Abstract: A hearing protective device consisting of a headset having opposite earmuff and/or earplug assemblies resiliently connected by a headband assembly including a normally open power switch closed when the headband assembly is put on the user's head. The switch controls the energizing of the electrical circuitry of the device. Each earmuff and/or earplug assembly has an outwardly-facing microphone and an inwardly-facing sound reproducer, connected to circuitry defining respective stereo channels, each channel including preamplifier circuitry and a respective power amplifier drivingly connected to one of the sound reproducers. The circuitry also includes a hyper AGC circuit which receives and sums signals from the amplifier chain and derives attenuation signals therefrom which are fed back to the amplifier circuitry and which reduces the gain of this circuitry when the input sound exceeds a certain level.

Abstract: An energizing circuit for applying maximum electrical power to a heatable element having, at least, a high initial thermal lag. By sensing the resistance of the element, which varies as a function of its temperature, a control voltage is obtained which is used to vary the effectiveness of the heating current so as to provide rapid initial heating without the danger of burn-out. More specifically, the circuit is useful in energizing the glowplug of a model airplane engine.Current pulses are obtained from a storage battery which is connected in series with the glowplug and a semiconductor switch. The switch is turned on and off by a multivibrator to produce a series of current pulses. The duration of each "on" pulse is controlled by a bridge circuit which senses the resistance of the glowplug, which is a function of its temperature, and produces a control voltage which varies the timing of the multivibrator.