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Frank Kelly (Alesis): The Science Behind Gear For Sound

Frank Kelly Of Alesis By Paul White
Published April 1995

Alesis, once the champions of budget effects, drum machines, and more recently the ADAT digital recording system, have now diversified into mixing consoles, amplifiers, and studio monitors, with every sign that the list will continue to grow. Paul White spoke to Alesis' Frank Kelly about some of the new directions the company is taking.

Before joining Alesis, Frank Kelly worked in a number of radio stations in southern California, where he gained his recording and production experience. He then moved into design engineering, and in 1973, started with UREI. He stayed with that company for 17 years, which taught him a lot about power amplifiers and loudspeakers. Around five years ago, Frank left UREI, and, after a short break, got together with Keith Barr and Russell Palmer (the Chairman and President of Alesis respectively), and was invited to join Alesis as Director of Engineering.

"When I first came on board, Alesis were developing the RA100 amplifier, after which we went onto the D4 drum module and ADAT. ADAT at that time was coming out of our R&D labs, where Keith [Barr] had spent several years perfecting the technical details of what made ADAT work, but we still needed to turn it into a product we could actually manufacture.

"About a year later, I got the opportunity to do what I really wanted to do, which was to be in charge of a larger business unit, as opposed to staying in straight engineering. So with Keith and Russell's wisdom and blessing, we started the Sound Reinforcement division of the company. The plan was to expand Alesis by taking advantage of our existing sales force and distribution network, and with products that were slightly different from those Alesis had produced for the studio market."

But the first loudspeaker product to come from your new division was the Monitor One studio monitor, not a PA system.

"We did that because we have the worldwide distribution and name recognition in the studio business, so it was a logical step. We had first to develop the manufacturing and marketing structure for the loudspeaker business to follow, and the easiest way to start that was with much smaller speakers."

Designing The Monitor One

How did you go about designing the Monitor Ones?

"This is where I need to introduce Walter Dick. He is the principal acoustic designer of the Monitor One, and was the Engineering Manager at JBL during the '70s, their golden period as a transducer manufacturer. In the '80s, Walter went to Gauss, where he developed some of the highest powered transducers you can buy today — he's a real force in the industry, and as you can imagine, he knows a lot about loudspeakers!

"The idea for the Monitor One came from Alan Wald and Keith Barr; I recall a meeting where we were considering where to go next. The Alesis vision of the dream studio was incomplete at that time, and the nearfield monitor was one of the missing pieces. Many people in the company — Walter, for example, who has a good common‑sense approach to design — thought that although there were successful products in the nearfield market, many of them still appeared to be compromises in terms of what could be done with existing technology. We didn't need to reinvent loudspeakers, or come up with new cabinet material — all that was needed was appropriate execution of existing technology.

"All of the drivers in our products are proprietary, but make use of existing materials. They're designed either by us or to our specifications, and made exclusively for us. The only material that's actually new is the synthetic rubber coating on the Monitor One. We didn't really copy anybody — I've been asked a number of times what sound we were after, and to what extent we analysed the competition. Certainly, we looked at and listened to our competitors. We considered Yamaha's NS10s to be among the principal ones, because they were a popular product, and we wanted to get a handle on what their users were hearing. But I can say without reservation that we made no attempt to copy any of our competitors; in fact, our goal was to come up with something better, and we learned early on that there were various deficiencies in existing products, both in their performance and their sound."

Touching Bass

I've used the Monitor Ones, and they give the impression of accuracy, but with a more extended bass response than you might expect from such a compact monitor. How was this achieved?

"The first point to make is that the bottom end response of the Monitor One at medium listening levels is really just a function of transducer design and a properly‑sized box — there literally is no magic in that. I'm surprised that other manufacturers haven't been able to get to that level of performance, because it's quite possible if you take care of all the things that contribute to that performance properly. The over‑sized port in the Monitor One, which we call the Super Port, largely affects the transient response at high levels, and its design was based on work that Walter has done over the past 30 years. Quite often, when people do solutions for box designs, they pick a port diameter based on existing materials, either for cosmetic purposes or cost restraints. The larger the port, the more you have to deal with; it can be unsightly, and the ports also need to be made longer to compensate, because there's a direct trade‑off between the diameter and the length. You also have the physical limitation of a box that's only 12 to 15 inches deep — you can't have a port that's 20 inches long without having a bend inside the box or some other mechanical accommodation.

