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Passive Crossovers

A passive crossover has no active filters as were used in the electronic crossover. It uses coils (inductors) and capacitors to cause a rolloff of the audio level above or below certain frequencies to prevent unwanted portions of the audio from reaching the various speakers. The simplest types use a single coil OR capacitor connected in series with the speaker. If a capacitor is used, the capacitor will tend to block the lower frequencies. If an inductor is used, it will tend to block the higher freqquencies. A capacitor would be used on smaller speakers and tweeters. An inductor would be used on larger speakers (midrange speakers and woofers). When using only an inductor or a capacitor in series with the speaker, the rate of rolloff is 6dB/octave. In a more complex configuration combining both coils and capacitors on each speaker, a steeper rolloff is obtainable.

The following crossover is one from a relatively high-end speaker system. It allows you to set the tweeter output to 3 different levels. It allows you to select either 0 or 180 phase for the low frequency speaker and has protection for the tweeters.

The next image shows the headers and shunt used to select the various tweeter levels. The shunt (on the 0dB header) shorts the two pins together making the connection between them. When the shunt is moved to the -3dB or -6dB headers, the tweeter output is reduced by the corresponding level (3 or 6 dB). This is used when the owner feels that the tweeters are a bit too loud (too bright).

Some crossovers include components that protect the tweeter. They generally do so by increasing in resistance as more current flows through the tweeter (and the device). When the resistance increases, the current flow is limited and the risk of blowing the tweeters is significantly reduced. In this crossover, 12v lamps are used. If you have a crossover with similar protection, you will see them begin to glow when you drive too much power to the tweeters. Other crossovers use either PTC thermistors or polyswitches.


6dB/octave High Pass Crossover:
The diagram below shows how capacitors would be used/connected in a high pass passive crossover system. Notice how one capacitor is in a positive wire and one is in a negative wire. Audio is an AC voltage which means that current flows both ways in the circuit. For a simple crossover like this, it doesn't matter which terminal you have the capacitor connected to. The same is true for the inductors in the next section.

6dB/octave Low Pass Crossover:
The diagram below shows how inductors would be used in a low pass passive crossover system.

6dB/octave High and Low Pass Tri-mode Crossover:
The diagram below shows how the inductor and capacitors would be used in a tri mode system.

Earlier, it was stated that it didn't matter whether the capacitor/inductor was in series with the positive or negative speaker wire. That's true but when using the amp in tri-mode, you have to make sure that the capacitor or inductor is only in series with the speaker that needs it. For example, if the capacitor were installed on the bottom speaker terminal of the amp above and the larger speaker (woofer) were tapped off of the negative terminal of the smaller speaker, the capacitor would be in series with the woofer and the low frequencies would be blocked from reaching the woofer.


Position the mouse cursor over the crossover component values on each side of the graph to see how the slope slides as you change the crossover point. You should also notice how the coils roll off the high frequencies and the capacitors roll off the low frequencies. The straight line is the audio signal level with no crossover components in series with the speakers. The crossover points are roughly based on a 4 ohm speaker. The first time through, you should hold your mouse over each value until the image loads completely.

CAPACITORS
100 microfarad
47 microfarad
22 microfarad
10 microfarad
4.7 microfarad
Flat response > no crossover COILS (inductors)
2.2 millihenry
3.3 millihenry
4.7 millihenry
5.6 millihenry
6.8 millihenry

This demo shows how the crossover point shifts when a 47 microfarad capacitor is used with different speaker loads.

Speaker Impedance
1 ohms
2 ohms
4 ohms
8 ohms
Flat response > no crossover

Calculations and review

This diagram shows the rolloff characteristics of 3 different passive crossovers each of which consist of a single coil (of different values) creating the rolloff. The vertical lines are one octave apart and horizontal lines are 3dB apart. The red line shows the frequency response of a filter with a 200hz crossover point. The green line has a crossover point of 600 hz. The purple line has a crossover point of 1600hz. The rate of rolloff is only 6dB/octave.

passrllo.gif


This section shows the math involved in selecting the proper coil for your particular speaker system. This is the formula needed.

lopsfmla.gif

Impedance = The rated impedance of the speaker
Pi=3.14....
Frequency = The desired 3dB down (crossover) point
Inductor = Value of proper coil - value given in henries (multiply by 1000 to convert to millihenries)

For this example, let's choose 4 ohms as the speaker's rated impedance. We will use 200hz as the desired crossover point.
Value of the coil=4/(2*3.14*200)
Value of the coil=3.18 millihenries

The output level of a 4 ohm speaker with a 3.18 millihenry coil in series with it, would follow (approximately) the red line. If it were a 4 ohm resistor, instead of a speaker, the frequency response would follow the line exactly. A speaker has inductance in it's voice coil which would make finding the exact response difficult but this equation will get you close to the desired response.


