1. Field of the Invention
This invention relates to audio power amplifiers and, more particularly, to a circuit for limiting the output level of an amplifier to prevent clipping and loudspeaker damage.
2. Description of the Prior Art
Audio power amplifiers in the professional sound field are generally used to drive loudspeakers for sound reproduction. Often the loudspeakers have power ratings less than the maximum power output of the driving amplifier. This does not present a problem as long as the amplifier is operated at a power level less than the rated power capacity of the loudspeaker. However, power amplifiers are generally fixed gain devices so that the output power of the amplifier driving a fixed resistance load is a fixed multiple of the input signal level. Consequently, a relatively large input signal can easily produce an output of sufficient power to damage the loudspeaker, particularly where the loudspeaker is a conventional horn drivers typically used with professional installations.
An associated problem occuring when a power amplifier is driven to its output limit is amplifier clipping. Basically, amplifier clipping occurs when the output voltage of the amplifier approaches the power supply voltage so that the output voltage can no longer increase in proportion to the input signal. Amplifier clipping causes two distinct problems. First, the clippinng distorts the input signal by generating harmonics of the input signal which greatly depreciate the quality of the sound emanating from the loudspeakers. Second, as clipping becomes more severe the output waveform approaches a square wave so that the average power of the clipped signal continues to increase thereby increasing the possibility of loudspeaker damage.
Amplifier clipping and its attendant consequences have long been a problem in the audio field and attempts have been made to eliminate or at least control it. One solution might be to limit the amplitude of the input signal to a safe value such as by shunting the input terminal with a zener diode. While this solution would to some extent alleviate the problem of loudspeaker damage, the shunted signal would itself be clipped so that the amplitude limited output signal would be extremely distorted and its power would increase as the clipping became more severe.
Another solution to the clipping problem taught by U.S. Pat. No. 4,048,573 issued to Evans et al. Evans et al compare the output of the power amplifier to its input to generate an attenuation command signal whenever the ratio of the output signal to the input signal does not correspond to the gain of the amplifier. The attenuation command causes current to flow through a light emitting diode of a light emitting diode/light dependent resistor pair with the reisistor shunting the input. Thus, whenever clipping occurs and the output is no longer a fixed multiple of the input, the input is shunted a sufficient amount to maintain a predetermined level of clipping. One of the problems with this circuit is that it requires distortion to produce attenuation, i.e. it can only function by allowing some degree of clipping, and clipping is distortion. Thus, the output signal will always be a distorted reproduction of the input signal when the circuit is functioning to limit the output level to the maximum output level. Additionally, the circuit lacks a great deal of flexibility such as the ability to limit the output level of the power amplifier to a predetermined value. For example, a 200 watt power amplifier may be driving loudspeakers having a rated capacity of 50 watts. While the Evans et al anti-clipping circuit will be effective in preventing clipping, the 50 watt loudspeaker will be destroyed long before clipping is a problem. Finally, the Evans et al circuit is relatively complex and it employs a fairly large number of components.