Smart voltage rail reduction audio amplifier

An audio amplifier is provided that adjusts the supply voltage rails communicated to a power amplifier circuit according to the circuit's output. The amplifier compares the output voltage from the power amplifier to a predetermined voltage range to determine if the output voltage is or is not within a predetermined range. If so, the audio amplifier reduces the supply voltage rails for the power amplifier circuit to reduced values. When the output voltage from the power amplifier moves outside the predetermined range, the supply voltage rails return to maximum levels. Whenever the output voltage from the power amplifier is not within the predetermined range, the audio amplifier maintains the supply voltage rails at maximum levels. By reducing the supply voltage rails for the power amplifier circuit, less power is dissipated by the power amplifier circuit, thereby generating less heat in the audio amplifier.

FIELD OF THE INVENTION

The present invention relates to an audio amplifier, and more particularly to an amplifier that adjusts the dissipated power in the amplifier's electronics at certain output power levels.

BACKGROUND OF THE INVENTION

Today's amplifiers are called upon to cover a broader dynamic range than previous amplifier generations. For this reason, amplifier circuits are designed to have their supply voltage large enough to handle a maximum input signal without clipping by setting the operating point of an amplification device at an optimum fixed value for such a supply voltage.

However, since the operating point and the supply voltage of such amplifier circuits are primarily set to handle such a large input signal level with minimum distortion, a large amount of operating current, which would not be needed otherwise, will flow even with a small input signal level. In addition, the period during which a maximum input signal is driven into the amplifier circuit is relatively short, thus a large amount of idle current may pass through the amplification device, and this results in unfavorable power consumption, poor amplifier efficiency and durability. Additionally, this situation can create an excessive amount of heat due that can damage the amplification device.

One solution to this problem is to include heat dissipation technologies, such as a heat sink, in the amplifier. Heat sinks may remove the excessive heat resulting from the increased power on the amplification circuitry, which results from the large current and supply voltage. However, heat sinks add weight and cost to the amplifier and are thereby less than desirable solutions for this problem. Plus, even with the implementation of heat sinks in the amplification device, the excessive power dissipated by the amplification circuitry can still result in amplifier damage.

Other solutions include constantly monitoring the amplifier output and then adjusting the supply voltage accordingly so as to keep the power dissipated in the amplifier circuitry within tolerable levels. To accomplish this constant monitoring, additional expensive and complex circuitry is required in the amplifier, which introduces more components that may fail. These additional components oftentimes introduce additional delay, as it is difficult for this type of amplifier to yield a fast transient response.

DETAILED DESCRIPTION

The drawings referenced herein are showings for the purposes of illustrating embodiments of the invention and not for purposes of limiting the same. In fact, this description of each preferred and alternative embodiment comprises but select embodiments among others, which one of ordinary skill in the art would know upon review of this disclosure.

FIG. 1is a diagram of an audio system10comprising a signal generation source12, an audio amplifier15and a load17. The signal generation source may be any electronic device capable of electrically communicating a signal receivable by the amplifier. As nonlimiting examples, signal generation device12may be a musical instrument (such as an electric guitar, keyboard, violin, etc.), a CD/DVD player, an MP3 player, a computer or other device capable of playing electronic music files, a magnetic tape player, etc. One of ordinary skill in the art would know that several types of audio signal generations devices could operate as signal generation device12.

Load17comprises one or more loudspeakers. The load relates to the impedance of the loudspeaker, which may be 2, 4, 8, or 16 ohms, as nonlimiting examples.

Amplifier15receives an input signal from signal generation device12. The input signal is amplified, and the amplified signal is output to load17, which is reproduced as sound waves by the loudspeaker.

As discussed above, amplifiers sometimes have difficulty handling input signals at certain levels because the supply voltage and current level associated with the input signal generates too much power for the components of the amplifier15to safely handle. In this situation, the power generates heat in the amplifier that oftentimes damages the electronic components (such as power amp42ofFIG. 3) in the amplifier15causing complete or partial failure of the amplifier15.

