An Interview with Doug Hurlburt of Dynamic Sounds Associates, LLC

LB: Doug, welcome to Dagogo. Please begin by telling us about your extensive background in electronics and physics.

DH: Both of my parents were scientists and it seemed that a career in science was inevitable for me. I did my undergraduate studies at Johns Hopkins where I concentrated on physics and mathematics and made friends with a number of audiophiles, who had similar interests to my own. I remain good friends with some of them to this day.

During the summer months, I was a student employee at the National Bureau of Standards where I worked in the acoustics laboratory designing measurement equipment and working with the full-time staff. This was “heaven” for a budding audiophile, as the majority of the staff members in the laboratory were also audiophiles, and we spent many hours discussing, and working with the high-end audio equipment of that time. It also gave me an early opportunity to learn about electronic circuit design from some very good design engineers.

After graduating from Johns Hopkins, I chose to pursue graduate studies at McGill University in Montreal where I enrolled in the physics department to pursue a master’s degree in Solid State Physics. After graduating, I decided to stay at McGill and pursue a Ph.D. in physics. However, I was offered a slot in the Electrical Engineering Department, and that’s where I received my Ph.D. even though my research topic was primarily physics-based in nature.

Throughout this time, I continued to experiment with audio electronic equipment, both for my enjoyment and to assist in the building of equipment for friends. In spite of the fact that I have a degree in electrical engineering, and because this degree was late in my education, virtually all of what I know today about the design of electronic circuits was self-taught. I never had any specific courses focused on electronic circuit design.

My professional career started at the R&D Labs of RCA Limited in Montreal where I had the opportunity to be exposed to cutting edge developments in semiconductor devices and their applications both at the renowned RCA Princeton Laboratories and elsewhere within RCA. Over the years, my professional career took me to MIT in Boston, and then back to Washington, D.C. where I provided consulting services in the areas of high technology development and program management. I continued to be passionate about fine audio and how it could be improved. I was always experimenting on my own system with new ideas and system configurations to see what worked best for my applications. This was somewhat frustrating for my wife Andrea, who was always hoping that the last modification would truly be the last.

This passion led me on a diversion that resulted in the publication of a paper in the March 2000 issue of the Journal of the Audio Engineering Society (JAES). This paper revised and expanded the theory on passive radiator loudspeaker systems, originally developed by Richard Small in 1974, and resulted from my desire to re-build my loudspeaker system.

This growing passion for exploring new possibilities in audio reproduction happily coincided with my transition to being a consultant and not having to keep regular work hours. Now that I am fully retired (I think), I have more time and energy to devote to what I’ve always loved – building the finest possible audio electronic equipment.

LB: Tell us more about how you become interested in audio, and when you first tried your hand at designing audio gear?

DH: I first became seriously interested in audio when in high school. Between science fair projects, being President of the Science Club and a semi-finalist in the Westinghouse Science Talent Search (now the Intel Science Talent Search), I assembled a Heathkit 20W all-tube power amplifier using 6L6 triodes for the output stage. Several of my high school buddies were also becoming interested in audio and we were in “competition” to see who could build the best system. After about a year, I purchased a Dynaco upgrade to the Heathkit that added the ultra-linear tap (going to pentode output tubes) and a new Dynaco output transformer for added power. When I went to John Hopkins, I built a smaller version of the power amplifier using a published Dynaco circuit diagram for use in my dorm room. I also built a tube preamplifier based on a design published by Stu Hegeman at Harmon-Kardon from their Citation line. I must have been prescient because I broke it into two parts with the phono stage in a separate chassis right next to the turntable and the line stage a few feet away. My desire was to reduce the cable length to the phono stage, and to have a low impedance output from it to the line stage. This was probably the first time that I began to seriously consider the multiple aspects of audio electronic design and how I would address many of the issues that were involved.

This system lasted until I went to McGill when I purchased the circuit diagram and the output transformer for the Dynaco 125W all-tube power amplifier. This was a mono unit that Dynaco would custom build on request, but was not a regular market item. I built mine using parts obtained at local electronics parts surplus stores, and I was able to purchase the only set of four KT88 tubes in all of Montreal for the output drive.

