Crossovers & Filters Guide

Link to Crossovers Link to Crossovers 12Db

Building your own passive Crossover network

If you don't want to have a separate amplifier for each separate frequency range (ie. active crossover network), you can use a passive approach, using Capacitors and Inductors.

You can put woofers, midrange horns and tweeters all in the same cabinet, or you can put them in separate cabinets with their crossover components; it makes no difference, as long as you provide the right components for the frequency range you want.

The following is for the simplest crossover network - a first order (6db per octave) crossover. The goal is to match the speaker to the task it was designed for and make sure that you never apply frequencies inappropriately to speakers that cannot handle it.

A first order crossover is often what you will find in PA systems. You will often find a second order (12db per octave) in studio monitors and many higher end home speaker systems. The reasons for this are many. There are major differences between the actual speakers found in a PA system versus those used in a Studio monitor or home speaker system. Studio monitor systems components are tightly controlled and everything is matched specifically to the enclosure and is made to give as flat a frequency response as possible; this is done using speakers that are lower efficiency. PA speakers tend to be very high output. You can use second order crossovers in PA systems, many of the integrated multi-speaker cabinets available today have them.

Studio monitors are made for a specific environment, and are focused on getting the most accurate representation of sound in a small stable environment. PA speakers tend to be far more efficient than those used in a studio monitor, and will be used to project sound to a large group of people in a random environment (small room, large room, outdoors, acoustically dead room, acoustically live room, etc.). The separate frequency components will not always be as closely matched as possible (you will probably be buying different brands). They must also be able to handle large transients and often substantial power.

This is not to say that some PA speaker systems are not highly accurate at reproducing sound, however, it is unlikely that you will find many pro-studios mixing down sound on their outdoor PA systems. The end functions are different, and you should always keep this in mind when designing speaker systems.

First order crossovers are fairly simple to implement. The following diagrams show the schematic. There is a single crossover component per device.

If wiring up separate components for specific functions

If enclosing separate components in a single unit

Why would you wire things separately? You might have a Sub-Woofer; you don't install Tweeters in those very often. You might also have a large Midrange horn - this is probably in its own cabinet. No matter how you need to wire it, its quite easy to select the right crossover components.

Since each speaker will provide load at the specified frequency range, you need to calculate the loads at the given frequencies to make sure that you don't overload you amplifier. Cabinets that have all the speakers in the same place make it easier to calculate the loading effect as a single speaker. If you have a number of cabinets, each load must be accounted for. High frequency loading does not provide as significant an effect as low frequency loading - always make sure that your Woofers are accurately accounted for.

Crossover Capacitor/Inductor Selection Guide - 6db (First Order)

When you are ready to choose your crossover point, this chart indicates what frequency that relates to both the Low Frequency (the Inductor) and the high Frequency (the Capacitor) components. Keep in mind that you may not be able to find an exact match when trying to purchase the Inductor or Capacitor. Try to find the closest matching parts (or use as many as needed) to get the frequency response you are shooting for. The chart lists values for 16 ohm, 8 ohm and 4 ohm cabinets/speaker systems.

Additional Information

Capacitor Values

Unlike speakers, putting capacitors in Parallel increases the capacitance. If you put 2 (or more) Capacitors in Parallel, the uf is additive. For example, if you have a 2 uf and a 2.5 uf capacitor, and you wire them accross themselves, the end result will be a 4.5 uf capacitor.

Inductor Values

Inductors put in Series increase the inductance. If you had a 1 mH and a 0.5 mH inductor and you wired them in Series (output of one goes to the input of the other), you get a 1.5 mh inductor

 

