Eminence/Evenstar D-fend Loudspeaker Protection Circuitry
Comparing pro audio systems is a lot like trying out new cigars. You can read about the “specs,” where it was made, what leaves and spices were used, etc., but you really have to try them before you know if you like them. For a cigar, I’ve got a soft spot for Rocky Patel 1961s. For Pro Audio, it’s for ribbon tweeters.
Ribbon tweeters can be very sensitive and expensive, so you typically find them in “high-end” enclosures. Hopefully, those who can afford these cabinets know how to properly set up processing equipment to ensure the best sound quality and performance within the parameters of their enclosures. Unfortunately, this is not always the case. Just because an individual can afford a high-end cabinet doesn’t mean they can set it up properly. You also have to consider that even the best sound engineers can’t control what everybody is doing on stage. I have personal experience with someone unplugging a microphone cable from a live mic and blowing a compression driver.
That’s really where D-fend begins— a guy in a room with some ribbon tweeters. This guy’s name is Joel Butler. He is the director of electronics and engineering for Evenstar, the technology division of SLS Audio. SLS has always been on the cutting edge of technology; by way of example, they use ribbon tweeters in most of their enclosures. Joel decided to create a “fail-proof” protection circuit for their ribbon tweeters. If you want to see the typical method for protecting tweeters, pop off the input cup on the back of most two-way cabinets and look for a light bulb or resistor bank.
As with all power-dissipative techniques, these methods work, but have some “baggage.” If the light bulb blows, the show is over until it is replaced. It may blow and then shunt the voltage through resistor banks, but with a noticeable difference in the SPL—even to the untrained ear. Think about the unplugged, live microphone... the bulb doesn’t have time to heat up and blow because the voltage spike is so fast. Bulbs and resistor banks just aren’t very musical and can’t always react quickly enough to protect from surges of power.
These issues are what Joel wanted to overcome. He desired a protection circuit fast enough to catch every possible over-voltage incident and something that was musical—unnoticed, protection utilizing attack and release times like a compressor/limiter. He needed something versatile that could be used in any enclosure. Oh yeah, and also something that could handle thousands of watts! This was a lofty goal considering protection circuitry had been around for a while and no one had been able to overcome the baggage. But, you know the old saying, “If you always do what you’ve always done, you’ll always get what you’ve always got.”
Keep in mind that the initial goal was to protect sensitive ribbon tweeters. Joel used that framework to begin experimenting. He had many trials and errors before discovering ways to get extremely fast reaction time. He found ways to make the protection work with music, yet still allow for dynamics. He discovered a way to monitor the audio signal to protect across frequency bands (different settings for different drivers in the same enclosure using one protection circuit). Once this method of protection was established, he realized it could be used for protecting large woofers at low frequencies from excess voltage.
How did he do it? Well, we can’t show all our cards now, but we can tell you this much: By creating a technique that is not power-dissipative, the baggage of a bulb or resistor bank protection can be overcome. D-fend, the official name of this protection circuit, is completely solid-state and works from the audio signal. D-fend doesn’t ask the amp for power because it dynamically (musically) raises the impedance that the amplifier sees. When the operator pushes the amp past the capabilities of the driver, the D-fend unit keeps the amp from sending the excess power. Instead of an 8-ohm load, the amp may be seeing an 80-ohm load. It tells the amp, “I don’t want anymore.” It doesn’t clamp the sound, but maintains it at a safe level. In turn, the amp runs cooler and easier, because it isn’t pulling the current from the wall.
SLS Audio realized that this product had greater potential than only being used in its product. In order to market this to the masses, SLS contacted Eminence Speaker to consider a partnership in the sales and marketing of D-fend technology. Eminence manufactures loudspeakers for most everyone in the audio industry, so we saw the potential for this product as well. Eminence and SLS have always had a great relationship, and it seemed like a natural fit for both.
Providing all that this technology offers at an affordable cost is difficult. We have been creative in our efforts to keep the cost around a 1:1 ratio with the driver it protects. We’ve found a good balance of quality and cost with the help of some industry friends.
“Revolutionary” is a label put on every new product entering the market. I steer away from that term as often as possible. I will say, there’s nothing else out there like D-fend: a protection circuit that is a speakerlevel compressor/limiter. Set it and forget it, knowing that your driver will live out its full life cycle…even those expensive ribbon tweeters.
Josh Martin serves Eminence Speaker as technology sales manager.