However, one embodiment disclosed herein among others relates to an amplifier15, which as a nonlimiting example may be a Class A/B amplifier, that is configured to reduce the power dissipated in the amplifier's power amplifier circuitry when its output tracks into a predetermined value range and remains within that range for an predetermined length of time. By reducing the supply voltage rails when the output voltage is within the predetermined range, the amplifier generates less heat and operates more efficiently.

FIG. 2is a diagram of the method20for reducing the supply voltage rails on the power amplifier circuit in the amplifier15ofFIG. 1based on preset conditions. This method20may be logically configured in amplifier15according to preprogrammed settings. As an alternative embodiment, one or more of the settings may be user adjustable.

As stated above, signal generation device12electrically communicates a signal, which in one nonlimiting embodiment is an analog signal, to amplifier15. One or ordinary skill would know, however, that the signal input to amplifier15could be a digital signal that is internally converted to an analog signal by the amplifier15. The input signal is amplified by the amplifier15and communicated to the loudspeaker load17. However, the output from the power amplifier circuit (power amp42ofFIG. 3) in amplifier15is also, as shown in step22, communicated to a circuit that sets the supply voltage rails for the power amplifier circuit. As shown in step23, the voltage of the amplifier output is reduced for further signal processing. The output from the power amplifier circuitry in amplifier15is converted to a DC voltage for further processing, as shown in step25.

After DC voltage conversion, the voltage level is measured to determine if it is within a predetermined range. In one embodiment among others, the predetermined voltage range is in the area between 1/10 and ½ of full power for amplifier15, which is the range where a typical Class A/B amplifier's output devices dissipate the most power. However, one of ordinary skill in the art would know that other ranges may be configured as the predetermined value range, such as between ⅛ to ⅓ of full output power.

According to this embodiment and nonlimiting example, a determination is made in step27whether the DC voltage is above or below a level that corresponds to 1/10 of full output power based on the known impedance of load17. When the amplifier is operating below approximately 1/10 of full operating power, the voltage across the load17is relatively low, which means that the voltage across the power amplifier circuit (i.e., transistors) is high. The voltage across the power amplifier circuit is the difference between the supply voltage and the voltage drop at the load17. In this instance of approximately 1/10 full power or less, the signal current is low, which means that the power dissipated by the amplifier circuitry of amplifier15is also low.

Low power dissipation in the power amp42(FIG. 3) of amplifier15means that the generated heat is within tolerable limits. Therefore, if the power amp42output voltage is not above 1/10 of full power (in this nonlimiting example), the amplifier15makes no changes to the supply voltage to the amplifier circuitry, as shown in step32.

As a nonlimiting example, for a 150-watt amplifier driving an 8-ohm load, the voltage across the load at 1/15 full power, or 10 watts, is approximately 8.9 volts. So if the supply voltage is 40 volts, that means that the voltage drop across the amplifier circuit is approximately 31 volts. In this nonlimiting example, one of ordinary skill would know that the signal current equates to be approximately 1.1 amps. If the supply voltage is 40 volts, power amp42(FIG. 3) must dissipate approximately 34.1 watts of power [(40 volts−8.9 volts)*1.1 amps)].

As shown in step30, the amplifier15also determines according to method20whether the amplifier's output power level is above ½ of full power (in this nonlimiting embodiment). At a higher power level, the voltage across the load is substantially greater, which means that the voltage across the power amp42(FIG. 3) is substantially lower. So even though the signal current is greater in driving the load, the power dissipated at the power amp42(FIG. 3) is still low.

In continuing the 150-watt amplifier nonlimiting example from above, the 8-ohm load has almost a 35-volt voltage drop across the load at full power. For the 40-volt supply, that means that the power amp42(FIG. 3) only has about a 5 volt drop. While the signal current is approximately 4.3 amps, (square root of the power times the load resistance), the power dissipated at the power amp42(FIG. 3) is approximately 21.65 watts. So at full power (150 watts in this nonlimiting example), amplifier15dissipates less power at power amp42(FIG. 3) than it does at less than 1/15 of full power.

Consequently, if the power amp42(FIG. 3) is operating above ½ full power, the voltage of the signal measured at step30results in advancement to step32whereby the supply voltage to amplifier circuitry in amplifier15is unchanged.

