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Thursday, December 19, 2013

Output Transformer Saturation



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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