Sample 3 Way Crossover

  First, what is a crossover? People can hear sound frequencies from 20-20000Hz. There is no one speaker capable of producing all frequencies throughout this range. Therefore, multiple speakers must be used. Usually, it is damaging for a speaker to produce frequencies lower than what it was designed for. Also, if two speakers produce sound at the same frequencies, then the sound at those frequencies will be louder. For these reasons, some type of circuit is necessary to make sure that each speaker only produces a certain set of frequencies. That circuit is a crossover.

  The basic components of crossovers are inductors and resistors. Inductors become more reactive (increasing AC resistance) as the frequency increases, and thus lower the sound pressure on the driver more and more as the frequency increase. Capacitors work just the opposite. They have higher AC resistance as the frequency decreases.

  No crossover can completely block out all frequencies beyond the crossover point. Instead, it filters the frequencies in greater amounts as the frequency moves away from the crossover point. How fast it filters the sound is determined by the order of the crossover. A 1st order crossover filters 6 db/octave, a 2nd order 12 db/octave, a 3rd order 18 db/octave, and so on. A logarithmic scale is used for the frequencies. An octave is the doubling (or halving) of the frequency. A 2nd order low pass crossover at 1000Hz will decrease the signal by 12db at 2000Hz, 24db at 4000Hz, 36db at 8000Hz...

  For reference, a 3db increase is twice as loud and requires twice as much amplifier power to create this increase. For people, however, a 3db change is the minimum noticeable change. It takes a 10db increase for a speaker to "sound" twice as loud. And this 10db increase requires over 8 times more power to create. 2x power for 3db, 4x power for 6db, 8x power for 9db, 16x power for 12db...

  For this example, I picked 3 ScanSpeak drivers for a 3-way speaker. These drivers were not picked because of how well the worked together, but rather because they have problems that can be solved with the proper circuit. The drivers I chose were:

Driver	Model	Frequency Range	Imped	Sensitivity	Fs
Tweeter	D2008/8512	2k-30k Hz	8 ohms	90 db SPL	1000Hz
Mid	13M/8636	200-4k Hz	8 ohms	88 db SPL
Woofer	18W/8543	35-3.2k Hz	8 ohms	89 db SPL

  All of the drivers are 8 ohms. There are no differences in output caused by different impedances with the drivers. The tweeter has 2db sensitivity over the mid, and the woofer has 1db sensitivity over the mid. Resistors will be used to balance out the sensitivity/load problems. A l-pad circuit will be used to lower the tweeter output by 1db and the woofer output by 2db.

  The Fs ( free air resonance ) of the tweeter is at 1000Hz. This is the frequency at which the tweeter will resonate, and produce a large positive spike in the frequency response. A series-notch filter will be used to remove this spike.

  You want to pick crossover points between the two drivers. Remember that it is a base 2 logarithmic scale. For the mid/woofer crossover there are 4 octaves between 200-3.2k Hz, 200-400-800-1600-3200. 800 Hz is the middle frequency, with 2 octaves flat in either direction. For the tweeter/mid crossover, there are only 1 octaves, 2000-4000. 3k Hz is the crossover point with 1/2 octave stable in either direction. These two drivers have little overlap, and normally would not be used together.

  In the mid/woofer combo, the frequency response is stable 2 octaves beyond the crossover point, and for the tweeter/mid, only 1/2 octave. Therefore, a higher order crossover must be used with the tweeter/mid than with the mid/woofer. A 2nd order, maybe even a 1st order crossover can be used with the mid/woofer combo, while a minimum 3rd order crossover should be used with the mid/tweeter.

  Some people believe that it is best to use a low order crossovers when possible, preferably only 1st order. This does have some benefits. With the greater frequency overlap, voices will not seem to jump from one driver to another as quickly as they would with a steep crossover. It also follows the minimalist approach where the simpler the circuit, the less distortion and modification of the signal is introduced. The problem with 1st order crossovers is that the frequency overlap in the drivers would have to always be at least 2 octaves (or more) in each direction from the crossover point. It would probably require at least 4 drivers.

  Another belief is that even order (2, 4, 6...) order crossovers should be avoided. Even order crossovers tend to have spikes or dips in the frequency response around the crossover point. These spikes can be as bad as -30db, but can easily be solved by reversing the polarity of only one of the speakers, limiting the spike to about +- 3db.

