Abstract:
A multichannel electrical power amplifying system constructed and arranged to distribute electrical power of amount P to a total load coupled thereto. The system includes a plurality n of power amplifier channels, coupling the power amplifying system to a portion of the electrical load. Each of the amplifier channels distributes a portion of the electrical power P to a portion of the electrical load. Each power amplifier channel has a power distribution capacity, at least one of the power amplifier channels having a power distribution capacity significantly greater than          P   n     .                           
     The sum of the capacities of the plurality of amplifier channels is significantly greater than P.

Description:
BACKGROUND 
     The invention relates to power amplifiers, and more particularly to multichannel power amplifiers. The power distribution capacities of multichannel amplifiers are typically “rated,” on a “per channel” basis, usually on an “all channels driven” basis; that is, each of the four channels has a power distribution capacity of the per channel rating when all the channels are driving an electrical load. The total power distribution capacity of the amplifier system (often unspecified in a conventional multichannel amplifier) is the sum of the “per channel” ratings. For example, a four channel amplifier may be rated as 100 watts per channel, that is, each channel has a power distribution capacity of 100 watts, and the total rating, or power distribution capacity of the multichannel amplifier is 400 watts. To use all 400 watts of power distribution capacity, each channel would have a total load of 100 watts, but no more. 
     It is an important object of the invention to provide an improved multichannel power amplifier. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the invention, a multichannel electrical power amplifying system is constructed and arranged to distribute electrical power of amount P to an electrical load coupled thereto. The system includes a plurality n of power amplifier channels, coupling the power amplifying system to a portion of the electrical load, each of the amplifier channels for distributing a portion of the electrical power P to a portion of the electrical load. Each power amplifier channel has a power distribution capacity, and at least one of the power amplifier channels is constructed and arranged to have a power distribution capacity significantly greater than        P   n                          
     and the sum of the capacities of the plurality of amplifier channels is significantly greater than P. 
     In another aspect of the invention, a multichannel electrical power amplifying system is constructed and arranged to provide a total amount of electrical power P. The amplifying system includes a plurality of electrically powered devices, the plurality of devices representing an electrical load. The amplifying system also includes a number n of power amplifier channels, each amplifier channel coupled to a portion of the electrical load, each of the plurality of power amplifier channels for providing a portion of the total amount of electrical power P to the portion of the electrical load. One of the portions significantly exceeds          P   n     .                          
     The amplifying system also includes a plurality of input channels for receiving a plurality of input signals and circuitry to selectively route the input signal from any of the plurality of input channels to any one of the power amplifier channels. 
     Other features, objects, and advantages will become apparent from the following detailed description, which refers to the following drawing in which: 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a block diagram of a four channel power amplifying system according to the invention; 
     FIG. 2 is a thermal model of the amplifying system of FIG. 1; 
     FIG. 3 is a top plan view of a heat sink for the amplifying system of FIG. 1, showing the footprints of heat producing elements of the amplifying system of FIG. 1; 
     FIG. 4 is a simplified schematic diagram of a circuit for implementing the power distribution portion of the amplifying system of FIG. 1; and 
     FIGS. 5A-5D are four schematic diagrams, which, combined show a circuit of a power amplifying system as in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     With reference now to the drawing and more particularly to FIG. 1, there is shown a block diagram of an audio signal power amplifier system incorporating the invention. A multichannel power amplifier system  10  includes a common power supply  13  (that is, a power supply that powers all channels of the power amplifier system) coupled to four high voltage, low current power amplifiers  12 - 1 - 12 - 4  (hereinafter power amplifiers, and sometimes referred to as constant voltage, distributed amplifiers) which are coupled to four audio output lines  14 - 1 - 14 - 4 . Each of the audio output lines  14 - 1 - 14 - 4  has audio devices  16 - 1 - 16 - 4  coupled to it. Audio signal processor and router  18  has a plurality of input terminals  19 - 1 - 19 - 4  and a plurality of multiple output terminals  20 - 1 - 20 - 4 . Each of the output terminals  20 - 1 - 20 - 4  is coupled to one of the power amplifiers  12 - 1 - 12 - 4 . The power amplifiers  12 - 1 - 12 - 4  are mounted on a single heatsink  22 . Optional fan  24 , if necessary, is positioned so that it directs a cooling airflow across heatsink  22 . 
     In operation, audio signals are received at input terminals  19 - 1 - 19 - 4  and routed to output terminals  20 - 1 - 20 - 4  and conducted to any one of power amplifiers  12 - 1 - 12 - 4 , which amplify the audio signals and transmit them to electrical loads  16 - 1 - 16 - 4 , typically electroacoustic transducers, which transduce the amplified audio signals to sound waves. Power supply  13  converts line AC voltage current to a DC voltage suitable for powering 70.7- or 100-volt low-current direct-drive power amplifiers; typically ±130 or ±170 volts, respectively, is suitable. The output of power supply  13  is conducted to power amplifiers  12 - 1 - 12 - 4 . Heatsink  22  conducts heat away from power amplifiers  12 - 1 - 12 - 4 . If passive cooling is insufficient to conduct the heat away from the heat sink, optional fan  24  directs heat away from heatsink  22  to the external environment. 
