Abstract:
An amplifier circuit is disclosed suitable for feeding a loudspeaker system having plural drivers. The amplifier circuit has an output stage and plural output connectors such that there is an output connector for each driver, wherein the output stage is connected separately to each output connector for independent connection to each driver. A method of feeding a loudspeaker system having plural drivers with an amplifier circuit having an output stage is also disclosed.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an amplifier circuit suitable for audio applications and in particular relates to an amplifier circuit for improved sound quality. 
       Definitions 
       [0002]    In this specification “amplifier” refers to a single audio power amplifier (i.e. one channel) and includes an electronic amplifier circuit and any peripheral apparatus associated with the amplifier in a consumer packaged form, including power supplies, protection devices, signal conditioning, switches, potentiometers, connectors and a case or enclosure. However it excludes other amplifier circuits that may be included in the same case or enclosure. Thus in the case of a stereo amplifier product, in this context there are two “amplifiers” to which the present invention may be applied. 
         [0003]    In this specification the “controlled output” of an amplifier denotes the final output amplification stage. Typically it is the point in an amplifier circuit where an output feedback loop commences. Accordingly this specification may use the terms “controlled output” and “feedback point” interchangeably. 
         [0004]    In this specification “passive loudspeaker system” refers to a loudspeaker system comprising more than one loudspeaker driver fed by a single amplifier and may include passive filter components. 
       BACKGROUND OF INVENTION 
       [0005]    In multi-band loudspeaker systems it is often observed that passive loudspeaker systems lack the sound quality of bi-amped or active loudspeaker systems. It is sometimes also observed that very low impedance loudspeaker cables provide an improvement in sound quality. Many theories are advanced as to why these results occur and in fact it is not uncommon to see engineers attempt to debunk these claims for theoretical reasons without seeing a need to perform a listening test. 
         [0006]    However loudspeaker drivers in passive multi-band systems are not “firmly” connected to the low output impedance “controlled output” or feedback point of the amplifier. Instead the drivers are isolated from the controlled output or feedback point to some degree by various impedances, including the loudspeaker cable impedance and the impedance of a damping output inductor. Even when the drivers are connected in a bi-wired arrangement there is still a damping output inductor partially isolating the controlled output of an amplifier from the loudspeaker drivers. The damping output inductor or at least the impedance associated with the damping inductor is one reason why bi-wired configurations do not sound as clean as bi-amped configurations. 
         [0007]    In passive systems all drivers connected to a single amplifier act as loads for the back EMF of all other drivers connected to the single amplifier regardless of any electrical filtering interconnecting the drivers. This is especially the case where there are no electrical filters or where the filters are of low order. If the loudspeaker drivers were firmly tied to a controlled output having a low output impedance, driver interaction would be negligible. However in consumer packaged amplifiers they rarely are firmly tied because damping output inductors and/or other components are typically present between the controlled output or feedback point and the amplifier terminals. 
         [0008]    Typically in a passive system, the damping output inductor or equivalent circuitry in the amplifier and cable impedances isolate the controlled output or feedback point from a branching point to individual loudspeaker drivers so that back EMF from drivers results in circulating currents between drivers causing audible corruption of the sound produced. 
         [0009]    A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge in Australia or elsewhere as at the priority date of any of the disclosure or claims herein. Such discussion of prior art in this specification is included to explain the context of the present invention in terms of the inventor&#39;s knowledge and experience. 
         [0010]    Throughout the description and claims of this specification the words “comprise” or “include” and variations of those words, such as “comprises”, “includes” and “comprising” or “including, are not intended to exclude other additives, components, integers or steps. 
       SUMMARY OF INVENTION 
       [0011]    According to one aspect of the present invention there is provided an amplifier circuit suitable for feeding a loudspeaker system having plural drivers, said amplifier circuit having an output stage and plural output connectors such that there is an output connector for each driver, wherein said output stage is connected separately to each output connector for independent connection to each driver. 
