I first met Doug Hurlburt, the man behind Dynamic Sounds Associates, some 20 years ago now; we were members of the same extended southern Maryland listening group. Doug has designed and built audio equipment for his own uses since his high school days. On my first visit to his home in (then) Potomac, MD, I was impressed by his clearly advanced comprehension of electronics and his passionate love of music. He had recently done an incredible mod, rebuilding his KEF CS5 loudspeakers, including totally redesigned crossover networks. This rebuild led to his rewriting Richard Small's theory of passive radiator loudspeakers, all published in the March 2000 issue of the Journal of the Audio Engineering Society. Further, and more to the point here, he had already built and installed his own end table sized amplifiers, that, to be blunt, sounded wonderful.
After earning his undergraduate degree in mathematics and physics from Johns Hopkins University (1962), he went on for his M.Sc. in Solid State physics from McGill University (1964), where he also received his Electrical Engineering Ph.D. (1972). He began his career at RCA Ltd. in Montreal developing microwave integrated circuits and surface acoustic wave (SAW) devices and systems. He moved to the MIT Lincoln Laboratory in 1976, continuing his work in advanced SAW devices. It was here that he got involved in the analysis, design, and development of ultra-wide band spread spectrum communications systems and airborne radar systems, leading him to both design and fabricate critical systems components for advanced concept communications and radar systems.
The late 1980's found him serving as a Program Manager at the Defense Advanced Research Projects Agency (DARPA) where he managed multi-million dollar programs focused on developing advanced radar systems and related technologies. After leaving DARPA he stayed in the Washington, D.C. area and continued to provide consulting services, radar system analyses, and program management assistance for the Department of Defense (DoD) and other Government agencies until he chose to retire to a beautiful gated community in Naples, Florida.
I must add that Doug's passions don't stop at fine audio; he also has an appreciation for automobiles. After taking care of the final arrangements following my father's passing in the summer of 2015 (he had lived in Fort Myers – just a 45-minute drive from Naples) I had opportunity to visit with Doug at his current home. His garage houses a Maserati GranTurismo (GT) cabriolet powered by a 4.7L Ferrari sourced V8 that puts out about 450HP, parked right beside a stately Bentley Flying Spur. The ride in the GranTurismo was without question the highlight of the visit (Thanks, Doug!). As an aside, when I sent this article for fact checking, I learned that the Maser is gone – supplanted by a Bentley GTC cabriolet. With the "legendary" Bentley W-12 twin-turbocharged 6L engine, developing some 600HP, with torque to match, and all-wheel drive, Doug said he finds it to be much superior (better performance and handling, and a joy to drive) to the Maserati. Some guys have it so rough...
DSA really began while he was still in Potomac, when, in 2004, he introduced the Phono ONE, Dynamic Sounds Associates first commercial product. I was the first to review that exemplary primer product, and it became my reference phono stage for nearly a decade. Its development came out of a desire to design the finest possible phono preamplifier without regard to the potential cost. Its no-compromise philosophy was advanced by the Phono II in 2012, a unit which weds first-class sonic performance with unprecedented versatility and ease of operation. The Phono II has been my reference phono stage since its arrival in the spring of 2015.
The subject of this audition, the Pre I linestage preamplifier, became a reality in the fall of 2014, following closely in the path of its Phono II sibling in both design and versatility. The latest DSA product, the Amp I was introduced mid-2016, and, no surprise, keeps both the familial look and superb performance. In its few public showings to date it has garnered tremendous positive press.
The front of the chassis is separated into three distinct sections, matching the Phono II, and Amp I's appearance. Starting with the left section, in a row across the upper third we have seven direct source selection buttons (A through F, and Aux), each with a corresponding indicator lamp immediately above. In a row beneath the source switches, centered vertically, are three indicator lamps for the selectable gain, 6dB, 12dB, and 15dB (controlled by switches at each input – read on). Below them, in the bottom row, is the mode (stereo/mono) and phase (normal or inverted) switches, each with two corresponding indicator lamps, one left and one right, of each switch.
