As known, a string musical instrument, such as an acoustic guitar or a violin, has a remarkable sound richness, depending on the construction features of the sound board which derive also from the fine skill of the lute maker which produces them. The acoustic power supplied directly by an instrument can be modulated by the music maker, but it cannot go beyond a certain level and in the cases of sonorisation of large indoor or outdoor spaces, it is certainly insufficient. From here the need to amplify the acoustic signal of the instrument.
A traditional system for amplifying volume is that of placing a microphone in front of the instrument and hence amplifying the sound with conventional amplifying equipment. This solution is not always satisfactory, because it is affected by microphone and environment quality, in addition to being prone to the problem of the Larsen effect, due to the return of the acoustic signal into the microphone, precisely in situations of great amplification; moreover, the microphone is able to pick a limited dynamic of the sound which comes out of the instrument, in addition to being strongly affected by environmental acoustics and by the microphone position with respect to the instrument.
Somewhat better results are obtained by positioning special microphones on the mouth of the sound board, below the strings. However, this solution is not fully satisfactory either.
In order to improve the quality of the acquired sound, freeing oneselves from the use of acoustic microphones sensitive to changes of acoustic pressure, it has also already been suggested to use special pickups (acquisition sensors), of the type found on electric guitars.
The pickups used in string instruments (guitars and double basses) are substantially of two types.
A first one is of a magnetic type and comprises a series of single or double coils of metal wire, which generate a magnetic field due to the presence of permanent magnets: this type of pickup is arranged in the proximity of the metal strings of ferromagnetic material, so that the magnetic field is “disturbed” by the vibration of the metal strings and the extent of this change is acquired as electric signal (the technical term is “variable-reluctance”). This signal detects well the vibrations of the strings which move in front of the pickup, but is unable to significantly appreciate any resonance of an acoustic board, which provides the timbre of a certain instrument.
A second one is of the piezoelectric type and comprises a piezoelectric pressure sensor: it is positioned below a bridge, for detecting the direct or resonance vibration in terms of variable pressure, consequent to the mechanical action of the strings vibrating on the support bridge. The nature of the signal acquired with this sensor, as can be guessed, is profoundly different from the one obtained with a magnetic pickup.
In order to acquire a signal from a piezoelectric transducer, it is normally resorted to preamplification circuits with a high input impedance, such as the schematised ones in the attached FIGS. 1A and 1B.
A first exemplifying circuit structure (FIG. 1A) is obtained through Q1 and Q2 and defines a discrete-component amplifier stage with a high input impedance, obtained with the use of a FET (Q1) in a bootstrap configuration. The second circuit structure (FIG. 1B) is obtained by means two operational amplifiers and defines an amplifier with a high input impedance. In both cases the circuits are adequate as voltage amplifiers.
With known electronic configurations, no perfect adaptation of the input stages towards the piezoelectric transducers is obtained and a significant reduction of signal quality is determined in case of parallel connection of multiple piezoelectric transducers. Moreover, since the mixing between signals coming from different, jointly-used transducers is critical, in the prior art no optimisation of the logarithmic response curve of the individual level controls for each signal is provided.
In order to supply an amplified sound as faithful as possible to the natural one, it has also been suggested to simultaneously acquire the signal deriving from multiple different sensors, arranged on a same acoustic instrument. WO2004/023454, for example, discloses a preamplification system for a pair of sensors arranged, on an acoustic instrument, one below a bridge for detecting the direct vibrations of the strings, and one on the instrument sound box, for detecting resonance vibrations. WO2011/003148 discloses a similar system, wherein a third sensor in the shape of a microphone is also provided.
Both these prior-art systems, however, still have significant management problems of the different signals. As a matter of fact, in the light of the different nature of the signals produced by the various sensors, as well as the different nature of the acquired sound (direct vibration, resonance vibration, acoustic wave in the air, . . . ), problems of electronic processing of the signals in the preamplification stage exist, to then obtain the correct output signal to be amplified in the final stage, without distorsions, without spectrum limitation of the signal, and having a final sound as natural as possible. It must be observed, moreover, that the magneto-dynamic inertia sensors have limited responses in frequency, remarkable mass and high sensitivity towards external electromagnetic fields, especially those at network frequency and harmonic frequencies thereof.
Finally, it must be considered that the reduced space available on the musical instrument also makes critical the physical configuration of the preamplifier which must be of excellent quality as well as having high immunity to electromagnetic fields of external sources.
U.S. Pat. No. 7,304,232 discloses a joystick gain control system for the amplification of a string instrument. Two equal magnetic-type or piezoelectric-type pickups are provided, with the volume potentiometers directly connected to the pickups. Such configuration in actual fact does not allow to obtain a good response from piezoelectric-type sensors, because it would require a load resistance of a few MΩ, necessary in order not to load the sensor, which would imply an alteration of the frequency response upon the varying of the potentiometer cursor position. Moreover, a change of the equivalent resistance seen from the piezoelectric sensor would imply a change of the frequency response on the low part of the spectrum (equivalent to a sort of response of a high-pass filter at variable frequency). The volume control is arranged between the pickups and the amplification stages. The structure is that of a generic amplification system, volume adjustment of a passive type and tone control, but it cannot be understood what the inner structure and the circuitry in the active version are. The volume controls are carried out directly on the sensors, changing the response thereof, increasing the equivalent electric noise thereof due to the additional resistance of the potentiometer circuit, virtually allowing the use of sole low-impedance sensors; an analogic→digital and digital→analogic chain is furthermore accomplished, obtaining the volume and tone adjustment functions numerically. The phase inversion of either one of the two sensors is obtained through an electromechanical-type switch (a double deviator with crossed connections).
In the document Jarmo Landevaara: “The Science of Electric Guitars and Guitar Electronics” some amplification circuits for a piezoelectric sensor are disclosed. This prior-art solution does not offer an ideal amplifier in the application considered by the present invention yet, because it is a FET impedance-adapter, follower circuit.