Building Custom Crossovers

7/30/2019 Building Custom Crossovers

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The easiest (and cheapest) crossovers to build are 6 dB/Oct, made of either an inductor or capacitorinline. While this might work as a simple fix, or at a crossover frequency that is not close to the speaker

frequency response limits, it is not the best solution. For higher slope crossovers, complexity and costadd up quickly.

Parts of a Crossover Network

1. Filter: This is the real crossover. It blocks undesired frequencies by increasing

impedance seen by the amplifier. Made up of capacitors and inductors. There are three

types: High pass, low pass and bandpass (high pass and low pass filters used

together).

2. L-pad: Attenuates the output of a speaker, while presenting a constant load to the

amplifier. L-pads are made using two resistors that dissipate power that would go to the

speaker. It is used to match "volume" levels of different speakers.

3. Impedance Stabilization: Commonly known as a Zobel circuit. It uses a capacitor and
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resistor to compensate for the inductive effects of the speaker coil, making the speaker

play in a more linear fashion. This also makes the amplifier see a more stable load

(speaker impedance varies with frequency). Zobel networks are used on speakers that

play lower frequencies, not tweeters.

How to Pick Crossover Components

1. Capacitors: If you can afford them, try to get Mylar or polypropylene capacitors, especially when

they are used in series (high-pass filters). For low pass filters, or high capacitance values non-polarized electrolytic capacitors have to be used.

2. Inductors: Most critical in low-pass filters, when they are placed in series. For audiophile quality

sound, CFAC (Copper-foil-air-core) inductors are probably the best choice, but at a high cost. Most

cost/quality effective option is air core inductors for high-end crossovers. At larger inductance

values, the series DC resistance in air core inductors becomes a problem. This is when iron core

inductors would have to be used.

3. Resistors: High-power resistors are bulky. Always pick a higher wattage than you would need on

average conditions. Get non-inductive resistors for best performance.

4. L-pads: There are commercially available variable L-pads, or a set can be fabricated using two

high-power resistors. If you buy an L-pad, it is very important that you get the right value. I.e: For 4-

ohm speakers, get a 4-ohm L-pad. A dual 8-ohm L-pad could be wired in parallel to operate with a

single 4-ohm speaker, at twice power handling.

5. Circuit Boards: Chose double sided copper boards. The thicker the copper coating and

insulating material, the better. You can either etch the board with chemicals, or with a Dremel tool.

Crossover Design Tips

Capacitor voltage/power ratings: Typically, 50-volt capacitors can handle up to 70 RMS Watts,

100v can handle 200w and 250v up to 300w.

Inductors gauge/power ratings: In inductors, the gauge of the wire used determines power

handling. Common values are: 20 gauge = 180 watts, 18ga = 250 to 300w, 16ga = 500w and 14ga

= 800w.

Series/parallel inductors add up in the same way as resistors, capacitors add up in the opposite

way (capacitance increases in parallel, decreases in series). For formulas, look at the "speaker/sub

wiring" page.

In crossovers with large non-polarized electrolytic caps, sound quality can be improved by bypassing

them with a small value (0.01 to 0.47 F) film or foil polypropylene capacitor in parallel (tip courtesy

of Parts Express).

Crossover Initial Design

Plan very carefully which frequency you are using for the crossover. Once you buy thecomponents, you can't change it. Consider car's response, speaker's response, slope,

acoustical effects, etc.


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Whether it is plugging numbers into formulas, or having a computer figure out the values, youwill come up with a set of inductor and capacitor values. Most likely, the values you have arenot commercially available. You will have to play around with frequencies and commercially

available values to compromise on a good design without much deviation from theoreticaldata.

Once you have your values figured out, you need to calculate power. Always over-engineercrossovers and pick your inductor's wire gauge and capacitor's max. voltage accordingly. Anyweak component will cause problems in the overall crossover design.

If you have the resources, try to run a simulation of the crossover's interaction with thespeakers parameters, see what comes up and how shifts in voltage and current will affect theresponse.

Building The PCB

Take your time figuring out the best component layout possible. A good layout takes up aleast amount of space while avoiding traces to cross each other. Two sided-copper boardsare easier to work with. If there is no way to avoid traces crossing, both sides can be used tokeep from soldering jumper wires.

