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Title:
DEVICE FOR AMPLIFYING AN AUDIO SIGNAL
Document Type and Number:
WIPO Patent Application WO/2017/081571
Kind Code:
A1
Abstract:
The invention relates to a device (12) for amplifying audio signals, comprising an amplifier (121) adapted to receive a first signal as input and to output a second signal capable of powering a loudspeaker (RL), control means (122) configured for detecting the current and voltage of the second signal and for controlling said amplifier on the basis of the voltage of the first signal and of selection information (k) that allows selecting whether the amplifier (121) is also driven on the basis of the voltage or current of the second signal, tuning means (123) configured for changing the output impedance of the device (12) so that, when the first audio signal has constant frequency and voltage, the voltage of the second signal will be independent of the selection information (k) used by the control means for controlling said amplifier (121).

Inventors:
ROMEO GIOVANNI (IT)
Application Number:
PCT/IB2016/056412
Publication Date:
May 18, 2017
Filing Date:
October 25, 2016
Export Citation:
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Assignee:
ST NAZ DI GEOFISICA E VULCANOLOGIA (IT)
International Classes:
H03F3/187; H03F1/56; H03F3/45; H04R3/00
Foreign References:
US7053705B22006-05-30
US4216517A1980-08-05
US20040178852A12004-09-16
Attorney, Agent or Firm:
BIANCO, Mirco et al. (IT)
Download PDF:
Claims:
CLAIMS

1. Device (12) for amplifying audio signals, comprising

- an amplifier (121) adapted to receive a first signal as input and to output a second signal capable of powering a loudspeaker (13),

characterized in that

it also comprises

- control means (122) configured for detecting the current and voltage of the second signal and for controlling said amplifier (121) on the basis of the voltage of the first signal and of selection information (k) that allows selecting whether the amplifier (121) is also controlled on the basis of the voltage or current of the second signal,

- tuning means (123) configured for changing the output impedance of the device (12) so that, when the first audio signal has constant frequency and voltage, the voltage of the second signal will be independent of the selection information (k) used by the control means (122) for controlling said amplifier (121) .

2. Device (12) according to claim 1, wherein the selection information (k) describes the configuration of a potentiometer (R9) .

3. Device (12) according to claims 1 or 2, comprising a signal generator configured for generating a tuning signal, and wherein the first signal consists of said tuning signal.

4. Device (12) according to claim 3, wherein the tuning signal is a signal having constant frequency and amplitude.

5. Device (12) according to claim 4, wherein the tuning signal has a sinusoidal shape.

6. Device (12) according to any one of claims 1 to 5, comprising a voltage meter configured for detecting the peak voltage of the second signal.

7. Device (12) according to claim 6, wherein the voltage meter comprises display means configured for indicating the detected peak voltage of the second signal.

8. Device (12) according to claims 6 or 7, comprising processing means and actuating means in signal communication with said processing means and configured for actuating the control means (122) and the tuning means (123), and wherein the processing means are configured for

- actuating the control means (122) through the actuating means, detecting, by means of the voltage meter, the variation in the peak voltage of the second signal as the selection information (k) changes when the first audio signal has a constant amplitude and a constant frequency, and

- actuating the tuning means (123), through the actuating means, on the basis of the peak voltage variation detected by the voltage meter.

9. Device (12) according to claim 8, wherein the processing means are configured for cyclically actuating the control means (122) and the tuning means (123), so as to search for the output impedance of the device (12) that makes the voltage of the second signal independent of said selection information (k) .

10. Device (12) according to claims 6 or 7, comprising

- processing means in signal communication with the control means (122), so that said processing means can read the selection information (k) , and

- actuating means in signal communication with said processing means and configured for actuating the tuning means ( 123 ) ,

wherein the processing means are configured for

- reading the selection information (k) ,

- detecting, through the voltage meter, a peak voltage of the second signal when the first audio signal has a constant amplitude and a constant frequency,

- calculating an impedance value of a load powered by the second signal on the basis of the selection information (k) and of the peak voltage detected by the voltage meter,

- actuating, through the actuating means, the tuning means (123) on the basis of the calculated impedance value.

