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Interview with
Bruce Thigpen from Eminent Technology
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Good afternoon Mr Thigpen
First, I have to tell you that within minutes of hearing the LFT-8b I decided to buy a pair and to apply to be the exclusive London retailer. Thus from the start, I'm a ribbon-enthusiast – and have been for years. That’s where I'm coming from. So my first question is this . . .
Can a full range pure ribbon loudspeaker (no cone drivers) be built that is nominally flat from say 30 Hz to 20k?
Thank you very much very for the nice compliment above, also thank you for giving me the chance to present my views, more realistically my biases, with respect to your questions, audio, and the audio industry.
The answer to your first question is yes – but it would be a very large and costly speaker.
Today the definition of these transducer types has been distorted because of marketing. I define a ribbon as a loudspeaker that uses a loose foil conductive diaphragm with magnets on either side, the foil has no substrate and is suspended end to end. Most of the time these are small single conductor devices. The ribbon diaphragm is “loose”, not under tension where the resonance frequency is well below the operating frequency. Ribbons are usually narrow.
Like the Decca Ribbon?
The Decca ribbon tweeter embodies these design elements.
In summary then, what are the distinguishing features?
The distinguishing features of a planar magnetic transducer are that it (a) has a diaphragm under tension and(b) can have multiple turns usually attached to a substrate such as mylar or polyester. The conductors are typically glued to a substrate which carries the tension. The magnets as opposed to being on the side are distributed across the back and/or front plane of the diaphragm.
How does that relate to the full-range issue?
There are two problems making a true ribbon speaker play full range. First, the diaphragm width is limited because flux from the magnets which defines efficiency roughly follows an inverse square law. As the ribbon gets wider to help push more air, the magnets are farther apart and the efficiency drops. Secondly, the tension of a diaphragm is used in a planar magnetic to boost the low frequency output typically 6dB at resonance. Tension provides a significant gain in low frequency extension because of the elasticity of the substrate.
Magneplanar commendably has held steady in their definition of the loudspeakers they manufacture calling them planar magnetics. In the LFT-8 we call the midrange panel a planar magnetic, our current tweeter shares some design features of a ribbon and a planar magnetic speaker.
So re my earlier question “Can a full range pure ribbon loudspeaker (no cone drivers) be built that is nominally flat from say 30 Hz to 20k?” – is that a yes, a no, an “I'm not sure” or . . . hey Howard, let’s not go there?
I’m definitely sure that it can be done although the raw material cost would be very high and I’m not sure that the low frequency range quality would be that good because of the loose foil. It would require about double the surface area of the largest existing full range planar speakers.
You are I believe not only the conceptualiser but the senior engineer, team leader and public face of the company. Have I got that right?
Yes that's right, I wear a lot of hats and frequently have reservations about my qualifications.
Are you by training and/or an electronic expert, mechanical engineer, acoustician or all of those, or none?
I’ve some experience in all of the above, in high school I repaired radios, in college I helped pay for school by repairing televisions and then obtained a job with a local engineering firm where I was lucky enough to get both mechanical and electrical engineering projects.
There is a growing consensus in the UK among open-minded audiophiles who take the time to think about these things that measurements are – at best – no more than a sign that one is or isn’t pointing in the right design direction. Thus, it’s possible to have speakers which measure brilliantly and are sonically un-engaging and in contrast, measure averagely and sound compelling. Can you offer us your thoughts re this?
Air is an imperfect medium to transfer sound, if you want to measure a loudspeaker and maintain signal integrity over distance we need something better!
The measurements are good at what they do. However, my guess is that we err in the interpretation of the measurements. For example not properly weighting a frequency response curve against our ability to hear what is displayed on the graph. Sharp dips or peaks would look bad on a graph, but we might not hear them very often – or at all – depending on the program material. A frequency response trend is very important but tends to be hidden by the display scales. The same applies to time, waterfall, distortion and impulse response measurements. Loudspeakers are far from perfect and in my view none of them are right.
In my experiences audio magazine editors are not as open-minded re the subjectivist versus objectivist debate as the aforementioned audiophile. By this I mean that a publication’s subjective conclusions might be skewed to ‘fit’ the measurements. I’ve not experienced this re your Hi-Fi World review, but have you seen examples of this elsewhere?