     In one embodiment, the total power rating of the power amplifier system is 400 watts; that is, the power amplifier system is constructed and arranged to have a power distribution capacity of 400 continuous average watts. The audio signal processor and router  18  may be a conventional analog or digital signal processor and router having multiple input and output channels, in this example four input and four output channels; the number of input channels does not have to be equal to the number of output channels. Power supply  13  is a conventional power converter, constructed and arranged to convert line AC voltage to ±130 or ±170 volts and to supply power equal to the total power rating of the power amplifier system, in this embodiment 400 watts (plus any power that is dissipated in the amplifiers). Power amplifiers  12 - 1 - 12 - 4  may be 70- or 100-volt high-voltage low-current power amplifiers, each rated at up to 400 watts. Power amplifiers  12 - 1 - 12 - 4  may be linear amplifiers, however in an exemplary embodiment, they are switching amplifiers, each containing two field effect transistors (FETs). Power amplifiers including switching amplifiers are advantageous over linear amplifiers in a system described by FIG. 1, because they are more efficient than linear amplifiers, particularly in the case of high power loads, and low power (or no) loads. Power amplifiers  12 - 1 - 12 - 4  are directly coupled to audio output lines  14 - 1 - 14 - 4 ; that is, there is no intervening transformer coupling power amplifiers  12 - 1 - 12 - 4  and audio output lines  14 - 1 - 14 - 4 . Output lines  14 - 1 - 14 - 4  couple power amplifiers  12 - 1 - 12 - 4  to audio devices  16 - 1 - 16 - 4 , respectively. Audio devices are shown as single transducers, however typically each power amplifier is coupled to a plurality of conventional loudspeakers constructed and arranged to operate at the output voltage of the power amplifiers. The power ratings of each of the individual power amplifiers  12 - 1 - 12 - 4  can be substantially greater (for example at least twice) the total power rating of the amplifier system  10  divided by the number of channels and up to a number equal to the total rating of the amplifier system. So, for example, in this embodiment, the power rating of the four channel power amplifier system  10  is 400 watts, the rating of each of the power amplifier channels is much greater than 100 watts, in this case 400 watts. Additional details of the power amplifiers  12 - 1 - 12 - 4  are shown in FIG.  4 . 
     The common power supply is advantageous because separate power supplies for each channel would require a 400 watt power supply for each channel, even if some channels may be loaded at substantially less than 400 watts. 
     The power rating of each of the power amplifiers does not have to be equal to the power rating of the amplifier system as in the embodiment described in FIG.  1 . Any significantly greater per channel power rating, such as two times the power rating of the amplifier system (in this case 400) divided by the number of power amplifier channels (in this case four) is advantageous. Nor is it necessary for all channels to have a significantly higher rating than the power rating of the amplifier system divided by the number of power amplifier channels. For example, in the embodiment of FIG. 1, it would still be advantageous if only one or two of the power amplifier channels of the embodiment of FIG. 1 were rated significantly higher than 100 watts. The principles of the invention are also applicable to multichannel power amplifier systems having more or fewer than four channels. 
     In a power amplifier system according to the invention, the total rated power distribution capacity can be apportioned to the individual channels in any desired combination. For example, the power amplifier system of FIG. 1 may be incorporated in a sound system for a restaurant. Coupled to power amplifier  12 - 1  by audio signal output line  14 - 1  may be ten loudspeakers tapped at 32 watts (320 watts)  16 - 1  which could be placed in the dining area of the restaurant. Coupled to power amplifier  12 - 2  by signal output line  14 - 2  may be a single loudspeaker tapped at 32 watts placed in a waiting area, for providing music, paging signals, and voice messages for waiting customers. Coupled to each of power amplifiers  12 - 3  and  12 - 4  by output lines  14 - 3  and  14 - 4  respectively, may be single loudspeakers  16 - 3  and  16 - 4  respectively tapped at eight watts (16 watts), which could be placed in the men&#39;s and ladies&#39; restrooms. The example of FIG. 1 show that the wattage of the loudspeakers on each of the channels does not need to be “balanced” and that a large portion (320 watts or 87% in this example) of the total wattage (368 watts in this example) of the loudspeakers in the sound system may be driven by a single channel. The sound system of FIG. 1 could be easily converted to a sound system in which six loudspeakers tapped at 25 watts (150 watts each, total 300 watts) are coupled to each of power amplifiers  12 - 1  and  12 - 2 , to provide different music to two dining areas of a restaurant, in which two loudspeakers tapped at 8 watts (16 watts) are coupled to power amplifier  12 - 3  to provide sound to the men&#39;s and ladies&#39; rooms, and a single loudspeaker tapped at 32 watts could be placed in the waiting area. 
     In the discussion of FIG. 1, the wattages, voltages, methods of specifying amplifier power capacity and the specific usage are exemplary. The principles of the invention may be applied to other wattages, voltages, methods of specifying amplifier power capacity, and usages. 
