Abstract: Compressor for use with an audio amplifier, and of the type wherein a junction FET connected to the input of an amplifying means, has its control terminal connected (a) to the output of said amplifying means through a control transistor, and (b) to either a reference voltage lead through a first resistor, or the input of said amplifying means through another resistor. With such an arrangement the control action of said junction FET is automatically adjusted and adapted to the operating range of output signals of said amplifying means, without the need of separate trimming means.

Abstract: A short-circuit protection device for an audio amplifier, comprising a preamplifier and an output amplifier including one or more current amplifier branches each having an output transistor included therein. A measuring resistor is connected between an electrode of each output transistor and the output of the amplifier. A feedback transistor is connected in parallel with at least one of the measuring resistors and is connected to the control electrode of a thyristor. The thyristor has an electrode connected to one or more current source transistors so as to block these transistors and to decouple the output amplifier upon the occurrence of an excessive current in one of the current amplifier branches of the output amplifier.

Abstract: A speaker system comprises an enclosure and at least one speaker mounted therein. The enclosure includes at least one duct connecting the interior of the enclosure directly with the outer air at the front thereof. Each duct is confined at least at one side by a member extending substantially normal to and running substantially the width of the front of the enclosure. The member has its front and/or rear edge rounded so that the cross-sectional dimension of the opening of the duct at the respective edge is greater than that of the central section of the duct.

Abstract: A system for reproducing phase stereophonic signals, in which the audio signals applied to one or more inputs of the system are split up at each input to pass along two parallel paths. Each of these paths includes a frequency-dependent, phase-shifting means, and the phase-shifting means of each pair of paths are interconnected through a potentiometer. The output signals of each pair of phase-shifting means are respectively applied to two reproducing devices. The phase-shifting means may be arranged for individually frequency-dependent operation.

Abstract: An electronic reverberation device comprising an input amplifier and an output amplifier connected in a circuit in which a delay device is connected between an output of the input amplifier and an input of the output amplifier, and a feedback path connects an output of the output amplifier to an input of the input amplifier. In order to render the operation of the device substantially frequency-independent, the delay line is a variable delay line the delay of which can be regulated by a regulator controlled by a control device which derives a control voltage from the output of the input amplifier. Preferably, the delay device is a bucket memory, the regulator a clock, and the control device comprises a diode having one terminal connected to the output of the input amplifier and another that is earth-connected through a capacitor and connected through a resistor to a control input of the clock.

Abstract: An electronic echo apparatus comprising an input amplifier, an output amplifier connected thereto through a transmission channel, a delay device connected in parallel to said channel, and a feedback path connected in parallel to said delay device, characterized in that said feedback path includes a control means, and in that there is connected to the output of the output amplifier a means for measuring the output signal of said output amplifier, comparing said output signal to a predetermined threshold value and, when said output signal reaches said threshold value, supplying a control signal to said control means for terminating the feedback.

Abstract: An electronic echo device comprising an input amplifier, an output amplifier connected therewith across a transfer channel, a delay device connected in parallel to said channel, and a feedback path connected in parallel to said delay device. According to this invention a control device is included between the input of the echo device and the delay device, to which control device the feedback path is also connected. The arrangement is such that the gain factor of the control device, upon a relatively sudden change in the signal level supplied to it, varies to counter-act said change.

Abstract: An amplifier having a plurality of cascaded stages each comprising an operational amplifier having an output connected to a first output terminal of the amplifier through two series-connected resistors and a capacitor, the junction point of the two resistors being connected to the negative input of the operational amplifier, and the junction point of the capacitor and one of the series-connected resistors being connected to the positive input of a next operational amplifier, the output of each operational amplifier being additionally connected to said first output terminal through a variable resistor having a movable contact connected to the second output terminal of the amplifier.

Abstract: A device for protecting an audio amplifier against overload or short circuit. The device includes an output amplifier comprising a plurality of branches having output transistors, a preamplifier to control the output amplifier, and means for limiting the current through the transistors at an allowable value. The device further includes means which, when the current through the output transistors exceeds this value, limit this current at this value, and means which, in the presence of such an unallowably high current in one of the branches, immediately disconnect the output amplifier irrespective of the fact that this current occurs in the one or in the other branch.

Abstract: A transistor amplifier comprising a first transistor having its base connected to the input terminal of the amplifier and having one of its further electrodes connected through a coupling capacitor to the output terminal of the amplifier. The transistor has the other one of its further electrodes connected to the base of a subsequent transistor having the corresponding electrode of its further electrodes connected through a second coupling capacitor to a second output terminal of the amplifier. The capacitance of the second coupling capacitor is higher than that of the first coupling capacitor.