the crossover frequency", but in an active system the crossover point tend to move around a lot. :)

[grumpy]

My 2 cents (possibly worth that much):

One could trust the operator, crossover, and amp and use no cap.
This is the extreme of negligible.

One could use the nominal "octave below crossover" rule of thumb for
a compromise.

One could use a cap -at- crossover and gain extra protection (steeper roll off, which affects system design, driver phasing).

... or anywhere in between. Designer's choice. For compression drivers, excursion limits are pretty important (destructive almost immediately). Clear as mud :)

If you just want to play with electronic crossovers, use the rule of thumb, don't play at concert levels, and power amps on last, off first.

[DingDing]

Thank you very much for helpful advice.

Been running without protection for almost a year now and knock on wood, nothing has broken, but it keeps me awake at night so I'm going to do something. :)

Capacitor voltage question
Although I don't play concert levels very often, I don't want the capacitor be a limiting factor. :D As we know these large format compression drivers can take a lot of beating and be EQ'ed pretty hard too. So what kind of voltage caps should be used to protect a 2446 (It can handle 150W cont.)?

[1audiohack]

For a 2446H it only takes 35V to deliver 150Watts and twice that for a 2246J so you don't need a 400V cap.

Food for thought, horn dependant of course, one usually ends up with a big hump of extra SPL in the mid band of a large format compression driver and a need to either kill some of that energy below 3-4kHz and or boost everything above that. A single capacitor in the passband can get about 6dB out of that hump for you for free and afford much more LF protection than a cap selected an "octave below".

Add a couple of resistors and another cap and you can level it out even more. Look at the Constant Directivitey compensation network for the JBL 2360 or 2380 for instance.

To me this often sounds better than the requisite 18dB of HF boost usually required to do it all in the EQ.

As an added bonus, moving the crossover points around on a more linear device is a lot easier.

Is this is just more dirt in the water, feel free to disregard. :)

Barry.