Yes, a magazine writer might like to pretend that they are unbiased but they’re human, regardless. I enjoy looking at the measurements and try to make my own interpretations which are sometimes entirely different than that of the writer. Measurements might not tell you everything about a speakers sound but you can learn quite a bit about how a speaker is designed. That’s interesting to me.
From the maker’s perspective, what are the sort of problems you encounter dealing with the audio consumer press? I'm thinking here, for example, that an editor has a method of measuring cabinet / cone speakers and insist on applying the same technique to measuring planar speakers. That kind of thing?
Generally you can measure a small box/cone speaker near-field, maybe 1 meter and get meaningful results. I’ve been surprised when a speaker with drivers farther apart or longer than 1 meter is measured under the same conditions. You wouldn’t get an idea of what the designer intended.
To a technical layperson like me it seems that your ribbon-drive shares many features with the Magneplanar system. Am I correct or am I missing something that’s in plain sight to everyone else?
You are correct. The design principles are loosely based on Magneplanar's planar magnetic loudspeakers. The sound of electrostatics made an impression on me, the first I listened to was the -Quad 57 in the late 60's and that many years ago its sound was radically different and I thought much better than the cone speakers of the era. So I built electrostatics for fun and began to understand the limitations.
I always looked at the original Magneplanar design being single ended and having a diaphragm driven by wires with distance between the wires as being a less uniform force than the electrostatic. I set out to make a planar driver that was push-pull and had a more uniform driving-force to distinguish ourselves in the market place.
The interface between your ribbon drive and the woofer on the LFT-8b is as effective and seamless as I’ve ever heard, bettering those used in state-of-the-art Martin Logans in my opinion. A terrific achievement. Was this interface a very challenging design exercise?
The interface took a long time, I finally settled on the opposite of what some might think. The first is a paper cone with extra mass, the goal was to design the woofer where it won't play into the midrange. The second is a low crossover frequency, choosing the materials and dimensions of the midrange panel so that it has a low resonance frequency for its size, between 90-110 Hz, remember the bump at resonance talked about above. Third is a very large overlap in the electrical crossover, for example the 3dB down point on the midrange panel is 30Hz, it does not play down there much if at all, but it helps the blend. That in a nutshell is the secret sauce.
To what extent, if at all, does attending live music impact on your designs and fine tuning?
Primarily through having musician roommates in college, hearing and measuring their instruments, also going to concerts. Then listening to the speakers I built, which were electrostatics at the time. If you take a good recording system and a pair of microphones to a concert, then run home and listen, you quickly realise how far away we are from live music!
No doubt your LFT-8b is designed to work with a comprehensive range of decent amps. May I enquire as to which type (valve and/or solid state) amp or amps you used during development of this product and possible the brands too?
It’s good practice to use as many different pieces equipment as you can. I used tube amps to make sure that the damping factor would work well with the woofer alignment. Solid state amplifiers helped most with the midrange / tweeter interface. Because of the relatively high impedance the LFT-8 it needs voltage-swing rather than current. A failing is that it’s not as efficient as I’d like.
What are the limiting factors on the construction of the ribbon itself currently?
In a word, efficiency. Ironically if efficiency were raised, then it would be more difficult to get a good blend with a woofer.
With regard to reliability, the midrange panel on the LFT-8 is fairly robust. Woofers and tweeters fail far more often that the midrange panel does.
Can you enlarge on that please? I'm thinking here, in part, about transit damage to delicate ribbons.
The most frequent failure of the midrange panel is a loss of tension during shipping. This can usually be rectified by adjusting what we call “cam spacers” which allow user-adjustment of the diaphragm tension. Its thermal capacity is about 3 watts per square inch which means the power handling is very good. The woofer and tweeter run out of output capability first.
Were time and money in infinite supply, what could you build ribbon-wise with your know-how, your expertise and so on?
