WO1999021273A1

WO1999021273A1 - Audio amplifier apparatus and a method for reducing an audible transient

Info

Publication number: WO1999021273A1
Application number: PCT/US1997/018681
Authority: WO
Inventors: Alan Anderson Hoover
Original assignee: Thomson Consumer Electronics, Inc.
Priority date: 1997-10-20
Filing date: 1997-10-20
Publication date: 1999-04-29
Also published as: AU4985097A

Abstract

An audio signal processing channel combines the Vcc supply filter capacitor with the amplifier output coupling capacitor in order to significantly reduce the audible transients or 'pops' which typically occur when a single-ended power supply used in connection with an audio amplifier is energized and deenergized. Particularly, two capacitors connected in series provide filtering for the supply and coupling of the amplified audio output signal to said producing means.

Description

AUDIO AMPLIFIER APPARATUS AND A METHOD FOR REDUCING AN

AUDIBLE TRANSIENT

Field of the Invention

This invention concerns an arrangement for suppressing audible transients, sometimes referred to as "pops", which may occur when a voltage supply for an audio signal processing channel is energized or deenergized.

Background of the Invention

When audio or video equipment is turned on or off, an audible transient, sometimes referred to as a "pop", may be produced and heard, for example through a loudspeaker, as the voltage supply for the audio signal processing channel is energized or deenergized. To avoid such "pops", circuitry may be provided for reducing or eliminating the resulting pop. However, many circuit options involve increased circuit complexity requiring additional components and cost.

One solution employs a split power supply wherein the positive and the negative Vcc supplies typically charge to their terminal voltages at approximately the same time thereby maintaining the amplifier output voltage at approximately zero volts during turn-on and turn-off conditions. Such a configuration typically eliminates the audible pop during turn-on and turn-off. However, split power supplies are typically more expensive than single ended power supplies (discussed below). Further, split power supplies are prone to a short-circuit failure condition which causes one of the two power supply voltages to be impressed across the loudspeaker causing potential catastrophic damage to the loudspeaker. One option to overcome such a failure condition is to include an additional coupling capacitor in series with the loudspeaker. However, an additional coupling capacitor adds significant cost and physical size to the circuit.

Single ended power supplies are inherently less expensive than split power supplies but are subject to producing audible pops. Various solutions have been developed for reducing or eliminating the pop when single ended power supplies are utilized. One solution involves use of an electromechanical relay in series with the loudspeaker which opens up the circuit during power-on and power-off intervals. Another solution involves adding a controlled bias circuit. However, both solutions involve extra components which increase the cost and size of the circuit.

Another solution to reduce pops is disclosed in US Patent 5, 199,079 to Anderson et al., issued March 30, 1993. Anderson discloses an audio signal processing channel having a bias network for deriving a bias voltage which is coupled from the supply voltage to a point within the channel. To inhibit audible pops due to the rapid reduction of the bias voltage when the voltage supply is deenergized, a diode is coupled between the bias network and the voltage supply. The diode selectively decouples a filter capacitor of the bias network from the voltage supply and associated loads when the voltage supply is deenergized. As a result, the bias voltage is gradually reduced at a controlled rate of change which is independent of the impedances of voltage supply and associated loads.

Summary of the Invention

Generally, the present invention provides an apparatus for processing an audio signal comprising a means for preventing a differential voltage from being applied across a means for producing an audio output signal during power-on and power-off operating conditions.

Particularly, the preventing means comprises means for filtering and coupling said supply voltage and said amplified audio signal to said producing means.

In accordance with one aspect of the present invention, the filtering and coupling means comprises a first and a second capacitor connected in series. The producing means is connected between the amplifying means and a junction between the first and second capacitors.

In accordance with another aspect of the present invention, the preventing means further comprises a means for biasing said amplifying means to provide a dc bias voltage. The biasing means further comprising a first and a second resistor connected in series such that the dc bias voltage is approximately equivalent to a voltage generated at the junction between the first and the second series connected capacitors.

In accordance with yet another aspect of the present invention, there is provided means for minimizing the dc offset of said amplifying means, means for providing high frequency bypass of the supply voltage and biasing of the amplifying means and means for decoupling said amplifying means from said supply voltage.

