Dynaco: High Quality Audio Components

Founded by David Hafler and Ed Laurent in Philadelphia, Pennsylvania in 1955, Dynaco was an American hi-fi audio system manufacturer popular in the 1960s and 1970s for its wide range of affordable, yet high quality audio components. The two men refined and developed the Ultralinear audio circuit pioneered by British audio electronics engineer Alan Blumlein, using taps from the output transformer to feed signal back into the output stage screen grid circuitry. The Acrosound transformer circuit was later used in many home-built and commercial hi-fi amplifiers in the early 1950s. In order to appeal to a wider consumer market, Hafler decided to design and build entire power amplifiers as build-it-yourself kits, complete with preassembled, tested circuit boards that only required the customer to wire the boards to the transformers, controls, and power supply to complete the project.

Today, Dynaco is best remembered for its highly regarded vacuum tube stereo amplifier, the Stereo 70 (ST 70). Introduced in 1959, the ST 70 was available as a kit (Dynakit) intended for assembly by the purchaser or as a complete factory-wired unit. The ST 70 used four EL34 output tubes, one GZ34/5AR4 rectifier tube, two 7199 input (driver) tubes, two output transformers, one power transformer, and a preassembled printed circuit board (PCB) containing the driver circuit. It produced 35 watts per channel (wpc). The driver circuit had a single 7199 pentode/triode tube per channel, and used the driver tube to handle both voltage amplification and phase splitting. The output transformers are an ultralinear design, whereby part of the primary winding is fed back to the output tube’s screen grid. This design reduced distortion and improved audio quality.

The popularity of the ST 70 contributed more than any other single product to continuing consumer interest in tube-based stereo amplifiers at a time of increasing market dominance by solid state audio products.