We’ve started a number of projects that failed. At some point you have to cut and run. Either I got tired of a project or we ran out of funding. You weigh how much the R&D will cost against the potential return. In a small company there’s always a noose around your neck. Eight years ago we started a speaker project that ended up on the shelf. It was very small dipole, half the size of the Quad 57. The irony is that the hard part, by which I mean getting low frequency performance out of a very small thin dipole, worked very well, to about 35Hz, but the midrange and high frequencies were not satisfactory.
Where that small dipole now?
It sits on the shelf for another day.
What’s the inside story re your Rotary Woofer?
A woofer's job is to move a lot of air. Volume displacement is the term. I was flying a model helicopter, watching the pitch of the blades as it was hovering on the ground. I was moving the collective lever (the control that pitches the main rotor blades) as fast as it would go and it occurred to me that it was a loudspeaker. Since I had crashed the helicopter more than once I had plenty of spare parts. I remember it took less than 24 hours to make the first prototype. When you try out-of-bounds ideas it is very rare that they work. Surprisingly the first prototype performed better than I expected.
Would you like to expand on that a bit?
Sure. The idea is to prove that we can hear well below 20hz and that part of the sound spectrum can have an effect on the overall reproduced sound, especially so with film soundtracks, to some degree with music. It’s been ignored because we’re taught that we cannot here below 20hz. This simply isn't true, and cone woofers quickly run out of the required volume displacement. The funny thing is people said the TRW-17 will never sell, the irony is its selling very well, just not where I had expected.
Are you at heart and in your soul a digital man, a vinyl man, and FM man – or are these considerations just plain irrelevant to you in the design process?
I'm a vinyl man at heart, but today the market is digital and you cannot bury your head in the sand and expect to survive. Modern high bit rate/bit depth digital recordings are extremely good.
If you were an automotive manufacturer, what one would Eminent Technology choose to be perceived as?
Lotus
Because of their outright innovation?
Yes.
But not the unreliability, right?
I drove a Lotus Europa as daily transportation for quite a few years and have first hand experience with that.
You’re probably best known, even today, for the air-bearing tonearm. What litmus test of success in sound quality did you set yourself in those early days?
The short answer is to get the physics right and everything else will fall into place. From my point of view the straight line tracking tonearm is the correct way to playback an LP.
When I was in college I was on a quest to build the entire audio chain, preamp, amplifier, speakers (home made electrostatics at the time) so I wanted to make a turntable. I was in a physics class and saw an air bearing demonstration. That lead to the development of a turntable which eventually became the Coloney AB-1 then the Mapleknoll and the ET-1.
Hey, I had a Mapleknoll. Terrific sound, except for the pump. What’s the current start of play with that brand?
Mapleknoll was a spin-off of the Coloney AB-1, to my knowledge, Mapleknoll is no longer in business.
Hmm. A pity. Anyway, moving on . . . what’s your litmus test for a system rather than a component?
If there is a litmus test of success in sound quality then it’s everything commonly understood when you appropriately use the word ‘perfection’.
Yes, but what does that really mean?
You’re always searching for perfection – but you never get there. You know when you hear a live acoustic guitar around a corner your brain tells you that's the real thing. Does your sound system do the same thing?
That’s it?
Yup, pretty much!
Simple!
Indeed!
How difficult, as a manufacturer is it to deal with reviewers?
Over the years I’ve found it easy to deal with magazines. We usually make unique products that differentiate themselves in the marketplace and the magazines tend to be interested in us. I try to put myself in their shoes.
In your experience, is there a relationship between advertising spend and editorial coverage?
Generally I’m not much of an advertiser. Magazines don't exist without advertising and because I'm not spending big bucks, I can’t reasonably ask or expect much.
Realistically, and without being specific if you don’t want to, have you been a victim of hi-fi politics do you think?
Actually no. My business model is to be very low key and invest profits back into R&D, very much less into advertising. Given that, surprisingly, publications like Absolute Sound, The Audiophile Voice, Audio, Hi Fi World, Stereophile, International Audio Review, Sound and Vision, Positive Feedback and many others have been good to us. Our products have been in the Guinness Book of World Records, in movies and on several TV shows. Writers regularly approach us about product reviews and the way we are treated is surprising considering our miniscule advertising budget. There are times where you may get a review with a glaring technical error or a comment that may be way off base.
