impedance of the driving source, as seen from the secondary, and the other is the impedance of the load, as seen from the primary. An example of output transformer properties is shown below. The open circuit impedance, at 1 kHz, of either winding is about 150 k S . Since the DC resistance is about 40 S per winding, if the primary is short circuited, the secondary impedance will be 80 S . If we place the transformer between an amplifier and a load, the amplifier will "see" the load through the transformer and the load will "see" the amplifier output impedance (generally tenths of an ohm for amplifiers with negative feedback) through the transformer. In our example, the amplifier would "look like" 80 S to the output line or load and the 600 S load would "look like" 680 S to the amplifier. If the load were 20 k S , it would "look like" slightly less than 20 k S because the open circuit transformer impedance is effectively in parallel with it. For most loads, this effect is negligible. An example of input transformer properties is shown below. The open circuit impedance, at 1 kHz, of the primary is about 2 M S . Because this transformer has a 4:1 turns ratio, therefore 16:1 impedance ratio, the secondary open circuit impedance is about 125 k S . The DC resistances are about 2.5 k S for the primary and 92 S for the secondary. Since this is an input transformer, it must be used with the specified secondary load resistance of 2.43 k S for proper frequency and time domain responses. We can calculate that this load will "look like" about 42 k S at the primary, which certainly makes it suitable for a "bridging" input stage. To minimize the noise contribution of the amplifier stage, we need to know what the transformer secondary "looks like", impedance wise, to the amplifier. If we assume that the primary is driven from the line in our previous output transformer example with its 80 S source impedance, we can calculate that the secondary will "look like" about 225 S to the amplifier input. REFERENCES: B. Whitlock, "Balanced Lines in Audio - Fact, Fiction, and Transformers", Journal of the AES, Vol 43, No 6, June, 1995. Copyright 1995, Jensen Transformers, Inc.
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