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Themoguide’s ZRS-1 Desktop Subwoofers

Design utilises motion feedback technology

Thermoguide Motion Feedback Subwoofers

Founded by electronics engineer Zami Schwartzman, Thermoguide have been providing design and analysis services to the professional audio industry for over 16 years. During the last three years, they have been focusing their attention on a project which aims to bring improved low-frequency monitoring solutions to engineers working with compact monitors in small rooms.

Relying on motion feedback technology, the company have developed a subwoofer system which they say is capable of accurately delivering the 30 to 60 Hz content missing in many small monitors, whilst avoiding intermodulation between low- and mid-frequency content. Perhaps more impressively, the resulting units are compact enough to be placed on a desktop or stand directly underneath a pair of nearfield monitors.

Project Inspiration

The company say they decided to pursue the project due to the prevalence of compact nearfield monitors in home and small professional studios meaning that many engineers are forced to work with significant compromises in their system’s low-end response.

A lot of smaller nearfield models are designed to roll off sharply below the 50 to 60 Hz mark in order to avoid issues that affect their overall clarity and stereo imaging; others employ ported designs to extend their low-end response, but this can often create resonances and result in sustained output at low frequencies, causing them to sound ‘boomy’ or ‘boxy’.

Another common practice amongst studio users is to add a floor-standing subwoofer, but that can also come with its own set of pitfalls: careful placement is required to avoid room mode excitation — something which can be difficult or impractical in small rooms — and they may not always ‘blend’ well with the main monitors.

Motion Feedback

The goal of the project was to create a system that could enhance the low-frequency performance of a compact nearfield system, but in a small enough form factor that it could be used seamlessly alongside existing monitors. After realising that the usual bass enhancement approaches would not result in a design that they felt would be compact enough, Themoguide decided to develop a system based on MFB, or motion feedback.

The technology was first used commercially by Philips in their RH544 and RH545 speakers in the 1970s, and the company had also published an article detailing its use in 1968, although the concept itself was developed much earlier. Thanks to the use of MFB, the speakers were able to achieve impressively low frequency responses — the RH545 was specified down to 20Hz using a 12-inch driver in a 50 litre sealed cabinet.

Speaker designs that utilise MFB use an accelerometer to monitor the movement of the driver, the information from which is returned to the amplifier via a control circuit. It is then compared with the original input and used to generate an error-correction signal that can used to modulate the amplifier’s output, reducing low-frequency distortion and thermal compression.

ZRS-1 Desktop Subwoofers

After settling on accelerometer-based MFB technology as the basis of their compact subwoofer design, Themoguide set about designing a control circuit and building prototype units. The resulting design employed fourth-order Linkwitz-Riley crossover filters, a switching power supply, six-inch driver (modified to incorporate an accelerometer) and a Class-D amplifier, all of which were integrated into an MDF enclosure measuring just 200 x 200 x 200 mm.

Six prototype versions were built, and the company say that every design exhibited a flat response from 30 to 400Hz without the need for any adjustments, as well as performing favourably in listening tests paired with a range of different nearfield monitor models.

The final design features an amplifier plate which fits comfortably in the 200mm cabinet, housing a 1kW (peak) Class-D amplifier, a 500W power supply, the MFB circuitry, a 24 dB/oct Linquitz Riley crossover, real-time voice coil thermal protection, and XLR input and output connectors. Its frequency range is specified as 30 to 300 Hz, and an adjustable crossover frequency of 100 to 200 Hz makes it possible to pass on a high-passed signal to connected nearfields.

The company say that in addition to their current design, the amplifiers can also be used alongside drivers of any size in a sealed enclosure.

To find out more about Thermoguide’s work, along with more details about their ZRS-1 Desktop Subwoofers, take a look at the link below.

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