Acoustic apparatus

Abstract: An acoustic apparatus comprises a cabinet, and a loudspeaker unit having a pair of input terminal, a first and a second driving systems disposed in the cabinet. The acoustic apparatus is driven by an external power amplifier which normally constant-voltage-drives a conventional loudspeaker. The first driving system supplies the entire of a input driving signal to one input terminal of a loudspeaker unit and the second driving system supplies component signals of the input driving signal other than ones in the specific frequency band. As a result, driving systems drive the loudspeaker unit with the component signals of the input driving signal in the specific frequency band in cooperation with the external amplifier.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acoustic apparatus which can supply a specific frequency band component signal of a driving signal to a loudspeaker unit and, more particularly, to an acoustic apparatus which is arranged without using a passive dividing network, and is driven by a conventional external power amplifier unit like in a conventional apparatus using the passive dividing network. 2. Description of the Prior Art As a conventional system for driving a multi-way loudspeaker system, e.g., the system having a woofer, squaker, tweeter, or the like, a dividing network system and a multi-amplifier system are known. However, the dividing network system requires large-capacity LC real elements. For this reason, this network system poses the following problems: (1) A reproduced sound is distorted by a magnetic distortion of an inductance element (in particular, the distortion is conspicuous when a cutoff frequency f c of a filter constituting a network is decreased). (2) An element inevitably becomes large in size since a core of an inductance element must be increased in size to reduce the magnetic distortion. (3) Serial resistances of a loudspeaker unit and a driving system are increased due to a resistance of a coil of an inductance element, and Q of the loudspeaker unit is increased and cannot be damped. (4) In addition, an AC nonpolarized (bipolar) capacitor having a large capacitance is required. In general, tanδ is small, and precision of a capacitance (i.e., dividing precision) is also low. (5) Since a load is not a pure resistance but a impedance of a loudspeaker is changed depending on a frequency, it is difficult to design network characteristics. (6) When an attenuator or the like is arranged to adjust, e.g., frequency characteristics of an output sound pressure, damping characteristics or the like are further adversely influenced. On the other hand, the multi-amplifier system can solve the problems in the dividing network system. However, the multi-amplifier system must systematically deal with the entire system including a channel divider, power amplifiers in units of frequency bands, and the like, and can never be a loudspeaker system in which a loudspeaker system and an amplifier can be arbitrarily selected, i.e., which can be driven by an amplifier selected by a user. SUMMARY OF THE INVENTION The present invention has been made in consideration of the conventional problems, and has as its object to provide an acoustic apparatus which constitutes a multi-way loudspeaker system without using an LC real element (passive) dividing network, and can be driven by a conventional power amplifier like in a conventional apparatus employing the passive dividing network system. In order to achieve the above object, according to the present invention an acoustic apparatus for supplying a specific frequency band of a driving signal to a loudspeaker unit having first and second input terminals comprises a first driving system for supplying the entire driving signal to the first input terminal; and a second driving system, having the entire driving signal as an input, for removing frequency components corresponding to the specific frequency band from the entire driving signal and producing a modified driving signal (Vi·T(s)), the second driving system supplying the modified driving signal (Vi·T(s)) to the second input terminal. More specifically, as a driving signal source, a conventional power amplifier is used, the loudspeaker is differentially driven by a path from the conventional power amplifier, and a path (auxiliary path) for an auxiliary amplifier, which branches from the former path, and driving