If the measurement of the signal voltage described at step30corresponds to an amplifier power level that is less than ½ power but greater than 1/10 power (resulting from a YES result in step27), the process advances to step34to reduce the supply voltage rails to the power amp42(FIG. 3). This reduction occurs because power amp42(FIG. 3) dissipates the most power in this range.

Returning to the nonlimiting example from above, if the amplifier15is operating at ⅓ of full power, or at 50 watts, in this example, the voltage drop across the load is 20 volts. The signal current at 50 watts is 2.5 amps. If the supply voltage is 40 volts, and there is a 20-volt voltage drop across power amp42(FIG. 3), then that also means that power amp42(FIG. 3) must dissipate 50 watts of power. So for this reason, the method20advances from step30to step34to reduce the supply voltage, which effectively reduces the voltage drop across power amp42(FIG. 3), thereby reducing the power dissipation in the amplifier as well.

It is well known that amplifiers generate the greatest amount of heat in the range of approximately ⅛ to ⅓ operating power for the reasons discussed above. As also stated above, heat sinks may be used and are typically configured to dissipate heat at ⅓ operating power. But, the method described above operates to reduce the supply voltage rails when the amplifier power output tracks within a predetermined range, such as the nonlimiting exemplary range of 1/10 to ½ of full power.

But rather than constantly tracking the output voltage and adjusting the supply voltage rails on a continual basis, adjustment is made when the output voltage tracks within the predetermined range and not when the signal is outside that range. This scheme provides for a faster transient response for the amplifier.

While the signal corresponding to the amplifier output is above or below the predetermined range, which in this nonlimiting example is 1/10 to ½, the supply voltage rails on power amp42(FIG. 3) remain at normal high levels (40 watts in the nonlimiting example above). However, when the signal tracks into the predetermined range for a predetermined time, the supply voltage rails are reduced to a level that decreases the power dissipated by the power amp42(FIG. 3).

This reduction does not affect the power delivered to the load. More specifically, the load on the loudspeaker remains constant (i.e., 4 ohms, 8 ohms, etc.). The voltage drop across the loudspeaker load remains the same as well. By reducing the supply voltage, the reduction is seen at the voltage drop across power amp42inFIG. 3. Stated another way, by providing a lower voltage that is still greater than the voltage drop across the load, the remaining voltage to drop across power amp42(FIG. 3) is lower. This reduction means that the dissipated power at power amp42(FIG. 3) will also be lower irrespective of the signal current.

The amount of time after the signal enters the predetermined range is a function of the amplifier's ability to dissipate heat. By reducing the output voltage of the power amplifier circuit, the amount of generated heat decreases and the output sound is not distorted. When the signal tracks back out of the predetermined range, whether on the low or high end, the output voltage rails return to normal values.

FIG. 3is a diagram of one embodiment among others for configuring a circuit40for implementing the smart voltage rail reduction method20in the amplifier15ofFIG. 1, as depicted inFIG. 2. The input signal is received from the signal generation device12. The signal is amplified by, in this nonlimiting example, linear power amplifier42, which drives load17.

The output from power amp42is divided by resistors44and46and communicated to buffer49. The voltage is divided so that smaller-sized components may be implemented in this embodiment, so one of ordinary skill in the art would know that the larger electronic components could be used in lieu of the voltage dividing resistors shown inFIG. 3. (The configuration ofFIG. 3is but one embodiment among others.)

The output from power amp42is an AC voltage. So rectifying diode51operates to convert the AC voltage to a DC voltage, which is filtered by capacitor53and resistor56.

The DC signal is communicated to a window comparator composed of comparators59and61. Comparator59determines if the DC voltage signal is less than a preset voltage value, and comparator61determines if the DC voltage signal is above a preset voltage value. In the nonlimiting example above, the preset voltage range boundaries are 1/10 to ½ of full amplifier output power.

Resistors64,66, and68set the reference value levels for comparators59and61so that if the voltage of the DC voltage signal is within a predetermined range, comparators59and61switch their output to high. Resistors71, and73are pull-up resistors that operate to charge capacitor78. As long as the output from comparators59and61is high, which corresponds to the DC voltage relating to a 1/10 to ½ power amp output level, capacitor78continues to charge positively.