The center section is built out roughly ½", with the volume knob centered in the top half, which also holds the remote-control receiver window. A flip-down door displaying the DSA logo, and concealing the auxiliary set of single-ended (RCA) input jacks, AND a ¼" headphone jack, occupies the lower half. A round 4 position, cycling display dimmer switch (from maximum intensity, to slightly reduced intensity, to greatly reduced intensity, to minimum intensity, back to maximum intensity) is cleverly secreted about two inches back, dead center, on the underside of the chassis as well. The right side features the two digital volume displays, L above R, with a vertical row of buttons to their right, consisting of Run, Mute, and Stand By (top to bottom), with their corresponding indicator lamps to the left of each. I find the appearance to be striking, embodying simplicity, elegance, and effectiveness.
The rear panel will get your attention. From left to right we have the IEC socket, which also houses the two AC line fuses, as well as the On/Off rocker switch. Next, the output jacks include a separate pair of single-ended record out jacks (volume independent), as well as two sets of balanced and one single-ended output. Between the single ended jacks is are a switch and some access points that allow for phase balance, if or when needed. Next are the inputs, A through F, you may recall, the Aux input, two ¼" jacks, are under the flip-down front panel. The inputs alternate between balanced and single-ended. So, inputs A, C, and E accept XLR jacks, while inputs B, D, and F accept RCA jacks.
Are you ready for this? Each of the 6 rear inputs has its own three position toggle switch positioned between the top (L) and bottom (R) inputs; the up position offers +3db of gain, center is unaffected, with normal throughput, and down attenuates, -6dB. Whichever is selected is displayed on the front panel display when that input is selected. So, why go to the bother? It is rare in my experience that all your sources, phono stage, DAC, open reel deck, tuner, optical disc player, etc., have exactly the same output voltage. Further, using balanced inputs on the Pre I will yield a 6dB increase in level as compared to the single-ended inputs. This unique added functionality, especially the use of the -6dB switch at the input allows for much a more even balance across all inputs, reducing the need for excessive level adjustment when switching between sources. Brilliant!
Those selectable throughput gains are achieved by use of a precision 6dB attenuator at the input, based on a T-pad constant impedance design, and is active on both the positive and negative sides of the input. The addition of 3dB of throughput gain, if required, is derived by changing the internal feedback in the first gain stage.
Honestly, its simple and elegant remote has been the first that any visitor to my room has ever chosen to comment upon. Roughly 6 3/8" long, 1 5/8" wide, 7/8" tall, and just over 8 ounces with its batteries, it is black, with silver end caps, slightly rounded sides, and fits extremely comfortably in your hand. The DSA logo and Pre I appellation appear etched into the lower quarter of the top of the remote, which also sports 13 rounded, dot-like buttons in the upper two thirds. Near the very top is a blue indicator lamp (that lights to tell you the remote is transmitting), with the Run button immediately beneath it, and the Phase toggle button to its left. Immediately below them is a matrix of arrowed keys, one each marked up, down, left, right, with another in the center of the diamond created by those four. Up and down control the volume, incremented in 0.5 dB steps. The left and right arrowed buttons control balance by changing the volume independently in the left and right channels. The beauty of this process is that as one channel is incremented upward by 0.5 dB, the other is decremented by an identical 0.5dB so that the total audio output remains unchanged. The button central to the volume and balance buttons is the mute trigger. Finally, below them in two vertical rows that are aligned with the Run, Right and Phase, Left buttons, are the source selections, A, C, and E to the right, B, D, and F, to the left. Doug designed the entire Pre I circuit, but the control logic it operates within the preamplifier is a result of the efforts of John Chapman of Bent Audio in Vancouver. John not only provides the shells of the control (which are engraved specifically for DSA), but also provided the micro-controllers — programmed to Doug's requirements. I find this remote to be among the very best available, and a beauty to hold and operate.