In accordance with yet another aspect of the present invention an apparatus for processing a stereo audio signal, comprises ( 1) a first and a second means for amplifying the audio signal to provide a first and a second amplified audio signal, (2) means for providing a supply voltage (Vcc) to the first and the second amplifying means, (3) a first and a second means for producing a first and a second audio output signals and (4) a means for preventing a differential voltage from being applied across the first and second producing means during predetermined operating conditions of the apparatus. Each producing means has first and second inputs, each first input being coupled to a respectively different amplifying means. The preventing means comprises means for providing filtering and coupling of the supply voltage and the first and second amplified audio signal, respectively, to the first and second producing means. Further, the preventing means comprises at least one means for biasing the first and the second amplifying means by providing at least one dc bias voltage.

In accordance with still another aspect of the present invention an apparatus for processing a multichannel audio signal comprises (1) a plurality of means for amplifying said multichannel audio signal to provide a plurality of amplified audio signals, (2) means for providing a supply voltage to said plurality of amplifying means, (3) a plurality of means for producing a plurality of audio output signals, (4) means for preventing a differential voltage from being applied across said plurality of producing means during predetermined operating conditions of the apparatus. Further, each producing means has a first and a second input that is coupled to said plurality of amplifying means by said first input thereof. The preventing means having means for filtering and coupling (Cl, C2) the supply voltage and said plurality of amplified audio output signals respectively to the producing means; and at least one means for biasing (R4, R5) the at least one amplifying means to provide at least one dc bias voltage.

In accordance with a method aspect of the present invention, there is provided a method for processing an audio signal during power turn-on and power turn-off operating conditions which comprises ( 1 ) amplifying the audio signal to provide an amplified audio signal to a means for producing an audio output signal and (2) preventing a differential voltage from being applied across said producing means by filtering a supply voltage provided for amplifying said audio signal and coupling the amplified audio signal to the producing means. The filtering means and said coupling means comprise a plurality of capacitors connected in series.

These and other aspects of the invention will be explained with reference to a preferred embodiment of the invention shown in the accompanying Drawing.

Brief Description of the Drawing

Figure 1 is a schematic diagram of a generic audio signal processing channel in accordance with the invention;

Figure 2 is a schematic diagram of an exemplary implementation of a monaural audio signal processing channel in accordance with this invention; and

Figure 3 is a schematic diagram of an exemplary implementation of a stereo audio signal processing channel in accordance with this invention.

Various component values are indicated in the Figures by way of an example. Detailed Description of the Drawing

In Figure 1, undesirable audible pops produced by an audio signal processing channel 10 may be reduced by amplifying an audio signal Vin to provide an amplified audio signal to a means for producing an audio output signal, for example a loudspeaker 20, and preventing a differential voltage from being applied across loudspeaker 20 by utilizing a first and a second capacitor connected in series for both filtering the supply voltage and coupling the amplified audio signal to the loudspeaker. Audio signal processing channel 10, utilizing a generic amplifier topology, is configured as a monaural audio signal processing channel powered by a single-ended power supply configuration (not shown) which may, for example, be incorporated in a television receiver.

The audio signal Vin to be processed is coupled, by capacitor C3, to the positive input 14 of a power amplifier 12. Amplifier 12 is operated between Vcc, typically positive 12 volts, and ground. One input (e.g., input 22) of producing means 20, typically a loudspeaker, is connected to the output 18 of amplifier 12.

A negative feedback loop, comprising resistors R2 and R3, connected between output 18 and negative input 16 of amplifier 12, essentially determine the ac and dc operating characteristics of amplifier 12. The value of capacitor C4 is selected to insure that the closed-loop dc gain of amplifier 12 is unity; this minimizes any dc offset at the output of amplifier 12. The dc bias point of amplifier 12, typically configured to be 1/2 Vcc, is determined by series connected resistors R4 and R5 and resistor Rl which is connected at the junction of resistors R4 and R5. Resistor Rl is connected to the positive input 14 of amplifier 12.

Typically for such audio amplifiers, because the positive bias current is very low the current through Rl has minimal effect on the voltage at the junction of R4 and R5. Thus, series connected bias resistors R4 and R5 would typically be of equal value to provide an input bias voltage equal to 1/2 Vcc.

The audible pop of audio signal processing channels utilizing single-ended power supplies is reduced to a negligible level by combining the Vcc supply filter capacitor with the amplifier output coupling capacitor. In Figure 1, the Vcc supply filter capacitor is configured as two series connected capacitors, Cl and C2, typically of equal value. The second input 24 of loudspeaker 20 is connected to the junction between the series connected capacitors Cl and C2, thereby doubling as coupling capacitors for the loudspeaker 20.

