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Dynaco ST-70 Series 3 Tube Power Amplifier

Dynaco ST-70 Series 3 Tube Power Amplifier

Back to the future. When Radial Engineering announced its plans to market a modern version of the Dynaco Stereo 70, the news struck me as a valiant time shift, aimed squarely at resurrecting the soul of one of the most influential amps of all time, and the component that enshrined Dynaco in audio’s Hall of Fame. Over a span of nearly 20 years in the 1960s and 70s, more than 400,000 ST-70s were sold both in kit and assembled forms, total sales by some estimates approaching half a million by 1977. That would make the ST-70 the most popular tube amplifier of all time. It sold well not only in the U.S. but also in Europe and Japan. I would venture to say that nearly every serious audiophile owned an ST-70 at some point in his lifetime.

Resurrecting a classic is a risky commercial proposition. Panor Corporation gave it a shot in the 1990s, manufacturing an improved version of the ST-70, dubbed the Series II, only to cease operation in the late 1990s. As it turned out, improving upon a classic was a lot more work than anyone at Radial Engineering expected. The design team approached the project with the reverence due a classic and embarked on a challenging but ultimately rewarding 18-month journey. The Series 3 may well be technically and sonically the finest ST-70 ever produced, and that should be exciting news for audiophiles and music lovers alike. Note that there will be no kit version available. This was debated in-house, but in the end, the legal risk of sending out a product with high voltages that could be worked on by customers without proper training was judged to be too great. And as far as looks, the new ST-70 has come a long way from the its humble 1950s utilitarian kit persona. The finish is improved and the curvy white tube cage on black base gives the amp a sexy noir look.

By his own admission, David Hafler considered the ST-70 his favorite by virtue of its unbeatable combination of sound quality, cost, and a power delivery sufficient to meet the needs of, as he put it, “about three quarters of the market.” To fully understand its appeal, please buckle up for a fast trip down memory lane. The year was 1955. Monophonic sound ruled the day. Fisher and H. H. Scott were major consumer audio brands, while Marantz and McIntosh were still years away from their heydays. The build-it-yourself movement was in full swing. Heathkit and EICO were busy selling various consumer kits, including hi-fi amps and preamps. You could purchase a Heathkit W3-M amplifier kit, a Williamson-type Ultralinear design, complete with an Acrosound output transformer, for $49.75. It was also the year that Dynaco came into being. David Hafler and his boyhood friend, Herb Keroes, had founded Acrosound transformer company in 1949 and successfully promoted and popularized the Ultralinear output stage. But when it came to business plans the two friends were rather incompatible. For example, Keroes was resistant to the idea of interleaving output transformers because of the added complexity during production; he wanted to keep things simple for the highest volume. Eventually, for Hafler it was not just about selling more transformers. Interacting with numerous customers he realized a clear need for offering complete amplifier kits on a cost-effective basis. His timing couldn’t have been more flawless at what turned out to be in hindsight the dawn of tube audio’s golden age.

Dynaco’s first viable amplifier design, the Mark II, quickly reached sales volumes of 1000 units per month. This was followed by the Mark III which offered 4, 8, and 16 ohm taps and the KT88 beam power tube for a mere $10 more. With the advent of stereo in 1959, Dynaco launched its first stereo amp, the ST-70, available as a kit for $99.95 (cover included). A review in the December 1959 issue of High Fidelity magazine concluded that its performance was only matched by manufactured amplifiers costing far more. It consisted, in essence, of two Mark II circuits on one chassis, and closely emulated Hafler’s U.S. patents for a high-fidelity amplifier and output transformer design. Hafler understood that a low distortion spec along with flat frequency response over the audible bandwidth did not necessarily correlate with audible performance benefits. His major design concerns were transient response and stability under feedback conditions, especially when driving difficult loads such as electrostatic speakers.