The power supply uses two matched, shielded toroidal transformers, whose primary coils are driven out of phase of each other, and are encased in separate magnetically shielded housings. Both are fully regulated with individual supplies isolated from each other, using separate ±65 rails for the amplifier gain stages, ±47V rails for the output section, regulated ±12VDC for internal use by the regulator, and +5VDC to drive the front panel display and functions. The power supply is designed with approximately 60 seconds' ramp time from ±5VDC at turn on to the full ±65VDC for running. As the voltage slowly increases, the value of both positive and negative rails are monitored, and only when both have achieved the proper final ±65VDC, and the regulator has "clamped," is a turn-on signal generated, permitting the Pre 1 to accept changes from the Mute or Run buttons. Doug also employs an internal RFI filter on the AC power line to help eliminate residual power line interference.
Doug explains that the Pre I is a "no-compromise," fully balanced, dual channel circuit, with no loop or global feedback whatsoever. When using an unbalanced input (single-ended), the negative side of the balanced input is grounded to prevent it from adding interference. Each gain stage employs its own internal feedback, eliminating transient inter-modulation distortion, a common by-product of configurations where throughput gain is achieved using intra-stage feedback.
Another benefit of controlling feedback internal to each stage is that it helps ensure that all forms of distortion are held to extremely low levels, allowing each stage to more fully realize a dynamic range capability that should greatly exceed that of the source material.
A single high quality film and pure tin foil coupling capacitor is used at the input to the level control. While Doug really dislikes using any coupling capacitors within the audio chain, this was required to prevent the DC voltage to the output of the first stage from causing a DC voltage at the output of the unit that varied with level control settings. If such a signal were fed into a DC coupled power amplifier, it would result in a displacement of the speaker cones with the level setting.
To preserve the very high dynamic range and low distortion inherent in the Pre I's basic design, a very precise (minimum step is 0.5 dB) 7-bit digitally controlled attenuator (based on T-pads or Pi-pads) was developed to provide constant impedance and the necessary precision for each attenuator step. The binary up/down counter configuration uses the following seven individual attenuator steps; 0.5 dB, 1.0 dB, 2.0 dB, 4.0 dB, 8.0 dB, 16.0dB and 32.0dB. The level control operates by using the attenuator steps in series, either switched into the circuit to provide attenuation, or removed completely by a short circuit. These values enable a range of 0dB to 63.5dB, all said to be accurate within 1%!
The switching is performed by high quality DPDT reed relays, chosen for their combined very low contact resistance and operational bounce time. One of the big advantages here is that, in the mid-range of the level control, only a few attenuator steps are being used, meeting one of Doug's design goals of having only a very few resistive elements in series with the audio path.
The output stage uses a separate high-bias current Class A output state for each polarity of the amplified audio signal, with output stage supply voltages fully regulated and powered separately from the voltage rails the provide power to the amplifier chain. Output impedance for each polarity at the output connectors is said to be 75 Ohms.
Because of the high rail voltages, and the inherent linearity of the design, the Pre I is claimed to have the capability of providing an output drive voltage of ≥40 volts peak-to-peak without any clipping. The output stages can develop up to 40mA of drive current without distortion, and will drive amplifiers with input impedances as low as 50 ohms, or cables with greater than 30nF capacitance (as may be found in long runs to an amp), with no difficulty.
Midrange is full bodied, with remarkably rendered tone color and texture. Though a purely solid-state linestage, there is the inescapable sense of instrumental body and bloom that makes the best tube gear so alluring. I had one visiting listener walk to my equipment stand to look for himself, as he couldn't believe such liquid and vivid mids could be produced without valves. Overall, spectral neutrality and mid-range richness are its hallmarks, with never a hint of over-ripe emphasis, just honest bloom, awash with texture.
Extension, air, and sparkle in the uppermost registers are breath-taking, replete with nuanced detail and rife with specificity and subtlety of intonation. The upper reaches of instruments like harp, piccolo, piano, violin, even the tempestuous contributions of cymbals, are accurately portrayed, with genuine verve, no noticeable slurring of attack, and uninterrupted, lingering decay. The way it reconstructs the highest frequencies is simply enchanting.
