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In Reply to: Here's some more clarification. posted by kurt s on October 22, 2006 at 17:32:47:
hey kurt,This reduces distortion in the core by having plenty of voltage swing up front with a high turns ratio - and this means the B field in the core is actually lowered than if the transformer has to "work" harder by using a smaller turns ratio.
could you please clarify this? if you have an inductive volume control and increase the signal level, the overall B field on the core goes up and since the B field is directly related to the distortion, i would expect core related distortion to go up too.
Follow Ups:
> > could you please clarify this? if you have an inductive volume control and increase the signal level, the overall B field on the core goes up and since the B field is directly related to the distortion, i would expect core related distortion to go up too. < <Well, yes, this does require some clarification because I did not say enough, and I am in error. I forget there is some actual load present, although a high resistance usually. When you just increase the input voltage to the transformer and do nothing to the volume control setting, the B field will increase. This is because the power applied by the energized core of the transformer has to go up to supply the added power that is delivered to the load since the load didn't change and the turns ratio didn't change.
But if you increase the voltage input and drop the turns ratio to maintain the same signal output then the power delivered to the same load is identical. This should maintain the same B field in the core, not decrease it.
The only way to decrease the B field in the core and get the same signal output is to do one or more of three things: increase the core volume, increase the number of primary turns, or decrease the permeability of the core material.
The limit to increasing core size and primary turns is that the B field can also become too small and begin a different form of distortion - the effect of stuck "domains" in the core that need enough energy to overcome it. Somewhere in this design there is some optimal size and turns ratio for a given signal level, load, and core material.
I have found I like a certain mu metal formula for the core laminations, 80% nickel, a little oversize for the core size, and lots of primary turns with small gauge wire. The mu metal type was selected for low distortion at low B fields. In other words, those "domains" unstick readily at small magnetic levels. These core types may all say 80% nickel, but they do have differences, and you can pick one that is lower in low level distortion than others. And those hysteresis patterns differ, too, for low level signals. Some are more rectangular, for example.
hey Kurt,This is because the power applied by the energized core of the transformer has to go up to supply the added power that is delivered to the load since the load didn't change and the turns ratio didn't change.
I still think you are in error. Lenz Law suggests that in an ideal lossless situation a transformer of any size could deliver infinite power (current). It is my contention that this means that power delivered to the load has zero practical effect on the B-field.
But if you increase the voltage input and drop the turns ratio to maintain the same signal output then the power delivered to the same load is identical. This should maintain the same B field in the core, not decrease it.Look at the formula for b-field, it simply states that if you increase the voltage, you increase the flux. Nowhere is there a provision for the power delivered to the load. In other word, if you believe Lenz, loading of a transformer has no effect on the B-field.
The only way to decrease the B field in the core and get the same signal output is to do one or more of three things:
i don't see where the "same signal out" comes in here, that should simply be a matter of turns ratio, and the associated impedances.
increase the core volume
yes, this is the Ac (AREAcore not Vac in the formula)
increase the number of primary turns
yes, but i would simply state increase the turns. Adding the word primary only holds for a stepdown. if it were a step up you would have to say decrease the secondary turns and then it just gets confusing.
or decrease the permeability of the core material.
the perm of the core has no effect on the flux in the core, so i'm not sure what you mean here.
that the B field can also become too small and begin a different form of distortion - the effect of stuck "domains" in the core that need enough energy to overcome it.
blasphemy, don't you know that the lower the signal, the lower the distortion :-) Seriously, i agree 100% with you on this one and have been trying to at really low level behavior of cores and lets just say it ain't easy. this is probably getting a bit too technical so lets just say i may be your only supporter in this claim.
To be complete, you can also decrease the B field by increasing the frequency. (what a can of worms that statement opens :-)
Somewhere in this design there is some optimal size and turns ratio for a given signal level, load, and core material.
absolutely, I would also include source impedance, and consider signal level and source impedance to be the two most important things to know when starting a design. Those two numbers tell you how much flux and inductance you need, then the rest needs to be fit into place.
You're right Dave. I have forgotten too much to comment on this. I was trying to do it from my old memory. Disregard my postings on this. I don't know what I was thinking anyway.
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