11. Device according to any one of claims 1 to 10, comprising a second amplifier (U3) configured for generating a signal having the same amplitude as the output of the amplifier (121) with an offset of 180 sexagesimal degrees.

12. Method for tuning an amplification device (12) according to any one of claims 1 to 11 to a loudspeaker (13) , comprising the steps of

a. inputting to said device (12) a tuning signal having constant frequency and amplitude,

b. changing, through control means (122), selection information (k) that allows defining whether said device (12) drives a loudspeaker (13) by current or voltage, and detecting any variations in the peak voltage of the amplified signal,

c. changing, through tuning means (123), the output impedance of the device (12), so that the voltage of the amplified signal is independent of the selection information (k) .

13. Method according to claim 12, wherein, during the step of changing the selection information (k), the variations in the peak voltage of the amplified signal are detected through the variation in the loudness level of the sound reproduced by the loudspeakers (13) .

14. Method according to claims 12 or 13, wherein, during the step of changing the selection information (k), the variations in the peak voltage are detected through a voltage meter .

15. Method according to any one of claims 12 to 14, wherein the steps of changing the selection information (k) and changing the output impedance are carried out cyclically until, during the step of changing the selection information (k), peak voltage variations are detected which are lower than a threshold.

16. Method for tuning an amplification device (12) according to claim 10 to a loudspeaker (13), comprising the steps of

a. reading selection information (k) that allows defining the type of driving of said device (12),

b. detecting, through a voltage meter, a peak voltage of an amplified signal outputted by the device (12) when an audio signal having a constant amplitude and a constant frequency is inputted to the device (12),

c. calculating, through processing means, the impedance value of the loudspeaker (13) on the basis of the selection information (k) and of the peak voltage detected by the voltage meter,

d. changing, through the tuning means (123), the output impedance of the device (12) on the basis of the calculated impedance value.

Description:
Title:

DEVICE FOR AMPLIFYING AN AUDIO SIGNAL

The present invention relates to a device for amplifying audio signals, in particular a device for amplifying audio signals for audiophile use.

As is known, a high-end audio amplifier is mainly used for reproducing contents stored in analog form on media such as vinyl records or magnetic tapes, whereon audio tracks can be recorded without adding sampling noise caused by the process for converting an audio signal from the analog format into the digital format.

In fact, it is one aim of audiophily to reproduce sound with as much fidelity as possible, i.e. in such a way that it can be heard and perceived by the listener with much the same quality as it would be heard and perceived live at recording time .

In order to meet these requirements at best, it is necessary that the reproduction system can reproduce sounds having different frequencies at the same audio level, so that the tracks recorded on the vinyl record or magnetic tape can be reproduced with as much fidelity as possible without any distortion .

To do so, the audio amplifier must have a frequency response, when coupled to loudspeakers, as constant as possible within the range used for audio reproduction (typically between 50 Hz and 15 kHz), i.e. a transfer function the shape of which is as parallel as possible to the axis of the abscissae.

American patent US 7,053,705 to TYMPHA Y CORPORATION (see in particular Fig. 6A) describes a feedback amplifier operating both as a voltage amplifier and as a transconductance amplifier, wherein the feedback paths comprise reactive components (capacitors and/or coils) that allow defining the frequency ranges in which the amplifier drives a loudspeaker by voltage and the ranges in which the amplifier drives said loudspeaker by current, so that the dynamics of the loudspeaker, i.e. its own oscillations due to the geometrical and structural characteristics of the loudspeaker, can be controlled in such a way as to achieve an audio reproduction as free from distortion as possible. This amplifier requires ad hoc engineering to ensure an optimal coupling with the loudspeaker, so as to reduce the latter' s typical oscillations during the reproduction of an audio track; as a matter of fact, such optimal coupling cannot be obtained by the user of the audio reproduction system by modifying the geometrical and structural characteristics of the preferred loudspeaker.

In addition, this amplifier does not solve the problem of impedance adaptation between amplifier and loudspeaker, because every amplifier is built for providing its best performance (i.e. for introducing as little distortion as possible into the amplified signal) when it supplies power to a load having a specific impedance (design impedance), while the loudspeaker is built for an impedance that is not likely to be equal to the design impedance of the amplifier; in fact, when the impedance of the load is different from the design impedance of the amplifier, the frequency response of the amplifier will not be constant because the audio level also depends on the frequency of the input signal, thus generating some distortion while reproducing an audio track.

