593aac@houxa.UUCP (S.JOHNSON) (10/17/85)
I read with interest a recent posting about crossover design circa 1952. Bob, you're certainly on the right track by using film caps instead of electrolytics, but unless you're very lucky, the design right out of the book is not going to work very well. The problem is that the book usually makes some simplifying assumptions: the drivers present constant, purely resistive loads, and that the acoustic frequency response of the drivers is perfect. In addition, most experts in the field consider that 18db per octave is the gentlest crossover slope to use with practical drivers (12 db networks put a nasty notch at the crossover freq. unless you reverse the tweeter's phase, and then you get some rather strange phase effects!) (6db networks are theoretically perfect for ideal, physically coincident drivers but don't work so well with most (all?) real drivers). So in the past, most designers have either just used the book values, or have empirically come up with other designs. Neither is acceptable from my view. What is the answer??? 1) Make each driver look like a constant, resistive load to the crossover. By the way, assuming that the impedance is 8 ohms because the manufacturer says so just won't cut it. It's almost certainly 6.2 or 5.1 or 9.7, etc. For the most part the Zobel network will do this! It involves a capacitor and resistor across the driver in the case of woofers to compensate for the voice coil inductance ( no attempt to compensate for the fundamental resonance) and a slightly more complex network for mids and tweeters to compensate for the motional impedance at resonance as well as the voice coil inductance. Note that ferrofluid often damps the fundamental resonance enough to simplify matching problems for mids and tweets. 2) Don't assume that the frequency response of the driver is perfectly flat. Tweeters do tend to rolloff at 1 Khz or 2 Khz, etc. Add the acoustic rolloff to the electrical rolloff of the crossover. For example if the tweeter is rolling off at 12 db per octave at 2000, a 6db electrical network at 2000 will yield an acoustic crossover of 18 db per octave. The other approach is to use a sharp rolloff rate (18 or 24db) per octave far from where the driver rolls off and ignore the driver's rolloff. Using these techniques, book designs can be made to work. And very well indeed! One last point, don't wind your crossover coils with small gauge wire. The resistance, in many cases, will degrade performance appreciably by throwing off your design. I use 18 guage in small coils, and 10 guage in my largest coils. Film caps are better than non-polar electrolytics, never try to make your own non-polars by wiring standard electrolytics back to back. Steve Johnson (The above opinions are definitely my own)
rdp@teddy.UUCP (10/18/85)
In article <808@houxa.UUCP> 593aac@houxa.UUCP (S.JOHNSON) writes: >I read with interest a recent posting about crossover design circa 1952. >Bob, you're certainly on the right track by using film caps instead of >electrolytics, but unless you're very lucky, the design right out of the >book is not going to work very well. > In most cases, they don't work at all! > >2) Don't assume that the frequency response of the driver is perfectly flat. >Tweeters do tend to rolloff at 1 Khz or 2 Khz, etc. Add the acoustic >rolloff to the electrical rolloff of the crossover. For example if the >tweeter is rolling off at 12 db per octave at 2000, a 6db electrical >network at 2000 will yield an acoustic crossover of 18 db per octave. >The other approach is to use a sharp rolloff rate (18 or 24db) per octave >far from where the driver rolls off and ignore the driver's rolloff. > One of the most ignored characteristics of wide band drivers (woofer-midranges) is the fact that the on-axis response oftyen shows a tendency to rise with rising frequency, due to the fact that the radiation angle is reduced, but show an integrated power response that drops with rising frequency. A network can compensate for this, but the choice of response is a compromise. Also (my favorite bitch), cones made from paper have the disconcerting tendency to change these characteristics with changing humidity! >One last point, don't wind your crossover coils with small gauge wire. >The resistance, in many cases, will degrade performance appreciably >by throwing off your design. I use 18 guage in small coils, and 10 >guage in my largest coils. Film caps