Once you have a tentative layout and have analyzed that the layout matches the schematics,

draw each component's outline with a pencil. Mark where component leads and input/outputwires need to be drilled in the board. Dri ll all component holes and test fit all the componentswithout soldering. Etch or grind away parts of the board to create traces. If there is apossibility of a component shorting out traces, etch an outline around the component to avoidproblems.

It is good practice to glue or wire tie (or both) components to the board. This way, rattles and

vibrations that can stress and break wire leads are minimized. Take one component at atime, glue it to the board (hot melt glue works great), and then solder the leads. Cut excesswire. Repeat the process for all the components and wires.

Testing

First, visually trace all the connections and junctions on both sides of the board. Make surethere are no short/open circuits.

The second step is to test the board. Check the board to make sure there are no DC shortcircuits. Connect the board to an amplifier and speaker. You can either use an RTA or testtones to determine the board's frequency response. Use a volume level a bit higher than the

background noise in your test environment. If the frequency response (crossover point andslope) do not match theoretical data, you might have a short or loose connection. Re-inspect

your circuit.

Quick Parts Reference

Guidelines For 6-dB/Octave Crossovers


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Frequency 2 ohms 4 ohms 8 ohms

(Hertz) L C L C L C

80 4.1 mH 1000 F 8.2 mH 500 F 16 mH 250 F

100 3.1 mH 800 F 6.2 mH 400 F 12 mH 200 F130 2.4 mH 600 F 4.7 mH 300 F 10 mH 150 F

200 1.6 mH 400 F 3.3 mH 200 F 6.8 mH 100 F

280 1.2 mH 300 F 2.4 mH 150 F 4.7 mH 75 F

400 0.8 mH 200 F 1.6 mH 100 F 3.3 mH 50 F

600 0.5 mH 136 F 1.0 mH 68 F 2.0 mH 33 F

800 0.41 mH 100 F 0.82 mH 50 F 1.6 mH 25 F

1000 0.31 mH 78 F 0.62 mH 39 F 1.2 mH 20 F

1200 0.25 mH 66 F 0.51 mH 33 F 1.0 mH 16 F

1800 0.16 mH 44 F 0.33 mH 22 F 0.68 mH 10 F

4000 0.08 mH 20 F 0.16 mH 10 F 0.33 mH 5 F6000 51 H 14 F 0.10 mH 6.8 F 0.20 mH 3.3 F

9000 34 H 9.4 F 68 H 4.7 F 0.15 mH 2.2 F

12000 25 H 6.6 F 51 H 3.3 F 100 H 1.6 F

Guidelines For 12-dB/Octave Crossovers


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Frequency 2 ohms 4 ohms 8 ohms

(Hertz) L C L C L C

80 5.6 mH 700 F 11 mH 330 F 22 mH 180 F

100 4.5 mH 550 F 9.1 mH 270 F 18 mH 150 F

130 3.5 mH 470 F 6.8 mH 200 F 15 mH 100 F

200 2.3 mH 330 F 4.7 mH 150 F 9.1 mH 75 F

280 1.7 mH 220 F 3.6 mH 100 F 6.8 mH 50 F

400 1.1 mH 140 F 2.2 mH 68 F 4.7 mH 33 F

600 0.75 mH 100 F 1.5 mH 47 F 3.0 mH 27 F

800 0.56 mH 68 F 1.0 mH 33 F 2.0 mH 15 F

1000 0.45 mH 55 F 0.91 mH 27 F 1.8 mH 13 F

1200 0.38 mH 47 F 0.75 mH 22 F 1.5 mH 11 F

1800 0.25 mH 33 F 0.50 mH 15 F 1.0 mH 6.8 F

4000 0.11 mH 14 F 0.22 mH 6.8 F 0.47 mH 3.3 F

6000 75 H 10 F 0.15 mH 4.7 F 0.33 mH 2.2 F

9000 50 H 6 F 0.10 mH 3.3 F 0.20 mH 1.5 F

12000 38 H 4.7 F 75 H 2.2 F 0.15 mH 1.0 F

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Building Custom Crossovers