Motional Feedback Loudspeakers

Motional Feedback Loudspeakers  545

Figure 19.2: Motional feedback of cone position using capacitive sensing: ServoSpeaker method.

Ever since I first heard of MFB, it struck me that laser interferometry would be the ideal method; no moving parts added and no extra wires attached to the cone. Laser diodes can’t be expensive, since there’s one in every CD player—how hard can it be? However, I know little of such matters; one person consulted told me that the laser required to get the necessary coherence length would be pretty big—a metre or more long for a frequency stabilised helium-neon laser, which I was told was about the minimum hardware I could get away with. That doesn’t sound too practical; I would be glad to hear if others agree on the possibilities of laser interferometry.

Feedback of Velocity

At first it looks as if this might be easy; a secondary sensing coil is added to the cone primary drive coil and will produce a voltage proportional to its velocity in the magnetic field. The main problem with this approach is transformer coupling between the main voice coil and the sensing coil, which will completely swamp the desired velocity signal. A solution to this which was perhaps clumsy but has a certain no-nonsense appeal was that adopted by Panasonic MF-800 in the early 1960s (see Figure 19.3). [10]Asecond voice coil and magnet were mounted at the front of the drive unit, much reducing transformer action. On reading the user manual it is claimed that MFB reduces “distortion”, but it is not clear if that refers to frequency response errors or non-linear distortion or both. The main aim is clearly to extend the LF response of the loudspeaker, and switches for controlling this were provided, labelled “Frequency” and “Damping”.

There seems no doubt that this method worked as advertised, though according to the manual some control settings gave bass extension combined with a +6 dB resonance peak, which probably did not sound great. This could be suppressed by increasing the damping setting, but this much reduced the bass extension. There are very few references to this system anywhere, which suggests it was not a marketing success. No pictures of the actual drive units have so far been found.

A more subtle and apparently superior approach is to use the same voice coil for both driving the cone and measuring its velocity. The movement of the coil in the magnetic field generates a voltage signal

546  Motional Feedback Loudspeakers

Figure 19.3: Motional feedback of velocity using a separate sense coil and magnet: Panasonic MF-800.

Vvel proportional to the velocity, but this has to be somehow separated out from the voltage driving the speaker. Figure 19.4 shows one way; a balancing impedance R1, L1 is put in series with the drive unit; typical values are shown. The transformer reads the voltage across this load and subtracts it from the main feedback signal so that the velocity signal is isolated and can be added to the feedback as required. Note that the balancing impedance is made equal to the blocked impedance of the drive unit, which is its impedance when the voice coil is fixed so that it cannot move.

This method has a disadvantage which I don’t think I have ever seen explicitly pointed out; as shown it wastes half of the amplifier power that you trustingly feed into it, dissipating it in the balancing impedance. To make this approach practical the balancing impedance must be scaled so it dissipates a small fraction of the power in the drive unit, by raising the transformer ratio or otherwise modifying the feedback arrangements. See [4] for other versions of this circuit.

Motional Feedback Loudspeakers  547

Figure 19.4: Motional feedback of velocity from the voice coil with R1, L1 and the transformer cancelling the drive voltage.

A transformer is not a welcome part in a negative feedback loop; it may have its own linearity problems at low frequencies, and HF phase-shifts are likely to cut down the stability margins.

Figure 19.5 shows a more up-to-date version, where the required subtraction is performed by opamp

A1. The balancing impedance has been scaled down by a factor of seven to demonstrate the process, but the power dissipated in R4 will still be substantial, and further scaling would be desirable. The scaling is embodied in the values of R1, R2, and R3.

This method can also be looked at as making the output impedance of the amplifier negative—it is equal and opposite to the blocked impedance of the drive unit. This should not be confused with current-drive of loudspeakers, where the output impedance is made very high.

This all seems very promising—you can use standard drive units and do the MFB with a few cheap electronic parts. But there are two big problems remaining with voice coil velocity feedback: First, the resistance of a drive unit voice coil changes significantly when it gets hot; a 40% increase is entirely possible with a 100°C temperature rise. This plays mayhem with the impedance balancing, and it would be all too easy to produce a design that became unstable as it heated up, banging the amplifier output against one of the rails. In practice this means that only a small amount of negative feedback can be applied safely.

Second, the uniformity of the magnetic field in which the voice coil moves is imperfect, and this reduces the accuracy of the velocity feedback signal.