Portable Transistor Music System
A transistor power amplifier circuit has been developed in which all transformers and interstage coupling capacitors have been eliminated by dc coupling, even to the loudspeaker. Its transistor output stage is operated in push-pull class B and delivers 20 watts from a standard 45-volt battery and four mercury flashlight cells. The amplifier uses 13 transistors. Because of its excellent overload characteristics the amplifier is capable of producing listening levels, with unnoticeable distortion, equivalent ot the output of a 100-watt amplifier. Distortion below the overload point is held to an extremely low value by means of 60 db of feedback around eight stages. The power amplifier is installed together with batteries, a tone control preamplifier unit, and an efficient 15-inch woofer and horn tweeter combination in a 2.3 cubic foot cabinet. Contrary to usual practice, the enclosure is deliberately made highly reverberant. The tone control amplifier has 11 transistors, operates on self-contained batteries, and may be used as a remote controller. It incorporates a four-channel mixer, provision for monitoring from tape while recording, a magnetic phone pickup preamplifier, tone controls, and equalization for the acoustic response of the speaker system. Noise and distortion are extremely low. The entire system is designed to operate at ambient temperatures from 0 to 125 degrees Fahrenheit. Its total weight is 57 pounds. For easy portability the cabinet is equipped with a handle and wheels.
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