QuestionQUESTION: Hi, I'm planning to build a small PA system and my question is: do you really need a crossover(passive)? (with 2 2-way speakers with a tweeter and woofer) Can you hear the difference or will you damage your speakers?
ANSWER: Hello Ewout.
You do need a crossover!
The tweeter will need a high pass crossover. Generally, the lower the crossover point the steeper the slope that is needed. That is, a 2nd order crossover (12 Db/Octave) is twice as steep as a 6Db/Oct one. I always try to stay at least half an octave from the speaker's low frequency limit for setting crossovers using 12Db slopes.
This image may help which shows 'BandPass filters' at 6 and 12 Db/Oct (it only allows the freqs between the two crossovers to 'pass' on to the speaker).
http://i5.tinypic.com/54d6dyw.gif
Remember, an octave is only a halving or doubling of frequency (440Hz is middle C; 880 would be a C one octave up from middle C).
As you can see thew speaker is allowed to pass more sound with a steep slope and adjusted crossover points (referred to as the '-3 Db points).
You can also see how they protect the speaker from unwanted frequencies and how the slope rate you choose affects that ad well.
So.. if your tweeter is rated to go down to 1200 Hertz, you should apply a HighPass crossover at no lower than about 1800 Hz.
Which values of capacitors and inductors you use will depend on the impedance of each speaker.
Most woofers will be fine a running full range signal.
It may seem overwhelming but these are just some basics as an overview.
If you send me the make/models/specs of the speakers and the power you plan to apply to them, I should be able to give you some more specific advice.
JM
---------- FOLLOW-UP ----------
QUESTION: hello again, here are some more details;
- Eminence Gamma 12A (12", 300W RMS, 8 ohms, 55Hz - 5000Hz )
- Eminence APT 80 ( 8 ohms, 3.5 kHz - 20 kHz, 45W RMS,aldough i found sites that said 85 watts RMS... )
I would use a two-way crossover, the Eminence PXB2 3K5 (cross at 3.5 kHz , 400W , 8 ohms, 21dB / octave ) because I found multiple sites that said that the recommended crossover freq. for the tweeter was 3.5 kHz.
I would also take an amp that can deliver 350 watts per channel on 8 ohms because the say that as soon as you put more power over your crossover than it can take (400W with this crossover), you will destroy it, is this correct?
AnswerThanks for the follow up Ewout.
While my experience would lead me to disagree on the crossover point for the tweeter I would use the suggested crossover, for sure.
As to over powering them, once they overheat they are done. I dunno about them blowing 'instantly' in all cases but it will not take long (especially when/if the amp is clipping).
JM