characteristics are set by the auxiliary path. With the above arrangement, the loudspeaker is driven by a difference between the entire driving signal and component signals other than specific frequency component signal, i.e., the specific frequency component signal of the driving signal. Therefore, the acoustic apparatus of the present invention is not particularly limited except that a frequency band of a driving signal source, e.g., a power amplifier includes all or part of the specific frequency band, and can be driven by a power amplifier desirably selected by a user. Since the auxiliary amplifier is operated in cooperation with a driving amplifier, e.g., the conventional amplifier, it can be a relatively low-capacity and compact one, e.g., a compact IC. This merit is conspicuous especially when transfer characteristics T(s) of the auxiliary path satisfies T(s) > 0, i.e., when the output from the auxiliary amplifier has the same polarity as that of the driving amplifier. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram showing a basic arrangement of an acoustic apparatus according to an embodiment of the present invention; Figs. 2A and 2B are circuit diagrams showing in detail an auxiliary amplifier circuit shown in Fig. 1; Figs. 3A and 3B are graphs for explaining the relationship between transfer characteristics T(s) of a transfer characteristic providing circuit shown in Fig. 2A and loudspeaker driving characteristics G(s) obtained thereby; Fig. 4 is a graph showing a transfer gain-frequency characteristics given to the auxiliary amplifier circuit by the apparatus shown in Fig. 1; Figs. 5A to 5C are voltage waveform charts of the respective portions in the apparatus shown in Fig. 1; Figs. 6A to 6C are graphs showing signals corresponding to Figs. 5A to 5C as transfer gain frequency characteristics; Fig. 7 is a circuit diagram of a three-way loudspeaker system according to another embodiment of the present invention; Figs. 8A to 8C are graphs showing transfer gain-frequency characteristics of auxiliary amplifier circuits in the system shown in Fig. 7; and Fig. 9 is a circuit diagram of a loudspeaker system with a resonance duct port according to still another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Note that the same reference numerals denote common or corresponding parts throughout the drawings. Fig. 1 shows a basic arrangement of an acoustic apparatus according to an embodiment of the present invention. This acoustic apparatus drives a loudspeaker unit 2 having a pair of input terminals S1 and S2 by a specific frequency component signal of a driving signal Vi supplied from a conventional power amplifier (constant-voltage-driving amplifier) 1 through a pair of connection terminals I1 and I2. One input terminal S1 of the loudspeaker unit 2 is connected to one connection terminal I1, and the other connection terminal I2 is connected to an operation reference potential (ground) terminal E of an auxiliary amplifier circuit 3. The input terminal of the auxiliary amplifier circuit 3 is connected to one connection terminal I1, and its output terminal is connected to the other input terminal S2 of the loudspeaker unit 2. The auxiliary amplifier circuit 3 has transfer gain-frequency characteristics T(s) corresponding to the specific frequency band, and generates an output Vo given by Vo = Vi·T(s) in response to an input signal Vi. The loudspeaker 2 receives the driving signal Vi at its one input terminal S1, and receives the output Vo = Vi·T(s) from the auxiliary amplifier circuit 3 at the other input terminal S2. Therefore, the loudspeaker unit 2 is driven by a signal VL given by: VL = Vi - Vo = Vi[1 - T(s)] From this equation, loudspeaker driving characteristics as a target for driving the loudspeaker unit 2, i.e., transfer characteristics G(s) from driving signal source connection terminals I1 and I2 to input terminals S1 and S2 of the loudspeaker unit 2 are given by G(s) = 1 - T(s). Figs. 2A and 2B show the auxiliary amplifier circuit having such transfer characteristics. The auxiliary amplifier circuit 3 shown in Fig. 2A is constituted by a loudspeaker driving auxiliary amplifier 31 of a gain "1", and a transfer characteristic providing circuit 32 connected in series with the input terminal of the auxiliary amplifier 31. Transfer characteristics of the transfer characteristic providing circuit 32 are set to be T(s). In the auxiliary