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Servo-Control in Mainstream Audio - From Motional Feedback to the Bend-Sensor

Imagine a new class of speaker diaphragm position sensors that enable audio engineers to reduce distortion and extend bass response from studio monitors, soundbars, smart speakers, and even smartphone microspeakers. With the precision of laser displacement instrumentation, this sensor is located between a stationary component of the speaker and an appropriate moving part to provide an error correction reference signal back to the amplifier. Well, that sensor has already been developed.

In this article, Mike Klasco takes us on a brief trip down the evolution of feedback speakers, before previewing SubVo's Bend-Sensor and Klara-T servo-control approach to feedback error correction in loudspeakers.

Most speaker engineers expect the amplifiers we use with our speakers to have a fraction of 1% distortion. On the other hand, our loudspeakers often have an order of magnitude more distortion than the amplifiers with which they are used, especially at their bottom-end. As to why speakers have more distortion, we could discuss the nonlinearities of the motor, the spider, and the surround... But instead of fixing each less-then-perfect aspect to achieve a linear transfer function, we could take a more elegant route - as is done with audio amplifiers to null out the nonlinearities of the transistors, caps, and so forth.

The reason amplifiers have a fraction of a percent distortion, even the less inspired designs, is due to negative feedback correction. Negative feedback occurs when the output of a system is fed back to reduce the deviation of the output from the input. By returning an out-of-phase output signal back to the input, errors can significantly cancel.

The "Holy Grail" is to get all the components/factors within the feedback loop. A tube amp's "Achilles heel" was that the output transformer was not in the loop. I remember when Marantz (Sid Smith) managed to get the output transformer into the feedback loop and that was a big thing. Actually, the feedback signal was taken not from the secondary tied to the loudspeaker outputs, but from an extra pair of secondary windings. The output transformer contributed more distortion than the rest of the amplifier and today we all live better for the lack of interstage and output transformers.

But the biggest remaining challenge is with including the speaker in the feedback loop. Anyone who thinks feedback correction (servo-control) is a no-brainer to instantly fix poor speaker performance is in for more than a few surprises. Servo control definitely offers performance improvements not readily achieved by tweaking physical construction, but there are many new design rules with which to wrestle. Nevertheless, finding a robust stable sensor and the right positioning location can be game-changing.

SubVo's Loudspeaker Sensor & klaraT Processing

We need a low-cost, precise, and stable sensor that will not degrade or interfere with the physical operation of the speaker. This requirement is just now reaching the audio industry with the launch of the patented bend-sensor - a unique screen-printed sensor that changes in resistance when bent. Recently adapted and optimized for speakers, founders Joe Harris and John Kelley first introduced the subVo technology at the 2020 CES, last January.

SubVo's klaraT is the speaker compensation algorithm that controls the feedback loop, enabling each speaker to self-calibrate. All speakers have slightly different specification characteristics as they roll off the production line and with self-calibration this disparity is eliminated, enabling higher production yield.

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