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, I will take you 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, promised for next week.
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.
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The Infinity Servo Statik 18" subwoofer is shown with optional Coke can for reference.
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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 (more about that later).
Yet perhaps what blocked making feedback speakers practical has been two-fold. One was that amplifier feedback loops are picky. To keep the "network" stable, an integrated solution is needed for fighting chance of a reliable design. While essentially all subwoofers today have the amp built-in, same for soundbars, smart speakers, and smartphones - all these "systems" are optimized and all variables predetermined by the design team before going into the shipping carton. But 20 years ago, the receiver was matched to the speakers in the retail store, the wire selection and wire run was also random. The unpredictable nature of separate components was confounded by servo-control - and integrated systems were just not popular in days gone by.
One of the first feedback network loudspeaker designs was introduced in 1965 by LWE, designed by geophysical engineer Louis W. Erath. Employing a back-EMF-based negative feedback network, enabled over-sized woofers to behave as if they were in a larger enclosure. The speakers were picky about what they connected to and were typically paired with McIntosh amplifiers but also worked well with many of the Japanese receivers of the day.
In 1968, Infinity, while still a start-up, launched its Servo-Statik 1 and created quite a splash in the audiophile world. The 18"woofer itself was built by Cerwin-Vega, facing downward from a 2' cubic enclosure, it uniquely boasted a sensing coil coincident with the main voice coil. The signal induced into the sensing coil was fed back to the bass amplifier to provide the "servo" correction. While dramatic, it wasn't a completely stable design and was eventually discontinued in 1972.
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A Philips MFB speaker system, the MFB block diagram, and detail of the MFB woofer with sensor assembly are shown here
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Motional Feedback (MFB) was introduced by then audio giant Philips in the early 1970s. There was a full series consisting of five models, starting with a 7" woofer + dome tweeter. The loudspeakers had built-in amplifiers and a feedback sensor on the woofer. As with the Infinity Servo-Statik, the sensor measured the output signal of the woofer and compared it to the amplifier input signal.
In 1978, AudioPro, a Swedish startup (still very much alive today) introduced its ACE Bass negative output impedance scheme. In an audio system, the damping factor is the ratio of the impedance of the loudspeaker to the source impedance. The amplifier feedback contributes to the low-source impedance of the output stage. High feedback amplifiers have almost negligible source impedance - but imagine if you could have less than zero - then you could negate the voice coil characteristics and you could start with a clean slate. In this way, ACE Bass electronically synthesized the attributes of an "ideal speaker," or at least enabled the speaker designer to achieve virtual Thiele-Small parameters without some of the physical implications and compromises. The network is connected in series with the actual speaker and the voltage characteristics between the two are compared in real time. The difference is sent back to the amplifier as an inverse corrective voltage and fed into the input of the amplifiers negative feedback output stage.
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The Velodyne DD-18 driver is shown with an accelerometer. (Image courtesy audioholics.com)
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David Hall founded Velodyne in 1983 as an audio company specializing in servo-controlled transducers. Back then, I had just moved to California and Velodyne was my consulting company's first client. Hall wanted to develop a full-range 8" servo speaker, essentially a next generation of the Philips design. My contribution was to suggest we pivot to a servo-subwoofer and use it only in its piston range (up to a few hundred Hertz). I wanted to avoid getting bogged down with wideband feedback and the problematic aspects of cone breakup. At my previous company, we had developed a low distortion 18" woofer. We used a 3" voice coil but undercut the pole piece down to 2.5". For the Velodyne ULD-18, we bonded a piezo sensor to the voice coil collar. And many other "secret" refinements were developed at Velodyne, which enabled "high gain feedback" - about 20 dB, which then buys almost that much distortion reduction. We found that stable operation was best achieved by limiting our feedback loop to a fraction of the otherwise usable bandwidth. But there were many tweaks and tricks used to get a bit more performance and stability.
In 1989, I began work with Yamaha in Japan on the Yamaha Active Servo Technology (YST). The efforts involved many similarities but also some differences with MFB and ACE Bass. Developed to expand low-frequency reproduction, it involved two technologies. One was the Air Woofer (port) and the other was Negative Impedance Drive, which drives the speaker to cancel the impedance of the voice coil so the apparent impedance of the speaker is zero. The former was performed by a speaker, and the latter by an amplifier. Thirty years later, Active Servo Technology continues to be a cornerstone of Yamaha's audio products.
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This Yamaha YST-SW012 8" subwoofer is already using Advanced YST II and QD-Bass Subwoofer technologies.
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Next week, we will take a close look at the subVo servo-pickup bend-sensor, a unique screen-printed sensor that changes in resistance when bent. It is durable, has a long stable life, and is inexpensive to produce in mass quantities. SubVo's KlaraT is a proprietary DSP algorithm that controls the feedback loop, offers over-excursion protection, and eliminates end-of-line calibration routines required with other feedback techniques. The Klara-T self-calibration also compensates for degradation of the loudspeaker (surround/spider wear and tear and magnet sensitivity losses) over time and any change in the loudspeaker's environment.