Hafler addressed these goals with a design paradigm that combined three distinct elements. First, use of a pentode-triode combination tube input stage providing a pentode gain stage directly coupled to the triode section configured as a cathodyne phase-splitter.  However, Hafler confided many years later that he selected the 7199 and 6AN8 combination tubes for cost and space-saving reasons, so there’s no reason to think that Hafler was wedded to such tubes for their technical advantages. Second, use of two feedback loops, one being a standard global loop from the output transformer’s secondary to the input stage, and a second taken from the screen of one EL34, to linearize the phase-splitter’s output impedance at high frequencies. Finally, use of an interleaved output transformer of unique design that offered excellent high-end bandwidth, and that remains 60 years later competitive with modern designs.

This is a good place to emphasize that the new ST-70 was not intended to be an exact copy of the original. The longer the contemporary design team listened to and measured the original, the clearer it became that there was no room for nostalgia—the Series 3 version would have to raise the sonic bar significantly over what had been expected in the early days of stereophonic sound to meet current audiophile expectations. Development was driven by the notion of what David Hafler would do today given modern components and available tube production. It seemed fairly obvious that he would opt for the very best parts and aim to deliver exceptional value. Every design aspect was examined under this conceptual lens with an effort to improve the performance of the original where possible. That this was accomplished with passionate attention to detail is evident from the time invested in the process.

The first challenge involved the obsolete 7199 pentode-triode. There have literally been dozens of DIY-inspired upgrades attempted over the years aimed at replacing the original front end. Radial Engineering selected three input/phase-splitter circuits for extensive testing using various tube configurations. The first recreated the original design using a pair of EF86 small-signal pentodes and a 12AU7 dual triode as the phase-splitter, even though there was concern over the limited availability of the EF86. The two other circuits evaluated were all triode, based on circuits designed by Hafler over his long career. The first of these used a 12AX7 voltage amplifier and a pair of 12AU7 as long-tailed-pair phase-splitters. The practical issue with the long-tail-pair phase-splitter was the need for a trimmer adjustment for proper balance. The third circuit was derived from a Williamson design by Hafler that was not sold commercially for cost reasons. It uses a 12AU7 dual triode as a voltage amplifier and cathodyne phase-splitter, and another 12AU7 as an additional voltage amplifier and output tube driver. That’s a total of four 12AU7 triodes for both channels, so not surprisingly this version was dubbed the Quad Triode by the design team.

Six months were spent evaluating prototypes of all three designs using the same output transformers and power supplies. During the listening phase of the testing process, the pentode-triode design ranked lowest. And although both triode designs were comparable in midrange and treble performance, the Quad Triode nudged ahead by virtue of its punchier low end. It’s worth noting that the lower gain 12AU7 was selected instead of the common 12AX7 as the driver tube to reduce dynamic distortion levels. It also offers a lower source impedance and thus less interaction with the capacitance of the EL34’s control grid. Many years ago, David Manley opined that he could tell the IQ of a tube-amplifier designer by his choice of driver tube; a 12AX7 would score poorly in this context. Another advantage of the Quad Triode circuit is that a second feedback loop is no longer required as the cathodyne phase-splitter is buffered from the power tubes by the driver stage.

Many consider the ST-70’s A470 output transformer to be a primary contributing factor to its sonic excellence. And this weighed heavily on the design team, which purchased various ST-70s, ranging from the original to updated versions, and began by taking these apart and then scouring the planet for possible solutions. They tested various “off the shelf” output transformers, but none could be found that matched the high-frequency response of the original. The A470’s design as detailed in Hafler’s U.S. patent shows interleaved primary and secondary sections, but what is most unusual is that some sections were wound with reverse sense, or backwards if you will, and then connected in parallel. I don’t envy the transformer winder who had to keep track of all this. Working for many months with their transformer vendor, a final design emerged that nearly matched the rise time of the original without waveform distortion or overshoot but featured much better bass response. The final output transformer is a point of pride for the design team, which feels that it may well be the finest output transformer anywhere near this price point.

