Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/3/84; site teddy.UUCP Path: utzoo!decvax!ucbvax!ucdavis!lll-crg!seismo!harvard!talcott!panda!teddy!rdp From: rdp@teddy.UUCP Newsgroups: net.audio Subject: subwoofers and xovers Message-ID: <1395@teddy.UUCP> Date: Mon, 7-Oct-85 14:48:19 EDT Article-I.D.: teddy.1395 Posted: Mon Oct 7 14:48:19 1985 Date-Received: Wed, 9-Oct-85 11:46:13 EDT Reply-To: rdp@teddy.UUCP (Richard D. Pierce) Distribution: na Organization: GenRad, Inc., Concord, Mass. Lines: 64 [] > I would like to build a sub-woofer for my stereo system. I know how > to make crossover networks for regular speakers, but I can`t figure > out how to make one work for a sub-woofer. The catch is that I want > bass from both right and left channels to driver the sub-woofer. > I can use caps to eliminate the low frequencies sent to my > regular speakers and chokes to eliminate the high frequencies for > the sub-woofer, but how do I combine the left and right low > frequency signals so they can be sent to one speaker? > > Thanks, > Walt Barnes > ...!ihuxn!wwb > First of all, several points. Making a passive crossover network for subwoofer type frequencies (50Hz - 250Hz) is a real bitch. The component values needed are astronomical. We need (say for a 200Hz cutoff frequency) capacitances on the order of 200 uFd and inductances in the neighborhood of 6 mH. While sitting at home and winding such and inductor is possible, the cost of reasonable to high quality caps of that value suitable for audio use is comparable to the ransom paid for kidnapped first-borns. Be that as it may, let's presume that the basic network is designed (no mean feat in itself). There are several approaches that cab be taken The idea of simply summing the outputs of two amplifiers after two networks can be disasterous. Here we have essentially two voltage sources trying to drive each other, and, occasionally, the load. The fact that much information recorded at these frequencies and lower is essentially monaural doesn't help much. One alternative, which, unfortunately, severely restricts your choice of low frequency drivers, is the dual-voice-coil drivers made by several manufacturers (JBL and EV come to mind). Most of these drivers are perfectly wretched, however. Another approach, one which I have quite succesfully used in the past many times, is to have two drivers in a single cabinet of twice the volume of a single cabinet. This has the disadvantage of taking up as much room as two separate enclosures, but ensures total electrical isolation between channels. The method I used was to construct a transmission-line enclosure designed for drivers with the equivalent radiating area of a 12-inch driver, and use two high-quality 8 inch drivers instead. This way, the enclosure could be used as either a common channel subwoofer or a single channel woofer, simply by changing faceplates and drivers. (For the self-appointed TL illuminate out there, this approach works just fine, If you disagree, then you have the wrong idea about what TL's are capable of doing. If you want to know why this is, just ask, but don't flame me, this is an area I spent considerable time, energy, and money researching). In any case, when designing this whole system, make sure you know completely what the various components of the system are really doing, as that severely constrains the crossover configuration. Note especially that your designing a network close to system resonances (a couple of octaves is close to me), and that you have to deal with the attendant load impedance variations. Good luck Dick Pierce