audioXpress readers are already familiar with the interesting concept of the small desktop Motion Feedback (MFB) subwoofers introduced in the article "Motion Feedback Desktop Subwoofers for Nearfield Studio Monitors" originally published in audioXpress, September 2022, and now available online.
Zami Schwartzman perfected the MFB approach used by Servobass, and originally developed by its electronics engineering services company Thermoguide. For over 15 years, Thermoguide provided design and analysis services to the professional sound engineering community, during which Zami Schwartzman realized that the inability for small near field monitors to extend the reproduction to critical low frequency notes was a significant issue, leading him to develop the Desktop Near Field Subwoofers. These compact subwoofers intended to be used in home and project studios are able to complement the most popular desktop studio monitors, providing an extended reference response in nearfield monitoring.
Standard low cost, compact monitors can be used with excellent results when complemented with Servobass desktop subwoofers, which are significantly smaller than traditional subwoofers thanks to the fact that they use MFB - a concept that has been explored since 1968, when Philips published a paper describing the benefits of MFB for low frequency reproduction. Since then, many MFB related patents were issued, most of them were abandoned, terminated or expired after 20 years from their issue date and can now be used by the industry.
Zami Schwartzman perfected a mature MFB technology based on its own custom designed acceleration sensor. The sensor monitors the woofer's voice coil dynamics and feeds back a signal to the amplifier's input thus rectifying the driver's non linearity and the sharp resonance created when using small sealed enclosure.
"The frequency response of a woofer in a small sealed enclosure is resonant. This is a simple physical fact leading to sustained, over hanged, "slow" bass response. No equalizer can fix that. However, adding an accelerometer bonded to that woofer's voice coil with Motion Feedback can suppress that resonance to yield "fast", tightly controlled bass transients," Schwartzman explains.
Other benefits of the technology result in extended and flat frequency response, with reduced distortion, while using a very small enclosure. The plots below were taken with a 6.5" long throw driver in a sealed 20x20x20 cm enclosure (4 liters internal air volume!!). The red plot is without Motion Feedback and the Green plot is with Motion Feedback.
Servobass’s MFB technology was successfully implemented in the latest Desktop Subwoofers, which will be demonstrated at CES 2023, adding powerful base notes down to 25Hz, positioned directly underneath small studio monitors, which allows a direct extension for nearfield applications, which also helps mitigate room reflection effects. According to Schwartzman, a perfect extended wavefront response can be attained at the crossover frequency, yielding superb clarity and stereo image.
These days Servobass is also focused on implementing the same MFB technology for under the seat subwoofers for the car and home theater applications, including extending the response of sounbars. Servobass is presenting this concept for the first time at CES, looking for cooperation with speaker designers and manufacturers, and bringing the technology also to the consumer market.
Part of the demonstrations include the Servobass ZRS-1 plate amplifier, a high power class D, MBF control circuit, and a powerful switching power supply, combined on a single plate, ready to be used with any driver with enclosure sizes from 20x20x20 cm to infinite baffle. Flat frequency response can be achieved by selecting only a few control parameters, and voice coil temperature is monitored in real time, preventing voice coil overheating damage.
The Servobass ZRS-1 plate amplifier offers volume control, selectable 100/200Hz, 4th order Linkwitz–Riley crossover, selectable full range 30-400Hz response for external crossover, and a bypass mode switch that routes the signal to the monitors while the subwoofer is muted. Other possibilities include Mono (L+R bass signals summed) or full Stereo operation, real time voice coil thermal protection, and light indicator for near clipping warning. A power saving mode is activated after 5 minutes without input signal.
For the Servobass desktop subwoofers, which use a 6.5" driver with the company's custom designed sensor bonded on the voice coil former, the ZRS-1 plate amplifier is directly incorporated in the 20X20X20 cm enclosure.
Servobass will be showcasing its solutions at the ALTI Association Hospitality Suite at CES 2023, Lido 3101B - Level 3 of the Venetian Expo and Convention Center, and at the Menlo Scientific suite also at the Venetian. Visitors can coordinate demonstrations and meetings directly at the ALTI Association suite reception. Requests can be directed to Zami Schwartzman, to this email.
www.servobass.com
