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Active Versus Passive Crossover Systems



Anyone whom thought that slapping a home brewed filter together to act as a loudpseaker crossover was a simple task should have his head read. Loudspeaker design is a science with very little fiction.

Recently I got caught up with building two loudspeaker cabinets for some odd drivers I had lying around in the garage. Having built the cabinets I was then caught up in the age old debacle as to whether to go active or passive crossover. This was actually a no contest, I had three amplifiers lying around as well, a Proton 60×60, an Akai 25×25 and a no-name brand home theater 60×60.  Now just to make the filters. Initially I thought it a great idea to buy a 3x active crossover but sadly this time of the year most of the shops don’t stock or can’t get stock. Now to the drawing board – why go active?

Passive filters are by their very nature quite sophisticated beasts if designed properly. Not only that, most commercial crossovers for a multiple driver setup is quite expensive and even if one had to go that route one needs to take into consideration the amplifier stability used to drive the speakers. I had no doubt the Proton would do the job but didn’t like the idea of losing power into the passives. A three way passive setup is a very sophisticated setup – I’m not talking about the economy crossovers one gets in some low end speaker systems, but one in which power loss would not be extravagant and the likelihood of serious mismatching at certain critical frequencies damaging the amplifier.

Years back a colleague of mine blew his amplifier when one of his wharfedales had a bad connection through the spider (smaller corrugated disk used for centring voice coil at base of speaker). The reason for this was while trying to find out where the poor connection was he left the ampifier running – the one channel blew a fuse and for the rest of the day he sat scratching his head as to why the speaker had stopped playing although he thought he found the fault. What had happened was his home made cheap and nasty crossover possibly had gone into resonance and I surmise the sudden drop in impedance overloaded the amplifier. None of us knew about this phenomena in those days – ignorance was on top of the agenda and who thought that a passive crossover could do damage? I also know of someone that complained about his amplifier getting hot although the sound was slightly muffled – ended up being an electrolytic bridging the voice coil and not in series – well it certainly cut out the high notes to the bass driver. Fortunately the amplifier lived to tell the tale.

Active Crossover Networks

Elliott Sound Products has an excellent article on activer versus passive – http://sound.westhost.com/biamp-vs-passive.htm – for anyone venturing into crossover networks this is a must read. Also I am of the firm belief that if one had to be patient it may be cheaper to purchase a good second hand amplifier than purchasing a high end passive filter.

I have built my own crossovers and when testing the speaker system afetrwards found no difference between with or without the crossover.  The main cause of this was inductance value – winding your own coils is often a tedious task, especially if you don’t have the proper test equipment. In my case it was always having too little inductance or too much loss. 10 AWG copper wire is very expensive and a passive designed for upwards of 100W will work out cheaper if purchased off the shelf then the time and costing to make it.

My own experience is that passives are OK with a low powered (25W continuous)  two way speaker system with small tweeters but as soon as one gets into the high powered arena (upwards of 100W continuous) and have more than two drivers per cabinet the party starts. Running a disco in my youth I used a Fane 12″ driver plus two horns per box. The recommendation was to put resistors in series with the horns. Let me tell you for disco use it sounded quite good but in reality it sounded horrendous. The boxes were a leftover project and made from pine. Advice: never use pine. Most enthusiasts here use superwood or MDF (medium density fiberboard) but I have used chip board with some success – bracing is however very very very important but what I don’t like with chipboard is that one slight knock leaves dents or chips all over the place.  Superwood is way more expensive but the results are worth it.

Also be cautious about being too liberal about the size of your cabinet – superwood is damned heavy. Don’t forget either that big boxes don’t mean more bass. I read regularly about audiophiles not liking Bose, this may be an American thing – personally they are out of my price range but they do sound great. A friend had KEF speakers as well as BOSE, the BOSE beats the KEF hands down. In actual fact if one had to read into this you’d find that the BOSE just sounded better on the amplifier. (I cannot remember the brand). I found this out when my brother, whom has the same set of KEF didn’t like the sound and exchanged them for an old studio set of Tannoys. Now of course we move on three years and find the same set of KEFs in a friends apartment – with a NAD amplifier. It sounded magnificent. The only catch was here that the NAD was the same my brother used. The reality was that the apartment furnishings were different and the speakers were not on the floor (read carpet/rug) but mounted on granite slabs.

Yes, I have deviated off the main topic of discussion but thought it relevant to the outcome – speaker systems always sound different in different surroundings. Welcome to accoustics, a science all on it’s own.

To close off this article I was delighted to find this schematic and write-up on the web of a 3x active crossover –  http://www.siliconchip.com.au/cms/A_30278/article.html – designed by Mick Gergos.

Now just to find out what shops are open. Who said Xmas holidays are the best?

Repeat of article posted on 28th December 2011.  The article ended up being corrupt in a switchover of platforms.

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