[Ian Bell[_2_]]

Ian Iveson wrote:
So my question is, how does the inductance vary with (dc)
current. If it measures higher with lower dc current then
if I use a 10W instead of a 5W but with a lower dc
current, do I get even higher inductance because of the
lower current?

This isn't a simple question, IIRC from longago posts. I
venture the following expecting a tongue-lashing.

The inductance varies with the slope of the BH curve. If you
look at one common, DC depiction of that curve, it appears
as a sigmoid, shallow at the zero crossing because the
magnetic domains exhibit what you might call stiction,
becoming steeper, and then shallow again as the majority of
domains reach their elastic limit, and some reach the yield
point.

The yielding dissipates power, resulting in hysteresis.
Consequently, for AC, the sigmoid becomes a loop, steepest
at the zero crossing, and shallow at both extremes of its
variation. What's more, any AC signal will generate a
sigmoid, with the magnetic bias affecting how flat or steep
the sigma-shaped loop is, according to where its centre is
on to the DC BH curve.

Break for ridicule.

Now, it seems from the DC curve that inductance should be
zero as B crosses the H axis. Much nonsense arises from this
interpretation. In fact, for pure AC the curve is steepest
there, both on the upward and downward journey.

However the sigmoid is flattish for small AC signals, so the
mean inductance is lowish. Then it gets steeper as the
signal increases, so the mean inductance rises, and then
begins to flatten out at its extremes and the inductance
falls again, as the core saturation approaches.

Still with me?

Add DC and small signals see a higher inductance, but the
average inductance for larger signals doesn't increase very
much, especially if the signal results in the core getting
to the sharper curve approaching saturation.

I would have thought that the great benefit of using a
transformer rated for high DC current, assuming the same
rated inductance, would be that it gives you greater
separation between roll-off and saturation. That means you
can use more feedback to extend its bass range at a given
max signal. Alternatively, some designs bias the SE
transformer and deliberately oversize it, to avoid the bass
disappearing at small signal levels. So they say, I think.

So, the answer is: yes and no.

Ian


"Ian wrote in message
...
Just wanted to add a related question. In looking up SE
transformers on the net many that quote inductance do so
at a specified dc current. So the difference between a 5W
and a 10W transformer might typically be the 5W has a 10H
primary inductance measure at a dc current of 48mA whereas
a 10W type would have an inductance of 15H at a dc current
of 64mA.


Cheers

Ian





Thanks for that. Not sure I understood it all. Magnetic theory I have always
found difficult.

From what I have read recently in RDH4 is seems that the more primary Henries
you have the better bass extension you get which makes sense sine the reatance
is in parallel with the load. Also the number of Henries you get depends on the
signal level being lowest at low signal levels and RDH4 recommends quoted
primary Henries should be measured at low levels. I have seem SET transformer
specs give values for primary inductance but not one mentions the level it is
measured at. I doubt it is safe to assume they are all made at low levels.

Cheers

Ian