Why don’t more speakers have servos?

September 7, 2017
 by Paul McGowan

2 comments on “Why don’t more speakers have servos?”

  1. There are a coupe of things to understand that are important about servo woofers. Designing one is not for tyros who can get away designing a lot of other things they know little about.

    The servo feedback loop compares something related to the motion of the cone that generates an electrical signal to the amplifier input signal and determines the difference between them. But it is necessary to compare apples to apples. What does that mean? The input to a loudspeaker is a variable voltage amplitude as a function of time. A device for measuring that is called a strain gage. A strain gage can be as simple as a piston on a spring pushing a dial or potentiometer or as complex as the time it takes for pulses of laser light to bounce off the speaker and reach a photo transistor. The first time derivative of position versus time is velocity. The measuring instrument for this is called a velocimeter for obvious reasons. the first time derivative of velocity and the second time derivative of amplitude is acceleration. The instrument for measuring that is called an accelerometer. Often these are mounted on the speaker dustcap. So an accelerometer output is not directly useful to be compared to the input voltage to the speaker. It must undergo two stages of integration which is the inverse of derivative functions in calculus. This will give a signal comparable to amplitude versus time.

    There’s another problem. I posted recently that while turning the wheel of your car may give you a nearly immediate response in the direction of a car or possibly a small speedboat, it won’t result in the same fast response for the Queen Mary. You turn the wheel and sometime later you get a response. The same thing happens with speakers. There is a time delay between the electrical signal input and the corresponding motion of a speaker. It’s called the group delay. This is the result of inertial mass that has to be overcome. Remember that a body at rest tends to stay at rest and a body of motion tends to stay in motion, so moving the cone or changing its direction takes time. But if the accelerometer responds almost instantly and it will be correcting an error at the wrong time. So it would be better to delay the accelerometer signal long enough for the woofer cone to catch up. As I said, if the time error is too great, you have an oscillator steering the cone in the wrong direction at the wrong time. This is the same problem as negative feedback in amplifiers.

    When Infinity Servo Static and IRS were designed, these circuits had to be built by hand from discreet parts. Today it’s a different story. The equipment of choice is the PLC or Programmable Logic Controller. This is a black box that has digital and analog inputs and outputs and what it does to relate them is programmed using software. The computer inside the PLC is very powerful and can perform many functions. PLCs are the basis for pretty much all industrial controls today. I’ve tried my hand at designing with one and while I can do it, it’s very tedious work and I hate it. There are people who enjoy this kind of work and I leave it to them. BTW, this is the basis for robotic control and building automation systems.

    What can’t servo control do? It can’t prevent cone breakup or resonances. It can’t get the speaker to exceed its limits. It cannot limit spurious resonances in the enclosure. And a bunch of other stuff. At best it can get the voice coil to move exactly as the amplifier voltage varies in time within certain limits.

    The first thing to do when considering whether or not to incorporate servo feedback control for a woofer is to optimize the design without it. When FR can be flattened with an equalizer and harmonic distortion is around 1% you don’t need a servo system. Unfortunately looking at Bascom King’s design of the servo amplifier for the Infinity IRS shows the speaker had an open loop gain resonance frequency of 60 hz. This is not optimum design and could have been done better. Had it been lower with the same efficiency, a much smaller amplifier could have provided the same low frequency extension. In IRS only one of six woofers per channel has an accelerometer. It is assumed that correcting one will correct all of them as they are expected to all perform the same way. Do they? That depends on a lot of factors. The latest Genesis I think has two accelerometers per channel. So equivalent performance can be achieved with much less cost and complexity.

    1. hi paul,

      question for your video series:

      you mentioned wireless (and self powered) speakers and i agree, it is certainly in our future for both items. i see a great area for development in the wireless direction being better and better units for this. do YOU see it that way? if there is impetus for better wire (yes, i am a believer), it seems that a better blue tooth transmitter and of course, receiver could make an audible difference. what do you think? a good avenue for you and/or bill low?

      ..hifitommy

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