The present invention relates to a sound effector for amplifying original sounds produced from an electric musical instrument such as an electric guitar and for outputting amplified sound signals to a speaker or the like, and more particularly, it relates to such a sound effector which uses semiconductor elements and which may produce aurally effective sound signals by arbitrarily distorting the sound signals in the frequency band to be enhanced.
In a conventional sound effector, amplifier circuits employing vacuum tubes have been employed to partially enhance original sounds generated from an electric musical instrument such as a guitar during amplification of original sound signals by suitably distorting the waveform in the frequency band of the original sound signals to be enhanced and thereby generating aurally improved instrument sounds. By the use of vacuum tubes for the amplifier means, it is possible to obtain delicate tones such as vibration sounds of a metal wire used for a grid and a filament in the vacuum tubes, that is, vibration sounds of a cathode filament in the vacuum tubes. Therefore, the amplifier circuits employing the vacuum tubes are widely used in the conventional sound effectors.
However, as the amplifier vacuum tubes are generally large in size, and are short in life, semiconductor elements having a small size have been used heretofore in place of the vacuum tubes to provide electronic circuits which may produce a comfortable delicate tones similar to those obtained by the amplifier circuits employing vacuum tubes.
FIG. 3 shows an example of conventional electronic circuits employing semiconductor elements. As may be seen, the vibration of the strings or the sound of an electric guitar or the like is picked up by a pickup coil 1, and an output sound signal from the pickup coil 1 is received by an amplifier circuit 2, where the signal is amplified to 50-60 dB. The amplified signal from the amplifier circuit 2 is clipped by two reversely connected diodes 5 and 6 connected after a capacitor 3 and a resistor 4, so that the original sound signal is distorted. The clipped signal is then passed through a filter 7 to generate a signal component having a desired frequency range. Such an electronic circuit 10 is called a "distortion circuit". The output signal from the filter 7 is amplified by a voice amplifier 8, and an amplified sound is outputted from a speaker 9.
However, in the above electronic circuit 10, the quantity of distortion is insufficient since the original sound signals are distorted only by clipping the waveform of the signal received by the diodes 5 and 6. For this reason, it has been attempted to approximate the characteristic of the sound effector to that of the amplifier circuit employing vacuum tubes, by changing the value of the capacitor 3 and the resistor 4 and the characteristic of the amplifier 2 and the filter 7, or by changing the combination of these components.
FIG. 4 shows an example of the amplifier circuit 2 employing semiconductor elements in the electronic circuit 10. The amplifier circuit 2 includes an operational amplifier 21 having external terminals 22 and 23. The output characteristic of the operational amplifier 21 is regulated by changing the capacitance of a capacitor 24 connected between the external terminals 22 and 23. A negative feedback circuit is formed by resistors 25 and 26 and a capacitor 27 to feed back a negative feedback signal to an inversion input terminal of the operational amplifier 21.
FIG. 5 shows the frequency characteristic of an amplified signal to be generated from the amplifier circuit 2 shown in FIG. 4 and having a like frequency characteristic of the amplifier circuit employing vacuum tubes. In FIG. 5, the abscissa denotes the frequency, and the ordinate denotes the gain of the amplifier circuit. The frequency characteristic curve 11 shows that a peak point P lies at an appropriate frequency Q1 to give a maximum gain G, and that the gain is attenuated in high and low frequency ranges across the peak point P.
It will now be assumed that the peak point P shown in FIG. 5 is identified by the frequency Q1=500 Hz and the gain G=50 dB. If the negative feedback quantity is adjusted by changing the values of the capacitors 24 and 27 and the resistor 25, to thereby adjust attenuation of the negative feedback circuit in the amplifier circuit 2 to 50 dB and phase of the amplifier circuit 2 including the negative feedback circuit to 0 degree with the resistor 26 reduced to below 100 .OMEGA., the amplification degree of the amplifier circuit 2 would become one time (0 dB), but the total amplification degree could be increased to 50 dB by adjusting the capacitor 24. Accordingly, it would be possible to produce stable oscillation and thence generate an output signal having a frequency characteristic with a sharp peak.
To adjust the position of the peak point P to Q1=500 Hz and G=50 dB, each value of the capacitors 24 and 27 and the resistor 25 is adjusted to obtain a gain (.omega..times.value of resistor 25.times.value of capacitor 27)=50 dB at Q1=500 Hz, where .omega. denotes 2.pi..times.frequency. Further, the operational amplifier 21 is adjusted to reduce the open loop gain in the frequency range higher than the target frequency of 500 Hz. On the other hand, in the frequency range lower than the target frequency of 500 Hz, .omega. will diminish and therefore, the amplification degree of the amplifier circuit 2 will decrease in proportion to .omega.. Thus, the frequency characteristic as shown in FIG. 5 may be obtained having a peak gain at the target frequency of 500 Hz.
However, when the musical instrument is played by using the above sound effector incorporating the amplifier circuit having the frequency characteristic shown in FIG. 5, there is a problem that too gentle or somewhat indistinctive sounds are felt by the player.