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Contact: Zami Schwartzman, CEO
Thermoguide Ltd-Servobass


About us

Thermoguide Ltd is a private owned company  providing electronics engineering services to the high tech community.

During the last 15 years Thermoguide provided design and analysis services to Waves Audio - a leading software and hardware tools provider  to the professional sound engineering community.

We realized that the 30-60 Hz  bottom octave reproduction by small near field monitor speakers is a significant issue for sound engineers while they do  their music processing jobs.

Thermoguide took the challenge and developed  a new listening  concept with our ZRS-1 Desktop Near Field Subwoofers placed underneath near field Studio Monitor speakers

 at Home and  professional studios.

With these subwoofers , low cost monitors can be used with excellent  results because they do not have to reproduce bass notes and can blend perfectly with the subs. 

Subwoofers can only  be made  small by using some active control like Motion Feed Back (MFB). 

In 1968 Phillips published a paper describing the benefits of MFB for low frequency reproduction by small speakers.

Since then, many MFB related patents were issued, most of them were abandoned, terminated or expired after 20 years from their issue date and now can be used by the industry.

This mature  technology was implemented in our ZRS-1 design using on our custom  acceleration sensors.

By monitoring the woofer's Voice Coil dynamics and feeding back a signal to the amplifier's input,  the driver's non linearity and the sharp resonance that are inherent when using small sealed box can be rectified .




ZRS-1 20X20X20 cm Desktop Subwoofers undrneath Neumann KH-120 monitors


  • Motion Feedback (MFB) Subwoofers.

  • ​Desktop Near Field Subwoofers 

  • Shallow under the seat Car Subwoofers

  • In Wall shallow subwoofers

  • Small Subwoofers for smart speakers 

  • Motion Feed Back speaker technology consulting, technology licensing  and custom design.

  • Tiny, low mass, low noise acceleration sensors for MFB 

  • We modify drivers by adding acceleration sensors, ready to be used in MFB systems.

  • Fully assembled Plate Amplifiers with Motion Feedback control, cross over and voice coil thermal protection that can be matched with any low frequency MFB driver.

Near Field Desktop Subwoofers

Shallow under the seat MFB Car Subwoofer

Thermoguide Ltd developed a shallow "Under the seat " car subwoofer. We use our Motion Feed Back ( MFB ) enabled 8 Inch driver in a sealed 32x24x7 cm Aluminum enclosure to reproduce powerfull bass with flat frequency response of 30-300 Hz.
With our MFB technoliogy we get accurate low distortion bass notes with fast transient response that blend perfectly with the original car speakers.
Other manufacturers simply use equalizers  and heavy metal wight bonded on the driver's cone. That method yields sharp low frequency resonance and produce overhanged sustained and boomy bass.


ZRS-1 underneath Genelec 810A

MFB enabled Mini Subwoofer

Servobass  MFB powered  bass unit  with a 4" driver in a 12x12x12 cm sealed box ( 1 liter air volume ) These tiny subs have flat response from 50 up to 300 Hz with impressive SPL- Ideal for integration into Soundbars and Smart Speakers. 

ZRS-1 20X20X20 cm  Subwoofers for Home Studios, Gamers and   Base Guitar players  


ZRS-1 Desktop Subwoofers- Front &  Back View

Design Considerations

The vast majority of small near field monitors do not reproduce the low notes between 30 and 60 Hz because low frequency Inter modulation with mid range frequencies degrade clarity and imaging , therefore the low frequency range is sharply filtered out.

Most near field monitor speakers use ported enclosure to enhance bass response but the result is overhanged sustained bass transient response.

Adding a subwoofer with well designed crossover filter can eliminate these issues thus much louder SPL with low distortion can be attained.

However, most commercial subwoofers have large floor standing enclosures,  they  heavily excite room reflections and therefore poorly blend with near field monitors.

Thermoguide's small size ( 20X20X20 cm ) near field subwoofers are intended to be placed on desktop with low cost monitor speakers placed on top of them, thus perfect acoustic wave front alignment can be achieved at listener's ear height.

With desktop Near Field Subwoofers, room reflection effects are significantly mitigated. 

Our Desktop subwoofers blend well with any near field monitors by using 4th order Linkwitz Riley cross over filters at 100 or 200 Hz where the monitors are relatively flat with smooth phase shift.

By using MFB, tightly controlled bass transients can be reproduced without the over hanged  sustained response that are so common with ported enclosures.

Our Subwoofers can compliment any commercial Near Field studio monitors with the much needed bottom octave of 30-60 Hz, with superb clarity and stereo image.


ZRS-1 Subwoofers underneath  KH M402 monitors 
( Photo by courtesy of Yuval Silberstein,



ZRS-1 Plate Amplifier

MBF control circuit, high power class D amplifier and a powerful switching power supply  are combined on our plate amplifier, ready to be used with any driver with enclosure size from 20x20x20 cm to infinite baffel. Flat frequency response can be achieved by selecting only a few control parameters.

Voice coil temperature is monitored real time preventing voice coil overheating damage.



- Subwoofer Volume control.

- Selectable 100/200 Hz 4th order linkwitz–Riley crossover.

- Selectable full range 30-400 Hz response for external crossover.

- Bypass mode switch, in bypass mode the input signal is routed to the monitors while the subwoofer is muted.

- Mono (L+R bass signals summed) or full Stereo operation.

- Real time voice coil  thermal protection.

- Red light indicates near clipping/overdrive.  

- Power saving activated after 5 minutes without input signal.

Driver and enclosure selection

For our desktop subwoofer we use an off the shelf  6.5" driver with our custom designed sensor bonded on the voice coil former.

This driver was engaged  with the ZRS-1 plate amplifier in a 20X20X20 cm enclosure that is small enough to fit under many near field monitor speaker.



ZRS-1 Subwoofers underneath  Neumann KH120 monitors
 ( Photo by  Yuval Silberstein,


ZRS-1 Desktop  Subwoofer –Test Results

Speaker acoustics theory teach us that voice coil acceleration is directly proportional to the acoustic output of a woofer with sealed enclosure when in piston mode  bellow the breakup frequency.
In the following chart Green plot shows  the Voice coil acceleration without MFB, Red plot shows the response with MFB activated .
It is also known that very close microphone measurement on the acoustic axis of  the driver is closely proportional to free field measurements.
In the second chart , Red plot shows microphone output with 100 Hz crossover filter , Blue plot  with 200 Hz crossover, Green plot with full subwoofer bandwidth .

Frequency response
Green plot - with MFB disabled
Red plot - with MFB enabled


Cross Over response
Green plot - cross over bypassed
Red plot- cross over set to
 100 Hz
Blue plot- cross over set to 200 Hz 

THD & 3rd Harmonics Reduction by MFB 
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ZRS-1 Block diagram

ZRS-1 Block Diagram



Zami Schwartzman , CEO
Thermoguide Ltd
Ben Yehuda Eliezer 14
Israel 7630101


The articles in the links below describe the history of passive and active bass enhancement methods and how our  ZRS-1 20X20X20 cm desktop Subwoofers was designed to complement any small size studio monitors with powerful tightly controlled bass range  of 30-60 Hz by using Motion Feed Back dynamic control of the woofer's voice coil.

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