Output Transformer Saturation
For single ended operation, there will be standing current
(DC) present on the primary of the output transformer. Due to this reason, we
have to gap it to avoid output transformer saturation. We do not need to do so
for push-pull operation since there is no standing current but we sometimes do
so by gapping it for maybe 5mA – 10mA to cater for unbalanced push-pull
operation to prevent premature output transformer saturation due to unmatched
tubes being used.
When we measure the output transformer with LCR analyzer or
any other measurement equipment, we need to bias the output transformer for
actual operation. Without that, we won’t know whether the output transformer
core size is big sufficient and what the effect is when it is at real operation
mode. This will be most impactful for the low frequency operation where
saturation tends to occur.
Try it out – when you bias your output transformer during
measurement, the impedance and the phase will both be affected – to match the
real operation scenario. If it can be biased to the real operating current,
that’s even more accurate, provided that your measurement equipment can take
such measurement or provide DC bias themselves.
If the impedance dropped too much and phase differences vary
substantially, it signifies that it has design problem that will lead to
audible issues. If it is due to
saturation issue, then increasing the core size will help. If it is other
design issues, then increasing core size might not help.
Back to push-pull operation, if push-pull output transformer
is not designed properly – having unbalanced windings (needed for push-pull
operation), problem will happen too even if matched tubes are used. Such
unbalanced winding will create difference on the V-drop and therefore cause
unbalanced current being present on both winding, making the transformer
saturate easily. Saturated transformer will produce unbalanced sound, where the
most obvious characteristics would be poor low frequency response even if it is
a high power push-pull amplifier.
Therefore, the importance of gapping push-pull output
transformer to avoid saturation issue needs to be highlighted again and again. If
your push-pull amplifier does not sound powerful, it could be output
transformer saturation problem it is facing.
So, it is all back to basics, the know-how on how to wind
the transformer – be it SE or PP. And most importantly, measure your output
transformer before you use them to avoid wasting your time. Your circuit could
be good, but your parts are not! Don’t waste time troubleshooting for nothing.
J&K Audio Design
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