Dynaco Mk III mono power amplifier (1957)
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Taken from Audio magazine, December 1957

Slightly over a year ago we reviewed the original Dynakit II power amplifier (September 1956) with its unusual - for then - 50-watt output. Now there are many 50-watt amplifiers on the market, and some of still higher power, and it is somewhat of a surprise to note how many of them employ the same circuit as that of the Dynakits.

The newest model, the Dynakit III, is rated at 60 watts and incorporates a few features that were not part of the earlier model. While the circuit is essentially the same, the output tubes are KT88s, which accounts largely for the increased output. In addition, a filter choke has been added, improving the signal-to-noise ratio, and a 11.2 ohm resistor has been added from the cathodes of the output tubes to ground - an improvement towards more output linearity that has already been recommended as a suggested change for earlier owners of Dynakit II.

The new unit is the same size as its predecessor - 9 by 9 by 6¾ in. - and is of similar appearance. rated output is obtained from an input of 1.6 volts, and the model tested (which was assembled in slightly less than two hours) reached 68 watts at 2 percent IM distortion. Output impedances of 4, 8 and 16 ohms are available, and the unit provides power for Dynakit and heathkit preamplifiers directly, or for most others if proper connections are made to the power socket.

The Dyna Preamp is an interesting design, conventional in some respects, unusual in others. The tone control action is obtained from a feedback circuit with somewhat less interaction than is usual for the average preamp. One useful feature is the 'Special' input which may be wired by the builder to be equalized for microphone or tape head, or it may be employed as an additional RIAA phono position, depending on the user's requirements. In the latter connection, it is possible to leave two pickups plugged in at all times, with front panel switching - accommodating, for example, a changer and a turntable at the same time. Another very desirable feature is the rectifier-filter circuit which permits the use of a 6.3 volt A.C. source to provide some 11.5 volts of D.C for heater supply. It should be noted that when this arrangement is used, the ground connection normally present in the power amplifier should be removed, since the ground on the heater circuit is best obtained from the potentiometer in the preamp. The Dynakit III is normally wired with the heater winding free from ground, and if it is to be used with any other preamplifier the centre tap should be grounded.. Similarly, if the Dyna preamp is used with any other power amplifier, the ground on the heater circuit of the other power amplifier should be lifted.

Figure 2 shows the external appearance of the Dyna preamp, while Figure 3 shows the internal arrangement. The use of pre-assembled etched-wiring panels reduces construction to a relatively simple operation. In spite, however, of the case of construction, it is likely that most users would be very well satisfied with the Dyna models.

Figure 2 (left). The Dyna preamplifier kit ends up as a neat and attractive unit with excellent performance. Note left slide switch which connects monitor circuit to tape recorder without need for changing plugs. Figure 3 (right). Inside view of Dyna preamp. Tubes are completely enclosed and shielded when cover is in place.