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.

InCar Entertainment  - - CONTENTS -

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.


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.

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