Audio transformers improve sound quality by removing interference from audio signals. This interference, or ground noise, is caused by voltages from other devices and produces a humming or buzzing sound. Typically, audio transformers are encased in a magnetic shielding which is filled with an epoxy resin that provides insulation, protects the windings, and prevents vibration of the core material. Some audio transformers do not have a center tap, while other devices have a center tap in only the primary side, only the secondary side, or in both the primary and secondary sides. For audio transformers, the impedance ratio equals the square-of-the-turn ratio.
Audio transformers vary according to output power, operating frequency range, and rated DC current. They also vary in terms of insertion loss, 3-decibel (dB) bandwidth, and direct current resistance. Insertion loss, the measured loss through a device excluding the power division factor, is the ratio of power output to power input. 3-dB bandwidth is the frequency range over which the insertion loss is less than 3 dB for mid-band insertion loss. Direct current resistance (DCR), the resistance of windings as measured in DC current, is often minimized in the design of audio transformers and specified as a maximum rating.
There are many ways to package audio transformers. Flat packs are integrated circuit (IC) packages with gull wings or flat leads on two or four sides. Modular jacks incorporate the RJ-45 form factor and ensure high common mode noise immunity while maintaining signal integrity. Waveguide assemblies, which may contain solid or gaseous dielectric materials, have a hollow metallic conductor and are used in microwave systems. With through-hole technology (THT), components are mounted on printed circuit boards (PCBs) by inserting component leads through holes in the board and then soldering. In surface mount technology (SMT), components plug into PCBs by soldering component leads or terminals to the top surface of the board. Other ways to package audio transformers include chassis, dish, or disk mounts. H-frame mounting is used in applications with high levels of shock or vibration.
Manufacturers use several methods to pack audio transformers for automatic assembly, shipping, and handling. The tape reel method packs components into a tape system and draws specified lengths or quantities into a reel. The stick magazine method packs components into a tube. Audio transformers that are distributed as individual parts are processed in bulk packs, while components that have leads in four-sides use trays.
Audio transformers are used in car radios and broadcast equipment, and in sound reinforcement applications. In terms of certifications, audio transformers are built and tested according to a variety of standards. For example, both Underwriters Laboratories (UL) and the Canadian Standards Association (CSA) provide marks. The International Electrotechnical Commission (IEC) also publishes applicable standards.
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