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Motional feedback in speakers
On Sunday, November 10, 2019 at 9:55:11 AM UTC-5, Trevor Wilson wrote:
On 9/11/2019 6:50 am, Peter Wieck wrote: Ummmmmm..... A speaker is a linear motor with a magnet, and a commutator (voice coil). Just as in a PM Motor, when current is applied, the motor spins. DC motors spin according to the polarity of the power applied. Speakers move in or out depending on the polarity of the current applied. And, PM motors do, also, have a fixed resistance across the commutator just like a voice coil. Now, when current stops being applied, the motor generates current - acts as a generator as it spins down. If it is unloaded, that current goes nowhere and does not add additional resistance to the motor spinning than normal bearing friction. However, if the motor is loaded, there will be additional friction. Similarly the (conventional) speaker. Try it some time with a sensitive VOM. The bigger the driver, the more easily this is observed. Just a few taps on the speaker cone will show you. All and at the same time, DF is only one (1) single factor in how amplifiers interact with speakers. And, today in 2019, the issues that drove speaker design in the era after field-coil speakers were dominant up until the development of acoustic suspension are not particularly relevant as much evolution is taken for granted (and usually is granted). However, as one who spends as much time with electronics from the 1930s as from the the 1970s and up, I see all sorts of variations on how to control large speaker overshoot, sagging, and similar problems. A 15" Zenith speaker driven by a single-ended 6F6 is an entirely different animal than a 12" Long-throw woofer from an AR3a. **And again: It is the output impedance that is important. The so-called 'damping factor' of an amplifier has (almost) nothing to do with damping a speaker. No, it is not the output impedance that's important it is the total loop resistance of amplifier-speaker system that determines damping. And to attempt to drive the point home, let me repeat these two very crucial points in an attempt to emphasize their importance: 1. It is THE TOTAL LOOP RESISTANCE in the amplifier-speaker SYSTEM that determines damping. And that, at the very least includes the DC resistance of the voice coil, the speaker leads and the amplifier. 2. And it is the RESISTIVE components of each of these that are the mechanism that determines damping. You and Peter keep saying "impedance", when, to be technically accurate, it is NOT the impedance, but ONLY the restive component of the impedance that is [part of the total loop resistance that determines damping (in the electrical domain). Reactive components to the impedance MAY change the frequency response, as you suggest below, but they DO NOT change damping. The reason why speakers sound different on high output impedance amplifiers (like most valve amps) is due to the frequency response variations, caused by the interaction of the output impedance of the amplifier and the impedance variations over the audible range of the speaker system. I'd not necessarily go so far as saying this is THE reason they sound different, but it certainly is one of the significant contributors to real differences in acoustic output. Locate a speaker that exhibits an almost resistive load and check for yourself. Maggies are a pretty good start. As is almost anything that uses ribbon HF drivers (obviously, LF variations will depend on what kind of bass driver/s is used). Maggies exhibit a highly resistive load from top to bottom: As do many KEF systems that incorporate complex conjugate networks do compensate for reactive impedance variations, resulting in impedances that are almost uniformly 4 ohms across the audio band (you give one example). Dick Pierce |
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