Moreover, the presence of reactive components (capacitors and/or coils) in the feedback paths, defining the frequency ranges in which the amplifier drives a loudspeaker by voltage and the frequency ranges in which the amplifier drives said loudspeaker by current, makes the physical construction of the amplifier even more complex; in fact, the presence of reactive components makes the amplifier more subject to phenomena of resonance and/or interference between components, thus requiring that the circuit be designed (by paying special attention to the layout of the components) and assembled more carefully .

As aforementioned, this amplifier according to the prior art does not solve the problem of impedance adaptation; as a matter of fact, the loudspeakers employed in this type of systems are very often self-built, and therefore do not have an exact design impedance (e.g. 2, 4, 8 or 16 Ohm); it follows that the coupling between such loudspeakers and an amplifier sized for a different impedance will certainly cause the generation of distortion while reproducing an audio track.

The problem of impedance adaptation is usually solved by the man skilled in the art by resorting to transformers having a suitable number of loops for treating audio signals (preferably with a frequency in the range of 50 Hz to 15 kHz) and having a ratio between the number of loops of the secondary winding and the number of loops of the primary winding preferably equal to the square root of the ratio between the impedance of the loudspeaker and the design impedance of the amplifier. It follows that such tuning often requires building an ad hoc transformer for each loudspeaker- amplifier pair, thus making it necessary to use impedance meters .

Furthermore, many loops are required for building these transformers in order to ensure a correct magnetic coupling between the primary winding and the secondary winding at the typical frequencies of analog audio signals; this often causes the presence of non-null reactance on the primary winding even when the load (i.e. the loudspeaker) is connected to the secondary winding, resulting in the current and voltage of the amplified signal being considerably out of phase, which leads to distortion during the reproduction of an audio track.

The present invention aims at solving these and other problems by providing a device for amplifying audio signals as set out in the appended claim 1.

The basic idea of the present invention is to change the output impedance of an audio amplifier by varying the transfer function of the feedback path, so as to cause said amplifier to switch with continuity from the behaviour as a voltage amplifier to that as a transconductance amplifier, while also causing the output voltage value to remain unchanged for a predetermined load. To this end, the amplifier preferably is provided with two controls, i.e. one for establishing the output impedance value, which simultaneously controls the amplification as well, so that the output can be kept constant on a predetermined load resistance, and one for defining the load resistance for which the voltage will remain constant as the output impedance changes.

In this manner, the tuning between amplifier and loudspeaker can be carried out by applying an input signal having a constant frequency and a constant amplitude (such as, for example, a sinusoid having a constant amplitude), by operating the controls, and by measuring (e.g. through an RMS voltage meter) the voltage of the output signal or by listening to the level of the audio being reproduced by the loudspeakers; in fact, when the amplifier is tuned to the loudspeaker, the level of the audio being reproduced is independent of whether the loudspeaker is being driven by voltage or current.

In this tuned condition, it is possible to achieve both the benefits of current driving (fast loudspeaker response) and those of voltage driving (damping of the oscillations peculiar to said loudspeaker, due to the geometrical and structural characteristics of said loudspeaker) without using any reactive components, which would inevitably make the system more prone to phenomena of resonance (e.g. self-oscillations) and interference between components, nor any transformers, which would induce distortion. Thus, the amplification device according to the invention can drive a loudspeaker in such a way as to further reduce the distortion produced during the reproduction of an audio track, because the amplifier- loudspeaker system will behave more similarly to an ideal audio amplification and reproduction system, i.e. a system in which the frequency response is homogeneous and the loudspeaker has no oscillations of its own.

Further advantageous features of the present invention will be set out in the appended claims.

These features as well as further advantages of the present invention will become more apparent from the following description of an embodiment thereof as shown in the annexed drawings, which are supplied by way of non-limiting example, wherein :

- Fig. 1 shows an audio amplification system that comprises an audio amplification device according to the invention;

- Fig. 2 shows a block diagram of the device of Fig. 1;

- Fig. 3 shows a circuit diagram of the device of Fig. 1;

- Fig. 4 shows a circuit diagram that represents a portion of a first variant of the audio amplification device of Fig. 1.

