GB2068680A

GB2068680A - Arrangement for driving speaker through constant-current power amplifier

Info

Publication number: GB2068680A
Application number: GB8101385A
Authority: GB
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1980-01-23
Filing date: 1981-01-16
Publication date: 1981-08-12
Also published as: GB2068680B; JPS56103592A

Abstract

A method and its circuit arrangement for driving a dynamic loudspeaker 22 by an audio signal through a constant-current power amplifier 21, in which there are suppressed particular frequency components of the audio signal, that is, at least the lowest resonance frequency of the speaker and frequencies in the vicinity of the lowest resonance frequency so as to provide a flat frequency characteristic which is equivalent to or better than that when the speaker is driven through a constant-voltage power amplifier, at a given electroacoustic conversion efficiency of the speaker. <IMAGE>

Description

SPECIFICATION Method and circuit arrangement for driving speaker by audio signal through constant-current power amplifier The present invention relates to a system of driving a speaker and more particularly concerns an improved method and its circuit arrangement for driving speakerthrough a constant-current power amplifier.

In a prior art system where a dynamic speaker is driven through aconstant-current power amplifier, since the mechanical impedance of the speaker becomes minimum at the lowest resonance frequency of the speaker, the speaker driven with the same force as that at the lowest frequency thereof with respect to the other frequencies will have too high acoustic energy of sound level at the lowest frequency of the speaker as compared with ones at the other frequencies.

In orderto avoid such a resonance peak of the acoustic level at the lowest frequency of the speaker in the constant-current driving system, on the other hand, there has been suggested and often used, so far, a constant-voltage driving system in which a constant-voltage power amplifier with a relatively low internal resistance is connected across a voice coil of the speaker so that counter electromotive force generated at the voice coil is dissipated as thermal energy from the voice coil and thus damping is provided for the speaker.

However, in the conventional constant-voltage driving system, there has been such defects that the employment of a speaker with a large power factor for an increased electroacoustic conversion effi ciencywill cause over-damping of the speaker, providing an insufficient sound level at and near the lowest frequency, though a proper sound level can be obtained at the other frequency range; whereas, the speaker driven buy a high poweramplifierwill cause increase of the temperature at its voice coil at the time of high sound level output. In the latter case, the resistance of the voice coil increases to 1.5 to 2 times that at its room temperature and under this condition, the output of the speaker will not increase in direct proportion to the increased output of the amplifier.In other words, the output of the speaker will not be linear with the output of the amplifier.

Accordingly, it is an object of the present invention to eliminate such defects as above in the case of the conventional constant-current driving system.

According to the present invention, the above object can be achieved by providing a method of driving a speaker by an audio signal through a constant-current power amplifier comprising the steps of suppressing the frequency components of the audio signal, the frequency being at least a lowest resonance frequency of the speaker and frequencies in the vicinity of the lowest resonance frequency, applying the suppressed audio signal to a constant-current power amplifier, and driving the speaker by an output signal from the constantcurrent power amplifier; and by providing a circuit arrangement for driving a speaker by an audio signal comprising an input terminal for receiving an audio signal, a speaker, a constant-current power amplifier for driving the speaker, and means connected to the input terminal for suppressing the frequency components of the audio signal, the frequency being at least a lowest resonance frequency and frequencies in the vicinity of the lowest resonance frequency of the speaker and for applying an output thereof to the constant-current power amplifier.

Other objects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompaying drawings, in which: Fig. 1 is an equivalent circuit of a speaker mounted on an inner wall of a bass-reflex enclosure, including the electric system and mechanical system thereof; Fig. 2 is an equivalent circuit of a speaker in the case that the speaker is mounted on an inner wall of a bass-reflex enclosure, showing a combined electric system thereof with the mechanical system thereof being put in the corresponding electric equivalent; Fig. 3 is a suppression circuit used in a constantcurrent driving system in which particular frequency components of an audio signal are suppressed to drive a speaker; and Fig. 4 shows, as an example, an entire connecting arrangement of the circuit of Fig. 3 in the case that the same circuit is actually connected to a speaker.

