THE CROSSOVER – PART 2
After last issue’s discussions on the role of the capacitor, Damon Greenwood now looks at the humble inductor; what it is, what it does and its actual influence on the speaker crossover.
In this second episode we look at the humble inductor and its less-than-humble role in a speaker’s crossover.
So from the last issue I expect that you are all complete experts on capacitors; and for those of you that have experimented with some on your system then give yourself a gold star. Believe it or not in most crossovers there are only ever three basic combinations (albeit in wildly varying configurations) made up of resistors, capacitors, and inductors (coils) on a circuit board – that is generally all you need.
Resistors are fairly self explanatory so this issue we get into inductors and how they work on their own in a circuit. We’ll leave it for the next issue to look into how capacitors and inductors interact in a circuit to give different characteristics as opposed to when simply on their own. And then nally we get into the good stuff; calculations for some simple crossovers and their applications in a real world car environment; giving you some starting points for your own systems and creations.
INDUCED & CONFUSED?
Ok, so where to start – what exactly is an inductor? Well for our purposes there are two types; iron core and air core and they are both extremely complicated and intricate bits of gear. Now this is crucial campers; you may need to read this more than once – the former is a coil of wire around a symmetric piece of iron and the latter is a coil of wire. Does your head in right? Sounds simple but they only work by utilising electromagnetism so some of the physics can need a little head-scratching.
At this point it would be easy to get bogged down in the ne points of iron versus air core coils and spend the next two pages waxing esoteric about which one is best. Suffice it to say that for the purposes of this article iron core coils are cheaper but can saturate at high power and therefore cause distortion at high levels and air core coils can take high power but cost more. Having mastered the vast complexity of a coil of wire in a circuit let’s move on to what they actually do in a circuit; and yes there will be another plumbing analogy to make things easier. In a way capacitors and inductors are similar and yet very different – both store electricity; one stores it as an electric charge (capacitor) and one stores it as an electromagnetic charge (coil). ese different storage methods are what create the differences between the two and are the reasons why capacitors allow high frequencies to pass and inductors allow low frequencies to pass.
So what are the tiny electrons (aka music) doing in a coil? Well when everything is in equilibrium all is ne; electrons move through the coil and a magnetic eld is set up around the wire in an equal and opposite direction to the induced current simply because that is the way magnetism works (opposites attract). But the electromagnetic eld acts like a virtual shock absorber; resisting a current increase or decrease throughout the whole circuit. Seriously this bit is a
little tricky; because of this characteristic in a DC circuit electrons will ow until they reach equilibrium state with the power source and the coil. So imagine the coil as a heavy water wheel in a plumbing circuit – when the water is turned on it takes a little while for the water wheel to reach its optimum speed; and any increase or decrease in water ow will have a belated effect on the wheel as it will take time to adjust.
But there’s more! at scenario was for a DC (direct current) circuit like your battery – what the hell goes on in an AC (alternating current) circuit, like music going to speakers? Well imagine swapping the terminals really, really fast – what is going to happen? Depending on how big the wheel is some water will get through so long as it is at a frequency that is lower than how quickly the wheel can change direction. For example, you have a water wheel that can change direction 10 times a minute then if you change the water direction nine times a minute then all of the water will move backwards and forwards throughout the system. However if you change the water direction 11 times a minute then only a percentage of all of the water will get through.
Wasn’t that hard was it?! When you correlate the water wheel analogy back to electronics the frequencies obviously get a lot faster but the percentage of by-pass I outlined still applies; for both inductors and capacitors. is is what gives you what is termed the crossover slope which is measured by dB per octave. I’ll be discussing these pointers in the next instalment.
Well that’s all for now punters – I need a nap! Until the next issue, that is, when we will be tying all of the information together and giving you the ammunition you need to start making some informed judgments on tweaking your own speaker systems.
A fully assembled crossover with a clear and efficient layout featuring inductors (top of the crossover left & right), capacitors and white ceramic resistors.
A typical inductor neatly wound and wrapped in cable ties to protect it and to allow convenient placement and coupling with circuit boards within the crossover.