A signal sensing circuit (10) receives an electrical signal on an input (18, 20) to connect power to a controlled device in the presence of an electrical signal and disconnect power in the absense of the electrical signal. The electrical signal is capacitively coupled (28) to an amplifier (30) and low pass filter (32) for amplifying the signal and filtering out high frequency components of the signal thereby eliminating the false triggering by transient noise on the inputs. The signal is rectified (34) and compared (38) to a predetermined set point. When the signal exceeds the set point, a timer (44) is triggered. The timer (44) produces a control signal when triggered for a predetermined time, and can be retriggered by subsequent control signals which resets the predetermined time. The presence of the control signal actuates a switch (48) to supply power to the control device, such as an amplifier.

An automatic turn-off assembly (10) for supplying and discontinuing power to a device (12) in response to an audio signal. The audio signal is rectified (22) producing a level signal which is compared by a window comparator (26). When the level signal is within the window, a trigger signal is produced to initiate and reset a timer (30), which in turn produces a control signal for a predetermined time while being powered. If the level signal is below the window, the timer (30) is not reset. If the signal is above the window, a muting circuit (44) is initiated to mute the device (12), which is preferably an audio amplifier. The assembly (10) includes a manual push-button switch (18) for initially supplying power to the device (12) and to the timer (30). A relay (RL1) maintains the supply of power in the presence of the control signal and disconnects power from the device (12) and timer (30) after the predetermined time. The device (12) may be manually turned off to disconnect power thereto prior to the expiration of the predetermined time.

A power supply circuit arrangement for electrical equipment, such as a loudspeaker with an integral power amplifier, which can be switched between an operational mode and a standby mode in response to a detected on/off signal, provides a desirable isolation of the main supply voltage from the detector. The arrangement comprises a transformer, having a primary winding and a secondary winding, and a switched impedance device between the transformer and the power supply input. The switched impedance device is switchable between high and low impedance states in response to the detected on/off signal. A control circuit for detecting the on/off signal and switching the variable impedance states is connected in common with a main circuit of the equipment to the secondary winding of the transformer, so that their respective supply voltages are both switched, in common, by switching of the impedance device. The control circuit and variable impedance device isolate the detector from the power supply. In the operational mode, the variable impedance is switched to its low impedance and the common voltage supplied to the main circuit and the control circuit is sufficient to operate both of them. In the standby mode, the variable impedance is high and the common voltage is inadequate for normal operation of the main circuit but adequate for normal operation of the control circuit.

A non-linear, order-statistic filter that is able to detect transients within an ambient signal environment without the transients affecting the setting of the transient detection threshold. The filter obtains a power spectral estimate (PSE) signal for each one of a plurality of predetermined bandwidth portions of the ambient input signal. Specific PSE magnitude values making up the PSE signal are sorted and ordered from smallest to largest. An average PSE magnitude value is obtained for a preselected center range of the specific PSE magnitude values, and this value is used to set the transient signal detection threshold for the filter.

A powered volume control is provided for a distributed audio system having a plurality of remote speakers. An amplifier or signal conditioner is provided having a dual channel amplified signal from a high impedance source. The amplified or conditioned signal is provided to a plurality of remotely located powered volume controls having high input impedance and further having internal amplifiers which provide a dual or mono-channel amplified signal having an amplitude or magnitude determined by a user variable adjustment device. The amplified signal is then provided to one or more remote speakers. Each volume control is designed for in-wall installation in a single-gang wall box and may be covered by an ornamental face plate. The system is also designed to utilize existing four conductor speaker wire for installation or retrofitting existing distributed audio systems.