  For this example, a 3rd order crossovers at 3000Hz and a 1st order crossover at 800Hz will be used. The Crossover Calculator was used to determine the crossover components. These are the results of the 2 crossover calculations:






  Now, these two diagrams must be combined into a 3-way diagram. When working with 3 or more speakers, at least one speaker must be bandpass. Bandpass means that the speaker has a high pass filter (HPF) that filters out low frequencies and lets high frequencies pass through, and a low pass filter (LPF) that filters out high frequencies and lets low frequencies pass through. In this system, only the mid will be bandpass. When wiring multiple speakers, you usually start with the largest speaker. All speakers above that one are run through the HPF. In our 3-way system, both the mid and tweeter are run though the HPF from the woofer/mid crossover. A possible 4-way system would look like this.


  This diagram has been simplified, and only the positive (+) lead is shown, but you get the idea. The reason for going woofer to tweeter is so that the HPF is before the LPF for each bandpass speaker. The inductors (coils) in a LPF have resistance. This resistance affects the impedance of the entire circuit. If you put the LPF before the HPF, the amp will not have a stable load to work with.
  Although the diagrams in this document show each the high speakers being run through multiple high pass filters, this is not necessary. In the above diagram, the input for the second and third crossover could be directly tied to the main input instead of the high output from another crossover.
  The next step in designing the crossover circuit is to design the l-pads to equalize the different driver sensitivities. 2db needs to be removed from the tweeter, and 1db from the woofer. The L-Pad Calculator was used to determine the l-pad components.






  The last step is designing the series notch filter. The Fs is at 1000Hz, and the crossover point is at 3000Hz with a 3rd order crossover. The resonance spike is over one octave from the crossover point, and may be damped enough that it will not be noticed, but it will be added to the circuit anyway. The Series Notch Filter Calculator was used to determine the necessary components.


  Now, the crossovers, l-pads, and series notch filter must be combined into one circuit. There is no standard as to which parts come first, but the common method is crossover then l-pad then series notch filter.
  This is the complete circuit for the 3-way system.


  Both the inductors and capacitors will have some resistance. Usually, it is small but sometimes it can be greater than the resistance of the speaker itself. Since the entire crossover network is based on the resistance of the speakers, this can be very bad. For an 8 ohm woofer with a low crossover point, the inductor in the LPF could be 16 ohms, or even higher. With a combined load of 24 ohms, the amp would not be putting out anywhere near as much power as it should. There are several ways around this problem. The first is to buy expensive components. For capacitors, an Electrolytic capacitor is your basic type. They are cheap, but do not pass high frequencies well. Mylar capacitors are more expensive, but they are better for audio because they work better at the high frequencies, and have less inductance and resistance. Polypropylene capacitors are the best, but are also much more expensive.

  As for inductors, they are usually just a coil of copper wire, sometimes hundreds of feet long. Copper is the only realistic material to use. The only way to lower resistance is to use thicker wire. You can purchase a more expensive coil that uses a heavy gauge wire. Moving from 19g to 14g increases the price by at least 5x's. Using silver increases the cost another 20x's. There are also copper foil inductors which are more expensive, but work somewhat better. If you want to try to make your own inductor, check out the Inductor Calculator for information on winding your own coils.

  When you buy inductors capacitors or resistors there are usually only certain values available. These values are referred to the E ranges are discussed in Resistor Colors. That is why the values in the crossover tables for 1st, 2nd, 3rd order Butterworth crossovers have slightly different values than what the Crossover Calculator produces. The tables use commonly available inductors and capacitors.

  When using more than one inductor, the electro-magnetic fields of the inductors can interfere with each other. That is why it is best to keep them as far apart as possible. Also, keep the fields out of phase with each other by rotating the inductors 90 degrees. It is possible to have 3 inductors out of phase, as shown below.



  The last problem in designing a crossover is Phase Shift. Phase is the timing of a signal, and the shift is the degree of the delay that occurs on the signal when passed through a crossover. Each order of crossover introduces a 90 degree phase shift. A 180 degree shift is an inverse of the wave. If 2 speakers are 180 degrees out of phase then they will cancel each other where ever they produce the same frequencies. Even with crossovers, both speakers will produce sound for several octaves beyond the crossover point. If this problem occurs, there will be a noticable dip in the frequency response at the crossover point. To solve this problem, wire one, but not both, of the speakers backwards (+ to -). Usually, phase shift problems only occur with 2nd order (or 6th order) crossovers, but can also occur when using multiple 2-way crossovers in a 3-way (or more) speaker system. The only way to really find and fix a phase shift problem is trying all possibilities in reversing the speaker leads. If reversing the leads makes the system sound louder, then you know you have fixed the problem.