LL1540 tolerate DC (no current)??? - diyAudio
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Old 12th May 2014, 12:21 PM   #1
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Default Lundahl LL1540 tolerate DC (no current)???

I want to use fixed bias for an output stage, so I dont know if the secondary can tolerate DC voltage bias?

I assume it can, cos there will be no current just voltage,
what you say?

Last edited by DIYspaceW; 12th May 2014 at 01:48 PM.
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Old 14th May 2014, 09:19 PM   #2
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That will depend upon the extent of the bias voltage. Each secondary of the LL1540 has 800 ohms static resistance and any DC across that will deliver a direct current through it that will, if great enough, saturate its core. Even low values will reduce the transformers AC linearity and headroom. The maximum AC headroom spec will give an indication of how much combined DC and peak AC voltage the secondary can tolerate before saturation, i.e. the peak of the B-H curve, but the distortion will increase as the voltage approaches this. Using an equal but opposite DC bias on the other secondary winding will prevent this and is what is done in push pull circuit inputs. In fact some quite good examples of this come up when you look at the google images for LL1540.

John
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Old 15th May 2014, 07:27 AM   #3
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Thank you John,
you proprably right!
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Old 20th May 2014, 08:50 AM   #4
oshifis is offline oshifis  Hungary
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I can't imagine a DC voltage without current on a transformer
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Old 20th May 2014, 06:45 PM   #5
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oshifis and the original poster: By 'DC voltage' this could be a DC voltage, relative to ground, that is common to both ends of a winding. In other words, the winding is floating at some voltage other than 0V, but there is no net DC across the coil, so there is no DC current through the coil. If that's the case, it's OK - all transformers are meant to have common mode isolation at DC.

If there is any DC potential _across_ a winding, then there will be DC current through the winding, and that's generally not good, unless the transformer is specifically designed for it (such as the gapped transformer used in the single ended Neve output stage) or if it is cancelled by an opposing flux induced into the core by another winding (as John Luckins described).
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