1. Field of the Invention
The present invention relates to a tone control circuit and, more particularly, to a tone control circuit having a high input impedance, a low output impedance, gain, and a relatively constant phase shift.
2. Description of the Prior Art
Most music applications require bass and treble tone controls and a number of tone control circuits have been developed. Typical tone control circuits are attenuation circuits and, due to the insertion loss of the circuit, it has been normal to use one or more preamplifiers therewith. However, a variety of other problems make such tone control circuits unsuitable for a variety of applications.
For example, in a stringed instrument, such as an electric guitar, it is usually necessary to sum the outputs of a plurality of pick up coils before application to the tone control circuit. Therefore, the tone control circuit must have a high input impedance to prevent distortion of the output of the coils. Furthermore, the input of the tone control circuit must provide a virtual ground to prevent interference between the multiple pick up coils. However, tone control circuits with high input impedances have not been generally available heretofore.
In the case of an electric stringed instrument, where the output of the tone control circuit is conducted over a long cable to an amplifier system, it is necessary that the tone control circuit have a low output impedance. If the tone control circuit has a high output impedance, the cable acts as an antenna and introduces a significant amount of noise into the signal. Furthermore, with a high output impedance, the gain control loads the output of the tone control circuit and affects the bass and treble boost and cut characteristics. Again, most available tone control circuits have a high output impedance.
Since most tone control circuits are attenuation circuits, there is a loss rather than a gain in signal strength, which degrades the signal to noise ratio of the system. Furthermore, most tone control circuits have a substantial phase shift variation as the control is varied. Since a music signal consists not only of fundamentals but many harmonics, this phase shift variation alters the phase relationship between the fundamentals and the harmonics. This substantially degrades the quality of the output signal.