3 Way Crossover Design Example
Note, this sample crossover makes use of many of the calculators found on the menu on the left. You should also review the Crossover Guide for help with this example.
For this example, I picked 3 ScanSpeak drivers for a 3-way speaker (the same 3 used on the Speaker Box Example. Note: This example old and the characteristics of these drivers have since changed. These drivers were not picked because of how well they 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||Response Curve|
|Tweeter||D2008/8512||2k-30k Hz||8 ohms||90 db SPL||1000Hz||Chart|
|Mid||13M/8636||200-4k Hz||8 ohms||88 db SPL||Chart|
|Woofer||18W/8543||35-3.2k Hz||8 ohms||89 db SPL||Chart|
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. An L-Pad / Driver Attenuation 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 range / 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.
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 of 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 / Driver Attenuation Calculator was used to determine the L-Pad components.
The last design step is 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. Note: A bi-amp/bi-wired system would look something like this.
With the crossover designed the next step is to procure the parts: the capacitors, resistors, and inductors. See the Crossover Guide for explanations on the different types of these components (Mylar vs. polypropylene capacitors...). In the end, it is about how much money you want to spend, which should be no more than half the cost of the drivers.
When you buy inductors, capacitors, and 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. A 16.58uF Capacitor (as required for the first crossover) is not something you can find in a store but you should be able to find something close. You can also use multiple different capacitors, inductors, and resistors in series or parallel to achieve the desired value.
With the crossover designed and parts in hand, the next step is to build the crossover. For this step, you will need a piece of wood to mount the parts to, a hot glue gun and some glue sticks, a soldering iron and solder, and finally some wire. Any piece of wood will work as a mounting board. You can even use the MDF for the speaker itself. First, layout the components on the board according to the crossover diagram that you have made. Try to place the components close enough to each other so that jumper wires are not required to connect the different components together. Cut the board to size once you have decided on the layout.
Once the components are in place, use your hot glue gun to mount them to the board. Be sure that the inductor coils are not near each other and that each one is on a different axis to eliminate "inductive coupling" (See the Crossover Guide for more info).
Now solder the different components together. If possible, solder the components directly to each other. Otherwise, use short jumper wires to connect them. I prefer using 12AWG for the crossover but it is not required.
Finally, mount the crossover in your speaker, connect the crossover leads to the back of your binding post, and connect the speakers to the crossover. Positive (+) to Red. Negative (-) to Black. When testing your speaker, pay attention to possible Phase Shift problems (See the Crossover Guide) where the sound volume dips significantly at one of the crossover points. If you suspect you have a Phase Shift problem, reverse the leads (+/-) on one of the speakers to see if the system gets louder. If so, then you have found and solved your problem.
The final step in any design is experimentation. Remember that every component (capacitors, inductors, and resistors) each exhibit all 3 properties (capacitance, inductance, and resistance). This is why thicker copper wire is desired for inductors - to lower its resistance. No design is perfect, and improvements can be made by making small changes to the crossover. This may not be possible if you don't have an electronics shop filled with parts. Ordering foil inductors one at a time can get expensive. The best alternative may be to wind your own inductor coils using the Inductor Calculator. Start large and then unwind (but don't cut) the inductor to experiment with different values.