As Andrea and I moved from Montreal to Boston, and later to Maryland (outside of Washington, D.C.), I was continuously building and re-building our home audio system. Even though by that stage in my career I could have assembled a system using commercially available high-end audio gear, I found that I enjoyed designing and building my own equipment and, because of this, I received greater pleasure from listening to it.

While in Boston, I decided to design and build a new power amplifier to meet my requirements. After considerable research into the factors that influenced power amplifier design, I embarked on the design and building of a 125W-per-channel amplifier that would operate in Class A mode at the 125W output level. It was to be all FET, use no coupling capacitors in the circuit (truly DC coupled), and employ only sufficient feedback to ensure total stability under any load conditions. It took about two years to complete, but remains my reference power amplifier to this day. It was during the design and building of this power amplifier that I started listing the design goals that were important to me, and that I would carry over into any new audio designs.

When we moved to a new house in Maryland with a much larger listening room, I began to plan my next audio system. I also re-established old audio acquaintances from my Johns Hopkins days and became a member of the southern Maryland “Backwoods Audio Society,” or the bAS to avoid confusion with the more famous Boston Audio Society (BAS). This disparate group encouraged me to continue on my path of designing and modifying my components, and it was this group that challenged me to design and build a better phono-stage preamplifier. The seeds were laid for the DSA Phono-ONE!

LB: When did you decide to commercialize the Phono-ONE, and what was the motivation?

DH: I really wanted to do the phono stage first, as I believe this to be the most critical electronic component in the reproduction of music from LPs. The large dynamic range that is available on a good LP presents a real challenge. The ability to amplify this range without adding or subtracting anything from the largest voltage to the smallest microvolt, while simultaneously preserving the distinctness and clarity of the recording across the entire audio spectrum, requires a preamplifier with extraordinary control, linearity, and dynamic range. If any portion of the recorded audio information is lost in this critical first step, it can never be recovered in later stages. My design goal was simple: to ensure that every note, nuance, transient, and minute piece of audio information on the LP was faithfully preserved and amplified.

My initial thought was to design and build a unit for my own purposes, but my audio buddies soon made it clear that, if I were successful, they would want their own Phono-ONE. So, without any real thoughts of mass-marketing, or how to proceed with commercializing my products, I formed Dynamic Sounds Associates, LLC as a small, sole proprietorship to leave all possibilities open.

LB: A key decision each designer must make is whether to use tubes, transistors, or a combination of the two (i.e., hybrid design). Your phono stage and line stage are entirely solid state. What factors went into that choice?

DH: I believe that the world of fine audio is large enough to accommodate a wide range of listener preferences, likes and dislikes. I am the last person to say that designs should only use one type of device or another. I have made my choices based on what I believe to be important to my overall design goals. When I designed my power amplifier many years ago, I was faced with the same dilemma – tubes or solid state? However, given what I was attempting to accomplish, I found that the use of solid state components gave me more flexibility to achieve my design goals. Having decided on solid state I had to choose between bipolar transistors and field effect transistors (FETs). I made the decision to go with FETs based on their operating characteristics and because FETs represent what is known as a “square-law” device, very similar to the situation with tubes. In fact, FETs can be viewed as solid state tubes in how they operate, but with improved linearity in most instances. I use both JFETS and MOSFETs in my designs and I base the specific selection on where the component is used in the circuit, so that I can optimize the performance by choosing the best component for the job. I do not employ “exotic” components in my designs; instead, I design systems that use high quality parts but which are insensitive to the minor variations that are standard in even the finest components. I believe that good design from the start eliminates the need for exotic parts which are often used to correct for failings in the overall design.

LB: Having heard the Phono-ONE first at shows and later in my own system, I can attest to its extraordinary performance. Please tell us a bit about its design, and how it has evolved since its inception.