amplifier circuit 3 shown in Fig. 2B, a voltage feedback amplifier 33 is added to the circuit shown in Fig. 2A. For example the voltage feed amplifier 33 can be used as a DC servo amplifier by constituting an integrating circuit in its inverting input side of the amplifier 33. In general, when T(s) gives quadratic or higher-order characteristics, it is difficult to directly generate the transfer gain frequency characteristics G(s) = 1 - T(s). For example, even when T(s) is expressed by quadratic high-frequency cutoff characteristics shown in Fig. 3A, characteristics G(s) make a complex change, as shown in Fig. 3B. In this case, the transfer characteristic providing circuit 32 can be realized characteristics T(s) and G(s) by an active circuit. When active characteristics are realized, the auxiliary amplifier 31 or feedback amplifier may also be used as an active element. Even when T(s) is expressed by linear characteristics, the transfer characteristic providing circuit 32 can be arranged in a feedback system of the auxiliary amplifier 31. An operation performed when transfer gain-frequency characteristic T(s) of the auxiliary amplifier circuit 3 is set as a band-elimination characteristic shown in Fig. 4 in the acoustic apparatus shown in Fig. 1 will be described bellow. In Fig. 1, when a signal including a low-frequency component f1 and a component f2 (middle-frequency component) in the attenuation band shown in Fig. 5A is applied across the drive signal source connection terminals I1 and I2, i.e., across the loudspeaker input terminal S1 and the ground terminal E, the auxiliary amplifier circuit 3 amplifies only the signal component f1 in a pass band with the gain "1", and outputs the amplified component. Therefore, a signal consisting of only the low-frequency component f1 shown in Fig. 5B as an output from the auxiliary amplifier circuit 3 is applied across the loudspeaker input terminal S2 and the ground terminal E. Thus, a signal obtained by subtracting the signal shown in Fig. 5B from the signal shown in Fig. 5A, i.e., a component signal consisting of only the middle-frequency component f2 shown in Fig. 5C is applied across the two input terminals of the loudspeaker unit 2. Figs. 6A to 6C show the transfer gain-frequency characteristics G(s) or T(s) of the respective portions corresponding to the waveforms shown in Figs. 5A to 5C. In this manner, in the apparatus shown in Fig. 1, a driving signal in the attenuated band by auxiliary amplifier circuit 3 is applied to the loudspeaker unit 2. By properly selecting the attenuation band of the auxiliary amplifier circuit 3, the loudspeaker unit 2 can be driven by a desired frequency component signal of the driving signal. Since the auxiliary amplifier circuit 3 has a small transfer gain in a band where a driving current of the loudspeaker unit 2 (i.e., an output current of the auxiliary amplifier curcuit 3) is large, an output voltage is attenuated and has a small amplitude. Contrary to this, in a pass band where the output voltage has a large amplitude, the driving current of the loudspeaker unit 2 is decreased. Therefore, the auxiliary amplifier circuit 3 has relatively low power consumption, and need only a relatively small-capacity and compact one. Fig. 7 shows an embodiment wherein the present invention is applied to a three-way loudspeaker system. In Fig. 7, an auxiliary amplifier circuit 3a is a high-pass filter (HPF) which has transfer characteristics T(s)w shown in Fig. 8A, i.e., has, of a driving signal Vi to be supplied to driving signal connection terminals I1 and I2, a driving signal band of a woofer 2a as an attenuation band, and other driving signal bands as pass bands of a transfer gain "1". Auxiliary amplifier circuits 3b and 3c are respectively a band-elimination filter (BEF) and a low-pass filter (LPF) having transfer characteristics T(s)s and T(s)t, as shown in Figs. 8B and 8C, i.e., having driving signal bands of a squaker 2b and a tweeter 2c as attenuation bands and other signal bands as pass bands, respectively. According to this arrangement, as described above, the driving signal Vi is supplied to the respective loudspeakers unit 2 (2a, 2b, 2c) with a transfer gain given by: G(s)x = VLx Vi = 1 - T(s)x (for x = w, s, t) More specifically, the driving signal Vi is divided into bands, and corresponding component signals VLw, VLs, and VLt are supplied to the respective loudspeakers. That is, a