[Object]To downsize a power amplifier, while keeping isolation between a transmission line and ground potential.[Means to Realize Object]Two terminals (2a, 2b) of an audio signal source (2) are connected to an input of an isolation input circuit (6). An audio signal developed, being isolated from the input-side, between two output-side terminals (6a, 6b) of the isolation input circuit (6) are inputted to a power amplifying stage (12). The power amplifying stage (12) amplifies the audio signal and outputs it at a high voltage from two output terminals (12c, 12d) to a plurality of loudspeakers (22) without using an insulation transformer. Operating power is supplies to the power amplifying stage (12) from two power supply terminals (20c, 20d) of a DC power supply (14). One (6d) of the terminals of the insulation input circuit (6), one (12d) of the output terminals of the power amplifying stage (12) and one (20d) of the power supply terminals of the DC power supply (14) are connected together to provide a common potential point different from the ground potential.

TECHNICAL FIELD

This invention relates to a public-address system and, more particularly, to such system having a high impedance output.

BACKGROUND ART

Some public-address system include high-impedance connection of a power amplifier to a plurality of loudspeakers, as disclosed in Non-patent Literature 1. In high-impedance connection, a voice coil of each loudspeaker is connected in a secondary side of a matching transformer. The impedance of the secondary side is made to match with the impedance of the voice coil. The impedance of the primary side of the matching transformer is chosen to be high. The primary sides of these matching transformers are connected in parallel via a set of transmission lines, and, then, connected to the output of the power amplifier. The output impedance of the power amplifier is made to mach with the parallel impedance of the parallelly connected matching transformers, and the power amplifier is adapted to provide a high output voltage. With this arrangement, loss in the transmission lines can be reduced.Non-Patent Literature 1: Revised New Edition of PA Audio System edited by Shigeo Fujioka, published by Kogaku Tosho Shuppan K K, Page 200

DISCLOSURE OF THE INVENTION

Problem to Be Solved by the Invention

Usually, an output transformer is used for obtaining a high output voltage from a power amplifier. The use of the output transformer can prevent large current from flowing to the ground through the power amplifier even when a transmission line is struck by lightning, for example. However, since the output transformer needs to reproduce audio signals having frequencies of from 20 Hz to 20 KHz, the core of the output transformer is large, and, as a result, the power amplifier must be large-sized.

An object of the present invention is to provide a small-sized power amplifier, while maintaining isolation between a transmission line and the ground potential, in a high-impedance public-address system.

Means to Solve the Problem

A public-address system according to an aspect of the present invention includes an audio signal source. The audio signal source outputs an audio signal between two terminals thereof. One of the two terminals is grounded. The two terminals of the audio signal source are connected to the input side of isolation input means. The isolation input means has two output-side terminals, between which the audio signal is developed, being isolated from the said input side. The audio signal is inputted to power amplifying means from the two output-side terminals of the isolation input means. The power amplifying means amplifies the audio signal inputted thereto and develops the amplified audio signal at a high voltage between two output terminals. The power amplifying means has an output side which is transformerless (i.e. uses no isolation transformer). A plurality of loudspeakers are connected in parallel with each other between the two output terminals of the power amplifying means. DC power supply means supplies operating power to the power amplifying means. The DC power supply means has two power supply terminals, through which operating power is supplied to the power amplifying means. One of the terminals of the isolation input means, one of the output terminals of the power amplifying means and one of the power supply terminals of the DC power supply means are connected together to provide a common potential point which is different from the said ground potential.

The power amplifying means of the public-address system with the above-described arrangement is isolated from the ground potential in its input-side since one of the terminals of the isolated input means, one of the output terminals of the power amplifying means and one of the power supply terminals of the DC power supply means are connected together to provide a common potential point, which is different from the ground potential. This can eliminate the need for using a transformer in the output-side, which makes it possible to use small-sized power amplifying means.

The isolation input means may be a transformer. The transformer has its primary winding connected between the two terminals of the audio signal source, and has its secondary winding connected to the two output-side terminals of the isolation input means. The isolation input means may include light-emitting means emitting light in accordance with an audio signal from the audio signal source, and light-receiving means providing an electrical signal in response to the reception of light from the light-emitting means.

BEST MODE FOR CARRYING OUT THE INVENTION

A public-address system according to one embodiment of the present invention includes an audio signal source2as shown inFIG. 1. A microphone, for example, may be used as the audio signal source2. The audio signal source2provides an audio signal between two terminals2aand2b. One,2b, of the terminals is grounded.