DH: When I accepted the challenge from my audio colleagues to design and build the Phono-ONE, I decided on several basic design principles that I would adopt. Some of them proved to be a challenge, but the basic principles are:

· Use JFETs and MOSFETs exclusively

· Design every gain stage to be a “stand-alone” stage with low distortion, very high dynamic range, and very high bandwidth

· Do not use any form of global or loop feedback

· Use only passive filtering to provide the correct compensation for LP playback

· Avoid using coupling capacitors in the audio chain

· Utilize separate, low impedance, output driver stages

I had already developed designs over the years to meet all of the above requirements and many were incorporated in my power amplifier. For the Phono-ONE, the most difficult to accomplish was the lack of coupling capacitors in the audio chain. In designing a preamplifier having more than 60dB of gain at the output relative to the input, it was necessary to prevent DC drifts and offsets from the early gain stages propagating to the output, resulting in large, and unstable, DC offsets. I worked on several designs using simulation and modeling software until I developed an approach that made all of my design goals possible.

Once all my design goals were achieved, the first Phono-ONE units were built and tested for both sonic performance and stability. Since that time the basic design concept of the Phono-One has remained unchanged.

The feedback that I’ve received is really quite interesting. Most listeners attest to the unusual combination of added richness of sound and enhanced detail; two qualities that, to the hi-end audio world, seem to be at odds. Delighted by the improvement, the oft-heard question is “what did you do to make my LPs sound so much better?” The answer is, “nothing.” The Phono-ONE is designed to do nothing other than replicate exactly what is on the LP. What people are hearing for the first time is the full-beauty of their analog recordings.

There have been some evolutionary changes and revisions that are incorporated in the latest version which you heard in both the audio shows and your system. However, the majority of them were involved more with the various user functions that the Phono-ONE supports and how they interface with the amplifier section, than the actual audio amplifier section. There have been a few changes in certain aspects of the amplifier design, all of which have served to enhance the performance, but they have been “around the edges” of the basic design.

With the sonic performance of the Phone-ONE now a fait accompli, I’m excited to announce that a new edition, cosmetically and functionally enriched, will be ready by autumn. We’re just not going to let externals keep audiophiles from considering the unit. For those who have systems that are capable of allowing the source material to be fully-expressed, we believe the next generation of the Phono-ONE will provide even greater satisfaction, both musically and aesthetically.

LB: The importance of the power supply cannot be overstated. Tell us about the one in the Phono-ONE.

DH: There are many different schools of thought on the design of power supplies. Some prefer power supplies built around “brute force” resistor-capacitor (R-C) or inductor-capacitor (L-C) filter stages. Others utilize voltage regulators in place of the R-C or L-C filters. I use a combination of both approaches in my power supply design to provide, I believe, the best of both approaches. I have three major goals in the designing of a power supply for audio applications:
· Provide a voltage, or current, supply for the individual gain stages that has no residual AC components

· Assure extreme stability in voltage and current with respect to temperature variations

· Provide sufficient power supply bandwidth to handle any transient demands

I do not rely on “exotic” approaches, or components, in my power supplies, only good solid design using high quality components.

The Phono-ONE has two totally separate, but identical power supplies, one for each channel. Each supply starts with a custom-made toroid transformer that is never operated near the design limits, and the primaries for both are connected out of phase to provide cancellation of any residual stray fields. They are also encased in a steel shield to further isolate the amplifier sections from stray transformer fields. These transformers are each followed by a bridge rectifier and a simple R-C input filter that drives a very stable high voltage regulator circuit which generates the plus and minus high voltage rails. I use a regulator circuit that I developed years ago which proves to be very stable, capable of very good regulation, and that also provides short circuit protection in case anything does go wrong. For the Phono-ONE, this regulator has a one-minute ramp from turn-on to achieving the final regulated voltages. The purpose of this slow ramp is to prevent turn-on transients from causing potential damage to the individual gain stages in the absence of any coupling capacitors. The output voltage rails of this regulator are then passed through another filter section which provides energy storage for transient demands on the high voltage rails.

Within the amplifier section, each gain stage has its own separate regulator that operates off the incoming regulated supply rails. This allows me to “tailor” the DC voltages and currents for each gain stage independently so that I can optimize the performance of that particular stage. It also helps me to avoid coupling capacitors between the gain stages. In addition, the low impedance output driver stage has its own regulated supply, separate from the amp supply.

LB: Your newest product is a line stage. Tell us about its design.

