Now that the speaker boxes are built and finished, I can move on to designing and building the crossover. Actually, I designed this prior to starting the build, since it’s a really good idea to work out all of the details before you begin cutting out parts.

“I’ll clarify – the title should read How To Make A (Simple) Speaker Crossover”

Before designing the crossover, you need to know the crossover frequency and how steep the drop off will be. In my case I have a woofer that has an effective frequency range up to around 4000Hz. The range of the tweeter I’m using is from 2200Hz up to 20,000Hz. So somewhere in the range of 2500Hz to 3000Hz should be the best choice for the crossover point. I opted for the lower, based on what I know about the woofer and the size of the tweeter dome (1 inch).

The slope is the order of the filter. First order is 6db per octave and a 2nd order is 12db per octave, and so on, and there are a few different alignments for these. I chose what’s known as a 2nd order Linkwitz – Riley and the benefits are a moderately steep drop off and the response is flat in the crossover region.

Head spinning yet? This is a dense subject and there’s a lot more to it than I have the time to go into here. I’ll clarify – the title should read How To Make A (Simple) Speaker Crossover, because I’m really just scratching the surface of the topic in this article.

If you really want to delve deeper into the intricacies of crossover design, brace yourself for many, many hours of reading. A good resource is from the man himself, Linkwitz Lab has a ton of accurate information.
And since this is an audio related subject, the deeper you go, the more familiar you will become with the overemphasis on the importance of everything involved, from component quality to the chemical makeup of the coating on the magnet wire used for the coils.

I’m not kidding.

Luckily, if you’re expectations can be set a notch or two lower, you can get perfectly adequate results using basic designs and good quality parts.

Here’s a plot of the crossover point for mine:

Not drawn to actual scale, but it shows how the low pass and high pass filters work to restrict the frequency range sent to each driver. The high pass supplies the tweeter with everything above 2500Hz, while the woofer gets everything below.

Now that I’ve settled on the crossover frequency and the alignment, I can design the crossover itself. I used a convenient online calculator to give me the component values:

Not shown in this schematic, but the tweeter (or woofer) must be wired in reverse, since the filters will output signals that are 180 degrees out of phase.

The tweeters I’m using has a higher sensitivity than the woofer, and normally you’d add what’s known as an “L-pad” to reduce that to match. But my hearing is not what it used to be, especially on the high end, so I’ll leave this as a bit of built in equalization. These speakers will primarily be used in my office to edit the videos I make.

It won’t come as any surprise to long time readers when I say that I’m going to use stuff that I already have to make this. I used to be much more “into” building speakers, so I have a large number of parts on hand already. A good example is this pair of 2.2 mH (millihenry – the measure unit of inductance) coils that I can split in two to make the smaller size ones I need:

I made a simple winding form from some hardboard and a 3/4″ dowel:

And put the full coil on a quickly made rig to hold it while it unspools:

Technically, I should measure the actual inductance of the coils I’m trying to make, but I figure weight will get me in the ballpark. I weighed the full coil and that’s 257 grams:

I wound off a little less than half, leaving 141 grams on the original:

I could then fine tune the weight of each coil to get it close to the target. And I should point out that there are a bunch of factors that affect how much inductance a coil has, from the tightness of the wind to the diameter of the hole, to the height of the wind. So yes, this is pretty rough, but I think it’s within my “close enough” range.

The four coils fine tuned and ready to use:

Each crossover gets one smaller and one bigger coil. And it also gets two capacitors, and fortunately I had ones that were again, “close enough” to use:

For the circuit board, I got fancy and used some pieces of clear 1/4″ thick acrylic.

The holes down the middle are for tie straps to hold the coils and capacitors in place. The holes on the edges are tapped for #8 – 32 machine screws that will be the connection terminals:

Low pass components in place:

While this looks extra pretty (with maybe the exception of the shaggily wound coil), it was a lot of work getting it all put together. One problem was short leads on the (used) capacitors that I had to extend:

Making it like this does make it easier to swap out components if you are not entirely happy with the results. Indeed, a high quality crossover designer will go through several combinations or small changes before settling on the final “voicing” for the speaker. I’m less demanding, though, and quite realistic about how easily even exceptional hearing can be fooled.

I like using banana jacks for the back to connect the speaker wire. They give a clean, low profile look and are fast and easy to install:

To keep the nuts that hold the jack from coming undone, I dripped on some hot melt glue:

Another neater looking option is to use thickened super glue, but this is inside the box and will never be seen again.

To mount the crossover boards inside the box I made standoffs from a piece of 1/4″ plastic tubing:

Then drove the four screws in, one in each corner:

On the right side you can see the wires coming in from the banana jacks, and on the left the leads that will go to the woofer and tweeter.

You might be saying “Hey, you can buy ready made crossovers online for peanuts! Why bother making your own?” And that comment could apply to everything you can find on this website – it’s not the having, it’s the doing.

And besides, the low cost ones don’t give you the option to customize and are made from low quality components, like coils that have a magnetic core. A way to increase inductance and use less wire is to add a ferrite core, but coils made that way can saturate and may cause audible distortion.

I bought these to try side by side with my homemade ones and possibly pull out the scope and function generator to measure how accurate the filter response is on both.

Onward! Stuffing the box will damp (not dampen – there’s no water involved!) some resonances inside the box and also make the box “seem” bigger. Without going too deep into this effect, it’s related to the stuffing material absorbing heat (in the form of sound energy) more efficiently than a box box filled with just air.
You can count on this increase in apparent volume being somewhere in the range of 15 to 25%, so this needs to be decided before designing the box so that you can make it the right size. The ideal “Qtc” for a sealed box is 0.7, and adding stuffing will lower the Qtc for the same volume box. I used just regular wall insulation – fiberglass – cut to size to fit neatly inside:

While designing the box, I considered the increase in volume as about equal to the volume reduced by the backside of the drivers and the crossover, so no net change in the calculated volume.

With the premium quality stuffing in place, I can get the drivers installed, starting with the tweeter. I had to make a gasket first to stop any leaks:

The woofer already has a gasket, a strip of self adhesive foam weatherstripping around the flange. That is what I normally use, but didn’t have any on hand:

Another note on the drivers: they normally come with just straight holes, but I like to countersink and then colour the fresh cut with a black marker:

That way, the screws sit flush and look much neater:

With the first one done and installed, I set up a listening test and you can hear the results of that at the end of this build video: