GB2142509A - Audio amplifier - Google Patents

Abstract

An audio amplifier having a single two terminal input 20, 22 for receiving both a signal for amplification and electrical power for use in amplifying a capacitor 28 for coupling the signal from the two terminal input to the amplifier for amplification and for blocking the electrical power therefrom, and a voltage regulator 26 for separating electrical power from the signal for amplification and for providing said electrical power to the amplifier. <IMAGE>

Description

SPECIFICATION Audio amplifier The present invention generally relates to audio amplifiers, and in particular, to audio amplifiers in combination with speaker for remote locations.

Public address systems and audio broadcast systems are well known and widely used. Generally, such systems include an audio source, an amplification means, and one or more centrally or remotely located speakers. One variation of this scheme includes a central audio source, which may or may not include some amplification, and one or more remote amplifier/speaker combinations. Such systems require the transmission from the central audio source to the remote amplifier speaker locations of both the audio signal for amplification and an electrical power signal for use by the amplifier in amplifying the audio signal. One embodiment of this arrangement is shown in U.S. patent No. 4,258,332 which uses a two wire transmission system to carry both the audio signal and a direct current electrical power voltage.The amplifier includes means for separating the audio signal and the electrical power.

The audio signal is separated by a capacitor and the electrical power is separated by a magnetic core inductor which passes only the DC voltage and blocks the audio signal. Unfortunately, such a system is subject to variations in the power output due to changes in the DC voltage transmitted for power amplification purposes. Further disadvantages may be encountered due to the size and weight of the magnetic core inductance used to separate the direct current voltage from the audio signal.

The present invention provides an audio amplifier of the type having an amplifier means, a single two terminal input means for receiving both a signal for amplification by the amplifier means and electrical power for use by the amplifier means in amplifying the signal, and circuit means for separating the signal from the electrical power, the audio amplifier further comprising capacitor means for coupling the signal from the two terminal input means to the amplifier means for amplification and for blocking the electrical power, and voltage regulator means coupled to the input terminal means for separating the electrical power from the signal for amplification and for providing the electrical power to the amplifier means for use in amplifying the signal.

There now follows a detailed description, to be read with reference to the accompanying drawing, of an audio amplifier embodying the invention. It will be realised that this amplifier has been selected for description to illustrate the invention by way of example.

The accompanying drawing is a schematic diagram of an audio amplifier embodying the invention.

In reference to the drawing an audio amplifier circuit 10 is shown which generally includes an input section 12, a filtering or separating section 14 and an amplification section 16. A speaker means 18 is shown connected to the output of the amplification section 16.

Generally, the input section 12 includes two pairs of input terminals 20 through 23. The second pair of terminals 22 and 23 are connected directly to respective terminals 20 and 21. The purpose for duplication of input terminals is to allow easy connection of the amplifier speaker combination to a transmission line with other amplifier/speaker combinations. Input terminals 20 and 22 are coupled to the filtering or separating section 14 while input terminals 21 and 23 are coupled to the ground of the circuit 10. Input terminals 20 and 21 are used to receive an input signal which is a combination of an alternating current audio frequency signal and direct current voltage for supplying electrical power to the circuit 10.

Terminals 20 and 22 are coupled to the anode of a diode 24, the cathode of which is coupled to both the input of a voltage regulator 26 and a capacitor 28.

The voltage regulator 26 and capacitor 28 make up the separating section which is used to separate the audio signal and the direct current voltage electrical power. Many different voltage regulators may be used for voltage regulator 26. One unit which works quite satisfactorily is made by National Semiconductor and is an integrated circuit, three terminal voltage regulator designated LM317. As specified by the manufacturer, National Semiconductor, this voltage regulator requires the addition of two resistors as shown at 30 and 32 for the purpose of setting the output voltage, Thurly connected, the voltage regulator 26 separates the direct current electrical power from the input terminals for use in powering the amplifier section 16. The LM317 voltage regulator which is used has a performance characteristic including an 80 decibel ripple rejection.That is, ripple or alternating current appearing on the input terminal to the voltage regulator 26 is reduced by 80 decibels at the output terminal of the voltage regulator. As mentioned, the separating section 14 also includes a capacitor 28. In the present instance, an electrolytic capacitor is used which has a positive terminal connected to the cathode of diode 24.

The other side of capacitor 28 is used for supplying the audio signal to the amplifier section 16 for amplification purposes. A plurality of resistors 34 to 39 are coupled in series, and the voltage divider formed thereby is coupled between the other end of capacitor 28 and the ground of circuit 10. This voltage divider provides a multiple of audio signal voltage levels which are used to allow variation of the output signal produced by the amplifier. The interconnection points of resistors 34 through 39 are each connected to one of a separate pair of jumper terminals located on a jumper terminal block 40. The other terminals of each of the pairs of jumper terminals are connected in common and through a capacitor 42 to the input of an amplifier means 44.By placing a jumper 41 across any pair of terminals of jumper block 40, the audio signal is coupled to the input of amplifier means 44. The specific pair of jumper terminals used determines the signal level coupled to amplifier means 44 and thereby the output signal level of amplifier means 44.