DH: I am having a great time with this. From a design standpoint it has the same overall goals as the Phono-ONE. The power supplies are virtually identical from a circuit design standpoint; however the line stage power supply is in a different physical format, and the high voltage rails are at an even higher voltage. As with the Phono-ONE, I wanted to ensure that there were no unwanted or varying DC voltages appearing at the output. However, to prevent a small DC voltage at the output that varied with the volume control setting, I had to use a coupling capacitor in the design. For this purpose, I chose a high quality “audiophile grade” capacitor that is located immediately prior to the volume control, and this is the only capacitor in the audio chain.

The feedback has been more than encouraging. The few listeners who have been able to couple the prototype of the Line Stage with the Phono-ONE report a sense of “infinite” dynamic expression and even greater detail. For me, it’s exciting to add another DSA design to the audio chain. When the line stage is complete, it will match the new edition of the Phono-ONE in its style and functionality and—most importantly—in its sonic attributes. I’m hoping that you will have the opportunity to review both units simultaneously!

LB: Many purist feel that remote operation compromises sonics. What kind of attenuator do you use, and how do you handle remote control?

DH: For the prototypes of the line stage, I have been using stepped attenuators by either DACT or Goldpoint. These are mounted internally to minimize wire lengths, and connect to a knob on the front panel via an extension shaft with a universal joint. To operate the stepped attenuator remotely, I use a stepping motor and remote control that I obtain from another source. I went down this path because I have not yet found another form of attenuator that provides the performance that I am looking for and also supports a remote control capability. The beauty of using the stepping motor is that the control circuit is only active when actually making steps—otherwise it is off and there are no other potentially interfering signals that I need to worry about.

I also have incorporated a remote input selection and mute capability in the prototype line stage. I will certainly carry the remote mute into the final design; however, it is not obvious that a remote input selection has much value.

LB: Can the line stage be ordered with built-in phone section? If so, how would it differ from the stand-alone Phono-ONE?

DH: I was asked about this by one of my audio friends, again in the form of a challenge since he knew I would dwell on it for some time. I do know that the design of such a unit would certainly be significantly larger than the upcoming Phono-ONE or the line stage. I’d have to incorporate separate power supplies for both the line stage and the phono stage. It might even evolve into a two-chassis design, which raises the question “why bother?”

LB: Your current business model is direct sales only, with each unit being custom built. Do you intend to change that in any way?

DH: In addition to direct sales, I am working with a marketing associate who has been very helpful in getting the Phono-ONE introduced to a much larger audience. We are looking for opportunities that include both direct sales and some dealer networks. My biggest concern with any future business model is being able to maintain the quality of the current products and any future models that I develop, while also meeting customer demands. I am not looking for large volume sales. If that happened, it would significantly impact the product quality and also take away my ability to have direct control over the development of new models. I prefer to stay as a small business making high quality products for discerning listeners.

LB: Are there any plans to introduce additional models, perhaps some a bit less expensive?

DH: There are very few options for making a DSA product that is significantly less expensive. Once the unit design is finalized, and this includes both the electrical and the mechanical designs, the only real savings would likely come from significant increases in production volume—which I said earlier was a quality control concern. I will not dilute the product line by introducing a unit that is “almost as good” as the flagship unit.

LB: What are your thoughts on the current state of high-end audio?

DH: I view high-end audio as a very dynamic and expanding area. New products, new designs, and new designers are entering the field and I think this is very exciting. Without new designers entering the field, and bringing their approaches to solving many of the vexing problems of high-end audio reproduction, the field becomes dominated by a few with designs that have been “static” for some time. I believe that there is always room for new approaches to old problems and only by encouraging these changes will high-end audio continue to evolve and improve.

LB: Doug, on behalf of Dagogo and its readership, I would like to thank you for taking the time to talk with us. We wish you continued success, and I hope to have an opportunity to spend more time with your excellent products.

DH: Thanks, Larry. I realize that some of my answers might have been difficult to follow. But believe me, a powerful passion is at the core of all I do. I love music and I live for the opportunity to take on new challenges, and I am forever fascinated by the universe of electrical circuit design. For me, high end audio is the perfect storm! There’s no better feeling than when a fellow-music lover listens to a DSA product and experiences a connection to the music they have never had before. Ultimately, that’s why I do this.