Dynaco ST 70 stereo power amplifier
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Vintage test report


Taken from 'Stereo Review' magazine, 1959

At a glance: The Stereo 70 is a dual 35-watt power amplifier, featuring the high quality and moderate price which has characterised earlier DYNAKIT amplifiers. Its electrical performance is matched only by manufactured amplifiers that cost far more. The Stereo 70 can be used as a pair of 35-watt amplifiers for stereo. or as a 70-watt mono .amplifier. Its power and distortion ratings are completely conservative. Its listening quality is unsurpassed.


In detail: Each channel of the DYNAKIT Stereo 70 employs a 7199 triode- pentode and a pair of EL-34 output tubes. The lower plate voltage used the Stereo 70 assures long life for tubes and filter capacitors. In fact this amplifier's components are operated more conservatively than those in any other commercial amplifier we have tested.

The circuitry of the Stereo 70 is very similar to that of the' older Mk II and Mk III amplifiers except for the smaller output transformers. Somehow, the two amplifiers and their power supply have packaged in a space not much greater than that of the older mono amplifiers, without crowding and apparently without running at a higher temperature.

The Stereo 70's frequency response is so flat that jt almost seems pointless to plot it. The power response, in either mono or stereo modes is nearly as good. The apparent discrepancy between the maximum power outputs shown in these curves and those suggested by the distortion curves is simply due to the different method of measurement. The power response shows the maximum power obtainable before some distortion of the output waveform can be seen on an oscilloscope. Very small amounts of distortion can be detected in this way. Acceptably low numerical distortion values can be obtained at much higher power levels.


As the curves show, the distortion each channel of the Stereo 70 is immeasurably low or approaching the limitations of the test equipment at powers of 10 watts or less. At 1,000 cycles each channel is capable of some 43 watts before distortion becomes appreciable. By paralleling the two channels with the switch provided on the amplifier, one has a superb 85-watt amplifier (though DYNA calls it a 70-watt amplifier).

The hum level when driven from a low-impedance source (such as any preamplifier) is better than 90dB below 10 watts. Less than 0.7 volts is required to drive each channel to 10 watts or both channels to 20 watts. The damping factor is approximately 9 (Dyna says 15) Outputs are provided for 4, 8 and 16-ohm: speakers. The Stereo 70 will develop the equivalent of 18 watts per channel into a 3-mfd capacitive load at 10 kHz, which makes it quite suitable for driving electrostatic speakers at high volume levels. It is absolutely stable and free from ringing under any conditions of capacitative loading.

Two power take-off sockets are provided to operate a pair of Dynakit preamplifiers. Each has its own heater winding to prevent interaction between channels.

A comparison of the performance of the Stereo 70 against the Dynakit Mk III shows the Stereo 70 to be definitely superior in all respects. Its distortion at all power levels is lower than that of the Mk III, and strikingly so near maximum power output. Its frequency and power response are better at the extremes of the audio range. Its sensitivity is about 15% higher. Of course, this improvement in flexibility and performance costs money, which, in many cases might not be warranted. But for the person who wants a very compact, superior quality stereo amplifier, or an extremely powerful mono amplifier at a remarkably low price, the Dynakit Stereo is a natural.


There seems to be only one important point which the Hirsch-Houck Labs. Did not include – the fact that the Stereo 70 that they analysed was a kit (although it is also available wired). We attribute the excellent performance which can be obtained from our kit to the use of a factory-assembled printed circuit board containing the the critical parts of the circuit, the quality of the parts used, and the factory testing and matching of tubes. The conservative operation of the components permits us to offer a one-year guarantee on all parts – a guarantee which we believe is unique in the kit field.