"Most manufacturers carry out a computer 'cookbook' calculation for their box dimensions and ports, and end up with a port that's around one or one and a half inches in diameter, because that's the way they've always done it. That makes the tube maybe three or four inches long, but with analysis, you see a couple of things that happen under heavy bass use. A great deal of air has to move in and out of that port — you can check that for yourself by playing a typical monitor loud and putting your hand near the port.

"In a nutshell, Walter has found that at high volumes, turbulence is developed within the port, and this reduces the port's effective diameter. This changes the box tuning, rather like sticking a sock in the port. It can also change the resonant frequency of the system, which in turn affects the load impedance the amplifier sees. At resonance, the loudspeaker load is largely resistive, but above and below resonance, it becomes reactive. We wanted to stabilise that, so we used a large‑diameter, folded port, giving a port size of around two and half inches in diameter by 13 inches in length. It isn't the prettiest thing in the world, so we put it on the back, but the net result is that the Monitor One performs better at higher levels both in terms of its bass performance, and in the way it handles fast attacks."

Enter Matica

How did you fire the imagination of the buying public with the new series of Matica power amplifiers? They're so often just boring black bricks...

"There were some corporate groans when I suggested that we should build a power amplifier, because they've come to be seen as a commodity item. I feel that the manufacturers have to take some of the blame for the 'Watts per dollar' war that's going on in the music stores today — it's perceived that there's no difference in the performance of amplifiers other than the amount of power that they put out. We don't agree with that argument, in the same way as we don't agree that all compressors, tape recorders and mixing desks are the same. We're in the music business because we like to listen, and we like the emotional experience of what good sound provides — so we believe we should pay attention to how our power amps sound, and how they perform."

On the face of it, a power amp takes a signal and just makes it bigger. Providing it doesn't distort the sound or run out of current, there should be no problem, so why are there so many imperfect designs?

"Under fairly undemanding conditions, at low to medium power levels, you could probably get away with using any number of power amplifiers, although there are amps out there from reputable manufacturers that don't sound as good as they should — you can hear the difference between power amplifiers if you know what you're listening for. Beyond that, in demanding applications, for example in live sound, where you may be on a hot stage, or out in the sun with a number of speaker loads of dubious impedance connected together — in these circumstances, the amplifier needs to be better designed. All the garage bands I know just add more speakers when they increase the size of their PA — they never seem to get rid of any of the old ones! These are real‑world situations, and we think it reasonable that anyone in live sound should be able to buy a power amp that not only doesn't cost a lot of money, but also has good, clean sound to go with it."

Engineering At Both Ends

I've looked inside your power amplifiers, and you seem to have approached the engineering from both ends; the unit is designed with a very effective‑looking cooling system, and yet the circuitry has been designed almost along hi‑fi lines.

"That's exactly right. Amplifiers have some interesting manufacturing‑related problems because of the size and weight of the materials used. The package must be designed both for ease of manufacture and for quality, and you have a number of forces that are working against you. I consider how the product is made to be just as important as the circuitry. Customer support must also not be overlooked, because it's a much bigger picture than if you were making something like a compressor.

"Unfortunately, there isn't much new in the way of linear amplifier circuits these days, and many people have either copied each other or designed products based on the semiconductor manufacturers' application notes. Even outright cases or piracy are not unknown. Without getting into the details of our circuit design, we took a different approach, and addressed the points that affect the amplifier's sound. The Matica amplifiers have a slew rate of 80 volts per microsecond, or 160 volts per microsecond in the mono mode, and to my knowledge, that probably makes them the fastest professional quality amplifiers out there. That directly affects the transient intermodulation distortion of the amplifier. You might still ask why we need such a wide audio bandwidth, when the human hearing range extends only up to 20kHz or below. A typical sound system comprises many components, each with its own bandwidth, and when these components are combined, you find that the effects of bandwidth limitation are cumulative. Add up all the odd dBs of roll‑off and you can end up with significant attenuation at 20kHz, and, perhaps more importantly, undesirable phase shifts occurring an octave or so below that.

