Yes, you’re right. I’ve said I’d be doing crossovers and I haven’t. But here we are, back in the frame. “Everything comes to him who waits.” Incontinence, overdrafts, road accidents, mummyfication. Even bird shit if we wait long enough in the same place. I could go on. And on….and….
Alright, the first question, which can be levelled at most things, is “Why do I need one?” Were I trying to sell you one, I might come up with something like:-
“Wow! I mean, so cool. That colour! It’s just you! It’s this week, I mean, well, what do I mean? But you gotta have one. Well, I mean, it’s the end, man. Plus its got the right name; y’know ‘CROSSOVER’ how cool is that? PPhhheeewww!”
Wow! I mean, its enough to send me running to my creche. Well, it just won’t do as an explanation. Here’s a more factual, and yet still emotive reason. Without one of these little wigetty crossover things in it, your nice hifi speaker will become an incendiary device. Or in extreme cases, a projectile. So they are (whatever colour) necessary.
The above schematic diagram (sounds good for start) is the simplest you’re going to find, and it consists entirely of the capacitor C1. So, we might ask, why do I want to spend a second mortgage on a crossover if I can buy a capacitor for 50 pence? This is where I do my ‘Old Git’ impression and say something horribly wise like “When the Sun is in the West, the rice pudding sinks fast.” Or some such.
The capacitor of a suitable size (this depends on a number of factors but it’s often between 6uF and 10uF), will allow high frequencies through to the horn, and block bass frequencies. So what else do we need? Well, nothing really, but the problem is that the bass speaker is still trying to deal with the high frequencies (because there’s nothing to stop them) and so it soaks the high frequency power away from the horn, but doesn’t actually do anything. Those frequencies are too high for the bass speaker to respond to. So a lot of the high frequencies are just dissipated in the bass speaker as heat. That’s where this simple crossover falls down. It’s just not efficient. It also has a slow roll-off per octave, which sounds vaguely erotic, but isn’t. I’ve led a sheltered life.
This is the next simplest crossover. We’ve added a coil, L1,(an inductor if you want to be posh) in the line to the bass speaker, and this does two things in this application. L is the letter that is usually used to signify an inductor, or inductance. It’s just a coil of wire, on some sort of former. It does the opposite to the capacitor. The capacitor passes high frequencies and blocks low, and the inductance passes low frequencies and blocks high. So, by putting L1 in line with the bass speaker, we block the frequencies that are meant for the horn, from getting to the bass speaker and being soaked away. This, then, is a much more efficient arrangement. The L1 does another thing as well as that. It builds up the resistance (‘impedance’ if you want to be even more posh) of the signal path to the bass speaker, and therefore pushes more signal up to the horn. A double whammy, then. The way the frequencies are dished out (high to horn, low to bass) form a ‘crossover point’, and its about here that things get paranoid. To enjoy doing graphs I think is the sign of a sick mind. Please have a look at the graph below that I drew. (YAAHHHGGGGHHH HEE<HEE<hee)
And just to show how far over the wall I’ve got, I’m going to EXPLAIN IT!!!! Woooo!
The bottom (horizontal or X axis) of the graph is marked off in frequencies. Notice that it doesn’t go 10,20,30,40 etc. It goes 50,100,200,400. So each marker is twice the previous one. This is a logarithmic scale, and is the only one that makes sense in this context, because the markings, when done like that, are in octaves. The blog previous to this one hopefully sorted out that issue. Down the left (Vertical or Y axis) of the graph we have markings in dB. It stands for DECIBELS and it’s a comparison thing and that’s also a logarithmic thing. Leaving that aside with the rest of the washing-up, the important part for us, is that 6dB is a half. So what?
Now we’ve got to look at the line marked ‘woofer’; it starts at 0dB and that means the full output of the amp. It’s a form of passive low pass filter, and theres a fair bit about filters here and there on the blogs. So the woofer gets all the output until it gets to the rolloff point, where it takes a sharp dip. When it dips past -6 dB, thats half the output level. -12 is a quarter; -18 an eighth. Each one of these points corresponds to a frequency on the X axis and the rolloff point we’ve selected happens at 400hZ. Beyond that it drops off (rolls off) at the rate of 6dB per octave, because it dips by 6dB for every doubling of the frequency, or octave. So we’re not going to hear much from the woofer beyond 1.6 kHz or so. However, the tweeter, which has the high pass filter, rolls off at 6.4 kHz and isn’t doing much at all until it gets to around -12 dB, which is the crossover point. This point happens where the woofer and the tweeter have the same power directed to each of them, and beyond that point the tweeter takes more and more of the power and the woofer, less.
That’s about it, except to make the point that these are idealised in this graph, and what really happens is a lot less clear cut. But the ideas are sound; it’s just that ideals are great but we’re stuck with reality. Never quite been able to hack that stuff.
TIME FOR TEA.