Crossover Frequency	16 ohm Capacitor C1	16 ohm Inductor L1	8 ohm Capacitor C1	8 ohm Inductor L1	4 ohm Capacitor C1	4 ohm Inductor L1
80 Hz	-	-	248.0 uf	15.92 mH	496.8 uf	7.96 mH
100 Hz	-	-	198.8 uf	12.74 mH	397.6 uf	6.37 mH
125 Hz	80.0 uf	20.40 mH	159.0 uf	10.20 mH	318.0 uf	5.10 mH
160 Hz		-	124.2 uf	7.96 mH	248.4 uf	3.98 mH
180 Hz	-	-	110.4 uf	7.08 mH	220.8 uf	3.54 mH
200 Hz	50.0 uf	12.74 mH	99.4 uf	6.37 mH	198.8 uf	3.18 mH
250 Hz	-	-	79.5 uf	5.10 mH	159.0 uf	2.55 mH
320 Hz	-	-	62.1 uf	3.98 mH	124.2 uf	1.99 mH
400 Hz	25.0 uf	6.36 mH	49.7 uf	3.18 mH	99.4 uf	1.59 mH
520 Hz	-	-	38.2 uf	2.45 mH	76.4 uf	1.22 mH
640 Hz	-	-	31.1 uf	1.99 mH	62.2 uf	0.99 mH
800 Hz	-	-	24.8 uf	1.59 mH	49.6 uf	0.79 mH
1000 Hz	10.0 uf	2.54 mH	19.9 uf	1.27 mH	39.8 uf	0.64 mH
1600 Hz	-	-	12.4 uf	0.80 mH	24.8 uf	0.40 mH
2000 Hz	5.0 uf	0.32 mH	9.9 uf	0.64 mH	19.9 uf	0.32 mH
2500 Hz	-	-	7.9 uf	0.51 mH	15.9 uf	0.26 mH
3200 Hz	-	-	6.2 uf	0.4 mH	12.4 uf	0.20 mH
4000 Hz	2.50 uf	0.66 mH	5.0 uf	0.32 mH	9.9 uf	0.16 mH
5200 Hz	-	-	3.8 uf	0.25 mH	7.6 uf	0.12 mH
6400 Hz	-	-	3.1 uf	0.2 mH	6.2 uf	0.10 mH
8000 Hz	-	-	2.4 uf	0.16 mH	4.9 uf	0.08 mH

What ratings should these parts have?

For Capacitors, the voltage that they can handle will determine how hard you can drive them:

• 50 Volt Rating: 70 watts RMS
• 100 Volt Rating: 200 watts RMS
• 250 Volt Rating: 300 Watts RMS

Capacitors used in crossover networks must be Non-Polarized. If they are marked with either a '+' or a '-', these are not suitable for this function, and they will fail very quickly.

Inductors for high powered systems will be made of solid copper wire, wrapped onto a plastic bobbin. For PA systems, they will be anywhere from 20 to 14 gauge wire. Always oversize inductors at least one size where possible.

• 20 gauge inductors cannot handle more than around 180 watts.
• 18 gauge inductors are usually good up to 250 watts.
• 16 gauge inductors are usually good up to 500 watts.
• 14 gauge inductors are usually good up to 800 watts.

Inductors have resistance, and can be checked with an ohm-meter. They typically run anywhere from 0.2 ohms (for .1 mH) to 1.3 ohms (for 10 mh).

Keep in mind that these power ratings relate the load an individual speaker will carry, not an entire system. If you have 4 speaker cabinets per side and 400 watts driving them, its likely that each cabinet will see 100 watts of power on average. This is what you need to scale for. Never drive your speakers harder than they are rated for - you will destroy them long before a properly sized crossover component.

What about 3 speaker systems?

The information given here implies that you can only use a low frequency speaker and a high frequency speaker. The truth is that you can add an additional higher frequency speaker by adding another inductor and capacitor, where the inductor now drives the midrange, and the capacitor drives the higher frequency speaker; in a sense, slapping another crossover network behind the first high frequency capacitor stage.

For example, you can have the woofer/midrange crossover at 1000 Hz and then add another crossover for a tweeter at 5200 Hz. This is how most pre-packaged 3 speaker crossovers are made.

Exceptions to the rules

Piezo Tweeters appear as a capacitive load to your audio power amplifier, and their actual load is a function of frequency. They don't need a crossover capacitor (it won't hurt to use one, but its not necessary). Piezo tweeters typically cannot handle more than 50 to 75 watts, so you normally wire them up in series until the needed power rating is achieved. Many people also wire a 20 ohm, 20 watt power resistor in series with the Piezo tweeters to protect them.

Since a Piezo tweeter can have frequency response up to 50 kHz (well beyond human hearing), they easily add sparkle to things that have high frequency overtones, like cymbals. Many people consider their sound harsh. I like them, but I don't use that many of them in my PA cabinets. Piezos can radiate high frequencies at high power so well, that they can damage your hearing without you noticing it. They are also highly directional - You may need a few to aim in different directions to properly disperse the sound.

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