As capacitor78is charged, the signal is communicated through impedance buffer81to integrated circuit85. In this nonlimiting example, integrated circuit85is a pulse width modulating controller that operates to reduce the supply voltage to the power amp42when it receives a signal that is higher than the reference voltage VREFof error amplifier87. If the signal communicated to integrated circuit85is lower than the reference voltage VREFof error amplifier87, such as when the power amp42is operating below 1/10 of full power or above ½ of full power (in this nonlimiting example), the integrated circuit85allows the supply voltage rails to remain at the maximum level.

Integrated circuit85does not operate to immediately reduce the supply voltage to the power amp42whenever the output power from power amp42is between 1/10 and ½ power (or according to another predetermined range). Instead, integrated circuit85is configured to reduce the supply voltage to power amp42when capacitor78becomes completely charged. There is a delay from when the comparators59and61output a high signal to when capacitor78is fully charged. This delay prevents the constant decrease and increase in the supply voltage to power amp42, which would otherwise decrease the efficiency and sound quality of amplifier15if the supply voltage were constantly adjusting. The input signal from the signal generation source12(FIG. 1) may constantly cause the amplifier output to repeatedly track in and out of the predetermined voltage range. So capacitor78takes an amount of time to charge, but once charged, it holds a high signal level to integrated circuit85until the output from power amp42tracks out of the 1/10 to ½ range. Yet, the repeated tracking in and out of the predetermined voltage range for short intervals does not permit the capacitor78to charge and thereby reduce the voltage rails supplied to power amp42.

When the output power from power amp42tracks outside of the predetermined range, which in this nonlimiting example is between 1/10 to ½ of full power, comparators59and61output a low signal value. Resistors73and74operate to quickly discharge capacitor78. In the discharging of capacitor78, a low signal value is communicated to integrated circuit85, which results in integrated circuit returning the supply voltage rails to maximum configured levels.

As discussed above, integrated circuit85is a pulse width modulating circuit. But one of ordinary skill in the art would know that other circuit combinations may be implemented to likewise achieve the result of adjusting the supply voltage to the power amp42.

Additionally, the value of resistors71,73, and75determine the value of the supply voltage rails, as the value of these resistors configure the voltage of the signal output by comparators59,61and maintained by capacitor78. One of ordinary skill would know that numerous resistor value combinations would operate to produce any number of desirable supply voltage rail value ranges.

FIG. 4is a graphical representation of the output power from power amp42ofFIG. 3and the supply voltage rails. In the first part of the chart where the output power is below 1/10, the voltage rails are depicted at their maximum outer limits because the corresponding voltage is below the threshold set according to circuit40(FIG. 3). In this situation, the power dissipated by power amp42is within tolerable limits.

However, in the middle section of chart86, the power amp output is shown to be between 1/10 and ½ of full power. As discussed above, the circuit40recognizes this event and operates to reduce the voltage rails, as shown in chart86. It should be noted that the +VRAILand −VRAILrail reduction is shown in chart86as delayed due to capacitor78.

Finally, the third section of chart86depicts the power amp output signal increasing beyond ½ power. It should be noted in this instance that the voltage rails return to their outer limits more quickly than as they were reduced.

The configuration described above and shown inFIG. 3is generally tailored for a predetermined impedance load17. However, amplifier15may be configured for multiple impedances. In this alternative embodiment the amplifier15may be configured with multiple outputs couplable to a load17wherein each output relates to a predetermined impedance load. As a nonlimiting example, one output jack may be for a 4-ohm load, another for an 8-ohm load, and another for a 16-ohm load. By physically coupling the different impedance load loudspeakers17to the different jack outputs, different resistor values are implemented for each of resistors64,66, and68.

Alternatively, amplifier15may also include an impedance selector switch that changes the values or resistors64,66, and68in relation to the dialed in impedance of load17. By adjusting the impedance selector switch, different resistors may become electrically coupled at the positions of resistors64,66, and68respectively with the correct resistance levels for the corresponding impedance level.