|The Crossover Design Cookbook|
Chapter 3: Speaker Motors and Crossovers
by Mark Lawrence
What are Crossovers?
1st order Crossover
2nd order Crossover
How Crossovers Work
Final Watt-V Crossover
What We've Learned
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In the cross over above, we see the original two inductors and capacitors from our first design. Also, we've added a Zobel R-C network to the woofer and the tweeter, and an R-L-C resonator to the tweeter. This collection of three resistors, three capacitors, and an inductor are present just to equalize the impedance of the two drivers so that our cross overs can do their job as designed. Finally, we've added two resistors to the tweeter to pad the treble down by 5dB.
When building a second order cross over, you must reverse the connections to the tweeter. In the picture above, the amplifier positive lead connects to the upper two open wires, and the amplifier negative lead connects to the lower two open wires. The woofer has its positive lead up and its negative lead down. The tweeter has its negative lead up and its positive lead down.
You might notice that the tweeter has a resonance compensation circuit, but the woofer does not. That's because the woofer resonance is going to be at about 30Hz - 70Hz, and the crossover frequency will be typically at 500hz - 2000hz. If the crossover frequency is ten times or more higher than the resonance frequency, we don't have to worry about the resonance. In a three or four way system, you would have to measure the resonance of each driver and compare them to each crossover frequency to determine when you needed a resonance compensator.
Is this cross over good? Yes. If you build this, you'll be happy. Is this the best we can do? No. There's more to this book.
Driver manufacturers change their production techniques from time to time. This cross over is based on one particular set of drivers. If you wish to make a Watt V alike, I strongly urge you to measure your own drivers, verify the driver efficiencies from the manufacturers specifications, and recalculate the values above. I expect you'll find some differences. My drivers are a bit old. I don't recommend that you build this cross over using exactly these values.
In our Watt V cross over, we decided to use a "Q" of .7. This means we built a Butterworth cross over. Some people prefer different cross over alignments, which means they like different Q's. If you re-do the math for a Q of .58, you'll have a Bessel cross over, which has optimum transient response. We decided to build a second order 12dB per octave cross over. Some people prefer first order, some prefer third order, and some prefer fourth order. Finally, we assumed that the drivers had flat frequency response, not only in their assigned bands, but, in order for this cross over to work, for about one and a half octaves beyond the cross over frequency.
Later, we'll examine these assumptions in detail, and perhaps decide to change our minds. However, I must emphasize that at this point we're chasing preferences, not major design errors. We have a functional, correctly engineered cross over for the Watt V.
We've come a long way now, so I think it's time to pause here and summarize what we know so far. Although we don't yet have the complete story, we now have enough to make perfectly serviceable cook-book cross overs, designed in about 5 minutes using only a pocket calculator with a square root key.
Second-order crossovers are pretty good. There are major trade-offs in time and phase performance when using third and fourth order crossovers which make these very tricky to design and fine tune. Personally, I prefer second order crossovers for almost all applications.
We've learned that when we design a crossover, we have to assume the drivers are pure resistors. Unfortunately, real live drivers typically have very complicated frequency dependant impedance which will cause poor performance of the crossover circuits. However, these frequency dependencies can be compensated with resonance and Zobel circuits. Also, different drivers have different efficiencies which can must be accounted for with an L-Pad.
Finally, we've learned a few construction techniques - it's important to orient your inductors correctly or they will generate cross talk. It's useful to build up your required capacitors from several smaller capacitors mounted in parallel.
Mount all power resistors with at least 1/4" of free air space on all sides. This means use the leads of the resistor to stand the resistor off the board, and allow air to flow under the resistor too. Make sure the resistors are non-inductively wound.
Orient inductors as shown below. Remember the speaker motor coils when considering orientation. Use air core, or 300 to 500 watt bobbin inductors. The inductors should have a DC resistance of no more than 1/2Ω.
Use capacitors rated for at least 100V AC, or 200V DC. Mylar or Polypropylene capacitors are the best for cross overs. If you must use electrolytics, use film capacitors for at least 40% of the total capacitance value. It's best to use several smaller capacitors in parallel to add up to your required capacitor values. For example, if you need a 10μF capacitor, it's best to use four 2.2μF and a 1.2μF capacitor in parallel. This will add up the capacitor values to the correct number. The variance in the capacitors will tend to cancel, and you'll get a more precise value. And the lead resistance and inductance of the capacitors are in parallel, so those cancel resulting in a capacitor with better high frequency performance.
Copyright © 2002-2019 Mark Lawrence. All rights reserved. Reproduction is strictly prohibited.
Email me, firstname.lastname@example.org, with suggestions, additions, broken links.
Revised Thursday, 15-Aug-2019 09:30:53 CDT