Now, the present invention will be explained in detail with reference to accompanying drawings.

Referring now to Fig. 1 of the drawings, there is shown an equivalent circuit of a dynamic speaker in the case that the speaker is mounted on an inner wall of a bass-reflex enclosure, in which reference numeral 1 denotes an audio signal source which supplies an output current 1o and has an internal resistance R0 (infinity), 2 denotes a voice coil of the speaker which has an internal resistance Rv, 3 denotes an electro acoustictransducerwhich has a power factor b I (where B is magnetic flux density in gap and I is the effective length of the voice coil, i.e. the whole length of wire of that portion of the voice coil which is located within the gap and actually works), 4 denotes a mechanical equivalent mass m0 of the speaker, including the mechanical equivalent mass of an acoustic inertance, 5 denotes a combined mechanical equivalent resistance rm of the mechanical and acoustic resistances, 6 denotes a stiffness SO of the speaker, 7 denotes a mechanical equivalent stiffness S of the acoustic stiffness of the enclosure, and 8 denotes a mechanical equivalent mass m, of the acoustic inertance of the port. Now, assume that only low frequency components of the audio signal are applied to the speaker. Then, the mechanical equivalent representation of the speaker is shown in Fig. 2 in which electric elements of the speaker are also converted to the corresponding mechanical equivalents.In Fig. 2, reference numeral 9 denotes a mechanical vibromotive force generator of an electric constant-current source whose vibromotive force is represented by B I 1o, and a resistance 5' is a sum of a mechanical resistance rm and a mechanical equivalent r0 of an electric resistance. Since the audio signal source has the electric internal resis tance Ro as described above, the sum resistance r0 expressed as follows.

The electric internal resistance R0 of the audio signal source is infinity and B I I is much smallerthan the resistance R2, so that the first term in the right side in Bl equation (1), that is ### is practically substituted R4 + R5 for zero and thus r0 = rrn.

On the other hand, an acoustic pressure P at a distance "d" from the speaker is represented by the following vectorequation.

Where, Po is the density of medium through which sound propagates, 5a is the effective emission area of a diaphragm in the speaker, k is 2## (X is wave length), oj is angular frequency, and v is the vibration velocity of the speaker diaphragm.

09 By substituting-(#0 is an angular frequency at #0 the lowest frequency of the speaker) for

(mechanical resonance sharpness) and

an equation obtained from the equlvalent circuit of Fig. 2, the equation is rewritten as

Substituting jwv in equation (2) for equation (3) results in

In order to make flat the frequency characteristic curve without peaks and troughs as possibre, it is desirableto place mO, SJ2 and 0.5 in the place of m1, St and O12 in equation (4),-respectively. However, Qm is usually much largerthan 0.5, for example, approximately 5. If Qm has such a large value as 5, the speaker used for a bass-reflex enclosure emits too high sound energy, i.e. too strong sound level from the diaphragm at upper and lower lowest resonance frequencies. On the other hand, when the speaker is employed for a closed enclosure, the speaker emits too high acoustic energy at the lowest resonance frequency. In either case, the above phenomena are not desirable.

Here, ifthe optimum value Qm' of Qm can be in fact obtained, then the acoustic pressure P' will be

The ratio of T of P' to P is given as follows.

Where,

This means that if it is possible to realize a circuit of a transfer function expressed by equation (6), then a frequency characteristic equivalent to equation (5) will be obtained by driving the speaker of the fre quency characteristic of equation (4) through the same circuit. The frequency characteristic of equation (6), for example, can be realized by a suppression circuit shown in Fig. 3.

In Fig. 3, a resistor 10 has a resistance of Rr, a resistor 11 has a resistance of R2, an inductor 12 has an inductance of L1, a capacitor 13 has a capacitance of C,, an inductor 14 has an inductance of L2, and a capacitor 15 has a capacitance of C2. The values R1, L1, C" L2 and C2 of the resistor 11, the inductor 12, the capacitor 13, the inductor 14 and the capacitor 15 are selected to satisfy equations (8) to (12), respectively.

