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The secret to using iron IT's (like Lundahl 1660)

Posted by Kurt Strain on May 11, 2002 at 19:47:40

Compared to some other cores, iron is not good for low signal and low flux density. They have a lot of distortion at tiny signal levels, with the tendency to mush out low level details. I missed this key aspect of the Lundahl interstage in a prior PP IT application.

It helps to drive it with a low impedance tube, but that's not the whole story I figured out. It needs a real load on the secondary, something to push some power through the core. The hysteresis is wide on iron cores compared to nickel and cobalt. It needs energy to push it off its static position and start climbing the B-H curve as soon as possible. This is still true of cobalt to a lesser extent, but not so true about nickel, the least hysteresis. As a result, my feeling seems to be that cobalt is ideal for OPT's and probably a wide swinging IT. Permalloy and such are more ideal for smaller signals, like volume step attenuators and MC step-ups, designed for less amplitude swings.

I like the sound of my Lundahl 1660 IT with a 6H30 driver going through the IT and into a 20K ohm load (47K on secondary with 1:2.25 step-up). It now sounds very detailed and no significant mushiness like I experienced before doing BOTH these things - low drive impedance, low resistance load on the secondary.

Just another tip. :)

Kurt

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