Protect & D-FEND: A Cure For The Common Blown Loudspeaker

A new attenuation and protection device for passive loudspeakers

Most people working in professional audio have two things in common.

One is our passion for the business, and the other is that most of us have blown up a loudspeaker (or two or three…) at some point in our career.

The former is great, the latter not so much, and it also begs a couple of questions.

If we’re professionals, surely we know how to properly interface amplifiers and loudspeakers, right? And why hasn’t “someone” invented “something” to help solve the problem?

The first question might best be answered with the stock favorite of weasely politicians everywhere: “no comment.”

But there’s now a potentially positive answer to the second, in the form of D-FEND, a new attenuation and protection device for passive loudspeakers that made its debut at the 2011 InfoComm show in Orlando.

Developed by Evenstar, a subsidiary of SLS Audio, D-FEND uses multiple on-board digital filters to monitor current and voltage from an amplifier with the purpose of protecting individual drivers (and crossover components) by attenuating the incoming signal.

The tiny unit can be mounted inside loudspeaker enclosures and needs no external power source because it operates from standard speaker-level signal.

Placed prior to passive filtration, the programmable digital circuitry allows the user customize attack, release, and threshold settings for each of its filters. The goal is to deliver a safeguard while also maximizing loudspeaker performance.

Typical passive loudspeaker protection circuitry has been a mixed bag, relying on switches, resisters and even light bulbs.

These approaches work by soaking up extra power from the amplifier, diverting it away from the drivers. It’s fine for protecting a component within limited parameters, but is usually limited to lower power full-range systems or high frequency components.

Scaling up this approach can incur significant cost, not to mention the added size and weight of the bigger components needed to absorb all the higher power.

And while the standard limiters and system controllers that are placed inline between a console and the amp racks do a great job at controlling the inputs to the amps, they don’t necessarily protect a loudspeaker if an amplifier’s gain is mistakenly raised, or the cabinet is connected to the wrong amplifier. 

Instead of absorbing the extra power from an amplifier, D-FEND is designed to “fool” the amplifier into thinking a higher impedance load is connected, with the amplifier then putting out less power. No large heat sinks are required, and very little heat is produced from the circuitry. 


As a result, D-FEND is capable of protecting high-power woofers and subwoofers from over power conditions as well as mechanical damage due to over-excursion. It can also protect against an amplifier delivering DC current.

The unit can be customized to protect individual components, multiple components in a box, or even easily retrofitted to existing designs and inventory. A computer GUI is under development to make for simple programming of the device.

Joel Butler, director of electronics engineering at SLS, who is heading up the development of D-FEND, notes that the company will be integrating this technology into their own products.

In addition, SLS has an exclusive license agreement with Eminence Speaker, which will also utilize the technology in its products and license it to other manufacturers, an effort under the direction of Josh Martin, technology sales manager at Eminence.

Read More
Do You Speak Geek? The Unique Language Of Audio Analysis

This is one of those “simple yet clever” pieces of technology that comes along from time to time to make a significant difference.

Craig Leerman is senior consulting editor for Live Sound International and has worked in professional audio for more than 25 years.