Magnetic Shielding of Transformers at Audio Frequencies

TN connection with the development of repeaters for long distance telephone lines it is found that noise is introduced into the telephone lines due to magnetic pick-up by transformers and coils in the repeaters. This applies also to sound pictures equipment, public and private address systems, etc., where high gain amplifiers are used. The stray magnetic field causing this pick-up may be produced by neighboring generators, transformers, rectifiers and other power equipment. It may also be produced by other amplifier transformers and coils or by relays located in the vicinity of the disturbed coil. The intensity of the disturbing field may frequently be of the order of 0.1 oersted at the point of pick-up. However, a field intensity of the order of 0.02 oersted often causes objectionable noise and under extreme conditions values as low as 0.001 oersted may be undesirable. As the gain of the amplifier increases and the demand for good quality becomes greater, it becomes increasingly important to control magnetic pick-up. The limiting of this pick-up is in fact today one of the important problems to be considered in the design of high-gain amplifiers. One method by which the magnetic pick-up can be decreased is by arranging the core structure and winding distribution of the transformer in such a way that the voltages induced by an external field are at least partially neutralized. In many cases, however, this MAGNETIC SHIELDING OF TRANSFORMERS 417 impairs other important characteristics of the transformer and is therefore undesirable.