sensitivity of our balanced system bridge to impedance unbalances by making

Sensitivity of our balanced system bridge to

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sensitivity of our balanced system (bridge) to impedance unbalances by making common-mode impedances very low at one end of our line and very high at the other. To deliver the maximum signal voltage from driver to receiver, the differential impedance must be low at the driver and high at the receiver. Most line drivers with low differential output impedance will also have inherently low common-mode output impedances, so the low impedance arms of the bridge are almost always at the driver. Generally, in professional audio, low impedance means under 100 S and high impedance means over 10 k S . Active circuits having high differential input impedance (sometimes called "bridging" since several can be bridged across a line with minimal level loss) are widely used, but the importance of the common-mode input impedances of these circuits has long been ignored by most designers. Almost all such "electronically balanced" inputs have common-mode input impedances in the 5 k S to 20 k S range, which seriously degrades their performance as a balanced line receiver. The common-mode input impedances of an input transformer are inherently about 1000 times that of most "actively balanced" inputs, giving the transformer about 60 dB better ground noise rejection in the real world, where significant source impedance unbalances almost always exist. WHAT’S THE DIFFERENCE BETWEEN "OUTPUT" AND "INPUT" TRANSFORMERS ? Most simply stated, output transformers are used at the low impedance or driven end of a balanced line and input transformers are used at the high impedance or receiving end. The technical requirements, and as a result, the designs and physical constructions, of the two transformer types are very different . An OUTPUT transformer is driven by an amplifier and typically loaded by several thousand pF of cable capacitance plus the 20 k S of a "bridging" line receiver. At high frequencies, most of the driver output current is used charging the cable capacitance. Occasionally, and usually because someone mistakenly thinks they must "terminate" a cable, a real 150 S or 600 S load must be driven, requiring even more driver output current. Therefore, an output transformer must have a low output impedance, especially at high frequencies. This requires low DC resistance windings and very tight magnetic coupling , since the sum of the winding resistances and the "leakage inductance" resulting from imperfect coupling are effectively placed in series between amplifier and load. To maintain
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  • Jensen Transformers

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