::: Fitting Capacitor Crossovers :::
This could very well be the best £1 you'll ever spend on your car. Before deciding if you're prepared to start shelling out this sort of money however, you need to ask yourself two questions
» One: Do some of your speakers break-up earlier than others when you wind up the volume on your stereo?
» Two: Does the illumination on your supposedly high powered head unit dim in time to the bass?
If the answer to either of these questions is yes - then it's time to dig deep and free up some loose change!
::: The Basics :::
In a nutshell, what we're going to do here is use a cheap electronic component known as a capacitor to cut off the lowest bass notes from our speakers to prevent them distorting under hard use.
What you'll need...
» Wire strippers
To make the most of this article it'd help if we gave you a basic understanding of sound waves. A vibrating speaker sends a wave of pressure fluctuations through the atmosphere. When the fluctuating waves reach your ears, they vibrate your eardrums backwards and forwards and your brains interpret this motion as sound. We hear different sounds from different speakers because of variations in sound wave 'frequency' - A higher sound wave frequency simply means that the air pressure is fluctuating faster. We hear this as a higher pitch (or higher note). When there are fewer fluctuations over a given length of time the pitch is lower. These fluctuations are referred to as 'cycles' and measured as 'cycles per second' or Hertz.
Most speakers are only capable of working well over a certain part of the audible frequency (for the purposes of this article we'll say this is 20hz to 20khz). Therefore most good installs will feature a variety of different types of speaker which are each optimised for a certain chunk of the audible spectrum.
Subwoofers are the biggest drivers and are designed to produce low frequency sounds. They tend to work best at frequencies between 30 and 100hz. The sheer enormity of their cones makes them unsuitable for the the fast changing notes that are typical of the higher frequencies and as such, they are used solely for bass.
Midrange speakers unsurprisingly produce a range of frequencies in the middle of the audible spectrum. For the sake of this example we'll say they produce their best quality of sounds between 100hz and 2khz.
Tweeters tend to be the smallest and lightest speakers and due to the limitations of their mass and size are totally incapable of producing sound frequencies much below 1khz.
To reproduce the full range of sounds produced by your CD head unit, you need to break the range into the specific chunks that each driver will handle best. This is usually the job of a crossover.
What we're going to do now, is make some assumptions...
We're going to assume that you've got a sub in the boot, powered by a bridged amp with a primitive low-pass crossover feature built in. Put simply this set-up gives you loud bass from your subwoofer from (for example) 30-100hz.
In the parcel shelf above the boot where the sub lives, you've either got a set of 6x9s or some aftermarket factory hole jobbies. (This advice will be applicable if you've still got the standard speakers - don't worry!) Your install is finished off with a set of uprated speakers which live in either the dash or the front door pockets. Both of these sets of speakers are driven by your 4x35watt rms head unit.
Chances are your 6x9s are 3-way (i.e they have 3 different speakers under their grilles) and your front replacements are 2 way (a full sized woofer and a tweeter on a stalk in the centre)
Here's how you can use 4 capacitors to dramatically improve the overall sound balance of your set-up.
Take the positive wire leading to your back left rear 'speaker' (6x9 or factory replacement) and cut right through it, about 5-10cms away from where it is joined to the speaker.
Now go to your amp, or dig around for the manual that came with your amp, and find out what frequency range the amp gives to your subwoofer. Lets say it cuts off at 100hz. If it is different to this consult the table at the bottom of this page!
Get your chosen capacitor from Maplins - in this imaginary case it would be a 400uf capacitor. Using a part of your terminal block, wire it in series on the positive leg of the capacitor. (ie the wire coming from the head unit going to the negative (shorter) leg of the capacitor and the remaining wire still connected to the speaker going to the positive leg). Repeat this process for the remaining back right speaker.
This will now prevent your rear speakers demanding huge voltages of your head unit, and will prevent them distorting as they 'flap around' trying to produce the bass notes that are best left to your sub. As a result of this mod, you'll notice that the head unit's illumination will cease to dim and the response from the speakers will be much less muddy. Now all you need to do is repeat this process with your...
What you need to bear in mind here is that the front speakers are A: Very small and B: Too close to you. Front speakers work best in a good set-up providing imagery or staging (in layman's terms they are there to complete the stereo effect - not provide huge amounts of bowel trembling bass). What we want to do here therefore is cut off any bass below (for example) 400hz. This stage can sometimes involve some trial and error, and a few different values of capacitors.
The process is exactly the same. Oncemore you are cutting the positive lead to the speaker, with the signal from the head unit entering the capacitor through its negative leg and leaving to go to the speaker down its positive leg.
Why this mod will work so well!
Although it may seem strange to spend money on expensive turbo-bass 6x9s and then cut how much bass they get, the logic is compelling. You have a high powered head unit, it is a good quality model and it is wired in properly - it is also more than capable of driving your speakers. Unless you have uprated the loom that leads to the head-unit you will still be using the factory power supply. This cable is perfectly good quality, but it was never intended to deliver the massive amperage needed by the head-unit when a driver starts to lose efficiency.
When you supply a full range signal to you 6x9's, they are going to try their best to reproduce the whole frequency range. To give subwoofer-like bass, demands increasingly larger voltages as the volume increases. As the speakers near their maximum power handling capabilities, they heat-up, become even less efficient and place even more demand on the head unit.
By preventing them ever having to try and reproduce the lowest notes, they are much more 'dynamically stable'. With this stability comes crisper delivery and better response. This means they can actually go louder, whilst sounding better, and these volumes can be complemented by your subwoofer, which is by its very nature a 'high power handling' device and can go LOUD.
Experimentation is the key here!. Don't be afraid to experiment with differing values of capacitors around the frequency mark you've got in mind. For instance if you followed the example and set your factory fit rears to handle everything over a mere 100hz, they'd still distort, You'd be better going for everything above 200hz or even 300hz!. The fact that there is (in theory) a small hole in the frequency range WILL GO UNNOTICED. The results will be far more impressive and even the standard factory speakers will sound great with your sub.
Once you have fitted this mod to you front speakers, tweak the fader settings on your head unit toward the rear - if the fronts sound louder than the rears, this set-up is wrong.
You want to hear the music next to you - not in front of you!
It is also worth bearing in mind that all aftermarket speakers that fit in factory holes (and 6x9s) have tweeters built into them - you may just have never heard them properly!. You will often find that you do not need additional tweeters if you cross-over your fronts properly (to reproduce only 500hz and above - for example)
Here's the values:
Disclaimer : Matey-Matey accept no responsibility for any of the information contained within this document or the accuracy thereof. It is intended as a helpful guide and is solely based on personal experience. The authors also wish to stress that the methods highlighted are centred around personal opinion and there may be other equally credible ways of performing this conversion.