Radial Engineering Relaunches an Audiophile Classic: Dynaco ST70 Series 3 Amplifier

In bringing back one of the audiophile world’s most popular amplifiers, the company says the updated version features revisions David Hafler would have made if he were still alive.


There are few names in the world of consumer audio more revered than Dynaco. Not too long ago the Canadian company Radial Engineering acquired the Hafler and Dynaco brands, and since then it has revamped the brands' products and just introduced new Hafler headphone amplifiers and phono preamplifiers products.

Following the new Hafler product releases Radial has just announced a brand new amplifier from Dynaco: the ST70 Series 3. Knowing the original ST70 is one of the most popular amps in audiophile history, Dan Fraser, senior engineer, Dynaco, says the new Series 3 amp balances the original amp's character with modern design elements that will endear the amp to today's consumers. 

“There are tens of thousands of the original Dynaco tube amplifiers still in use around the world. The original Dynaco Stereo 70 is a true classic that is still regarded with awe by audio enthusiasts worldwide. The ST70 Series 3 marks the introduction of a third generation of vacuum tube products bearing the Dynaco name,” states Fraser. “This 'new 70' reflects the illustrious history of Dynaco's involvement with vacuum tube designs, as well as the latest advances in vacuum tube technology. We like to think of the Series 3 as the update that David Hafler would have made if he were alive today.”

Commenting on some of the updates that were made to the new Series 3 version, Fraser says Radial made some revisions to the ST70's circuit design and its industrial design in order to take advantage of advancements in raw materials and audio technologies.

“Through its long history, the Stereo 70 has proven to be a robust, reliable design of exceptional value. We have sought to preserve the considerable virtues of the original 70 while making fullest use of much better and more modern parts, plus advanced circuit design techniques,” notes Fraser. “When comparing 'old 70' with 'new 70,' one cannot help but notice the external similarity [the metalwork is of similar dimensions, but with an entirely new connector array and top cover]. Internally, however, the two units differ radically from each other in both general appearance and execution of design.”

Keeping in the spirit of the original ST70 amps, the new Series 3 features an all-tube design that delivers the sound tubes are known for, including that soft clipping phenomenon that results in, “more pleasant natural compression that is both pleasing to the ear.”

Getting into some of the design detail of the Series 3, Radial says the new Dynaco amp utilizes the familiar ST70's open-loop gain design concept, as well as updated versions of David Hafler's output transformer designs. Updating the amp's power supply, the new amp features a larger dual-primary power transformer that offers improved line regulation, and a solid-state rectifier that replaces the old amp's 5AR4 tube rectifier. Explaining further, Radial says the Series 3 offers more power supply filter capacitance, and the company removed the selenium rectifier used in the bias circuit and replaced it with a solid-state version that includes addtional supply capacitance. 

Dynaco emphasizes these changes, as well as other design revisions improves the “stiffness” of the power supply by lowering the effective power supply impedance, while improving regulation to help the amp produce tighter bottom end. 

The amp also includes revisions to the open-loop design that affected frequency response. The new amp utilizes an input frequency limiting filter network that removes “tailoring” components from the original design's open loop's active circuitry to produce a flatter frequency response. 

Summing up the amp, Dynaco adds the amp now offers users 4- and 8-ohm operational switch, and a high-pass, three-position filter switch that includes bypass and two different frequencies that offer 6dB octave roll off.  Dynaco also notes the new ST70 Series 3 incorporates a pair of EF86 pentodes in the imput stage, a 12AU7 dual triode in the driver stage, and EL-34s /6CA7 tubes in the output stage.

About the Author

Robert Archer
Robert Archer:

Bob is an audio enthusiast who has written about consumer electronics for various publications within Massachusetts before joining the staff of CE Pro in 2000. Bob is THX Level I certified, and he's also taken classes from the Imaging Science Foundation (ISF) and Home Acoustics Alliance (HAA). Bob has also served as the technology editor for CE Pro's sister publication Commercial Integrator. In his personal time beyond his family, he's studied guitar and music theory at Sarrin Music Studios in Wakefield, Mass., and he also studies Kyokushin karate at 5 Dragons and Brazilian jiu-jitsu at Binda Brazilian Jiu Jitsu.