The terminals2aand2bof the audio signal source2are connected to isolation input means of a power amplifier4, e.g. two input terminals6aand6bof an isolation input circuit6. The input terminals6aand6bserve also as input terminals of the power amplifier4. The isolation input circuit6has two output terminals6cand6d, too.

The isolation input circuit6may be composed of a transformer8like the one shown inFIG. 2. The transformer8has a primary winding8aand a secondary winding8b. The two ends of the primary winding8aare connected to the input terminals6aand6b, respectively, and the two ends of the secondary winding8bare connected respectively to the output terminals6cand6d. Neither of the output terminals6cand6dis connected to the input terminal6aor6b. Accordingly, an audio signal is outputted between the output terminals6cand6d, being isolated from the primary winding8a.

Alternatively, the isolation input circuit6may be composed of a light-emitting device, e.g. a light-emitting diode10a, and a light-receiving device, e.g. a photodiode10b, as shown inFIG. 3. Specifically, the light-emitting diode10ahas its two ends connected to the input terminals6aand6b, respectively, and emits light in response to the flow of the audio signal therethrough. The emitted light is received by the photodiode10bconnected between the output terminals6cand6d, causing current to flow therethrough. Neither of the output terminals6cand6dis connected to the input terminal6aor6b. Accordingly, an audio signal is outputted between the output terminals6cand6d, being isolated from the primary winding8a. It should be noted that, although a bias circuit is actually provided for each of the light-emitting diode10aand the photodiode10b, they are not shown inFIG. 3since such is known. A photocoupler may be used as the light-emitting diode10aand photodiode10b.

The audio signal developed between the two output terminals6cand6dof the isolation input circuit6is applied to two input terminals12aand12bof power amplifying means, e.g. a power amplifying stage12. The power amplifying stage12is of an OTL configuration, which does not include an output transformer, for power amplification and develops a high output voltage between two output terminals12cand12dthereof. The output terminals12cand12dare also output terminals of the power amplifier4, and the output terminal12dis connected to the input terminal12b. A power amplifying stage of various types may be used as the power amplifying stage12. For example, an analog power amplifying stage, e.g. a Class AB or Class B power amplifying stage, may be used, or a Class D switching-type power amplifier may be used.

The power amplifying stage12has power supply input terminals12eand12f. A DC power supply14is provided for supplying operating power to the power supply input terminals12eand12f. The DC power supply14has a transformer16for transforming commercial AC power. The commercial AC voltage is voltage-transformed, and the voltage-transformed commercial AC voltage is induced in the secondary side of the transformer16. The induced commercial AC voltage is isolated from the ground potential, and is applied between two input terminals18aand18bof rectifying means, e.g. a rectifying bridge circuit18, which results in a rectified voltage developed between two output terminals18cand18d. This rectified voltage is applied between two input terminals20aand20bof a smoothing circuit20including a plurality of smoothing capacitors and smoothing reactors, for example, so that a smoothed DC voltage is developed at a power supply terminal, or between two power supply output terminals20cand20d, for example. The power supply output terminal20cof the smoothing circuit20is connected to the power supply input terminal12eof the power amplifying stage12, and the power supply output terminal20dis connected to the power supply input terminal12f, which, in turn, is connected to the output terminal12dand the input terminal12b. Thus, the smoothing circuit20and the power amplifying stage12operate on the output terminal20dserving as a reference potential, and, thus, does not operate on the ground potential serving as a reference potential.

A plurality of loudspeakers22are connected in parallel between the output terminals12cand12dof the power amplifying stage12via a pair of transmission lines24. Although not shown, the loudspeakers22have their voice coils connected to the secondary sides of their matching transformers, and the primary sides of the matching transformers are connected together via the transmission lines24. Since one of the terminals of the primary side of each matching transformer is connected to the output terminal12d, each loudspeaker22operates on the power supply output terminal20dof the smoothing circuit20serving as a reference potential and, thus, does not operate on the ground potential serving as a reference potential.

The transmission lines24, the power amplifying stage12and the smoothing circuit20of the public-address system with the above-described arrangement do not operate on the ground potential, and thus are isolated from the ground potential by the isolation input circuit6disposed in the input side of the power amplifying stage12. Accordingly, even when the transmission lines24are struck by lightning, it does not occur that large current flows to the ground potential, so that a required breakdown resistance can be secured. In addition, since the securing of this breakdown-withstanding voltage is realized by the use of the isolation input circuit6disposed on the input side of the power amplifying stage12, there is no need to use a transformer in the output side of the power amplifying stage12, which makes it possible to downsize the power amplifier4using the power amplifying stage12.