Any reference to "an embodiment" in this description will indicate that a particular configuration, structure or feature is comprised in at least one embodiment of the invention. Therefore, the phrase "in an embodiment" and other similar phrases, which may be present in different parts of this description, will not necessarily be all related to the same embodiment. Furthermore, any particular configuration, structure or feature may be combined in one or more embodiments in any way deemed appropriate. The references below are therefore used only for simplicity's sake and do not limit the protection scope or extent of the various embodiments .

With reference to Fig. 1, an audio reproduction system 1 will now be described, which comprises the following parts:

- a reading unit 11 (e.g. a record player or a magnetic-tape reader) comprising reading means (e.g. a phonographic or magnetic pick-up) capable of reading a medium, preferably an analog one (e.g. a vinyl record or a magnetic tape or the like), whereon audio tracks have been recorded, and of outputting an audio signal, preferably a low-power one;

- a device for amplifying audio signals 12 according to the invention, which is inputted the audio signal generated by the reading unit 11 and outputs an amplified audio signal, i.e. having preferably an average power which is greater than the average power of the signal generated by the reading unit 11;

- a loudspeaker 13, preferably of the type for audiophile use, which receives the amplified audio signal and transduces this signal into sound waves that can be heard and perceived by the listener, i.e. that can be preferably felt by the listener not only by the sense of hearing, but also through the rib cage and the skin, which allow perceiving the pressure wave caused by the reproduction of the lowest frequencies.

It must be pointed out that the reading unit 11 may comprise, as an alternative to or in combination with the above-described reading means, audio synthesis means capable of converting a digital signal (e.g. generated by a processor by reading and decoding a file stored in a memory) into an analog audio signal, preferably a low-power one, and of outputting said analog audio signal.

Also with reference to Fig. 2, the following will describe the block diagram of the amplification device 12 according to the invention.

The device 12 comprises the following parts:

- an amplifier 121 (also referred to as power amplifier), preferably of the operational type, adapted to receive as input the audio signal (also referred to as first signal), which is preferably generated by the reproduction unit 11, and to output the amplified audio signal (also referred to as second signal) for powering the loudspeaker 13;

- control means 122 (e.g. a feedback chain) configured for detecting the current and voltage of the amplified audio signal outputted by the power amplifier 121 and for controlling said amplifier on the basis of the voltage of the audio signal received by said amplifier 121 and of selection information that will be further described hereinafter, and that allow selecting whether the amplifier 121 is also driven on the basis of the voltage or current of the amplified audio signal;

- tuning means 123 (e.g. a potentiometer, a variable resistor, or the like) , configured for changing the output impedance of the device 12, i.e. the behaviour of said power amplifier 121, as will be described hereinafter.

The power amplifier 121 is configured in such a way as to have a voltage gain A preferably greater than or equal to ten; moreover, the signal inputted to said amplifier is the difference between the input signal, i.e. the signal entering the device 12 through the port IN, and a control signal CTRL that, as will be further described below, is a function of the amplified audio signal exiting through the port OUT and of the selection information. Therefore, the transfer function of the direct path G d (s) of the device 12, i.e. of the amplifier 121 alone, is as follows: where R L is the impedance of the load, i.e. the impedance of the loudspeaker 13, and R s is the open-loop internal impedance of the device 12, indicated in Figure 2 as a variable resistance .

It must be pointed out that the impedance R 3 is represented as a variable resistance only for the purpose of representing the current-voltage transducer required by the device 12 in order to behave as a transconductance amplifier, i.e. the resistance indicated by the symbol R s does not correspond to an actual resistor that transforms the current flowing therethrough into heat, since it is only a way to designate a factor that relates the current outputted by the amplifier 121 to the signal that is present in the amplification block indicated as k within the control means 122.

In order to make it possible to control the amplifier 121, the control means 122 are configured for carrying out the following steps:

a) detecting the current of the amplified signal exiting through the port OUT by means of a current sensor (e.g. a shunt resistor or a Hall-effect sensor or another type of sensor) ;

b) detecting the voltage, preferably to ground, of said amplified audio signal, e.g. by connecting, preferably by means of a voltage divider, the inverting port of an operational amplifier to its output port;

c) generating a control signal CTRL on the basis of the voltage and current of the amplified signal and of the selection information.