And . . . ?
You just have to let that go.
What is wrong with cone drivers and cabinet speakers in your view?
Cones have too much dispersion in some listening environments and most lack the degree of resolution that a good planar or electrostatic exhibits. It’s much more difficult to make a cone loudspeaker have low coloration. Given my obvious bias, what I can in all honesty say is that cone loudspeakers are ubiquitous, low cost, well tooled, efficient, and very good – which is why they own the lions share of the market. I'm very impressed and have a lot of respect for speaker designers who are able to design an excellent sounding cone loudspeaker.
The same with electrostatics?
With an electrostatic or planar you barely have to concern yourself with colorations or sound quality in the design. You spend your time working a little on the crossover but mostly on the delicate balance of tradeoffs.
The limit of the electrostatic loudspeaker is the dialectric strength of the air. The theoretical efficiency is very high, but cannot be realised in practice. The result is that a small drive unit cannot play loud or go low. When designing one you have to take your pick, size, how loud, how low. Other than that, on average the electrostatic is a very special form of loudspeaker. It’s hard to design one that doesn’t sound good. Most people will know it when they hear one.
Who's driving the industry today Mr Thigpen, the music publishers or the hardware makers?
I would say that software and the distribution of products and content (the internet) is driving the industry. The music publishers and hardware makers have to respond to that process.
Are you an advocate of multi-channel, given how unsubtle most of the public demonstrations have been up till now?
Ultimately yes because it’s the future.
I sense an underlying sadness in that answer. Correct?
Yes. The sad part is the market provides a multi-channel home theater in a box product whose audio quality is far from what you can get from a small two channel integrated amplifier and a good two way bookshelf speaker.
Digital Room Correction is a hot topic in some quarters. Any thoughts on the viability of the technique?
The purist in me wants the speaker to be right and not require any correction. Clearly the technique is viable, for technical and sound-quality reasons. However, we need to be careful about how much correction is used.
What are the current limitations re approaching 100% accurate voice and music reproduction in the home?
This question shorts circuits my brain and a circuit breaker trips. Can we go on to the next question?
Certainly. Okay . . . what’s the global audiophile market going to look like in five years?
Hmm. (cue: visualise his thoughtful expression and gentle stroking of chin. No reply though.)
Err, okay. How’s about in ten years time?
Let’s group the five and ten year questions together. Please take into account that my crystal ball is foggy; sometimes completely obscured. My perspective is from living in the United States.
Over the last 30 years We’ve seen a transition of the market and I believe that trend will continue. The audio retailers, the group that got me interested in this as a hobby, are going away. Unless you’re a company large enough to produce products for a big box retailer you have no choice but to direct market through the internet. Sadly for the under thirty crowd, the replacement for an audio system is an MP-3 player and ear buds.
Fifty years ago your consumer electronics choices consisted of a TV, table radio or some form of audio components. Today the internet, computers, software, peripherals, games, digital cameras and cell phones dominate the consumers disposable income and audio becomes an afterthought. To some degree Home Theater has added importance to audio – but the point is that the disposable income slice of pie available to audio equipment is significantly smaller today that it was 50 years ago. It shows in the number of retail outlets that sell just audio products and home audio's position at the consumer electronics shows.
The killer app then?
Unless there is a killer app that brings audio as a medium back into the forefront I see audiophile market continuing as small and specialised. High resolution audio content will be delivered via the internet.
Okay, moving on . . . what's your business philosophy Mr Thigpen?
To differentiate ourselves in the marketplace and invest heavily in R&D. We’ve a disparate approach to product design and tend to be very low-key. Our products follow good physics principals. We are weakest at industrial design and marketing, the net result being low brand recognition.
You didn’t mention revenue, profit, market penetration, glory!
No, I didn’t did I?
And your personal philosophy?
I probably work too hard and too many hours – but I really enjoy it. Right now my hobbies are flying airplanes, gliders, riding dirt bikes and working on cars. Good diversions from running a small business.
Finally, any message you’d like to give the readers?
I'd really be surprised if anyone made it this far into the interview.
Thank you Mr Thigpen
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