Dynaco ST-70 Series 3 Tube Power Amplifier

A Classic Resurrected

Equipment report
Tubed power amplifiers
Dynaco ST-70 Series 3
Dynaco ST-70 Series 3 Tube Power Amplifier

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

1588 Kebet Way
Port Coquitlam, BC
Canada V3C 5M5

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-4x12 speaker cable
Accessories: Sound Application power line conditioners, Monarchy Audio AC-Regenerator