The value R2 of the resistor 11 is arbitrary.

In this embodiment, it is assumed that the input impedance when viewed from terminals 16 and 17 is zero and the output impedance when viewed from terminals 18 and 19 is infinity. Though the suppres- sion circuit of Fig. 3 is made up of all passive ele ments, some or all of the passive elements may be replaced with proper active elements, as necessary.

Fig. 4 shows, as an example, an entire connecting arrangement of the suppression circuit of Fig. 3 in the case that the same circuit is actually connected to a speaker. In the arrangement of Fig. 4, a constant voltage signal source is denoted by 20, a constant current power amplifier by21 and a speaker housed in a bass-reflex enclosure by 22. As an alternative, the internal negative feedback circuitforthe constant-current power amplifier 21 may be provided with a proper frequency characteristic so that the constant-current power amplifier 21 itself has the same transfer function as the circuit of Fig. 3. In this case, it will be obvious that the constant-voltage signal source 20 can be directly connected to the constant-current power amplifier 21.

The above description has been made in the case that a speaker is housed in a bass-reflex enclosure.

However, the same holds true in the case thatthe speaker is housed in a closed enclosure, exceptthat m, is replaced with infinity ( ) in Figs. 1 and2 and equations (3) to (5), (7) and (12).

Even in the case that a speaker is housed in any type of enclosure other than that of bass-reflex type and closed type, the method according to the present invention can be applied, as long asian electric circuit with the same frequency characteristic as the electric admittance-frequency characteristic of the speaker in each of the bass-reflex and closed enclosure can be realized.

With the arrangement as has been described above, the present invention has such advantages that a flat frequency characteristic which is equivalent to or better than that when the speaker is driven in a constant-voltage systems can be obtained in a constant-current driving system at a given electroacoustic conversion efficiency of the speaker; and that critical damping, over-damping and underdamping can be selectively set to the speaker, whereby the system according to the present invention provides excellent performance in its lowfre- quencies, that is, excellent low tone quality, as well as increased freedom of the design, as compared with a constant-voltage driving system.

Claims

1. A method of driving a speaker by an audio signal through a constant-current power amplifier comprising the steps of: suppressing the frequency components of said audio signal, the frequency being at least a lowest resonance frequency of said speaker and frequencies in the vicinity of the lowest resonance frequency; applying the suppressed audio signal to a constant-current power amplifier; and driving said speaker by an output signal from said constant-current power amplifier.

2. A circuit arrangement for driving a speaker by an audio signal, comprising: an input terminal for receiving an audio signal; a speaker; a constant current power amplifier for driving said speaker; and means connected to said input terminal for suppres singthe frequency components of said audio signal, the frequency being at least a lowest resonance frequency and frequencies in the vicinity of said lowest resonance frequency of said speaker and for applying an outputthereofto said constant-current power amplifier.

3. Acircuit arrangement according to claim 2, wherein said speaker is fixedly mounted on the inner wail of a bass-reflex enclosure, and wherein said means comprises: an input and an output terminal; aicommon terminal; a first inductor(L1), a first capacitor(C1), a first resistor (R,), and a series con nectionof a second inductor (4))and a second capacitor (C2) which are connected between said input andoutput terminals; and a second resistor (R2) connected between said outp-ut andc-ommon termii3als.

4. A circuit arrangement according to claim 3, wherein said first resistor (R1), said first inductor(L,), said first capacitor (C1), said second inductor (L2) and sai'dseaond capacitor are calculated by the following expressions (8) - (12), correspondingly and respectivelys

5. A circuit arrangement for driving a speaker by an audio signal, such circuit being substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

6. A method of driving a speaker by an audio signal through a constant-current power amplifier substantially as hereinbefore described with refer ence to the accompanying drawings.