Speaker Crossover

Over 10 years ago I made a center channel speaker for my home theater sound system.  “Home Theater” is over selling it a bit but it was my center channel speaker.  I made it from some scrap plywood my dad had I think along with some 4″ woofers and a tweeter that Radio Shack was clearancing out.  The tweeter came with a capacitor attached to it to filter out low frequencies which I considered good enough for a crossover at the time.  I assembled it and used it for several years.  Eventually, we moved and the center channel got shelved as we were using the TV’s speakers.

A couple weeks ago we decided to move the TV into one of the spare bedrooms and create a game room.  I grabbed a surround sound receiver a friend had given me and decided to pull out the center channel again.  Here’s a pic of the speaker as it sits now.

Co1

At some point some kids (not mine) had poked in and ripped the old tweeter as you can see below.  I picked up the one you see above from Parts Express and fit it to the cabinet.  Note that the old tweeter had four screws holding it on while the new has three.  This and the larger back of the tweeter required modifying the cabinet a little bit.

Co1a

Before I get into making the cross over, I should probably explain what a crossover is.  As I’m sure you know, speaker drivers (the actual transducer not the box) reproduce sound.  They do this by moving in and out at a specific frequency to reproduce a specific tone.  Put a lot of movement together and you get music or the spoken word.  There are numerous types of drivers.  Some are made for very low frequencies known as subwoofers.  Some for low frequencies called woofers, higher up the frequency spectrum comes midrange drivers, and at the top of the spectrum are tweeters.  This leaves out some but covers it for the most part.  Drivers work best when they’re reproducing the frequencies they were designed for.  Trying to make a driver produce other frequencies can result in poor, quiet, or no sound at these frequencies.  In the case of tweeters, passing them the high power signal that should go to the woofers can burn them up.  Some speakers, typically poor ones, have one driver.  Others have two, three or more driver.  A speaker with two different drivers is typically known as a two way system.  A three way system has three different drivers.  In a two way speaker there’s a driver for producing lows/midrange and one for producing highs.  A three way splits up the work even more.

This is where the crossover comes in.  The crossover filters out certain frequencies for each driver letting pass what’s best for that driver.  My center channel is a two way speaker despite having two woofers.  They’re the same and in this design the two woofers correct an audio issue I won’t bore you with.  In a two way crossover, frequencies below a certain point get sent to the woofer(s) and those above the point get sent to the tweeter(s).  Once again, I’m glossing over details, but thats the basic idea.  For my speaker I set the crossover frequency at 2000 Hz.  I found a website that calculates the values of the required electrical components to filter the signal.

When crossovers filter a signal they don’t just chop everything else off.  Instead, they slowly decrease the signal outside of the bounds.  The rate at which they decrease the signal is known as the crossover order.  The crossover I chose attenuates the signal at 12 dB/octave.  The crossover frequency is actually the frequency at which the signal has been attenuated by 3 db.  If the crossover is designed correctly the two drivers will complement each other and there won’t be a noticeable frequency range that is attenuated around the crossover frequency.   I don’t feel like putting the effort into verifying this for my speaker though.  So, I’ll just cross my fingers and be ok if everything isn’t perfect.

With the theory out of the way, we can turn to the actual crossover.  The calculator asks for the impedance of the drivers and the crossover frequency desired.  With this information, it generates a diagram with some capacitors and inductors and the value of each.  Each capacitor/inductor combination will filter the signal for a speaker or speakers depending on how it’s wired.  The picture below shows the diagram of the crossover and the parts I’ll make it out of.  In the diagram the signal comes in from the left and gets spit up into the crossover components.  The signal is routed up towards the tweeter and to the right to the woofer passing through the cross over components first.  Capacitors let high frequencies pass and inductors let low frequencies pass when wired in series with a driver.  That’s the role of the C1 and L2 components in the diagram below.    Without L1 and C2 I’d have a first order cross over but the addition of these components results in a second order crossover.

Co2

One of the fun parts of electrical work is figuring out how to actually wire things together.  The diagram has lines connecting all the components that you might think of as wires.  In reality there doesn’t have to be any wire and you can connect the parts together directly.  To figure it out, I first laid all the parts out on a piece of wood I’d build the crossover on.  Then I made all the connections to assemble the crossover taking time to triple check the connections.  Here’s what it looks like on it’s own.  The capacitors are the black cylinders and the inductors are the wound copper loops.

Co3

Happy with how it looked, I pulled out the hot glue gun and stuck the components to the wooden base.  This keeps everything from rattling around in the speaker cabinet and putting strain on the wires.

Co4

Next, I added all the leads and soldered it all together.  I hooked up all the speakers and checked the resistance value of the crossover at the input leads.  All I wanted to check was that there wasn’t a short.  A multimeter checks resistance with DC which you can think of as a 0 Hz signal.  So, this signal is blocked by the capacitor from the tweeter and passed trough the inductor to my two woofers.  I got a reading of around 4 ohms which makes sense for my two woofers wired in parallel.  With this check and a final review of the whole circuit I hooked it up to a small amp.  I ran music through it for a bit and everything seemed to be working well.

Co5

Next, I clipped the extra leads off of the components and taped them up with electrical tape.  Not pretty, but who cares.  No one else will ever see it…other than you of course.  Finally, I glued it in the bottom of the cabinet, stuffed my fiberfill back in and put it all back together.

Co6

 

Hooray sound.  But not good sound.  Something is off but what.  I put my ear close to each speaker and as expected heard lows out of the woofers and highs out of the tweeter.  For some reason though the speaker sounded odd like it was missing some lows and like there was re-verb when listening to voices.  Music sounded like it was missing the lows as well.  After a little bit, I decided to place a book over one of the woofers with some music playing.  The lows instantly came back and I knew what had happened.  I swapped the book to block the other woofer to confirm and heard the same results.  Somehow, one of the woofers was out of phase and canceling out some of the sound from the other woofer.  If you remember your wave theory, two waves that have peaks and valleys that occur at the same time are said to be in phase.  When you combine them they add together.  If they are out of phase a little bit then the frequency can shift.  If the waves are opposite, known as 180 degrees out of phase, they will cancel each other completely.   This is what my woofers were doing.  They didn’t cancel completely of course but they were fighting each other.

I pulled the woofers to see what was going on and realized how I’d messed up.  I reused the leads with the crimped on connectors that I’d made years ago.  The woofers have a large and small tab for the positive and negative terminal respectively.   When I went to wire the driver leads up I only looked at the stripe on the wire sets to determine polarity.  I’d put the wrong connectors on of the woofer back then and had never fixed it.  I suppose that when I’d wired it up years ago I hooked the wires up intentionally backwards instead of recrimping the connectors since I didn’t have the odd sound before.  Anyways, I changed out connectors and put everything back together correctly this time.  Much better!  Low and midrange frequencies were back.

Speaking of phase, here’s another note.  In a second order crossover system one part of the crossover will lag by 90 degrees while the other will lead by 90 degrees.  This results in a phase difference of 180 degrees.  To correct for this, I wired the tweeter “backwards” so that it will be in phase with the woofers.

 

PS – I picked up an old Sony Cybershot camera at a thrift store for $10 and tried it out on this post.  I don’t think it’ll replace the D50 though.  So, no bounce flash like normal this time.

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