During power-on operating transients, as the Vcc supply rises to its nominal operating voltage, but before the amplifier reaches equilibrium, the two filtering/coupling capacitors Cl and C2 charge equivalently and the voltage at the junction of the series connected capacitors Cl and C2 is approximately equal to the dc bias point, typically 1/2 Vcc. This maintains a differential voltage of about zero volts across loudspeaker 20 which substantially reduces the audible pop to zero.

During power-off operating conditions, a zero differential voltage across loudspeaker 20 is maintained because the series connected Vcc filter capacitors discharge at a rate equivalent to that of the Vcc supply. Similarly, this reduces the pop to a negligible amount.

Figure 2 shows an exemplary embodiment of a monaural audio signal processing channel 10' in accordance with this invention. Features in Figure 2 that are the same and/or functionally equivalent to those in Figure 1 are numbered with the addition of a suffix indication (e.g., 12 in Figure 1 and 12' in Figure 2). In this embodiment, the preferred audio amplifier 12' is part number TDA7235 available from SGS-Thomson Microelectronics. (Audio amplifier TDA7235 is packaged as a 8 pin minidip integrated circuit (IC).) Two of the bias resistors, R4 and R5 of Figure 1, are contained within the amplifier IC and thus are not shown. Similarly, negative feedback resistors R2 and R3 are also contained within the amplifier IC and are not shown either. The operation of the circuit is not altered by the integration of these components in the IC.

A typical value for the last component of the bias circuit, Rl', is 1 ohm. Typical values for the audio signal coupling capacitor C3' and the closed-loop dc gain capacitor C4' are 10 and 47 microfarads, respectively. The two filtering/coupling capacitors, C l ' and C2', are typically 470 microfarad, 10 volt capacitors.

An optional power supply decoupling resistor R6 (typically, 0.47 ohms) is attached between Vcc power and the audio amplifier.

Further, an optional stability network, comprising resistors R7 (typically, 4.7 ohms) and C5 (typically, 0.22 microfarads) is utilized to provided a resistive load on the amplifier output at high frequencies. Still further, an optional high frequency Vcc bypass capacitor C6 for the amplifier is also utilized, typically 0.1 microfarad.

Figure 3 shows an exemplary embodiment of a stereo audio signal processing channel 100 in accordance with this invention. Features in Figure 3 that are the same and/or functionally equivalent to those in Figure 2 have the same reference numerals or are numbered with the addition of a suffix letter (e.g., 12' in Figure 2 and 12'a and 12'b in Figure 2). Similar to the monaural configuration shown in Figure 2, each of the two loudspeakers 20'a, 20'b is connected between the junction of the common series connected capacitors Cl " and C2" and the output of separate audio amplifiers 12'a and 12'b. However, the capacitance of each of the series connected filtering/coupling capacitors Cl " and C2" is greater, possibly at least twice as great, as the equivalent capacitance of the monaural system. For example two, 1000 microfarad, 10 volt capacitors may be used in the stereo configuration. Please note, a separate stability network (i.e., R7'b and C5'b) and a separate optional high frequency Vcc bypass capacitor C6'b is usually used for each channel.

The present invention has been described in terms of an embodiment in which the bias resistors for the audio amplifier are integral with the minidip integrated circuit. However, the bias resistors may be optional if known in the art power OP-amp is utilized. Further, optional externally mounted discrete resistors may be substituted.

An alternate embodiment of the present invention pertains to an apparatus for processing a multichannel audio signal. In such an embodiment means for filtering and coupling said supply voltage and said plurality of amplified audio output signals respectively to said producing means may be shared amongst the various required audio amplifiers (i.e., all the audio amplifiers share the means for filtering and coupling.

While the invention has been described in relation to a positive voltage supply, it will be appreciated it is also applicable to a negative voltage supply. Further, while the invention has been described in detail with respect to numerous embodiments thereof, it will be apparent that upon a reading and understanding of the foregoing, numerous alterations to the described embodiment will occur to those skilled in the art and it is intended to include such alterations within the scope of the appended claims.

