Putting a tweeter into a cabinet

I'll give you a quick over-simplification to lay the groundwork for folks with more experience in this area ...

The crossover works like a valve that changes impedance (resistance) at different frequencies. A true crossover would have a low pass filter (feeds lows to the woofer) and a high pass filter (feeds highs to the tweeter).

In the case of a high pass filter, when you push low notes at it the filter has a high impedance (and doesn't let much sound in). So it's like your 8 ohm woofer and say a 1000 ohm tweeter in parallel- not much gets to the tweeter and it doesn't really change the impedance your amp sees.

In the case of a piezo tweeter, they naturally have a high impedance at low frequencies and so they often don't bother with a crossover.

If I understand it correctly, Avatar drops in a capacitor in series with the horn, and that capacitor works as a simple high pass filter. The 15" woofer is probably not really presenting an 8 ohm load at those frequencies so your amp doesn't mind.

 

Most passive x-overs are designed to work with 8-ohm speakers. The x-over routs the high sounds to the 8-ohm tweeter and low sounds to the 8-ohm woofer so the impedance stays roughly the same or 8 ohms. This routing comes at a price though. Usually about 3db loss in efficiency. With most 2 way bass cabinets though all that is used is a capacitor on the tweeter. This capacitor will have a real high impedance value at frequencies below a certain value say 1K. Once the sound reaches 1K the impedance drops and lets the signal pass into the tweeter. At frequencies above 1K the average impedance of the other speakers is probably so high that the amp stills see 8 ohms as the average. This type of sudo-crossover isn't as hi-fi as the true crossover but it is usually good enough for bass applications.

 

Since you mentioned Avatar...Dave has the best prices for Foster/Fostex horn tweeters. He'll also sell you a complete wiring harness with jack plate, crossover cap and tweeter level control for a very reasonable price. All you'd need to do is cut two holes in your cabinet (one for the tweeter, one for the input cup) make four solder connections, install 8 screws and you're ready to go.

 

There is probably one at http://www.partsexpress.com

However, you can calculate it easy enough...

For a 6dB/octave cross over, the formula for the capacitor is C = 1/(2*pi*F*Z) where:

pi = 3.14
F = the cross over frequency in Hz
Z = the impedance of the tweeter, in ohms

Remember that if the exponenent (power of 10) is -6 then the number is in microfarads

So, based on this...an 8 ohm tweeter crossed over at 5000 Hz will need a 4 uF (microfarad) capacitor.

 

Speaker cable is fine; noise isn't an issue there.

 

Sorry. No, you don't want to use an instrument cable to carry speaker signals; you will indeed fry the cable over time and possible cause amp damage as well. They are shielded but the big problem is they are designed for low voltage (< 1v) and current (probably 100ma or less as a guess. Your power amp puts out 15-50v and probably 1 amp+ .

 

Yep, that's correct for a 6dB/ octave rolloff.