The selection information is preferably represented by a gain k variable from 0 to 1, which the user of the device 12 can set by operating a potentiometer.

Those skilled in the art may nevertheless use selection information of a different type (e.g. numerical selection information usable by control means 122 comprising a processor configured for carrying out the above-described steps a. - c. by executing a set of control instructions), without however departing from the teachings of the present invention.

In the preferred embodiment of the invention, the control signal CTRL is generated by multiplying by a gain B the result of the addition of the current detected and multiplied by the gain k and the voltage detected and multiplied by the gain 1- k.

Therefore, the transfer function of the feedback path R(s) is as follows : In other words, the selection information allows selecting whether the voltage of the second signal is a function of the voltage of the first signal or the current variation in the second signal is a function of the voltage variation in the first signal, or a combination of both options. This means that the control means 122 can control the amplifier 121 by voltage when k=0, thus creating a voltage amplifier, or they can control the amplifier 121 by current when k=l, thus creating a so-called transconductance amplifier; when 0<k<l, the amplifier 121 will have a hybrid behaviour, because it will be driven both by current and by voltage.

The closed-loop transfer function Go(s) of the device 12 can be calculated as follows:

By substituting in the above formula the definitions of G d (s) and H(s) respectively contained in the previously mentioned formulae (1) and (2), the closed-loop transfer function Go(s) can be rewritten as follows:

Assuming that the function resulting from the product of the transfer functions of the direct path G d (s) and of the feedback path H(s) has always a value much greater than 1 (see formula 5 below), the closed-loop transfer function G 0 (s) can be simplified as follows (see formula 6 below) :

G d (s)H(s) » 1 (5)

The tuning between the device 12 and the loudspeaker achieved when the following condition is met:

R L = R (7)

In this case, i.e. when the impedance R s of the device 12 is equal to the impedance R L of the loudspeaker, it can be noticed that the closed-loop transfer function Go(s) can be written as follows :

i.e. it becomes a constant, thus becoming independent of k.

It can also be noticed that, in the tuned condition, the voltage of the amplified signal is independent of the selection information (i.e. of the gain k) used by the control means for controlling said power amplifier 121. Therefore, if the loudspeaker 13 is connected to the device 12, and said device is perfectly tuned to the loudspeaker 13 (¾=¾) r then the reproduction level of an amplified signal obtained by inputting to the device 12 a signal having constant frequency and amplitude (e.g. a sinusoid) will remain constant as the gain k changes.

When the device 12 is in an operating condition, a signal, preferably a low-power one, enters said device 12 through the port IN to be then amplified by the power amplifier 121, which produces an amplified signal that exits said device 12 through the port OUT, said amplifier 121 being controlled via a feedback path that comprises the control means 122, so as to amplify the input signal without it becoming distorted during the amplification and reproduction process.

As far as tuning is concerned, a method for tuning the amplification device 12 to a loudspeaker 13 comprises the following steps:

a. inputting to the device 12 (i.e. the amplifier 121) a signal having constant frequency and amplitude (e.g. a sinusoid) ;

b. changing the selection information k in such a way as to change the mode in which the amplifier 121 is controlled and to detect any variations in the peak voltage of the amplified signal or in the effective value of said output voltage (e.g. by detecting variations in the audio reproduction level);

c. changing the impedance of the device 12 (by increasing or decreasing it) by a certain quantity, preferably determined on the basis of the peak voltage of the amplified signal or of the effective value of said voltage detected during step b . ; in other words, changing, through the tuning means 123, the transfer function of the control means 122 that control the amplifier 121.

In order to carry out the procedure for tuning the device 12 to the loudspeaker 13, the user of the device 12 can execute step a. by connecting to the input of the device 12 a signal generator (e.g. a waveform generator) configured for generating a tuning signal having constant frequency and amplitude .

