BACKGROUND ART Power supplies for digital audio amplifiers are usually unregulated or regulated to a voltage or current supply level causing the output power of the amplifier to vary with the load.

Sometimes the power supply, such as in the case of power supplies from Yamaha, switch between discrete power supply levels to achieve efficiency in the power supply and amplifier stages. The wide ranging load impedance on existing systems often results in excessive power output to a low impedance load and less than maximum power output on high impedance loads.

In the case of D-class digital audio amplification in particular, the voltage rails can vary greatly without the need for"bias adjustment"to compensate and remove, or at least decrease, distortion. This process can be applied to normal A/B class amplifiers with extra compensating circuitry. Regulation of power supplies is known in the art. Normally, a parameter is set, such as voltage, and the current is adjusted to suit the load such that the voltage remains substantially constant. As a result, the power output varies with the load. This is a common situation in audio power amplifiers, where more power is delivered is delivered to a higher load (having lower resistance and/or impedance). Thus, if such a system requires full power to be supplied to a high impedance load, the output voltage needs to be raised, which is commonly done using a transformer on the

output. However, measuring the load and varying the voltage-to- current ratio to that the appropriate power level is delivered to the load has not been appropriate for audio amplification purposes.

The present invention aims to provide a power supply apparatus which alleviates one or more of the shortcomings of the prior art, or at least provides an alternative to existing systems. Other aims and advantages of the invention may become apparent from the following description.

DISCLOSURE OF THE INVENTION With the foregoing in view, the present invention in one aspect resides broadly in power feedback apparatus for a power supply for an amplifier including: power output measuring means operably associated with the amplifier for measuring the output power thereof; output power adjustment means operably associated with the power supply for adjusting the output power thereof; and calculation means operably interposed between the power output measuring means and the power output adjustment means and operable to calculate the output impedance and adjust the voltage to current power ratio of the power supply.

Preferably, the power output measuring means includes a microprocessor for calculating the output impedance and providing a calculated answer for adjustment of the voltage to current power ratio of the power supply according to a predetermined function. Preferably, the output power adjustment means includes pulse-width modulation means and subsequent filtering means operable to adjust the voltage output to substantially match the load.

In another aspect, the present invention resides broadly in a method of controlling voltage to current ratio of a power supply for a digital audio amplifier including: measuring the output power of the amplifier; calculating therefrom the output impedance of the power supply ; adjusting the output of the power supply as a function of the output power according to the calculated output impedance.

Preferably, the calculation of the output impedance is performed in a microprocessor to provide a calculated answer for adjustment of the voltage to current power ratio of the power supply according to a predetermined function. Preferably, the voltage output is adjusted to substantially match the load by pulse-width modulation and subsequent filtering.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate a preferred embodiment of the invention and wherein: Fig. 1 is a circuit diagram showing power feedback apparatus according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS In the circuit diagram 10 shown in Fig. 1, an audio input signal is received along an audio signal line 11 into an audio amplifier 21, the output of which includes a feed signal to a voltage measuring device 22 and a current measuring device 23 in parallel with one another and electrically connected therefrom to ground. The voltage measuring device provides a voltage

signal along a voltage signal line 14 and the current measuring device provides a current signal along a current signal line 13.

A processor 24 receives the voltage and current signals and provides a power feedback signal along a power signal line 16 to a power supply 25. The power supply, which receives power along a power supply line 12, adjusts its power output according to the power feedback signal to provide an adjusted power output along an amplifier power supply line 15 to the amplifier.

In use, the power feedback apparatus of the present invention may be used to deliver maximum power into a high impedance line (for example at 100 volts) whilst maintaining safe power levels into low impedance speakers. It is believed that the application of the method of the invention, being specifically directed to audio amplification, provides greater versatility insofar as the range of audio amplification applications to which apparatus according to the invention may be applied.

It will be understood that the example described is merely illustrative of the invention and that such modifications as would be apparent to those persons skilled in the art may be made without departing from the broad scope and ambit of the invention as claimed in the following claims.