Without question, among this linestage's most significant achievements are its degree and quality of transparency and resolution. One contributing factor is clearly its supremely dark and near complete silence; its noise floor is virtually inaudible. In fact, its vanishingly low noise floor, taken in combination with its remarkably natural, detailed, and deep bass, have its performance knocking on the doors of linestages costing twice as much.
It has the ability to render dynamics of all variety, from transient attack that can clobber you with punch and power, to engaging you with the finesse of subtly shaded, effusively bared, microdynamic events. This graceful dynamic prowess, when combined with its ebulliently rhythmic drive, give it an ability to render nuanced performance that is nothing short of intoxicating. Listening to complex classical passages like the veritable cacophony of orchestration from the opening of Prokofiev's Scythian Suite [Mercury], is a revelation, as it is so cleanly portrayed and thrown into near-perfect focus.
As previously hinted at, another of its sublime accomplishments is its ability to span the chasm between valve and transistor. The result is soundstaging and image focus dimensionality that are reach-out-and-touch real, in size and specificity. Layering, and the generation of the sense of the space "between" and "around" instruments is exemplary. And there is an unmistakable sense of "big tone," beautifully dense tone color, and dynamic integrity that are so exciting with well executed tubed linestages.
The DSA Pre I has such addictive qualities, all of which combine to more effortlessly exposes the natural mellifluousness of instruments, or more clearly illuminate minutiae such as the weight of an artist's touch on their instrument. Its overall articulation of detail, harmonic neutrality, and tonal density and color, allows for the natural beauty of strings, horns, winds, even percussion, to reveal their native flowing splendor and bloom.
While most of my notes were based on listening with my reference Pass Labs XA160.8 monoblocks, they remained equally true of the sonic's I experienced with some large Class-D (Channel Islands Audio D-500 Mk IIs) and tubed (PrimaLuna DiaLogue Premium HP) monoblocks as well. Clearly the Pre I drives a wide variety of amplifier designs, with varying input impedances (from 50 to 100KOhms), without concern or any apparent disadvantage.
The DSA Pre I's performance is smooth and utterly non-fatiguing, yet richly detailed, with deep, articulate, and muscular bass, magically engaging mids, and sparklingly clear, delicately delineate highs. Its soundstaging and imaging abilities are pushing world class. It can recreate dynamic contrasts that can evoke a fight or flight response. Its ability to render any source it is fed accurately approaches that of the best available. Is its performance the equivalent of some of the pricier competitors I've singled out? Of course not! But it can afford the listener a lion's share of the exhilaration and expressiveness one experiences at the feet of those loftier priced, über preamplifiers.
This is one stirring performer; one that is laden with truly useful features like the variable gain selection, phase inversion, and a headphone jack. Younger readers should be aware that there was a time when every preamp and receiver on the market included one. My hat is off to DSA for its inclusion. Further, it is a pure joy to operate. At this price point, one would expect it to perform well, but, sadly, not all gear that sounds this remarkable, has such stately and pleasing aesthetics or offers such a high degree of functionality.
I'd like to make it clear that the way this linestage gets the job done is not just relevant, it borders on revelatory at its asking price. Much like the PhonoOne that I lived with for nearly a decade, and the more recent Phono II that has been my reference phonostage for the last 18 months, the Pre I offers extraordinarily refined sound that, while it is unmistakably exceptional, never calls attention to itself as being sonically spectacular. It delivers a presentation in which you find yourself utterly and inescapably immersed in musical expression, not caught up in "the sound."
Finally, I'd like to make sure that this product be considered in its proper perspective. While the DSA Pre I may have the appearance of being a new product, given its recent appearance on the market, I assure you, it should be considered a fully realized product from a mature and seasoned designer. Doug has been doing what he has done with the Pre I in some form or another for decades. With the Pre I, DSA has set an extraordinarily high bar for a linestage preamplifier that will only be marginally bested at considerably greater expense. I know I will be deeply saddened the day the call tag for its return to Florida arrives. Most enthusiastically recommended.
Custom design attenuator employing seven (7) precision attenuator sections:
Balance control in 1dB increments (0.5dB/channel)