As an alternative, the signal generator may be included in the device 12 and activated (e.g. by means of a switch or the like), so that the tuning signal generated by it will be inputted to the power amplifier 121, e.g. by excluding the signal coming from the reading unit 11. In this manner, the tuning of the device 12 is simplified and, most importantly, it is not necessary to disconnect and reconnect the connectors that connect the device 12 to the reading unit 11; in fact, these connectors are often plated with alloys of noble metals (e.g. containing gold, silver and platinum), which for this very reason cannot be subjected to many connection/disconnection cycles (typically a few tens at most) without impairing their electric properties (e.g. resistance), resulting in distortion or attenuation effects adversely affecting the audio reproduction quality.

Step b. of the tuning method can be carried out by the user by operating the control means 122 (e.g. through an analog or digital potentiometer) and by perceiving with the sense of hearing any variation in the loudness level of the sound being reproduced by the loudspeaker 13, so as to detect if the peak voltage (or the effective value) of the amplified signal coming from the port OUT of the device 12 is a function of the selection information k.

As an alternative, the user may perceive a possible variation in the peak voltage (or its effective value) of the amplified signal with the sense of sight; to this end, the device 12 may comprise a voltage meter configured for detecting the peak voltage (or its effective value) of the amplified signal (e.g. the RMS voltage), which may preferably comprise display means, such as, for example, a pointer or an electronic display, configured for indicating the effective value (e.g. the RMS value) or the peak value of the voltage detected, so that a user can detect a condition of imperfect tuning by observing, for example, the motion of the dial or the variation in the value shown on the electronic display. In this manner, it is possible to achieve a more accurate tuning than by using the solution based exclusively on the sense of hearing, because the voltage meter allows for direct detection of voltage variations without the user having to listen to the sound being reproduced by the loudspeaker 13. Due to this, the tuning procedure can be carried out also by a user whose sense of hearing is not very sensitive to volume changes.

Step c. can be carried out by the user of the device 12 by operating the tuning means 123 (e.g. by operating an analog or digital potentiometer) . It must be pointed out that it is useful to execute step c, during which the internal impedance of the device 12 is adjusted, when a variation in the peak voltage (or its effective value) is detected, during step b., in the amplified signal when changing the mode in which the amplifier 121 drives the load (voltage or transconductance) . Steps b. and c. are preferably carried out cyclically until, during step b., variations in the peak voltage (or its effective value) are detected which are lower than a given threshold when the user operates the control means 122.

It must be pointed out that the amplification device 12 can be configured to execute the tuning method in a partially or fully automatic way; to this end, the device 12 may comprise processing means (e.g. a CPU, a microcontroller, or the like) and actuating means (e.g. servomotors coupled to the rheostats/potentiometers, or digital potentiometers, or electronically controlled DAC multipliers) in signal communication with the processing means and configured for actuating the control means 122 (i.e. for changing the coefficient k) and/or the tuning means 123 (i.e. for changing the impedance R s ) so as to implement the tuning method either upon the user's request (e.g. when a push-button comprised in said device is pressed) or automatically (e.g. every time the device 12 is turned on) .

When the device 12 executes the tuning method, the processing means are configured for carrying out the following steps:

- activating (optionally) the signal generator (step a.);

- actuating the control means 122 through the actuating means, and detecting, through the voltage meter, the variation in the peak voltage (or its effective value) of the amplified signal as the selection information changes

(step b . ) ;

- actuating the tuning means, through the actuating means, on the basis of the variation in the peak voltage (or its effective value) detected during step b. (step c.) .

In this manner, the tuning can be carried out autonomously, avoiding that the different hearing or visual sensitivity of the user of the device 12 might affect the final tuning result, thereby ensuring a more accurate tuning than the solution based on the hearing or visual sensitivity of the user of the device 12.

Furthermore, the processing means can be configured for executing steps b. and c. cyclically until the tuned condition is reached, e.g. by executing an optimization algorithm, such as, for example, the gradient descent method or the like.

In other words, the processing means are configured for cyclically actuating the control means 122 and the tuning means 123 to search for that output impedance of the amplifier 121 which makes the voltage of the second signal independent of the selection information.

