the impedance balance of the output line, the transformer must also have balanced output capacitances . This requires a uniform distribution of primary to secondary capacitance across the windings. All these goals can be met in high performance output transformers built using bifilar (primary and secondary wound as if they were a single wire) designs and precision winding techniques.
Jensen AN-002 2 Jensen JT-11-DM 1:1 Output Transformer Jensen JT-10KB-D 4:1 Input Transformer Jensen Transformers, Inc. 7135 Hayvenhurst Avenue Van Nuys, California 91406 Van Nuys, California 91406 Tel (818) 374-5857 Fax (818) 374-5856 An INPUT transformer is driven by the balanced line and is typically loaded by the input of an amplifier stage. Its primary must have a high impedance to the differential voltage between the lines and this requires more turns of smaller wire producing relatively higher resistance windings . The transformer must also suppress any response to the common-mode voltage. A Faraday shield , connected to ground, is used to prevent capacitive coupling of the common-mode voltage from primary to secondary. Placing this thin copper foil between windings also reduces magnetic coupling, resulting in increased "leakage inductance" . To maintain impedance balance of the input line, the capacitance of the primary to the Faraday shield must be uniformly distributed across the winding. Because of its generally higher impedances and relatively high leakage inductance, the secondary load on an input transformer must be carefully controlled . The recommended load resistance and/or RC network must be used and load capacitance kept to a minimum. Generally, this means physically placing the input transformer as close as possible to the input amplifier stage. For example, the capacitance of 2 feet of shielded cable, about 100 pF, on the secondary of some input transformers will degrade bandwidth and transient response. WHAT’S THE "IMPEDANCE" OF A TRANSFORMER ? The "impedance" specification of audio transformers seems to confuse many engineers. Although they tend to produce optimum results when used with specified external impedances, the transformer itself has no intrinsic impedance . It simply reflects impedances, modified by the square of the turns ratio, from one winding to another. Keeping in mind that input and output power are equal, some simple application of Ohm’s law will prove this. The confusion probably stems from the fact that transformers can simultaneously reflect two different impedances . One is the
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