This diagram shows the rolloff characteristics of 3 different passive crossovers each of which consist of a single capacitor (of different values) creating the rolloff. The red line shows the frequency response of a filter with a 3200hz crossover point. The green line has a crossover point of 1200 hz. The purple line has a crossover point of 400hz. The rate of rolloff is only 6dB/octave.

passrlhi.gif


This section shows the math involved in selecting the proper capacitor for your particular speaker system. This is the formula needed.

hipsfmla.gif

Impedance = The rated impedance of the speaker
Pi=3.14....
Frequency = The desired 3dB down (crossover) point
Capacitor = Value of proper capacitor - value given in farads (multiply by 1,000,000 to convert to microfarads)

For this example, let's choose 4 ohms as the speaker's rated impedance. We will use 1200hz as the desired crossover point.
Value of the capacitor=1/(2*3.14*1200*4)
Value of the capacitor=33 microfarads

The output level of a 4 ohm speaker with a 33 microfarad capacitor in series with it, would follow (approximately) the red line. Again, if it were a 4 ohm resistor, instead of a speaker, the frequency response would follow the line exactly. A speaker has inductance in its voice coil which would make finding the exact response difficult but this equation will get you close to the desired response.

Check your math here
High Frequency Impedance? Ohms
Low Frequency Impedance? Ohms
Crossover Frequency? Hertz
Capacitor value = Microfarads
Inductor value = Millihenries


Note:
If you cannot find the correct value for a crossover, you can combine different values to make up the proper value.

This is how the series/parallel configurations effect the values of the components.

  • Inductors in series:
    Add the values together. It works like speakers and resistors in series. Two 4mh inductors in series is 8 mh.
     
  • Inductors in parallel:
    The total inductance is less than the smallest inductor's value. If the values are equal, simply divide the value of a single inductor by the number of inductors in parallel. Two 4mh inductors in parallel is 2mh.

This program calculates the value
of 2 parallel connected inductors.
Data Input:
Value of inductor 1? millihenries
Value of inductor 2? millihenries
Data Output:
Value of inductors in series: millihenries
Value of inductors in parallel: millihenries
  • Capacitors in series:
    The total value is less than the value of the smallest capacitor. If the values of all of the caps are equal, simply divide the value of a single capacitor by the total number of caps in series. Two 10uf (uf=microfafads) capacitors in series is 5uf total.
     
  • Capacitors in parallel:
    The total value is the value of the individual capacitors added together. Two 10uf capacitors in parallel is 20uf total.

This program calculates the value
of 2 series connected capacitors.
Data Input:
Value of capacitor 1? microfarads
Value of capacitor 2? microfarads
Data Output:
Value of capacitors in series: microfarads
Value of capacitors in parallel: microfarads

This diagram shows 2 different ways to cross subs at 100hz. All of the subs are 4 ohms.


Capacitor Voltage Ratings:
All capacitors have a maximum voltage rating. Capacitors used for crossovers are no different. Capacitors rated for use at 100 volts are OK to use with virtually all car audio amplifiers. For amplifiers rated to produce more than 2500 watts into 4 ohms, you will need higher voltage capacitors. 50 volt caps are good for amplifier rated up to 600 watts into 4 ohms. There is a calculator on the 'damping factor' page of this site which will calculate the voltage needed to produce a given power into a given load. Use capacitors rated higher than the voltage given by the calculator.

Polarized vs Non polarized Capacitors:
Most capacitors are non polarized (bi-polar) but there are a few that are polarized and have a positive and a negative terminal. Polarized capacitors should never be used for audio transmission (or coupling). The most common and least expensive capacitors used in crossovers are 'electrolytic' capacitors. Electrolytics can be found in both polarized and non polarized versions. When you buy electrolytic capacitors for use in passive crossovers, make sure that they are non polarized. Other capacitors used in passive crossovers such as Mylar, polypropylene and polyester are non polarized.

For more information on higher order crossovers, go here.


The following demo shows how the crossover's output signal level changes with a change in input frequency or a change in the crossover point. The output sine waves indicate power output. A sine wave half as tall as the original indicates that the speaker would receive only half power at that frequency and crossover point.


Click HERE to make this applet fill this window.


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You should remember:
1.A passive crossover requires no external power source to operate.
2.A passive crossover uses caps, coils and resistors to reduce the signal level above and/or below a certain frequency.


 

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