As an alternative to this solution, the processing means may be configured for not executing steps b. and c. cyclically. More in detail, the processing means may be configured for executing the following steps:

a. reading the selection information (i.e. the parameter k) , e.g. by detecting the configuration of the control means 122. To this end, the control means 122 may be in signal communication with the processing means;

b. detecting, through the voltage meter, a peak voltage (or its effective value) of the amplified signal when the audio signal that enters the power amplifier 121 has a constant amplitude and a constant frequency;

c. calculating (estimating) an impedance value of a load powered by the second signal (e.g. the impedance of the loudspeaker 13) on the basis of the selection information and of the peak voltage (or its effective value) detected by the voltage meter (e.g. by using the formula 9 shown below) ;

d. actuating the tuning means (123), i.e. changing R s , on the basis of the calculated impedance value, so that the device will be in a tuned condition (¾=¾) .

R _ k R S B V stim (9)

L 1 + (k - 1) B V stim

In this manner, the processing means can determine the load impedance R L , since they can control k and R s directly, and can therefore know (or estimate) the value thereof, while B is a constant defined when designing the device 12; by so doing, the tuning process can be carried out in less time than with the above-described solution based on the search algorithm. With reference to Fig. 3, the following will describe one possible physical implementation of the amplification device 12 according to the invention.

The power amplifier 121 may comprise an operational amplifier U2, preferably of the OPA541 type manufactured by Burr-Brown, configured for receiving at the non-inverting port the signal coming from the port IN of the device 12 and at the inverting port a control signal generated by the control means 122, which will be further described hereinafter. The non- inverting port is protected against overvoltage and overcurrent by a pair of resistors R5 and R6 that also compensate for any bias currents generated during the operation of the amplifier U2, wherein said resistor R5 is located between the port IN, and said non-inverting port has a resistance value preferably equal to 10 kOhm, whereas the resistor R6 is located between the port IN and the ground and has a resistance value preferably equal to 100 kOhm; the signal generated by the control means 122, prior to entering the inverting port of the operational amplifier U2, is made to pass through a voltage divider composed of at least one pair of resistors R8 and R10, preferably having respective resistance values of 90 kOhm and 10 kOhm, so that the operational amplifier 121 will have a voltage gain (i.e. the gain A shown in Fig. 2) of at least 10. More in detail, the first terminal of each one of the resistors R8 and R10 is connected to the inverting port of the amplifier U2, the second terminal of the resistor R10 is connected to ground, and the second terminal of the resistor R8 is connected to the control means 122, from which it receives the control signal.

The signal produced by the port OUT of the operational amplifier U2 passes through a resistor R7, preferably having a resistance value of 1 Ohm, and then exits the device 12 through the port OUT of said device 12; said resistor R7 is used by the control means 122 for detecting the current of the amplified signal on the basis of the voltage drop across said resistor R7. As aforesaid, the current of the amplified signal can also be detected by means of other systems (e.g. a Hall- effect ring or the like) .

The control means 122 comprise a pair of operational amplifiers U1A and U1B, preferably of the TL07xx type manufactured by Texas Instruments. The first operational amplifier U1A is configured for detecting the voltage drop across the resistor R7; in particular, the non-inverting port of said amplifier U1A is connected downstream of the resistor R7 (i.e. it has the same potential as the port OUT of the device 12) through a voltage divider composed of a pair of resistors R3 and R4, preferably having both a resistance value of 1 kOhm, which adapt the signal level to that of the non- inverting port and which protect the amplifier U1A against overcurrents ; on the other hand, the inverting port is connected upstream of the resistor R7 (i.e. it has the same potential as the port OUT of the operational amplifier U2) through a voltage divider composed of a pair of resistors Rl and R2 having the same resistance value, preferably equal to 1 kOhm, and wherein Rl is connected between the inverting port and the output port OUT of the amplifier U1A, and R2 is connected between the output OUT of the amplifier U2 and the inverting port of the amplifier U1A, so that the operational amplifier U1A will behave as a unity gain differential connected across R7.