low-frequency component as a component signal in a band which is not attenuated by signal bands from the HPF 3a is supplied to the woofer 2a, a middle-frequency component as a component signal in a band which is not attenuated by signal bands from the BEF 3b is supplied to the squaker 2b, and a high-frequency component as a component signal in a band which is not attenuated by signal bands from the LPF 3c is supplied to the tweeter 2c. In the system shown in Fig. 7, the transfer gains T(s) of the auxiliary amplifiers 3a, 3b, and 3c are set to be "1" in the pass bands, are set to be "0" in the attenuation bands, and become positive over all the bands. However, the gains may be set to be values other than 1 and 0 in correspondence with efficiency of each loudspeaker unit 2. A variable element such as a variable resistor (attenuator) may be arranged at the input side of each of the auxiliary amplifier circuits 3a, 3b, and 3c or in the feedback loop to vary the transfer gain T(s), thereby allowing adjustment of, e.g., frequency characteristics. Furthermore, the transfer gain T(s) may be set to be negative in the attenuation band (i.e., a speaker driving band). In this case, the auxiliary amplifier circuit 3 and the conventional power amplifier for supplying the driving signal Vi perform negative impedance driving (disclosed in European Patent Application Publication No. 0322686) in cooperation with each other, thereby improving reproduction characteristics of the loudspeaker unit 2 as compared to that in so-called constant voltage driving. Fig. 9 shows an embodiment wherein the present invention is applied to a loudspeaker system with a resonance duct port. In a system shown in Fig. 9, a loudspeaker unit 2 and an auxiliary amplifier circuit 3 as the characteristic feature of the present invention are housed in a cabinet having a resonance duct port 61. A DC power supply circuit 7 for operating the auxiliary amplifier circuit 3, and a protection circuit 8 for protecting the respective portions of the circuit from being deteriorated or broken due to an overload or an abnormal operation are also incorporated in the same cabinet 6. The auxiliary amplifier 3 negative-impedance drives the loudspeaker unit 2 in cooperation with an external power amplifier 1 as a driving signal source. In Fig. 9, the auxiliary amplifier circuit 3 comprises a driving amplifier 31, a transfer characteristic providing circuit 32, a feedback circuit 37, and a loudspeaker current detection resistor Rs. In the transfer characteristic providing circuit 32, a voltage dividing circuit including resistors R1, R2, divides a driving signal Vi at a voltage dividing ratio k (k = R2/(R1 + R2)). An equalizer circuit 35 provides transfer characteristics T(s) to an output k*Vi from the voltage dividing circuit. A buffer amplifier 36 amplifies an output k*Vi*T(s) from the equalizer circuit 35 with a gain (R3 + R4)/R3 ≃ 1, and amplifies the driving signal Vi supplied to its inverting input terminal through a coupling capacitor C1 and a resistor R3 with a gain -[R4/(R3 + R4)] ≃ -k, thereby generating an output given by: k*Vi*T(s) - k*Vi = k[T(s) - 1]*Vi More specifically, the transfer gain of this transfer characteristic providing circuit 32 is k[T(s) - 1]. The driving amplifier 31 amplifies the output from the transfer characteristic providing circuit 32 with a gain -[(R5 + R6)/R5] ≃ -(1/k). Thus, the transfer gain of the auxiliary amplifier circuit 3 is 1 - T(s). 1 - T(s) is caused to coincide with desired loudspeaker driving characteristics G(s), thereby applying a desired frequency component signal of the driving signal Vi to the loudspeaker unit 2. In the system shown in Fig. 9, a current flowing through the loudspeaker unit 2 is detected by the loudspeaker current detection resistor Rs connected in series with the external power amplifier (constant-voltage-driving amplifier) 1 and the loudspeaker unit 2, and is applied to the noninverting input terminal of the driving amplifier 31 via the feedback circuit 37 of a transfer gain β. In this manner, a voltage across the current detection resistor Rs is multiplied with β, and the product is added (positively fed back) to the output [1 - T(s)]*Vi, so that an output impedance Zo of the auxiliary amplifier circuit 3 is given by: Zo = Rs(1 - Aβ) (for A = (R5 + R6)/R5) Since β ≃ 1 at a sufficiently low frequency, Aβ >> 1 is satisfied, and the output impedance Zo