Pat Quilter: Sound Reinforcement and Amp Guru, Part Two

Pat Quilter: Sound Reinforcement and Amp Guru, Part Two

Written by John Seetoo

In Part One (Issue 118) Pat Quilter and John Seetoo talked about the beginnings of Quilter Audio Labs and QSC Audio, the history of recorded sound, non-amplified vs. amplified live acoustic music and more. The interview continues here.

John Seetoo: What do you see as the differences between pro and home audio? There are obvious ones like the room sizes you have to deal with and SPL requirements – but let’s say if QSC decided to enter the home audio arena, how would you approach it?

Pat Quilter: The home market is a lot different than pro audio. We always like to use the metaphor that pro audio is like an 18-wheeler and home audio is like a pickup truck. They can both haul stuff and most peoples’ needs can be met with the pickup truck. But if you’re going to carry heavy tonnage on the highway, you’re going to need an 18-wheeler. You need a heavy duty rig that can not only carry a heavy load but do so reliably day in and day out. That’s the main difference. Pro audio gear is designed to perform at a high average output level, reliably, for long periods of time; way more than what you would ever need in the home.

Pat Quilter and some of the company's creations.

At the same time, in order to get that performance, especially with loudspeakers, you have to resort to things like horn-loaded transducers that don’t necessarily sound as good as the best direct radiators, but can project a large amount of sound over great distances. With the right design, you can get them to sound pretty good, which is where I think QSC’s acoustic design team has done very well. We try to aim for that quality of “natural presence” which is at the heart of any good hi-fi system. But at the end of the day we have to make it go loud without breaking up or breaking down. This imposes some design decisions that you would never really use in the home.

For example, my own personal listening system uses a couple of large-format air motion transformers that were made by an obscure little company in Redondo Beach quite a few years back. If you’re familiar with the air motion transformer concept, it’s basically a way to take a ribbon tweeter [with a relatively large surface area], and by folding it up into kind of an accordion, you can compress the frontal area down to, in my case, about five inches tall and two inches wide. Yet it has the surface area and power handling of a 5-inch by 20-inch Mylar film.

So you get that wonderfully liquid, non-diaphragmatic reproduction of an almost infinitesimally lightweight ribbon transducer, but coming out of a smaller area, so it has a little more dispersion. At higher frequencies it can get fairly loud but not nearly as loud as a large compression driver, but at “polite” levels, I think the speakers sound great. I can listen to a performance and not really be distracted by the feeling that it’s coming through something. But it’s not very practical for a pro audio application.

In fact, some of QSC’s smaller 5-inch 2-way speakers with dome tweeters are wonderful-sounding speakers for any normal utility purpose. My brother uses a pair of them in his Volkswagen bus. I could enjoy listening to music through QSC’s larger powered PA speakers and not feel like I was grinding my teeth, but it wouldn’t be as good as an optimized home audio system designed for maximum liquidity at a living room level.


A selection of legacy Quilter and QSC products.

JS: Since many Copper readers are gear aficionados, do you mind giving some details on your home audio setup?

PQ: Although I have a CD player hooked up, keep in mind that my rig is kind of optimized for playing the vintage 78s (and some vinyl) that I collect. In my humble opinion, power amp technology was pretty much perfected in the 1980s. It wasn’t easy, but (by then), any good designer could design a clean power amp whose distortion products were orders of magnitude lower than anything else in the chain and certainly below anything at the speaker end of the chain, or even the microphone. But the intervening electronics have become essentially transparent. I know arguments rage on about that, but realistically, the various transducers in the chain are much more difficult to get right than ordinary, competent electronics. So my amplification is just straightforward solid-state stuff.

My homebuilt speakers are interesting. They are self-powered 2-way systems with a 9-inch woofer and a 50-watt Avlar air motion transformer that will go down to 400 Hz at polite living room levels. I use a low-order 400 Hz crossover to a woofer that’s a quarter-wavelength away from the high-frequency transducers, so there is minimal off-axis cancellation or lobing, and thus they practically form a point source. The woofers use ELF (Extended Low Frequency) processing in a sealed box to go down to 25 Hz, again only practical at living room levels. The tweeters have a definite sweet spot, but not as severe as a large-area planar driver. The minimal off-axis lobing makes the room reflections benign, and in the sweet spot there is a distinct feeling of “connection,” almost like wearing headphones.

I used early QSC DSP-3 digital signal processors to obtain a detailed flat response, but more importantly (in my opinion) the waterfall plot of these drivers is superb – no lingering resonances; every frequency just stops the instant it leaves the surface, so that the waterfall plot looks like a uniform cliff. Tricky sounds like violin glissandos that often elicit a telltale “chirp” from even good dome tweeters, thus drawing attention to the speaker, pass fluidly through these drivers, so the sound is very relaxed and distraction-free. I have heard other speakers which match the overall frequency response but aren’t quite as transparent. But again, it’s a system that’s optimized for nice living room listening levels.