         [0012]    The amplifier circuit may include an independent damping output inductor and/or other components or circuits that add an impedance between the output stage and each output connector. Other components may include a common mode filter. Each damping output inductor may comprise an air cored inductor in parallel with a damping resistor. Each damping output inductor may include a circuit that terminates at the respective output connector. Values of each damping output inductor may be determined to suit the respective frequency band served by the respective driver. In some embodiments, values for all damping output inductors may be identical. Each output connector may be labelled with one of the following names: tweeter, midrange, woofer and/or translations thereof into other languages. In some embodiments, the amplifier circuit may be embedded in an integrated circuit package. 
         [0013]    According to a further aspect of the present invention there is provided a method of feeding a loudspeaker system having plural drivers with an amplifier circuit having an output stage, said method comprising providing said amplifier circuit with plural output connectors such that there is an output connector for each driver, and separately connecting said output stage to each output connector for independent connection to each driver. 
         [0014]    The present invention may allow the bi-wiring principle to be extended to the feedback point of an amplifier containing a multiple output filter. It may be implemented as forked outputs commoned at the feedback point to ensure that current flowing through an output damping inductor is substantially the same current flowing into the loudspeaker driver to which it is connected. It may improve sound quality in the same way as bi-amping does but without the expense of additional amplifiers. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]    A preferred embodiment of the present invention will now be described in detail with reference to the following diagrams wherein: 
           [0016]      FIG. 1  shows a schematic representation of an amplifier according to the present invention; 
           [0017]      FIG. 2  shows a typical prior art passive multi-band loudspeaker system; 
           [0018]      FIG. 3  shows a typical prior art passive multi-driver loudspeaker system; 
           [0019]      FIG. 4  shows a prior art bi-wired passive loudspeaker system; 
           [0020]      FIG. 5  shows a passive loudspeaker system according to the present invention; and 
           [0021]      FIG. 6  shows a representative back panel for the amplifier of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  shows an overview of the present invention in schematic form. Amplifier  1  contains an amplifier circuit G with feedback originating from feedback point  2 . Output impedances Z 1  to Zn, which may vary from each other, represent independent damping output inductors and include any other circuitry added to the output of amplifier G past feedback point  2  for feeding a load such as loudspeaker drivers  22 ,  23  as shown in  FIG. 2 , connected to amplifier G. 
         [0023]      FIG. 2  shows an amplifier  11  in a common prior art configuration including connector  12  which may comprise a pair of binding posts or a connector socket. Internal to amplifier  11  is a pair of active output devices comprising bipolar devices  13 ,  14 . Bipolar devices  13 ,  14  are exemplified for illustrative purposes only, but serve to identify the controlled output or feedback point  15  of amplifier  11  from which a feedback signal is typically taken. In the forward direction a damping output inductor is shown comprising an idealised inductor  16 , having internal resistance  17  and damping resistor  18  in parallel. The damping resistor  18  is relatively large by comparison and may be of no consequence. 
         [0024]    Beyond amplifier  11  is a loudspeaker cable  24  with total resistance including the sum of resistances  19  and  20 . Cable  24  terminates at connector  21  which may include binding posts or connector sockets. For illustrative purposes, filtering of the signal at loudspeaker drivers  22 ,  23  is performed via a capacitor C feeding tweeter  22  and inductor L feeding woofer  23 . In some instances the filtering may be significantly more complex. In the example of  FIG. 2  the number of loudspeaker drivers and associated frequency bands is shown as two but in general may be more than two. 
         [0025]    In any event it may be seen that the controlled output or feedback point  15  of amplifier  11  is isolated from loudspeaker drivers  22 ,  23  by the damping output inductor represented by idealised inductor  16  and resistance  17  as well as the resistances  19 ,  20  of cable  24 , but loudspeaker drivers  22 ,  23  are relatively directly connected and can interact electrically with each other. In particular the back EMF of each driver  22 ,  23  is to some extent cycled through the other driver giving rise to both harmonic and non-harmonic distortion. For discerning listeners seeking a clean sound from amplifier  11  with a relatively low level of distortion (e.g. 0.01% or better), this prior art configuration cannot come close to providing it. 