are better than non-polar >electrolytics, never try to make your own non-polars by wiring >standard electrolytics back to back. I just spent the last ten minutes searching my office for the notebook holding about 100 AES and JAS reprints, and, to my anger, discovered that someone has walked off with it. DAMN! But, I will try to relate this as best I can from memory. In the late '70s an article appeared in the Audio Engineering Society about inductors for use in crossovers. In it the author, A. N. Thiele (yes, that Thiele) described a novel and simple method for designing and producing coils for crossover use. The name of the article is "AIr Core Inductors for Audio Use), or something very much like it. Without the article in hand, it is difficult to summarize things, but the method described allows accurate production of coils where all parameters (inductance, resistance, etc.) are controllable as needed by the user. When I find the b*****d who took my articles, I will post more information. Dick Pierce
rdp@teddy.UUCP (10/18/85)
In article <1452@teddy.UUCP> rdp@teddy.UUCP (Richard D. Pierce) writes: > >I just spent the last ten minutes searching my office for the notebook >holding about 100 AES and JAS reprints, and, to my anger, discovered >that someone has walked off with it. DAMN! Well, sure enough, I'm getting to the bottom of this heinous crime! I just discovered that the culprit did, indeed, steal my book, but he had the chutzpah to sneak an EXACT DUPLICATE into my briefcase! The nerve of some people today!! He even went so far as to cover it with MY fingerprints! >But, I will try to relate this as best I can from memory. In the late '70s >an article appeared in the Audio Engineering Society about inductors for >use in crossovers. In it the author, A. N. Thiele (yes, that Thiele) >described a novel and simple method for designing and producing coils >for crossover use. The name of the article is "AIr Core Inductors for >Audio Use), or something very much like it. Without the article in hand, >it is difficult to summarize things, but the method described allows >accurate production of coils where all parameters (inductance, resistance, >etc.) are controllable as needed by the user. When I find the b*****d >who took my articles, I will post more information. > The exact title of the article is: "Air-Cored Inductors for Audio", initially printed in Proceedings of the I.R.E.E (Australia), vol. 36, pp. 329-333 (Oct. 1975), later reprinted in the Journal of the Audio Engineering Society, vol. 24, no. 5, pp. 374-378 (Jul. 1976), with a followup in the Journal of the Audio Engineering Society, vol. 24, no. 10, pp. 830-832 (Dec. 1976). Dick Pierce
mohler@drune.UUCP (MohlerDS) (10/20/85)
I agree whole heartedly with your posting, however I thought I might add one quick comment. If the driver whoose impedance you are trying to linearize, (with a shunt) is mounted in the enclosure it will be used in while testing the effectiveness of that shunt. You will get a more accurate compensation effect. David S. Mohler AT&T - ISL @ Denver drune!mohler
rdp@teddy.UUCP (10/21/85)
In article <63@drune.UUCP> mohler@drune.UUCP (MohlerDS) writes: > >I agree whole heartedly with your posting, however I thought I might add >one quick comment. If the driver whoose impedance you are trying to linearize, >(with a shunt) is mounted in the enclosure it will be used in while testing the >effectiveness of that shunt. You will get a more accurate compensation effect. > > David S. Mohler > AT&T - ISL @ Denver > drune!mohler
rdp@teddy.UUCP (10/21/85)
In article <63@drune.UUCP> mohler@drune.UUCP (MohlerDS) writes: > >I agree whole heartedly with your posting, however I thought I might add >one quick comment. If the driver whoose impedance you are trying to linearize, >(with a shunt) is mounted in the enclosure it will be used in while testing the >effectiveness of that shunt. You will get a more accurate compensation effect. > No, this is not so. The shunt capacitor/resistor method of linearizing impedance takes care of the driver's voice coil inductance. This is pretty much unaffected by mouniting/enclosure, as it is well above the region affected by the enclosure. For example, one notices a (nearly) montonic rise in impedance in a typical 8" woofer above 500-1000Hz due to the voice coil inductance, where the enclosure affects the impedance curve due to the resonance, which is usually restricted to below 100 Hz. Dick Pierce