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
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.