The signal that exits through the port OUT of the amplifier U1A is then inputted to the inverting port of the operational amplifier U1B; along the connecting line that connects the two amplifiers there is a resistor R13, preferably having a value of 1 kOhm, which is thus interposed between the port OUT of the operational amplifier U1A and the inverting port of the operational amplifier U1B. The operational amplifier U1B is in the inverting configuration; in fact, the output port OUT of said amplifier U1B is connected to the inverting port through two resistors Rll and R12, wherein R12 has a resistance value preferably equal to 500 Ohm and Rll is a variable resistor (e.g. a potentiometer with the central terminal connected to one of the other two terminals) having a maximum resistance value preferably equal to 4.5 kOhm, while the non-inverting port is connected to ground through a resistor R14, preferably having a value of 1 kOhm, which is adapted to reduce the overcurrents that might damage the operational amplifier UIB and to compensate for the bias currents generated during the operation of the operational amplifier UIB.

In this exemplary embodiment, the operational amplifiers U1A and UIB, together with the resistors R1-R4 and R11-R14, implement the gain B shown in Fig. 2; furthermore, the tuning means 123 comprise the variable resistor Rll. In fact, by changing the value of the resistance of Rll it is possible to vary the (negative) gain produced by the operational amplifier UIB: the gain of the amplifier UIB will be -0.5 when the value of Rll is zero, whereas the gain will be -5 when the value of Rll is greatest (4.5 kOhm) .

The signal coming out of the port OUT of the operational amplifier UIB enters a potentiometer R9 preferably having a maximum resistance value of 1 kOhm, which performs the tasks of the adder node and of the gains k and k-1 shown in Fig. 2. More in detail, the potentiometer R9 comprises three terminals (two outer terminals and a central terminal), wherein the first outer terminal is connected to the port OUT of the amplifier UIB, the second outer terminal is connected downstream of the resistor R7 (i.e. it has the same potential as the port OUT of the device 12), and the central terminal is connected to the second terminal of the resistor R8.

By changing the position of the potentiometer R9, the gain k will be changed as well; in particular, when the potentiometer R9 is in a first extreme position in which the electric resistance between the central terminal and the outer terminal connected downstream of the resistor R7 is substantially zero, the amplifier 121 will be a voltage amplifier (k=0), whereas when the potentiometer R9 is in the second extreme position, opposite to the first one, the amplifier 121 will be a transconductance amplifier (k=l) .

It must be pointed out that, when the amplification device 12 is in a tuned condition (R S =R L ) , the potential difference at the outer terminals of R9 is zero, and therefore the position of R9 will not affect the peak voltage value or the effective voltage value of the amplified signal.

Of course, the example described so far may be subject to many variations.

A first variant is shown in Fig. 4; for brevity, in the following description only those parts that differentiate this variant from the above-described main embodiment will be mentioned; for the same reason, wherever possible the same reference numeral will be used to designate those elements which are common to both embodiments.

In this second embodiment of the invention, an audio amplification device similar to the above-described device 12 comprises, in addition to those parts already described, also an operational amplifier U3, preferably equal to the operational amplifier U2, configured for increasing the power of the amplified signal without requiring any modifications to the parts 121 and 122.

To this end, the operational amplifier U3 is in the inverting configuration: its non-inverting port is connected to ground, whereas its inverting port is connected to a voltage divider made up of two resistors R17 and R18. The resistor R17 is connected between the output port OUT of the amplifier U3 and the inverting port of U3, whereas the resistor R18 is connected between the output OUT of the device 12 and the inverting port of U3; such resistors have the same resistance value, preferably equal to 1 kOhm, so that the operational amplifier U3 will have a gain substantially equal to -1. Therefore, the output signal exiting through the port OUT of the amplifier U3 will be offset by 180 sexagesimal degrees with respect to the output of the operational amplifier U2 and will have the same amplitude as said signal produced by the amplifier U2. For this reason, the output of the amplifier U3 has been labelled -OUT.

In this embodiment, the loudspeaker 13 is connected between the ports OUT and -OUT, i.e. downstream of the resistor R7 and to the port -OUT of the operational amplifier U3.

Although this description has tackled some of the possible variants of the invention, it will be apparent to those skilled in the art that other embodiments may also be implemented, wherein some elements may be replaced with other technically equivalent elements. The present invention is not therefore limited to the illustrative examples described herein, since it may be subject to many modifications, improvements or replacements of equivalent parts and elements without departing from the basic inventive idea, as set out in the following claims.