Specifications

5
stereo operation, current draw conditions in both channels would rarely be identical, so given
that understanding, the common cathode connection would generally work to hold the overall
bias steady between both channels. In that light then, the connection is more than just an eco-
nomic move. In this case, it really does improve output stage performance when the amplifier is
used for its intended purpose. However, with mono operation (which is a major component of
stereo signals), current draw would be very similar in both channels. In that case, the advantage
of the common connection disappears, and performance suffers as a result.
Of course in the end it was all a compromise, because price point matters as well. Cathode bias
was deemed an acceptable one, and the rest is history. In retrospect, it is so Dynaco in their
attempt to squeeze so much out of so little. That it came off so well is so Dynaco as well. But
wouldn’t it be nice if the disparity in performance between single or both channels operating
could be resolved so that operating both together would still achieve the same great performance
as either does on their own?
Careful analysis of measured performance from all conditions of use gives a more complete
understanding of the operating conditions in the SCA-35’s output stages. That information also
clearly defines what needs to be done to gain the improvements sought.
The Plan
When all of the forgoing is taken into account, the plan for improvement boils down to:
1. Convert each output stage to fixed bias operation. With the Z-565 transformer, this will mini-
mize shifts in the operating point, which minimizes distortion and maximizes power output in the
process.
2. Minimize the effects of power supply sag between quiescent and full power output conditions
when both channels are driven. This also works to minimize shifts in the operating point, adding
to the benefits noted above.
3. By virtue of achieving the first two goals, the influence of one channel on the other will be
eliminated.
If these goals could be achieved, it would effectively cause both channels -- when operating at
the same time -- to achieve the same excellent performance that single channel operation pro-
duces from the original design. That would surely seem to be a worthwhile goal.
The classic way to achieve these goals is to add a negative bias supply along with its various
adjustment pots, install current monitoring points, and add additional power supply capacitance
to provide for a greater power reserve under large signal dynamic conditions.
All of this can be done, and it will achieve the goals and performance improvements sought. But
using such an approach would undoubtedly require significant physical modification to the little
Dynaco, and surely take away from the charm of its simplicity in design and operation. Hafler
and Laurent championed the KISS principle to the max in all of their pieces of equipment, so