Specifications

be mated to a specific amplifier cir-
cuit. We went through dozens of de-
velopmental samples in the case of our
new MC 2205 amplifier. What we
ended up with it shown schematically
in the diagram of Fig. 9. The trans-
former is trifilar wound to provide
coupling between sections. It takes 23
individual windings to make this out-
put transformer. There are five differ-
ent winding sections, all five of which
are connected in parallel. We use
grain oriented silicon steel core lamin-
ations because that kind of core means
less iron-and less iron in turn means
tighter coupling. It also means lower
winding resistance for a given size
wire. The grain oriented silicon steel
means that it has a higher magnetic
saturation point-about 17 kilogauss
versus 12 to 13 kilogausses for the
non-oriented variety. There is there-
fore less core loss, or, to put it simply,
we end up with a more efficient trans-
former-one which couples more of
the available amplifier power to the
speaker loads. To further improve cou-
pling, we don't use any interlayer in-
sulating paper in a power transformer
that might pose a breakdown problem.
But since our polyurethane insulated
wire is rated at 4000 volts per mil (and
since the highest voltage we're talking
about for an audio transformer is
about 56 volts), this really is no prob-
lem at all. All of our output trans-
formers arc potted with material
which has especially high thermal
conductivity. Besides helping to keep
operating temperatures within the
transformer down, this compound re-
duces lamination buzz to inaudible
levels. We figure you'd rather listen to
your speakers than to our transformers!
Our Transformers Are Only Part
of The Story
Whether an amplifier uses an out-
put transformer or not, its output de-
vices must be designed to work into an
optimum load so that maximum cur-
rent delivered by the output transis-
tors never exceeds the safe operating
area specified for the transistor. Fig.
11 shows current versus voltage limita-
tions for the epibase type of output
transistor used in our MC 2205 ampli-
fier. If all amplifier loads were purely
resistive, staying within the safe oper-
ating area would be relatively simple,
but the fact is that speakers often pre- Fig. 9 - MC 2205 output
auto-transformer schematic diagram
Output volts
Fig. 10-Load and limiting data of
the McIntosh MC 2205 measured at 8 ohm output
11