Speaker filters - crossovers, easier than you think

 To a lot of people, crossovers seem like some alien technology. They are simple devices once you understand the basics behind. The challenge is with the drivers, and your ears, will discuss this below.

Disclaimer: this article does not try to cover the exact technical aspects of the subject. It is designed to make you, your kids and Grandma understand the principles. Here at The Audiophile DIY we are also not big fans of lazy people. We can help, but if you want to master the subject, now that you understand how it works, and hopefully you have an idea of what you want, Google is your best friend. 

 Basic knowledge required: the standard audio spectrum is 20Hz-20KHz.  From 20Hz to 250Hz is the bass area, 3KHz to 20KHz is the tweeter area. Everything in between is the mid area. The middle area can be broken into many pieces and is the main part of everything we hear, I usually use low mids, mids, and high mids.

 Why do we need crossovers?

Easy explanation: there is no universal driver. 

Proper explanation: each driver has limits on what frequencies can reproduce with good efficiency. Based on construction and materials, drivers will perform better at low frequencies, mids or highs. We are not going to discuss full-range drivers as they are a separate field and never really full range.

A 10-inch bass driver receiving a full signal will most likely not be able to output anything above 1KHz. If you feed a tweeter a full signal, it will just blow up.

What crossovers do ?

In order to feed the drivers with the frequency range they enjoy, we need to filter out the rest; this is where crossovers come into play. We are going to consider a 3-way speaker, the bass driver will require a low pass filter that does … you guess … passes low frequencies and stops the rest.  The mids are the biggest pain as they require a bandpass filter that will cut the bass at the beginning of the audio spectrum, let a specific middle frequency pass and then cut again at the high end so it does not go over the tweeter. The tweeters are wrongly considered easy to work with when it comes to crossovers (just put a capacitor and is done), somewhat true, but considering that the top end defines the music (in my opinion it does), it can get complicated really easily. The tweeters use a high-pass filter that will pass only the high frequencies to the driver. Tweeters are also delicate devices that can be easily destroyed. In another post, I will discuss how a 10-watt amplifier can ruin big speakers and why tweeters go first.

Crossover types

You already know that there are 3 types: low pass, band pass and high pass.. Each of these has 3 levels. You will find them many times specified as 1^st order 6dB, 2^nd order 12 dB, and 3^rd order 18dB.  

What does it mean?  The simple explanation: let’s imagine we are working on a low-pass filter that will filter anything above 300Hz (the values are fiction just for explanation). The bass driver will only receive frequencies under 300hz, the filter order determines how hard the filter will cut out the rest, a 1^st order 6db circuit will gradually cut down the higher frequencies on a 6dB slope. The bass driver will play 350Hz, 400Hz will be lower in volume, 500Hz lower, and it will continue until, let’s say, 800Hz is barely passing through.  A 3^rd order 18db filter will be much more abrupt in the cut off, at 450 Hz, the filter will have already cut everything else.   Filter orders dictate the blending and resonation points for each driver. Let’s say a 2-way speaker where the mid-bass and the tweeter must cover the full frequency range. Based on the driver’s specs at some point in the middle you will have the blending point, where they meet, you can have sharp cuts at 18dB or softer cuts at 6 dB, and let them go over each other a bit.

   You will find many schematics and calculators online, but as much information is out there as little real-life information is. What do I mean by this?  Well, after 20 years of building amplifiers, building speakers, setting up clubs or stages, repairs and restorations … I learned that real-life scenarios rarely fit the tons and tons of theory that I personally spent many nights reading. One of my missions is to always find the sweet spot, tweak it until it delivers what you like.

 Seems rather easy, right? Well, it is, as we said at the beginning of the article, the drivers and your ears present the bigger challenge. Everyone hears differently, top end slowly fades off with age, the sound is created in our brain and this makes us unique even when it comes to listening to music.

  Not all drivers behave or sound the same. Many times, we got inspired by certain circuits, but we had to adapt and tweak for available drivers. There are many complete speaker projects out there and sometimes people keep the design and try to use cheaper drivers with disastrous results. This creates a rather false mentality that only a $$$ driver sounds good. Tweaking the crossovers can give amazing results even with cheap drivers.  And you have some options: get expensive measuring equipment, or improvise yourself. Although useful, the measurements are sometimes off, or the manual process, tweak and listen, tweak and listen. You will also see in future articles why frequency response does not guarantee high-end sound.

In future articles you will find some of our projects and see the filters designed after hours of listening test to find the sweet spot for the drivers and our ears.