The original power and bias supplies have always been weak links, and these were completely overhauled. A bigger dual-primary power transformer was designed that by virtue of a larger iron core features better AC line regulation. Solid-state rectification is now used instead of the 5AR4 tube rectifier. The filter capacitance has been greatly increased, basically using up available space while observing required safety clearances. The EL34 bias supply now uses a modern solid-state rectifier with additional supply capacitance, and its reference voltage is derived from an IC voltage regulator for improved bias-setting stability. Individual bias adjustments are now provided for each EL34 power pentode. This is done by tweaking a set screw so that that each pair of indicator LEDs is at equal brightness. A small Phillips-head screwdriver is helpful in reaching the recessed trimmers. Of course, any bias adjustment should be performed after a warm-up period—I would suggest 20 minutes—and with no signal playing.

Most of the hand-wiring has been eliminated by using a military-grade, double-sided, epoxy-fiberglass printed circuit board for the internal components and output tube connections. The PC board has been optimized using circuit development software to ensure that ground return currents from the power supply remain separate from the filtered power for the driver and output stages. The goal was to reduce channel crosstalk and lower overall noise. Output impedance taps of 4 and 8 ohms are selectable on the back panel via a switch. There is no longer a 16-ohm tap. In the original ST-70, feedback was always taken from the 16-ohm tap regardless of which tap was actually loaded. In the new ST-70, the feedback loop is tied to the impedance tap in use by the double pole impedance selector switch. One side of this switch connects either the 4- or 8-ohm tap to the output terminals. The other side connects the feedback loop to the tap selected and changes the feedback divider ratio so the overall gain remains the same. The payoff is reduced high-frequency distortion.

The original ST-70’s input stage incorporated filters designed to limit and roll off the amplifier’s frequency response. The subsequent loss of open-loop gain at the frequency extremes resulted in reduced feedback and thus additional distortion in the bass and treble. The new ST-70 implements a passive high-pass network, located after the input volume trimmers, to contour the bass response. The roll-off is a gentle 6dB per octave, and there are two selectable –3dB points at about 35Hz and 70Hz. The low-cut filter takes the form of a three-position switch on the front panel with the top position being a bypass. It’s perfectly safe to switch positions during play. I found this feature to be extremely useful in controlling the Quad 57 ESL’s excessive midbass. And I imagine that it could possibly be useful in taming room modes, which are typically below 200Hz in most listening spaces.

Prior to shipment, each amplifier is burned-in, power tubes are matched and individually biased, and a critical listening test is conducted for audio fidelity. A product report accompanies each amplifier that includes Audio Precision plots of frequency response from 10Hz to 20kHz and total harmonic distortion as a function of output power. The passing grade for THD plus noise is less than 0.03% at 1W. Dynaco anticipates a life span of 10,000 hours for the 12AU7 tubes and about 3000 hours for the EL34 power tubes under normal conditions. Since the new ST-70 sounded terrific right out of the box, my advice is to forget about tube substitutions and simply enjoy the music.

The ST-70 was my first tube amp. It replaced a Harman Kardon Citation 12 solid-state amp that had pretty good credentials and excellent benchtop specs. The resultant sonic transformation gave rise to the first “Wow” moment of my audio career. The second such moment, by the way, was hearing voice reproduced through the Quad 57 ESL. These events opened my eyes and ears to a conceptual world well beyond that captured by mundane engineering specs. That’s about when I started subscribing to both Stereophile and The Absolute Sound, the so-called audio alternative press that has made a living off the notion that not all amplifiers that measure alike sound the same.

The original ST-70 generated a spatial impression with believable depth perspective and image outlines. And then there were those liquid, pure, and sweet textures, in stark contrast to the insufferable grain of most solid-state amplification of the early 1970s. The music flowed with a verve that kept me glued to my sound system like never before. Unfortunately, the party stopped after a couple of days when I came to realize that low-level detail was being lost or homogenized into oblivion. Nor was I happy about the soft bass lines that lacked the crunch and punch of the real thing. It was Audio Research with its beam power amplifier designs, featuring rather sophisticated power supplies, that showed that excellent low-level detail retrieval was not the exclusive domain of transistorized designs.