         [0026]      FIG. 3  shows another prior art configuration with a single amplifier  31  feeding two different drivers  32 ,  33  operating in the same frequency band. In the example of  FIG. 3  two drivers are shown for illustrative purposes, but in general the number of drivers may be higher. Nevertheless the same problems arise as for the amplifier in  FIG. 2  and for the same reasons the configuration of  FIG. 3  may result in corruption of sound unless the drivers are identical. 
         [0027]      FIG. 4  shows another prior art configuration with a single amplifier  40  feeding a bi-wired passive loudspeaker system comprising drivers  45 ,  46 . The filtering shown is for representative purposes only and may differ from that shown in  FIG. 4 . As may be seen there is now more resistance between loudspeaker drivers  45 ,  46  than in  FIGS. 2 and 3 , but less resistance between the common output point  41  of the circuit and the controlled output or feedback point  43 . Accordingly currents circulating between loudspeaker drivers  45 ,  46  are now impeded by cable resistances  47 - 48 , and the controlled output  43  has more control over the common output point  41  of the circuit. Nevertheless that control is substantially weakened by damping output inductor  44  and the configuration of  FIG. 4  still cannot deliver clean sound. 
         [0028]      FIG. 5  shows an amplifier  50  feeding a passive loudspeaker system comprising drivers  56 ,  57  wherein amplifier  50  is configured according to the present invention. As in  FIG. 4  the filtering shown in  FIG. 5  is generic and for representative purposes only. In particular amplifier  50  is configured such that controlled output  51  of amplifier  50  feeds loudspeaker drivers  56  (woofer), and  57  (tweeter) via separate damping output inductors  52 ,  53  and via a separate set of output posts or sockets  54  and  55 . The separate set of output posts or sockets  54 ,  55  are connected via separate cables  58 ,  59  to the respective loudspeaker drivers  56 ,  57 . Accordingly circulating currents between loudspeaker drivers  56 ,  57  are substantially eliminated and the controlled output  51  of amplifier  50  substantially directly controls the current to each loudspeaker driver  56 ,  57 . This arrangement may be suitable for any passive multi-driver loudspeaker system and may provide relatively cleaner sound. 
         [0029]      FIG. 6  shows a representative stereo amplifier back panel  60  including the configuration of  FIG. 5 . Back panel  60  includes stereo (left and right channel) outputs for a two way passive loudspeaker system, including two tweeter outputs  61 ,  62  and two woofer outputs  63 ,  64 . 
         [0030]    In particular the configuration in amplifier  50  replaces a single damping output inductor as in  FIGS. 2 to 4  with separate damping output inductors  52 ,  53  for each electrical circuit feeding a respective driver  56 ,  57 .  FIGS. 5 and 6  show a configuration adapted for a loudspeaker system having two frequency bands including a tweeter and a woofer. Nevertheless, the principles of the present invention may also be applied to loudspeaker systems having three or more frequency bands. 
         [0031]    The damping output inductors  52 ,  53  may be formed from an air-cored coil in parallel with a damping resistor and the associated circuits may be terminated at respective output posts or sockets  54 ,  55 . The damping output inductors are representative only and alternative circuits may include transformers, shunt components, protection devices, common mode filters, and the like. 
         [0032]    The inductors  52 ,  53  may optionally have different values to suit the different frequency bands and may be labelled tweeter and woofer or translations hereof in the case of a two band system. In the case of a three band loudspeaker system, three damping output inductors may be used to feed a tweeter, midrange driver and woofer respectively via separate output posts or sockets. 
         [0033]    In a further embodiment an amplifier circuit according to the present invention may be embedded via in an integrated circuit package wherein multiple damping output inductors are also embedded and their respective circuits terminate via separate output pins. 
         [0034]    Care should be taken to avoid crosstalk between the respective output damping inductors. This may be done e.g. by (a) separation, (b) shielding and/or (c) orthogonal placement. However a preferred method may be to wind the damping inductors as toroids and to mount them vertically. Care should also be taken to avoid crosstalk between the damping inductors of different amplifier channels of multichannel amplifiers such as stereo and/or surround amplifiers. 
         [0035]    Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.