The amplifier means of the present embodiment is an integrated circuit, operational amplifier also manufactured by National Semiconductor and de signated LM383. A comparable substitute is manufactured by Fairchild and designated TDA 2002. The capacitor 42 is coupled to the non-inverting input thereof. The amplifier means 44 includes power input terminals 46 and 48, the latter of which is coupled to the circuit 10 ground and the former of which is coupled to the output of voltage regulator 26. Thus, the voltage regulator 26 supplies the electrical power for the amplifier means 44 to perform its amplifying function. A capacitor 50 is also coupled to the output of voltage regulator 26 and has its other end connected to the amplifier ground for the purpose of filtering out any spurious signals in that line.Another capacitor 52 is used for the same purpose between the output of amplifier means 44 and the amplifier ground. A feedback path is used for amplifier means 44 between its output terminal and its inverting input terminal and includes a resistor 54 connected in series with a capacitor 56. The interconnection between resistor 54 and capacitor 56 is coupled to ground through an additional resistor 58.

It should be noted that the LM383 and this type of amplifier means has the advantage of a performance characteristic of an 80 db power supply rejection ratio. In other words, signals appearing at the power supply input terminals 46 and 48 are reduced by a factor of 80 decibels at the output terminal. This characteristic further enhances the separating of the audio and DC power signals.

The output of amplifier means 44 is coupled through a DC blocking capacitor 60 to one terminal of the driving coil 62 of speaker means 18. The other end of driving coil 62 is connected to ground.

In operation, the circuit 10 performs in the following manner. An input signal is received on terminals 20 and 21 which includes a direct current voltage and the audio signal superimposed on the direct current voltage. The direct voltage is generally positive on terminal 20 with respect to terminal 21.Thecom- bined signals pass generally as is through diode 24, except for approximately a 0.6 volt forward bias voltage drop thereacross. The capacitor 28 separates the audio signal from the combined signal and passes it on to the input of amplifier means 44 while blocking the DC voltage from continuing likewise.

The voltage regulator 26 which is also coupled to the cathode of diode 24 conversely allows the direct current voltage to pass therethrough while blocking the audio signal. The direct current voltage is not passed in its entirety; but rather only that portion representing the regulated output voltage is allowed to pass. This is connected to the power input for amplifier means 44 for use in amplifying the audio signal. As mentioned, the audio signal is connected to the voltage divider including resistors 34 to 39, from which various voltage levels thereof may be connected to the non-inverting input of amplifier means 44. This connection is made by means of jumper 41 which may be located at any one of the pairs of jumper terminals of jumper block 40. The amplifier means 44 uses the DC voltage supplied thereto to amplify the audio signal and thus power the speaker means 18 for audio broadcast of the audio signal.

Thusly constructed, the output of the amplifier through speaker means 18 is allowed to remain constant during fluctuations in the DC voltage supplied thereto. Further, the need for unnecessarily bulky or heavy magnetic core inductance located in the amplifier/speaker enclosure is obviated.

It should be noted that the embodiment of the present invention described above is to be taken in an illustrative and not a limiting sense. Various modifications and changes may be made to the above described embodiment by someone skilled in the art without departing from the scope of the present invention as defined in the appended claims.

Claims (6)

1. Audio amplifier of the type having an amplifier means, a single two terminal input means for receiving both a signal for amplification by said amplifier means and electrical power for use by said amplifier means in amplifying said signal and circuit means for separating said signal from said electrical power, the audio amplifier further comprising: capacitor means for coupling said signal from said two terminal input means to said amplifier means for amplification and for blocking said electrical power; and voltage regulator means coupled to said input terminal means for separating said electrical power from said signal for amplification and for providing said electrical power to said amplifier means for use in amplifying said signal.

2. An audio amplifier according to claim 1,where said voltage regulator means includes an integrated circuit voltage regulator.

3. An audio amplifier according to claim 2, wherein said voltage regulator has a ripple rejection ratio of approximately 80 decibels.

4. An audio amplifier according to claim 2, wherein said amplifier means includes an integrated circuit, operational amplifier means having a power supply rejection ratio for enhancing separation of said audio signal and said electrical power.

5. An audio amplifier according to claim 4, wherein said power supply rejection ratio of said operational amplifier means is approximately 80 decibels.

6. An audio amplifier constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawing.