Building The Crossover!
A brief tutorial on how to assemble custom crossovers

The heart and soul of every loudspeaker is in the crossover network that distributes the audio signal between the drivers, i.e., the woofers, tweeters, midrange drivers, subwoofers, etc., that make up your speakers. All too often, people write to me and ask whether they can substitute "stock", i.e., off the shelf, crossover networks for the custom-designed crossovers that I post with the speakers I've designed (or for other speaker "kits" they've seen and want to try.) My answer is always the same -- the crossover is what makes the speaker sound the way it does. You can have the most expensive and best sounding drivers in the world, but if you do not integrate them properly with the crossover, you'll have a very expensive, lousy speaker. Do stock crossovers make lousy speakers? No, not necessarily... but a well-designed crossover that is specific to a set of drivers and the enclosure they're in is all part of what comes together to make a truly exceptional set of speakers. For more information on stock vs. custom crossovers, see my article "How Phat is Flat". (I'll pause while you read it)

Back? Good, you must have read the article and realized that building your own crossover from a published schematic is the way to go when you want speakers that don't just reproduce sound... but actually present sound in a manner that's not much unlike an orchestra. The drivers are your musicians and the crossover is the conductor. A skilled conductor that has a deep understanding and knowledge of the musical arrangement will leave the audience in tears. A competent conductor who is looking at the arrangement for the first time will merely make sure that the musicians are in the right seats. Think of the custom crossovers published with well-designed "kits" as highly skilled conductors... and stock crossovers as junior high-school band teachers who are hoping everyone is in tune and is trying to keep the kids in the back from throwing spitballs while the 2nd-chair clarinet player gropes the 3rd-chair flute.

As many of you have expressed in the email I've received, the idea of actually building a crossover from 'scratch' is a bit daunting at first. The truth is, though, that building a crossover from a schematic is a piece of cake. All you need are a few simple tools and the ability to read a schematic. You're on your own for the tools -- go down to radio shack and get a 40W soldering iron (about $8), some "fine" electronics solder (get the stuff with silver in it, it's excellent), and a hot-melt glue gun with a few glue sticks (about $6 at KMart) or some "Goop", an adhesive sold at many hardware stores, if you want to mount your components as securely and permanently as possible. For mounting the components on the board, get a piece of MDF, pegboard, or even heavy cardboard that's large enough to fit all the components. If you're going to mount the crossover internally in your speakers (as most of you are), just make sure you decide where the boards will go (or ask the speaker designer!) Assuming you have all your components, now is a good time to lay them all out, check their values to make sure everything is right, and making sure you have everything you need to get started. When you're going to make a capacitor value from two separate capacitors, i.e., when they tell you to "parallel capacitors", now is a good time to put them together as shown below. Paralleling capacitors is no harder than putting them side by side and twisting the leads together. In the Eros crossover, all the capacitors are made by paralleling two capacitors of smaller values. For the 22.8uF caps, I put a 6.8uF cap next to a 16uF cap and twist the leads together as you see below (not that the resistors are not parallel, they're just sitting next to each other), as shown in Figure 1.

Ok, lets assume you've got your soldering iron and hot melt glue gun all heated up and you're ready to start building. What now? ALWAYS start with the schematic. If this part is scaring you, get the book "Electronic Formulas, Symbols, and Circuits" by Forrest Mimms (about $6) when you're at Radio Shack. It has excellent instructions of how to read schematics, how to solder properly, etc. For my tutorial, I'm going to use the very simple (6 component) crossover I designed for the VSS-1 "Eros" speaker using a pair of Vifa PL18's in parallel and a Scan Speak 9500 tweeter. The schematic is shown in Figure 2.

This speaker is an excellent example of why you would want to use a custom-designed crossover rather than a stock crossover. First of all, the drivers, alone, cost about $400 for a pair of these. Obviously, you could go down to a stereo store and buy a pretty good set of speakers for $400. When you factor in the cost of the crossover components, you're looking at around $500 for the pair... and with good quality, nicely finished enclosures (shown below), you'll spend over $800 on raw materials (mostly the birch plywood and fancy wood veneer) alone for these. I upgraded my set of these to use 12awg Alpha Core inductors, making the total cost about $900/pr. What can you get for $900/pr in the store? Something that sounds pretty good. But if you ever get a chance to hear the Eros (Figure 3) live, you would find that it compares very, very nicely to speakers in the $4000-$6000 price range. Through a 1962 Fisher 800C tube amp I've been restoring, these speakers provide some of the most captivating and engulfing sound presentation I've ever heard... but (there's always a but!)

As I was developing the crossovers for these speakers, I found a few things that clearly emphasize why a custom crossover is so important. First, look at the lowpass section of the crossover. That's the section that starts at the "source" and ends at the 2 Vifa woofers. See that 22.8uF capacitor in there? I had tried a slightly larger capacitor (24uF) originally because it gave flatter response. When I switched to 22.8uF (using a 16uF and a 6.8uF capacitor in parallel with each other), the soundstage improved dramatically. It was as though the room became 10 feet wider and 5 feet taller. Now look at the highpass section. That section starts with the source and ends with the SS9500 tweeter (it includes the 18uF capacitor, the 220uH inductor and the 5 ohm resistor). I had originally chosen a 6 ohm resistor for the "shunt" resistor (a "shunt" component is any component that connects across the positive and negative leads in parallel with the driver, whereas a "series" component is one in which both leads are in the same path, + or -). I experimented with a variety of values, but it was clear that the 5 ohm resistor made the highs sparkle with a realism that words can't describe. When you put it all together, it was magic. With a stock crossover? No magic... in fact, even with numerous crossovers that provided very flat frequency response, there was no magic. This crossover combined such technicalities as baffle step compensation, acoustic rolloff rates, crossover point, acoustic rolloff shapes, transfer function shapes, impedance phase angle, driver phase tracking, and a few other things that you don't need to worry about... but in the end (about 3 months of computer modeling, building new variations, listening, tweaking, etc.) the final result was simply astonishing.

So, getting back to the schematic, we have two sections -- the lowpass made up of the 820uH (.82mH) inductor, a 22.8uF capacitor, and a 1.5 ohm resistor, and the highpass made up of the components I mentioned previously. Study the schematic until you see all this clearly. Once you do, look at the signal path for each section. The signal path is just a fancy terms for what goes through the + lead, in most cases. In the lowpass, the signal comes from the source and enters one lead of the inductor. The signal exits the inductor through its other lead and goes to the speaker. So far so good? Ok, then there's the little catch -- at the same place where the signal goes from the inductor to the woofers, it also branches and goes to "ground" (or the - lead). So what does this look like with real components?  Take a look at Figure 4.

Figure 5 shows the .82mH inductor (on the right) with its leads stretched out. At the bottom of the photo you see a 16uF capacitor in parallel with 6.8uF capacitor to make a 22.8uF capacitor. The black thing on the bottom left is the 1.5ohm resistor in series with the 22.8uF capacitor. Now that everything is laid out, I can use my hot melt glue gun to firmly attach the components to the board (which is made from a scrap piece of Baltic birch plywood I found in the garage that was leftover from making the Eros enclosures -- these boards will actually be mounted on the back side of the Eros enclosures in case I want to tweak them or experiment with different brands of components in the future.) If you are going to put both the highpass and the lowpass on the same board (not necessary, but since the Eros crossovers are so simple, I'm going to) you should layout the highpass section before gluing (see below). I like to mark the board with a pen to show where the tweeter + and - and woofer + and - leads are going to connect. Why? Just to make sure everything is hooked up properly and to make it easier when I go to hookup the speaker leads at the end. One trick is to keep the highpass and lowpass + input and - output near each other when you mount the crossovers on a single board. This lets you get away from fewer jumper wires. You can see, if you look carefully, that I put the + inputs for both sections together and the - outputs for both sections together (look for T- and W- on the bottom left of the board.)

Now that the components are glued to the board (no tricks here, just use plenty of glue), you need to solder the components to one another (the speaker input and output leads are done last). Start with the capacitors. You should have twisted the leads together (carefully) earlier. See Figure 6 for what they should look like.

To properly solder this joint, you'll need to learn how to make a "hot joint". This is where the metal is heated up and the solder melts directly onto it and "flows" across the joint. Never touch the solder to the tip of the iron -- always touch the iron to the metal and the solder to the metal nearby. The metal, once hot enough, will melt the solder which will then flow across the joint, making a solid contact. It should look as shown in Figure 7.

The same thing applies when you are joining components in series. Maximize the metal-to-metal contact when you twist the leads and then make a good, hot solder joint. Figure 8 shows what it looked like when I joined the 1.5 ohm resistor to the paralleled capacitors.

If there are any tricks to this (there really aren't), the one thing that might hang up a few of you is soldering the wire leads from the capacitors or resistors to the inductor foil. I've found that using an alligator clip to keep them firmly in contact is very helpful (shown in Figure 9).

I'll use the clip to hold the wire to the foil and touch the tip of the iron to the foil. Then I'll touch the solder to the wire leads from the component (a capacitor is shown here). When the solder starts to flow, it makes a nice, clean joint between the two. If I want this to be very permanent, I'll fold the foil over the wire lead and solder that as well.

Once you've got all the soldering done, you can attach the board to your speaker (or put it in the enclosure) and solder the speaker leads to the appropriate points that you marked. In the case of the Eros crossover, it looks as shown as figure 10.  Now, this doesn't reflect any attempt to arrange the components on the crossover board to minimize space requirements jumper lead usage (although none were needed here), you might want to consider such factors as you become more comfortable with the construction process.

A few last things to note. First, see the jumper wires that connect the (+) and (-) posts of the speaker terminal to the + input of the lowpass and highpass and the (-) junctions? The (+) lead (comes from the red terminal) is a bit hard to see, but the flat 14awg wire from the black terminal is pretty clear and it joins the (-) lead, or ground, from both the highpass and the lowpass and also connects to the wires leading to the (-) terminals of all the drivers in the enclosure. The wires leading to the bottom left corner of the board (where the inductor and capacitor are joined) are a pair of 16awg shielded wires that go to the (+) terminals of each of the woofers. Last thing -- notice the alignment of the inductors? one is on its side with the hole facing the other inductor to eliminate "inductive coupling". They're probably far enough apart not to worry about this, but it doesn't hurt to be safe.

That's all there is to it. All that was left to do was for me to check the impedance of the speakers and the frequency response, to make sure everything was working right and it all checked out A-OK! Then I sat back and listened to a few hours of music enjoying what I had accomplished.

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C2001, W. Jaeschke
Unauthorized Reproduction Prohibited