Claims

1. An apparatus for processing an audio signal comprising:

means for amplifying said audio signal to provide an amplified audio signal;

means for providing a supply voltage to said amplifying means ;

means for producing an audio output signal, said producing means being coupled to said amplifying means; and

means for preventing a differential voltage from being applied across said producing means during predetermined operating conditions of the apparatus, wherein said preventing means comprises; means for filtering and coupling said supply voltage and said amplified audio signal to said producing means.

2. The apparatus recited in Claim 1 wherein said filtering and coupling means comprises a first and a second capacitor connected in series, and further wherein said producing means is connected between said amplifying means and a junction between said first and second capacitors.

3. The apparatus recited in Claim 2 wherein the means for preventing further comprises a means for biasing said amplifying means to provide a dc bias voltage.

4. The apparatus recited in Claim 3 wherein said biasing means comprises a first and a second resistor connected in series such that the dc bias voltage is approximately equivalent to a voltage generated at the junction between said first and second capacitors.

5. The apparatus recited in Claim 1 or 4 further comprising means for minimizing the dc offset of said amplifying means.

6. The apparatus recited in Claim 5 wherein said predetermined operating conditions are power turn-on and power turn-off conditions.

7. The apparatus of Claim 6 further comprising a means for providing high frequency bypass of the supply voltage and biasing of the amplifying means.

8. The apparatus of Claim 7 further comprising a means for decoupling said amplifying means from said supply voltage.

9. An apparatus for processing a stereo audio signal, comprising:

first and a second means for amplifying said audio signal to provide a first and a second amplified audio signal;

means for providing a supply voltage to said first and second amplifying means;

first and a second means for producing a first and a second audio output signal, said first and second producing means each having a first and a second input and being respectively coupled to said first and second amplifying means by said first input thereof; and

means for preventing a differential voltage from being applied across said first and second producing means during predetermined operating conditions of the apparatus, wherein said preventing means comprises; means for filtering and coupling said supply voltage and said first and second amplified audio signal respectively to said first and second producing means; and at least one means for biasing said first and second amplifying means, said at least one biasing means providing at least one dc bias voltage.

10. The apparatus recited in Claim 9 wherein said filtering and coupling means comprises a first and a second capacitor connected in series, and further wherein said first and second producing means are each respectively connected at said second input thereof to a junction between said first and second capacitors.

11. An apparatus for processing a multichannel audio signal, comprising:

a plurality of means for amplifying said multichannel audio signal to provide a plurality of amplified audio signals;

means for providing a supply voltage to said plurality of amplifying means;

a plurality of means for producing a plurality of audio output signals, said plurality of producing means each having a first and a second input and being coupled, one to each of said plurality of amplifying means by said first input thereof; and

means for preventing a differential voltage from being applied across said a plurality producing means during predetermined operating conditions of the apparatus, wherein said preventing means comprises; means for filtering and coupling said supply voltage and said plurality of amplified audio signals respectively to said producing means; and at least one means for biasing said plurality of amplifying means, said at least one biasing means providing at least one dc bias voltage.

12. The apparatus recited in Claim 11 wherein said filtering and coupling means comprises a first and a second capacitor connected in series, and further wherein said plurality of producing means are each respectively connected at said second input thereof to a junction between said first and second capacitors.

13. An apparatus for processing a signal comprising:

an amplifier having at least one input, an output and a first and a second voltage supply points;

means for coupling an input signal to said input of said amplifier;

means for coupling a first capacitor to said first voltage supply point of said amplifier;

means for coupling a second capacitor to said second voltage supply point of said amplifier;

a point of reference potential located at a junction of said first capacitor and said second capacitor; and

an output signal of said amplifier being developed between said output of said amplifier and said point of reference potential;

14. The apparatus recited in Claim 13 further comprising means for biasing said amplifying means to provide a dc bias voltage.

15. A method for processing an audio signal during power turn-on and power turn-off operating conditions, said method comprising:

amplifying said audio signal to provide an amplified audio signal to a means for producing an audio output signal; and

preventing a differential voltage from being applied across said producing means by filtering a supply voltage provided for amplifying said audio signal and coupling the amplified audio signal to said producing means, wherein said filtering and coupling means comprise a first and a second capacitor connected in series.

16. The method recited in Claim 15 wherein the step of filtering the supply voltage and coupling the amplified audio signal are carried out by connecting a first input of said producing means to a junction between said first and second capacitors, and connecting a second input of said producing means to an output of said audio amplifier.