I have some AVI three ways. They come with no passive crossover.

If I go active how much power should I give the tweeters. I've read that if I give them to much power I could damage them.

( Would need that like a hole in the head, recently a small screw found its way into my PG zx500 so now have a destroyed pg and kicker sub)

Anyway trying to get over that :-( back to the tweeters. I've also read somewhere that I will need another cap inline with them for protection. Does anyone know the formulae needed for this.

To recap,
how much for tweeters 20, 50rms ??

cap inline for power protection?

go here --> http://www.avisound.com/ and see what they manufacturers say...

Remember, the tweeter will only use what you feed it, and your amp will only output what you tell it to... if you tune it up sensibly, you have little to worry about, you'll hear it overpowering long before you do any long term damage to it

If you want to be safe, i normally install a cap HPF set to one octave below the normal HPF point you have the tweeter set at, has never failed me yet

ok thanks for that. Just trying to plan things in advance before I even start to fit the three ways to the doors.
Active sounds ok, but still have to work out where to put the extra amp.

Not sure if I should attempt to build three way passive crossover.

so if the tweeter normally plays to 2500Hz, you set a HPF filter to 1250Hz?
i wouldn't advise that, i think you mis-worded your post!

To my understanding:

passive Xovers are calculated using the "roll off point".
this means that the frequency used to calculate the caps and coils is where the cap or coil STARTS to affect the frequency response.

when you use an active crossover, the crossover frequency is actually the -3db point (explanation further down)

on this basis, you'd be wise to pick a HP point a little higher, and a LP point a little lower than recommended, to account for this.
also, with a 1st order HP filter, i recommend you chose a frequency higher than usual, to help reduce the low frequencies to the tweeter

now... since you're goign 3 way, if you intend to build filters for each driver, then i suggest you use 2nd order.

crossovers cause phase shift at 90 degrees "per order"
so a 1st order crossover has 90 degrees shift
a 2nd order has 180 degrees shift etc.

now for your midrange, you'll need a HP AND a LP. this means the phase shift will be exactly double that of the tweeter (as it will only have a HP)

if you use 2nd order filters, you'll have:
midbass (lp only) 180degrees
midrange (LP + HP) 360 degrees
tweeter (HP only) 180 degrees

now, wire your tweeter and midbass in reverse polarity and all 3 components will have 360degrees of phase shift.

this will aid you greatly in imaging (which is what 3 ways are all about)

(explanation from above)
using the -3db point for crossover points is quite important IMO,
if your midrange and tweeter are both crossed over at 3Khz, (where each driver is at -3db at 3khz) both drivers will assist each other to provide a flat overall response (on paper)

if this sounds too complex, then you should be doing more research before you embark OR get 6 channels of amplification, and an active crossover.


there are some crossover calculators at http://www.carstereo.com/help/Articles.cfm
scroll down to "Crossovers & Filter Calculator"

I thought that passive Xovers were the same as actives in that the Xover point was the -3dB down point? Are you sure the passive Xover point is the start of the slope and not the -3dB point Ben?

as i said, that's how i understand it. i'll try to find something to back this up.
even if it's the -3db point, it still isn't a great idea to select a crossover frequency which is an octave lower than the optimum frequncy setting, especially with a shallow filter slope.
if the frequency is the rolloff point, then making it an octave lower will introduce even more overlap too, creating a peak in output (or dip if there's phasing issues) and muddiness where you have imaging cues coming from 2 different places at full output levels.

OK, cool... i wasnt questioning your view, just i always thought they both were -3dB points

I only use the extra cap as seperate precaution in case the onboard amps crossover packs a sad, and sonically i havent had too many problems besides the theory, but you are right on the phasing issues, i'll have to look into it

ok.... i may have missed something here.
are you using the caps as protection incase the amp's crossover fails,
or are you using them as the primary crossover.

i don't recommend using them as a failsafe incase the amp's crossover fails. due to phase shift.

Failsafe... hence why its set so much lower than the amps crossover point, the sound should be at least -24dB down by the time it reaches the cap, so i've never noticed the phase shift being an issue, at least as far as i can hear anyway

if it's -24 at the cap's crossover point, then if the amp's crossover fails, it'll gain 24db at that frequency... which is the lowest a frequency that the tweeter can handle.
couple that with the fact that things most often fail when they're at their limits, and it'll be like adding sticky tape to re-inforce your seatbelt.

if i ercall correctly, the phase shift doesn't ONLY occur at the crossover point, it affects all frequencies after the filter. if your tweeters are closer than your midrange this might even benefit your imaging, as a crude time delay.

OK, cool... so what can you do to ensure the tweeter doesnt get too-low frequencies in the event of an amp crossover failure (it happens )

For the mid if I put a first order lpf followed by a first order hpf will that cause a phase shift?