serves as a negative impedance. Thus, in a low-frequency band, negative resistance driving disclosed in European Patent Application Publication No. 0322686 is executed, and the loudspeaker unit 2 is very strongly driven and damped, thus improving reproduction characteristics of the loudspeaker unit, in particular, bass-band characteristics. In addition, the cabinet 6 and the entire loudspeaker system can be rendered compact without impairing reproduction characteristics of the loudspeaker system. As the protection circuit 8, there can be used a known circuit having known functions, such as a DC protection function for turning off a relay contact Ry when a DC current beyond a predetermined value flows through the loudspeaker unit, an overcurrent protection function for turning off the relay contact Ry when an overcurrent flows through the loudspeaker unit, a heat sink temperature protection function for turning off a relay contact Ry1 when a heat sink temperature exceeds a predetermined value, a power supply muting function for turning on the relay contact Ry after a predetermined delay time upon power-on to prevent noise generation due to a transient response upon power-on, or a circuit or the loudspeaker from being deteriorated or broken, and the like. Alternatively, a protection means such as a primary fuse, an intra-transformer temperature fuse, or the like may be arranged.

Claims: An acoustic apparatus for supplying a specific frequency band of a driving signal (Vi) to a loudspeaker unit (2) having first and second input terminals (S1, S2), characterized in that said apparatus comprises a first driving system for supplying the entire driving signal (Vi) to the first input terminal (S1); and a second driving system (3), having the entire driving signal (Vi) as an input, for removing frequency components corresponding to the specific frequency band from the entire driving signal (Vi) and producing a modified driving signal (Vi·T(s)), the second driving system supplying the modified driving signal (Vi·T(s)) to the second input terminal (S2). An apparatus according to claim 1, wherein said driving signal (Vi) is supplied by a constant-voltage-driving amplifier. An acoustic apparatus according to claim 1, which has a pair of driving signal source connection terminals (I1, I2) for supplying said driving signal (Vi) having a plurality of frequency bands, and wherein said first driving system comprises a wiring by which said first input terminal (S1) of said loudspeaker (2) is substantially directly connected to one (I1) of said driving signal source connection terminals (I1, I2); and said second driving system comprises an auxiliary amplifier circuit (3) having an output terminal which is connected to the second input terminal (S2) of said loudspeaker (2), and an operation referenc potential terminal which is connected to the other one (I2) of said driving signal source connection terminals (I1, I2), for outputting a signal containing components outside a specific one of said plurality of frequency bands, thereby said loudspeaker unit (2) being driven by a signal containing components within the specific frequency band of the driving signal (Vi). An apparatus according to claim 3, wherein said pair of driving signal source connection terminals (I1, I2) are provided by a driving signal source comprising a constant-voltage-driving amplifier. An apparatus according to claim 3, wherein said auxiliary amplifier circuit (3) comprises a transfer characterisitic providing circuit (32) having a filtering characteristic. An apparatus according to claim 5, wherein said transer characteristic providing circuit is an active circuit. An apparatus according to claim 5, wherein said auxiliary amplifier circuit (3) comprises a driving amplifier (31) for driving said loudspeaker unit (2), a current detection means (Rs) for detecting a current flowing through said loudspeaker unit (2), and a feedback means (37) for feeding back the detected current to said driving amplifier (31). An apparatus according to claim 3, wherein said auxiliary amplifier circuit (3) and said loudspeaker unit (2) are disposed in a cabinet (6). An apparatus according to claim 8, wherein said cabinet (6) constitutes a resonator with a resonance duct port (61).

Inventors: YOKOYAMA KENJI, YOKOYAMA, KENJI
Applicants: YAMAHA CORP, YAMAHA CORPORATION
Application year: 1990
Main classification: H04R3
IPC codes: H04R3