The Dynaco ST-70™ Series 3 tube amplifier is a modern version of the most popular stereo tube amplifier of all time. A medium power Class-AB push-pull amplifier, the ST-70 has long been recognized as a robust, reliable design of exceptional value. The ST-70 Series 3 builds upon the proven philosophies of the original David Hafler-designed ST-70 and uses contemporary design techniques to implement performance improvements that meet the requirements of modern audiophiles.
Dynaco has made extensive use of high-quality parts that were unavailable at the time of the original design, including precision metal-film resistors, poly-composition capacitors and miniaturized high-capacity power supply electrolytic capacitors. In addition, many of the newer parts are also built to tighter tolerances than the originals. The PCB is constructed of military-grade, double-sided epoxy fiberglass PC material. In compliance with modern practices, the speaker binding posts have been updated, and a heavy-duty, three-pin detachable grounded power cord is provided. To minimize noise and achieve electrical compliance, the signal ground is the same as electrical ground and chassis. To optimize signal transfer, all connectors are gold-plated.
For the ST-70 Series 3, Dynaco has substantially overhauled the power supply from the original version, utilizing a larger dual-primary power transformer with improved line regulation, increased filter capacitance, and a solid-state rectifier that results in a much “stiffer” response. The resulting bottom end is a dramatic improvement over the original ST-70’s low-end response. A three-position high-pass filter switch is provided to allow listeners to tailor the low-end response of the amplifier to their personal tastes.
The redesigned front end of the Series 3 amplifier replaces the obsolete 7199 pentode signal tubes from the original ST-70 with more commonly-used 12AU7 dual-triode tubes as part of a configuration that harkens back to the classic Williamson amplifier, which itself has a long-storied history in the audiophile community. The output stage and EL34/6CA7 output tubes are unchanged from the original ST-70, and the bias adjustment for the output tubes can be individually set for each tube. After a warm up period (and with no input signal present), the bias is easily set by adjusting recessed potentiometers to match the brightness of front-panel LEDs.
The original Dynaco A470 output transformer is revered for its role in creating the classic ST-70 sound. For the Series 3 amplifier, tremendous effort was applied to painstakingly meet and in some cases, exceed the performance standards of the A470 transformer in an effort to better adapt to today’s improved loudspeaker technology and more refined listening tastes. A switch allows the listener to select between 4 and 8 Ohm speaker loads. The feedback location is also configured by this switch, and the gain is adjusted accordingly. The improvements result in an improved transient response when compared to older ST-70 implementations.
The new ST-70 Series 3 incorporates advancements in component quality as well as improvements to the design, building on the legacy of David Hafler’s enormously successful Stereo 70 with a product that is sure to impress audiophiles familiar with the original, along with those who are hearing an ST-70 for the first time.