Marinair vs St Ives- BH Curve Testing-
There is a big difference in the B-H curves between these two Line In transformers. I do not know how this affects the sound as I have not done a listening test on these two, but I would think you would notice something different on the bottom end.
The B-H curve is a combination between a magnetization curve and an integrated curve. They can be taken with one on the primary and one on the secondary, or both on the primary or secondary. Either way tends to give the same results, as you can saturate the core from either end of the transformer. The amount of steel dosen't change. More turns will require less current to saturate, and vice versa.
I usually use a test setup like this:
It's nice to have the scope grounds isolated by the transformer. Here is a picture of both waveforms before they are combined with the X-Y display function on the scope. The sawtooth is the integrated wave which is taken across the cap, and the other wave is the mag wave, which you might recognize from the previous post. It is taken off the resitor in series with the primary. You must adjust both amplitudes to be the same in order to get a correct BH curve. This is made easier if the scope has fine adjustments on the amplitudes:
Here is what the waves look like when they are assigned X and Y inputs on a non time domained sweep. The first rising edge is the transformer getting energized by the signal. When the core can not handle any more magnetic flux caused by an increasing voltage and thus current through the primary windings, the voltage on the other side of the transformer will no longer rise along with the primary voltage. No matter how much juice you put into the primary, the secondary voltages stays about the same because the magnetic flux in the core is at it's maximum. The rest of the energy is dissapated as a magnetic field out into free space, away from the core. The flat spot on top of the BH curve represents this maxed out voltage.
When the voltage through the primary decreases, the voltage on the secondary decreases also. But not right away. There is a slight delay, which is seen as the space between the first and second vertical lines. This is because the core is still magnetized in a North to South direction, and now you want to magnetize it from South to North. But it has a little memory which must be overcome. Reversing the polarity of the voltage does this, but with a slight delay. The energy lost in switching the magnetic flux around in an opposite direction is called core loss. This is one of the things that chews up your signal in a transformer, especially at the low end.
As the frequncy decreases, this gives the core a longer time to get magnetized. Thus, the lower the frequency, the more core loss, and therefore, the wider the area between the two vertical lines.
Here is the St Ives at 25 hz, and then 15 hz showing the increase in core loss. You have to click on these, don't want to load up the post to much:
http://vacuumbrain.com/The_Lab/TA/St_Ives/B-H_Curves/ives_25.jpg
http://vacuumbrain.com/The_Lab/TA/St_Ives/B-H_Curves/ives_15.jpg
So it is desirable to have core material that can have it's magnetic flux switched back and forth without using up a lot of energy. You can see this loss on the following BH curve. Remember that the delay in getting the core flux reversed is what seperates the two vertical lines. So the more distance between these lines, the more core loss. In fact, you can calculate the core loss by measuring the area inside the BH curve:
http://vacuumbrain.com/The_Lab/TA/St_Ives/B-H_Curves/bh_demo.jpg
OK, so now that your totally confused, lets look at some Marinair vs St Ives Neve Line In transformer curves.
Here is a side by side shot of the Marinair on the left, St Ives on the right, taken with a 20 Hz sine wave at 7.24 volts on each. Don't ask me what that weird lookin curve is from on the St. Ives, as I don't know:
http://vacuumbrain.com/The_Lab/TA/St_Ives/B-H_Curves/marives_20hz_sine_7.jpg
Here is a side by side of a 20 Hz square wave, at 7.28 VAC rms input, which is slammin line input iron pretty hard. Not a huge difference here:
http://vacuumbrain.com/The_Lab/TA/St_Ives/B-H_Curves/marives_20hz_sq_7-28.jpg
Another set of curves at 20 Hz square and 9.8 volts
The MArinair on the left needs a little vertical adj.:
http://vacuumbrain.com/The_Lab/TA/St_Ives/B-H_Curves/marives_20hz_sq_9.jpg
OK, next up- phase shift. Then I have to ship this St Ives outta here.
cj