Hi pass filter question

[woody]

I need a 12/db oct hi pass xover for my tweeter amp. My amp requires
an output cap and my tweeter is a resistive load. So I was thinking
if I select the output cap value so that it is a 6db octive crossover at
2000 hz and another 6db octive 2000 hz 6db oct. hi pass filter at the
amp input. This should sum to 12/db oct. But will the xover point still be 2000hz and what will the Q be?

[sregor]

Yes, the output will be second order (12 db per octave) and Q will be .5 at 2000 Hx. Q of .5 gives you - 6 db at the Xover point. A very popular xover is the second order L/R, which involves inverting the tweeter (or woofer) and using two first order (or second order Q of .5) filters for both tweeter and woofer.

[woody]

Thanks that answers my question. Now I can use a small film cap at the output of my amp instead of a big electrolitic and as a bonus not have to worry about the turn
on thump blowing my tweeter.

[djk]

There will still be a 'crack' when the output cap charges up.

[tvrgeek]

Welcome to looking at things as a system. A luxury OEM's do not have.

It is not quite that simple, as if you look at the tweeters curves, you will find it has a natural roll-off somewhere around 2K, so one can combine your passive inter-stage filter with the natural roll-of of the tweeter and may not even need the cap at the speaker. Depending if the tweeter is a chamber or not, it's roll-off varies. If you don't line up the various poles, you wind up with very bumpy responses.

So, you need some information on the tweeter.

[djk]

"may not even need the cap at the speaker."

Don't think tweeters handle DC very well.

"My amp requires an output cap"

[AndrewT]

the low end roll off of a tweeter often has quite high Q resonances around those frequencies.
If you use the natural roll off, in combination with a DC blocking cap, you must find a driver that is well behaved or one that is easily corrected with a Zobel. A DC blocking cap rolls off the signal @ -6dB/octave. This low slope filter passes a lot of mid frequency signal to the driver and some of the bass signal gets through as well and may drive the tweeter beyond it's linear excursion range.
If you don't do this then a two pole filter consisting of the DC blocking cap and a single pole filter should roll off the speaker response at least one octave above the ripply part of the frequency spectrum.