"We employed bi‑polar transistors, because there's no doubt in my mind that we can make a better quality power amplifier in terms of sound, price and reliability using a bi‑polar design. This is a good opportunity to talk about the relationship we developed with Motorola, the semiconductor manufacturers. Until we started working with them, power amp manufacturers were forced into using what are really consumer audio transistors developed in Japan over the past 15 or 20 years. We felt a need to develop a new generation of high power, high 'safe operating area' transistors. The co‑designer of the Matica amps was actually a consultant to Motorola in the development of these parts, and this gave us a bit of an inside track. I wish I could say that it was an exclusive arrangement, but Motorola made a substantial investment in this as well, so they have now made these parts available to other people.

"Another thing you might notice is the A‑link multipin connector on the back of the amplifier. What we've done is to keep our options open for adding accessories such as active crossovers, rather than limiting ourselves to what we can physically put into the amplifier case. The architecture also allows you to implement remote control schemes."

These new products obviously bring your Dream Studio concept, and Dream PA for that matter, a little closer to fruition, but there are still obvious gaps to be filled if you're to present an entirely Alesis system. On the sound reinforcement side, it seems logical for you to move into live sound consoles and PA loudspeakers.

"Let's just say that we won't do things the way you might expect other manufacturers to do them. We like to look at the fundamental science behind the ways things are done, and that causes you to question some assumptions that other people may have made. We have a few new things cooking in our labs , though for obvious reasons, I can't tell you what they are right now."

Given the needs of the typical gigging musician, the aim must surely be to come up with smaller, more portable sound systems that still deliver the required level of performance?

"I think that's a good way to define our aims, yes. The bedrocks of sound reinforcement are amplifiers, loudspeakers, mixers and so on, so it's safe to assume that we'll be going in that direction — but with unique products."

Alesis Monitor Twos: You Read It Here First...

At the time of this interview, I'd just had a sneak preview of the prototype Monitor Two three‑way midfield monitor, and I have to admit that it sounded very impressive. It had the same sound character as the Monitor One, but with an obviously extended bass response. Given the projected price range (sub‑£1000 per pair), this should also turn out to be a popular design, and of course we'll bring you a full report just as soon as we can get our hands on a pair.

The Matica Amp: Staying Cool

The Matica amplifier is fan‑cooled, yet I believe you've devised a system to ensure that fan noise isn't a problem in studios.

"Many studios avoid using fan‑cooled amplifiers altogether, in favour of convection‑cooled models. Unfortunately, convection‑cooled amps are a little more expensive, and there's a practical limit as to how much heat you dissipate that way. Our CoolSink thermal management system starts with the amplifier circuit design, which is optimised to produce as little heat as possible, though that still leaves a significant amount to deal with.

"Our fan speed control circuit is unique in that the fan speed depends on the level of signal going through the amplifier. If the signal stops, the fan runs at a very low, idle speed, and because it's shock‑mounted, it's very quiet anyway. When music is played through the amplifier, the fan speed comes up immediately, but of course any fan noise is masked by the sound coming from the speakers.

"Heat is the enemy of power amplifiers, it's what causes power transistors to fail — but one of the main failure modes is thermal stress caused by constant temperature changes. That means it's not so serious that things get hot, but it's important to reduce thermal cycling to a minimum. Because our fan comes on as soon as a signal is present, the transistors are being cooled before they've had time to heat up, and this keeps them at a more even temperature than a traditional system, which relies on temperature sensors located on the heatsink. Our tests have shown that you can put these amplifiers in a studio control room and noise is not a problem."