QSC SDP-3 Digital Signal Processor module.

I made myself a scratch-remover preamp optimized for mono 78 records that can detect clicks and pops and cut them out on the fly. Today, it’s being done digitally more miraculously than my old analog scheme, but mine works pretty well, partly by taking advantage of a stereo needle playing a mono record. The noise is on one side-wall or the other, so you have some additional information to help separate noise from program material. I use a scope to dial in the particular cartridge so the noise impulse doesn’t ring and can be “snipped out” without leaving a noticeable gap.

Of course, anyone who collects old records knows you need a set of different-sized styli to play them correctly. So I get the right size needle and dial in the scratch remover threshold, and actually get pretty creditable performance, at least from the electrically-recorded 78s after 1925. Some of the earlier acoustic recordings can be considerably improved with heavy EQ but they always sound more artificial due to the musicians having to crowd around a very directional recording horn. Once they were able to put a decent condenser mic in a studio with enough room to allow the band to play at a comfortable distance and just go at it, you do get a very nice, natural sound quality out of these records, although technically, the fidelity is still somewhat limited.

JS: Are there any home audio systems or products that influenced your design work?

PQ: Keep in mind my early career was in electric guitar amps – that was all about making sound, not reproducing sound. Then we went through two decades when QSC was strictly an analog electronics company. Only in the last few decades have we gotten into loudspeakers and digital technology. When we started back into speakers, we hired a designer, Paul Hales, from a hi-fi background, but with a sound knowledge of acoustic science. He’s got his own company now, Theory Audio Design, in Southern California. He brought a very high standard of listening and a meticulous attitude to the work, which are qualities that QSC has built upon ever since. I’m happy to say, on Paul’s behalf, that he used acoustics and science in his work to try to get the best results. That got our program off to a good start.

As time went by, especially when we got into powered loudspeakers, we brought more and more people into the team. So it’s a much larger part of the organization now, with, golly, dozens of engineers handling various parts of the job. We’re working on multiple speaker projects at any given time.

My personal experience with sound systems has generally consisted of raising my awareness of what might be possible, as opposed to learning something that we could actually use. For example, one time at a high-end show I went and visited a room that was demonstrating a plasma tweeter.

To explain briefly, all normal transducers move a surface, which then moves the air to create sound. But any intervening surface, whether it’s a cone, a diaphragm or even a sheet of Mylar, inevitably imposes some limitations on actually getting the air to move as perfectly as you would want. The plasma tweeter ionizes a stream of gas, giving it an electric charge that can be acted upon directly by an electric field, and literally moves the air itself with no intervening mechanism.

It was the clearest, most “present” tweeter I’d ever heard; the only other one coming close being the air motion transformers. It was not very loud and it struggled to keep up with crescendos, but it raised the bar for impeccable high-frequency sound that has been on my mind ever since. It’s a fascinating bit of technology, even if it uses a tank of helium at the rate of 25 cents an hour as the ionizing medium.

JS: You talked earlier about Class D amplifiers. QSC was an early advocate. They’ve become common in bass amps, but are now also prevalent now in home audio and home theater. What has QSC done to make them better-sounding and more user-friendly?

PQ: Class D is a tool, the third generation of amplification after tubes and linear solid state. Ideally, it’s simply part of the overall piece of equipment that helps the user do their thing without having to worry or be aware of what’s under the hood. Class D amps are lighter and more efficient, so we can put more power in smaller spaces, and we can do a better job of smoothing out AC power and operate anywhere in the world. The technology involves extra steps of “mapping” the analog signal onto continuous changes in the pulse width, which requires more expertise to get right. But semiconductor devices and knowledge have improved to meet the challenge. One of my personal design goals is to make technology practical, and free of “pitfalls” so users can focus on their work without worrying about the gear withstanding predictable hazards. After getting a sufficiently high-fidelity transfer function, most of my design effort went to providing safe “guardrails” that kept the equipment from damaging itself when driving abnormal loads, with the least possible impact on the music even when pushed. This is another aspect of pro audio technology. Like race cars, the gear is expected to handle predictably even when pushed beyond normal limits.

As far as portable equipment goes, speakers should have handles in the right places, be as light as possible, have proper rigging points, that sort of thing. And nowadays, setup and operation is largely driven by onscreen user interfaces, which of course, is an art form in its own right, and an area in which QSC pays as much attention to as anything else.

JS: You have stated that QSC initially eschewed speaker manufacturing because of the larger space needs, heavy equipment, need to deal with sawdust, and other considerations. When QSC made the leap into speakers in the 1990s, did you already have preconceived designs before Paul Hales was involved, or was it more a trial and error process?

PQ: Even in the 1970s, when we made the conscious decision to get out of building wood cabinets, you needed a lot of automated equipment to be competitive. We just didn’t have that kind of capital. By the time the late1990s rolled around, when we needed to get back into speakers, we had the capitalization and the resources to get serious. But yeah, we were beginners, with some rather off-beat design concepts. I like to think that even our earliest products sounded pretty good, but in looking back, I realize that we had a lot to learn about efficient manufacturing, practical form factors, and selling at a competitive price. But we got there eventually. It helped that our power amps were reasonably profitable, which let us go the distance until we got through the learning curve.

