21 TRANSMITTERS, TRANSCEIVERS, RECEIVERS, AMPLIFIERS, PREAMPLIFIERS, TUNERS, SPEAKERS, MULTIPLEX ADAPTORS, STEREOPHONIC CONTROLS, REVERBERATION UNITS, ANTENNAS, PUBLIC ADDRESS SYSTEMS, INTERCOMMUNICATION SYSTEMS, TRANSFORMERS, CHOKES, TWINLINE, MICROPHONES, HEADSETS, MICROPHONE STANDS, DIRECT CURRENT POWER SUPPLIES, AND CODE PRACTICE SETS