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!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!petrus!bellcore!decvax!ucbvax!ucdavis!lll-crg!seismo!harvard!talcott!panda!teddy!rdp From: rdp@teddy.UUCP Newsgroups: net.audio Subject: Re: on crossovers - active vs. passive Message-ID: <1460@teddy.UUCP> Date: Mon, 21-Oct-85 10:34:48 EDT Article-I.D.: teddy.1460 Posted: Mon Oct 21 10:34:48 1985 Date-Received: Thu, 24-Oct-85 00:15:25 EDT References: <1259@tekgvs.UUCP> Reply-To: rdp@teddy.UUCP (Richard D. Pierce) Organization: GenRad, Inc., Concord, Mass. Lines: 117 In article <1259@tekgvs.UUCP> keithe@tekgvs.UUCP (Keith Ericson) writes: > The discussion of passive crossover design and the problems >involved in modelling the driver(s) is getting lots of net.audio space >lately. But, friends, it's *old technology* and not really current in >todays audio market-place, especially for those capable - as I believe >lots of net.audio reader are - of some relatively simple electronic >construction. (Right now you're saying 'what IS he talking about! - >bear with me...) > OK - the problem in a conventional (i.e., passive-crossover) >system is that you've got this monstrous power amplifier, with a >fraction of an ohm output impedance, trying to control an >electro-mechanical motor. The motor - the speaker - has a "mind of it's >own" as to what it wants to do with the signal thrown at it and it >will, if left alone, proceed to do whatever it thinks best with the >signal. One way you can start to "leave it alone" is to interpose a >crossover network between the amplifier and the driver. This prevents >the amplifier from damping out the extraneous motion that the driver >wants to make. It's like this: > Put a long-duration pulse to the speaker (long duration being >several times longer than 1 over the resonant frequency). The cone >moves out, then moves back. But when it moved out, it probably >over-shot the "correct" location and oscillated into place, finally >getting to where it was supposed to be. It will only overshoot the position if it is both electrically (as your are alluding to) and mechanically underdamped. It is interesting to note that total system Q does not change dramatically with source impedance (note Small, et al, JAES). The chances of this "overshoot" occuring are usually only found at the system resonance, see further... > Thes oscillations generate a >"back emf" (remember that from your power engineering classes?) that >tries to drive the amplifier. But if the amplifier (1) has a low >output impedance and (2) is connected pretty close (i.e., through >a very low impedance connection) then the back-emf will have to generate >a lot of current to exist. Instead, the back emf "gives up" and the >speaker doesn't oscillate around the resting location (yeah, I know >that's really simplified, but it gets the point across). With the >passive crossover there's lots of circuitry that can allow the current >from the back-emf to flow and not be damped out by the low output >impedance of the power amplifier. I's like trying to drive your car >with a spring between your hands and the steering wheel. You can steer, >but bumps will throw the car all over the place 'cause you can't correct >for them with the springs in the way. This is not only simplified, it's wrong. Look again at the situation of a woofer at resonance. Sure, it may want to overshoot, and it generates it's back EMF but there is NOT a lot of circuitry in the way. There might be a big mother inductor or two, but at these frequencies, the presented impedance of these inductors is damn near zero, else the woofer is putting out an attenuated signal. At higher frequencies, yes, the inductors now have a significant impedance, but this is unimportant for several reasons. 1), there is usually a shunt capacitor in the network, and it's impednace is now approaching 0, 2) the output of the woofer now is attenuated, so it's anomolies are becoming less and less important (that's the idea of a crossover, yes?) and 3) at these frequencies, the driver is being comtrolled by it's readiation chacteristics, not by it's second-oorder mechanical characteristics. The same holds true for tweeters. The big killer to the "crossovers are evil because they have high impedance and therefore stifle back-emf" argument is th fact that a typical "8 ohm" driver has a pure and simple resistor in series with it on the order of 7 to 7.5 ohms. This is the DC resistance of the voice coil. This resistance is always there. So it don't make diddly-skwat whether there is a .01 or a 1 ohm resistance due to the network (or lack of network). It is an interesting experiment to watch what hppens to the impedance curve at resonance of a woofer when it's hooked up to an amplifier with a source impedance of .01 ohms versus one with a source impedance of, say, 10 ohms. The impedance curve changes are damn near unmeasurable, indicating little, if any change to the second-order electro-mechanical characteristics of the driver. > The best way around this is to use a power amplifier for each >speaker, and put active crossovers between the pre-amp and the power >amplifiers. The advantages include the following: > 1) The crossovers are in a well controlled impedance. (You > could even use a passive implementation - if you really > wanted to for some reason.) ????? > 2) The crossovers work with low-level signals instead of > having to be capable of handling tens (hundreds?) of > watts of power. This is not a problem, given reasonable passive component selection. I have yet to see a crossover inductor saturate. Although I have seen capacitors explode! (Maybe the subject of another "Audio anecdote of the ") > 3) The speakers are driven directly by the (low output- > impedance) power amplifier. As seen above, the same is true of properly designed passive networks. > 4) Active crossovers completely do away with inductors, the > hardest part of any passive, high-level crossover to > fabricate. I have found it a lot easier to make an inductor of audio-use size and accuracy than a capacitor. I mean, I don't even know how to make a capacitor of the sizes I need :-) > > In the "old days" when good power amplifiers were prohibitively >expensive it was important to reduce their number. That just isn't the >case now days. Today almost anyone can afford a multiple power amplifier >system. Except that to *buy* the active crossovers is prohibitively >expensive. But they are quite simple to build, and there is really no >"magic" involved. Just check into National Semiconductor's "Audio/Radio >Handbook" publication (circa 1980). (But use the Signetics 5534 op-amp.) > Well this has gone on long enuff. If you haven't hit the 'n' key >by now you must !really! be interested in this stuff... > >keith > Add to the cost of such active crossovers, another disadvantage is that they can add quit measurable noise and (possibly) distortion to the signal. Dick Pierce