JS: In Copper Issue 103, John Strohbeen of Ohm Acoustics touched on how some Japanese customers’ sonic preferences – somewhat brighter with tighter bass – might require tweaks to Ohm’s speaker designs. Since QSC sells its products worldwide, have you found that various countries’ cultural tastes in sound have required any modification to your designs or installation parameters, compared to what you would implement in the US?

K12.2 active loudspeaker, rear panel.



PQ: I agree that there are regional differences in peoples’ expectations of sound. Europe and North America are probably pretty unified in what we want to hear. Some cultures seem to want less bass. The main problem with customers in India and China is that they often don’t have the budgets to buy top-of-the-line stuff, so they need cheap gear that projects impressive amounts of sound but without some of the nuances we expect in our high-end systems. There are a lot of outdoor village movie experiences in India and China that still use PA horns like we used to have here in the 1950s.

Frankly, QSC can’t compete at this price level, so our strategy is to offer much higher fidelity at the best price we can, and hope the customers eventually work their way up. The audiences now routinely hear full-range music of at least “fair” quality via streaming and computer/TV speakers, so they appreciate having a better quality at a public event. I’ve had many people tell me that QSC speakers “don’t hurt our ears!”  I think it was Pete Townshend who said, back in the day, “We mix our music to sound good through a four inch phonograph speaker!” (laughs) Because that was their expected audience, you know? Teeny-boppers listening through a little portable phonograph. But today, even tiny smart speakers actually have notable bass response.

JS: QSC’s online webinars and material about sound reinforcement, acoustics, physics, and music sound reproduction are excellent and detailed. Is the strategy behind this comparable to what Apple does in targeting its computers to schools, to create an educated market that will be comfortable and conversant with its products?


PQ: There’s obviously an element of that. QSC is able to take advantage of our size to underwrite the cost of this educational material. But it’s also in our direct interest, because the more customers understand what’s possible and how to get good results, particularly from our equipment, the more they appreciate the efforts we put into making good stuff, and understand why it is worth investing at a higher level.

JS: In designing and testing QSC equipment, do your R&D (research and development) and QC (quality control) departments use pre-recorded music and other sonic references? Can you tell us what that process is like and what your criteria is to meet your standards?

PQ: For electronics, we pretty much qualify everything on the bench using meter readings and a good understanding of where the pitfalls lie. It’s electric in/electric out, the device has a transfer function, so it’s easy to explore its full range, measure results, and thus know if it’s delivering flat response and low distortion into all the expected loads. We also have a long list of stress tests to validate the robustness of the design.

C-Q Series network amplifiers.



Transducers have a lot more going on, and voicing loudspeakers is still an art form. Yes, there are certain measurements that get you in the zone. QSC has invested in sophisticated speaker measurement capabilities, including an anechoic chamber and a concert-sized listening room, with an array of microphones spaced every five degrees so we can capture the entire sound field of a speaker, not just its front-facing response. The question still remains why two “reasonably flat” speakers can actually sound remarkably different in use. Off-axis response is one variable, because the off-axis sound bounces around in the room, gets to your ears eventually and affects your perception of the on-axis sound. Professional speakers are expected to have definite coverage areas and output volumes, so these constraints affect the frequency response and need to be artfully balanced. There are also possible resonances and breakup modes that don’t obviously affect the frequency response curve but are still audible to our ears, which need to be chased down. And I’m always amazed that our ears can listen through levels of loudspeaker distortion that would sound terrible in an amplifier.

So there are tradeoffs. You can’t get perfect off-axis and on-axis response from any practical loudspeaker. There are lobing effects and cabinet diffraction and various other side effects that prevent you from getting perfect coverage in the room, even with theoretically perfect drivers. So inevitably, there comes a point where balancing the tradeoffs involves listening tests, comparing A vs. B vs. C, using a panel of people in the company who have been in the field long enough to have good perspectives on sound quality and practical constraints. We have a professional audio crew that takes our stuff out and does roadshows with it. So they know what works and what doesn’t in a real-life environment. They’re part of the auditioning team, along with the product designers.

QSC E Series passive loudspeakers.


Although pink noise will quickly disclose resonances and other colorations, we assess dynamic range and the overall “tone quality” using a library of familiar music of various popular styles, that we’ve heard through a zillion different speakers. I also insist that the speaker be pushed to ensure that it doesn’t exhibit any distracting or even self-destructive behaviors even outside [its] comfort zone. And of course, our products also undergo long-term life testing and rigorous safety-agency approvals to ensure that they will provide long-term reliability and safe performance.

Part Three will conclude with discussions on musical instrument amps, vacuum tubes, the mysterious Optigan, the evolution of musical styles and more.

Header image: QSC Touch Mix 16 mixer.

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