     Referring now to FIG. 2, there is shown a thermal model of a portion of the audio signal power amplifying system of FIG.  1 . Power sources  30 - 1 - 30 - 4  represent the heat to be dissipated by each of the power amplifiers. Junction nodes  32 - 1 - 32 - 4  represent the junctions of the main output FETs in the power amplifiers  12 - 1 - 12 - 4 , respectively. Case nodes  33 - 1 - 33 - 4  represent the cases of the respective FETs and are coupled to the junction nodes  32 - 1 - 32 - 4  through thermal resistors  34 - 1 - 34 - 4 , respectively; thermal resistors  34 - 1 - 34 - 4  represent the thermal resistance between the junctions and the cases of the respective FETs. Sink nodes  36 - 1 - 36 - 4  are coupled to case nodes  34 - 1 - 34 - 4  through resistors  38 - 1 - 38 - 4 , respectively. Resistors  38 - 1 - 38 - 4  represent the thermal resistance between the case and the heat sink. Sink nodes  36 - 1 - 36 - 4 , represent the interface between the resistors  38 - 1 - 38 - 4  and the single heatsink  22 . Sink nodes  36 - 1 - 36 - 4  are all connected to common heat sink node  39 , which is coupled to thermal ground (ambient) node  40  through a single resistor  42 , indicating that all four heat pairs of FETs are thermally coupled to a single heat sink and (as will be shown in FIG. 3) the thermal resistances of the single heat sink for the four pairs of FETs are substantially similar. The heat sink dissipates all the heat produced by the FETs that is not dissipated by the resistors ( 34 - 1 - 34 - 4  and  38 - 1 - 38 - 4 ) that are associated with the individual FETs. A typical thermal resistance value for resistors  34 - 1 - 34 - 4  is 0.7° C./watt, and is determined by the characteristics of the chip embodying the FET and its package. A typical thermal resistance value for resistors  38 - 1 - 38 - 4  is 0.3° C./watt and is determined by the elements between the chip package and the heat sink, such as thermal washers and insulators, thermal grease, and the like. A typical thermal resistance for resistor  42  in a 400 watt power amplifier system is 2.0° C./watt, which represents the heatsink to ambient thermal resistance. 
     Referring to FIG. 3, there is shown a physical implementation of portions of the audio power supply of FIG. 1 that are represented by elements of the thermal model of FIG.  2 . FET footprints  44 - 1 - 44 - 4  are the footprints on heatsink  22  of the pair of FETs associated with power amplifiers  12 - 1 - 12 - 4 , respectively. FET footprints  44 - 1 - 44 - 4  correspond to the interface represented by sink nodes  36 - 1 - 36 - 4  of FIG.  3 . FET footprints  41 - 1 - 44 - 4  have a similar orientation relative to heatsink  22  and to each other. If heatsink  22  is substantially rectangular and center lines  46  and  48  are drawn perpendicular to the sides, each of the footprints  44 - 1 - 44 - 4  are in a different quadrants. No three of the footprints are collinear, so that the thermal path between any of the footprints  44 - 1 - 44 - 4  and any point of heatsink  22  passes at most through one other footprint. The thermal path between each of the footprints and the two closest edges of the rectangular heat sink passes through none of the other footprints. Mounting the pairs of FETs on a common heatsink, and in a substantially symmetrical pattern (that is, the footprint of each pair of FETs has the same orientation relative to the other pairs of FETs and to the boundaries of the plane of the heat sink on which they are mounted), with no three pairs of FETs mounted collinearly is advantageous because it enables adequate heat dissipation with a minimum of heat sink material. Since each of the channels is capable of a 400 watt capacity, each pair of FETs may dissipate heat resulting from the transmission of 400 watts. 
     Referring to FIG. 4, there is shown a schematic diagram of a simplified circuit for implementing and testing two output channels of the amplifier system of FIG.  1 . Implementing a four channel output could be done by a second essentially identical circuit. Module  46  is a model TA0105A module available commercially from Tripath, Inc. of Santa Clara, Calif. (www.tripath.com). The circuitry in sector  48  optimizes the protection circuits of module  46  for 70 Volt or 100 Volt operation. The circuitry in sector  50  is for inputting signals into module  46  and to control the DC offset at the output. The remainder of the circuit of FIG. 4 implements two of the channels of the power amplifier system. Power input terminals  52  and  54  represent the inputs from power supply  13  of FIG. 1 to two of the power amplifiers such as  14 - 1  and  14 - 2  of FIG. 1. 12 volt power supply  58  and five volt power supply  60  (neither shown in FIG. 1) supply power for some of the devices of the circuit. FETs  62 ,  64 ,  66 , and  68  represent the FETs referenced previously in the discussion of FIGS. 1,  2 , and  3 . 
     Referring to FIGS. 5 a - 5   d  there is shown a schematic diagram of a circuit implementing the power amplifying system of FIG.  1 . 
     It is evident that those skilled in the art may now make numerous uses of and departures from the specific apparatus and techniques disclosed herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features disclosed herein and limited only by the spirit and scope of the appended claims.