At your risk.
A use is for reducing bass or mid bass, mid heaviness to listener's ear brain when for example recording vocals or in a live concert.
It is an inductor L in series with a resistor R and the resultant connected across pins 2 and 3 of microphone output (or preamp or mic booster input).
Both L and R can be continuously adjustable. Varying L adjusts turnover or cutoff frequency, varying R adjusts slope. Obviously a high R leaves microphone output unchanged.
R can be a simple potentiometer of say 1KΩ but how can L escpcially a desirable toroidal inductor be variable?
The answer was found yesterday by almost chance on US2762020 and US3100882, J. F. Gordon and Norman Burnell patents respectively.
The toroidal core is sandwiched between 2 longitudinal magnets allowed to rotate relative to each other.
When they attract each other magnetic lines are flowing perpendicular to core and its inductance index remains the same.
But when they repel each other magnetic lines of force (repelling ones) are flowing in the direction of the toroid and its inductance index becomes almost 0. Similar action to the Fostex printed ribbon microphone operation with repelling magnets.
Between these 2 extremes the inductance index can be smoothly varied.
It was tried today on a tape (mumetal?) tape wound core coming as a separately bought spare for a Coles 4038 core for input transformer. Mr Beanland kindly supplied this on around 2000.
Such core with a small quality of wirewrap wire turns has an inductance of 40mH. This inductance was gradually brought to almost 0 by rotating one of the magnets. 2 much longer magnets that the core were used covering all its are, these magnets came from a Meazzi ribbon microphone which has now Neodymium replacements but still is not re ribboned.
It has also been observed that if a thin foam material (the protective wrapping material of the magnets) in placed between each magnet the inductance can be changed from 10mH to 0 just by the amount of hand pressure between the 2 magnets making the sandwich with the mumetal tale wound core. The may be another method of adjusting bass cutoff frequency.
It has not been tried yet connected to a mic.
Hopefully it will aid flat frequency response from producer's brain to listener's brain. [Lowe, Morgan]
Would it be feasible to make such a variable filter inside Neutrik cylindrical XLR inline Modules using the rotating switch disk mechanism as a means to rotates a magnet?
How would a microphone input transformer sound like by adding a secondary winding?
Reference:
Sound Picture Recording and Reproducing Characteristcs - D. P. Lowe, K. F. Morgan - Journal of the Society of Motion Picture Engineers
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