Speaking of The Good, The Bad, and The Ugly of tube amplification, what the new ST-70 is able to do so well is capture what is good about vintage tube sound while avoiding its bad and ugly aspects. Harmonic textures are consistently sweet and refined without sounding overly liquid. It seems that many modern tube amps convey a much brighter harmonic tapestry, a case in point being Linear Tube Audio’s ZOTL40, a David Berning design I happened to review. The Berning sounds faster, is more aggressive in dishing out transients, and shifts the presentation slightly forward. The new ST-70 offers a middle-of-the-hall perspective consistent with a slightly recessed treble range. It didn’t complain loudly about bad recordings and tonally reminded me of the Quad 57 ESL.

Its ability to resolve individual threads within a complex musical passage was highly evolved over that of most vintage tube designs. And it was able to latch onto transient decay down to what is an impressively low noise floor for a tube amplifier. Its spatial presentation and microdynamic conviction equaled or exceeded that of any comparably powered push-pull tube amp I’ve auditioned over the years, even when driving a 96dB-sensitive loudspeaker. That’s quite a testimonial for the purity of its first watt. In fact, I had just spent some quality time with a new 300B amp acquisition, and it hit me that the Dynaco approached the sound of a fine single-ended triode amp, actually sounding better at the frequency extremes, and nearly as refined in the midrange. I can say with confidence that the bass demons that afflicted the original ST-70 have been exorcised. Upright jazz bass was nicely defined with good pitch definition. However, deep bass performance was typical for a low-power tube amp, lacking current delivery into low-impedance loads.

Nailing the sound of any amplifier is much like trying to hit a moving target. An amp may sound terrific with Speaker A but fall flat on its face with Speaker B. The root cause is the interaction of the amplifier’s source impedance with the speaker’s impedance magnitude, which can modify the overall tonal balance. With few exceptions, speakers have far from flat impedance curves. That’s why I try whenever possible to include several speakers in any amplifier evaluation, including at least one electrostatic load. The ST-70 performed well with typical dynamic type loads but did have difficulties with the highly capacitive Electrostatic Solutions Quad 57 ESL. It rolled off the Quad’s treble and emphasized the bass range. My recommendation is to experiment with the impedance switch and try both the 4- and 8-ohm positions whatever your speaker’s nominal impedance rating may actually be. Let your ears decide which tap is better.

To my mind the ST-70 Series 3 represents a major achievement in tube amplifier design. It retains the sonic soul and virtues of the original, while injecting a dose of steroids into the mix. The bass range is much improved while detail resolution and microdynamic intensity are significantly sharpened. This is an amp that David Hafler would have definitely been proud of. And since it has no peer at its price point, it continues the Dynaco tradition of offering superb value. Do yourself a favor and check it out.

Specs & Pricing

Circuit type: Class AB push-pull Ultralinear
Power output: 35Wpc into 4- or 8-ohm loads
AC Power: 110/230 V
Maximum power consumption: 345 watts
Dimensions: 17″ x 7.5″ x 9.25″
Weight: 32 lbs.
Price: $3000

RADIAL ENGINEERING LTD.
1588 Kebet Way
Port Coquitlam, BC
Canada V3C 5M5
dynaco.com

Associated Equipment
Speakers: Innersound Isis Mk 3.5, Quad ESL 57, Basszilla Platinum Edition Mk2 & OB3 DIY
Preamps: Lamm Audio L2.1 Reference, the horn shoppe Truth
Digital front end: Apple Mac BookPro running Sonic Studio’s Amarra Version 3.4 software, DiDit 212se DAC, Sony XA-5400 SACD player with ModWright Truth modification
Analog front end: Kuzma Reference turntable, Kuzma Stogi Reference 313 VTA tonearm, Clearaudio da Vinci V2 phono cartridge; Sound Tradition MC-10 step-up, TPAD 1000 phonostage
Cable: Acrotec 6N, Kimber KCAG Select interconnects; Acoustic Zen Hologram II & Schmitt Custom Audio T-4×12 speaker cable
Accessories: Sound Application power line conditioners, Monarchy Audio AC-Regenerator

Dick Olsher

By Dick Olsher

Although educated as a nuclear engineer at the University of Florida, I spent most of my career, 30 years to be exact, employed as a radiation physicist at the Los Alamos National Laboratory, from which I retired in 2008.

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