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May 26, 1970 S. CUTLER 3,514,723 TONE CONTROL CIRCUIT COMPRISING A SINGLE POTENTIOMETER Filed June 23, 1966 2. Sheets-Sheet l -2w— s OUT - 2do CPS eOU ATTENUAT on FRECUENCY sf- 7 it INVENTOR 97am/4 a Y Cluttlee "62-6 7-ol May 26, 1970 S. CUTER 3,514,723 TONE CONTROL CIRCUIT COMPRISING A SINGLE POTENTIOMETER Filed June 23, 1966 2. Sheets-Sheet 2 FeCuen CY FREQuency INVENTOR. S7am/Alas Y Cutta-la a "sy- -67. 7all 3,514,723 United States Patent Office Patented May 26, 1970 2 That is, the harmonic content, rather than the funda 3,514,723 mental seems to play the key role in identifying the TONE CONTROL CIRCUIT COMPRISING A type of instrument being played. SINGLE POTENTIOMETER Stanley Cutler, Los Angeles, Calif., assignor to Warwick The circuit of the present invention provides a single Electronics Inc., Chicago, Ill., a corporation of Dela control means for selectively attenuating or boosting Ware either the high or low frequencies of the audio spectrum Filed June 23, 1966, Ser. No. 559,847 with respect to a given reference or "pivot' frequency. int, C. H03: 7/10 The circuit to accomplish this comprises a novel four U.S. C., 333-28 4. Claims terminal LRC network which may selectively be adjusted O to perform the functions of a low-pass filter, a fre quency-flat transmission channel, or a high-pass filter, ABSTRACT OF THE DISCLOSURE with a continuously variable transition from one mode A tone control circuit is described having a single to another. The high-pass and low-pass gain-versus-fre potentiometer for selectively attenuating or boosting quency curves are characterized by relatively steep rates either the high or low frequencies of the audio spectrum of attenuation in the cut-off region. With respect to a given reference frequency. The circuit It is therefore, an object of this invention to provide comprises a four-terminal LRC network and a common a novel and improved circuit for differentially control control potentiometer which differentially adjusts the ling the relative bass and treble frequency response of network's transmission characteristics to function as an audio amplifier. either a low-pass filter, a high-pass filter, or a frequency 20 It is another object of this invention to provide a flat transmission channel with a continuously variable novel and improved single-control audio equalizing cir transition in the gain-versus-frequency response. cuit which may differentially adjust the frequency re sponse of either the high or the low end of a given ackasaasa. is remarr pass band. This invention relates to tone control circuits and 25 Yet another object of the invention is to provide a more particularly to circuits for selectively modifying novel and improved four-terminal LRC network for use the relative gain of an audio amplifier over a given fre with electric or electronic musical instruments. quency range. Other objects and features of the invention will be Various types of tone control circuits have been pro come apparent from the following specification and posed heretofore, many of which satisfactorily modify 30 drawings which show a preferred embodiment of this the frequency response of an audio amplifier in a de invention. sired manner. Typical ones of these prior circuits em In the drawing: ploy separate controls for boosting or attenuating the FIG. 1 is a schematic circuit diagram of a preferred so-called bass frequencies, at the lower frequency end embodiment of the invention; of the audible spectrum, and the so-called treble fre FIG. 2 is an equivalent circuit illustrating the func quencies at the upper end of the spectrum. Additionally, tioning of the apparatus of FIG. 1 in its low-pass filter single-control circuits have been proposed which modify mode; tonal response in various ways some of which roll-off FIG. 3 is a graphical plot illustrating the frequency the high frequencies in some given manner. Although response of the circuit of FIG. 2, wherein relative gain these prior circuits are suitable for music reproduction 40 is plotted along the axis of the ordinates and the fre and public address systems, they suffer from certain quency is plotted along the axis of the abscissa; shortcomings which detract from their use in connec FIG. 4 is an equivalent circuit illustrating the func tion with electrical or electronic musical instruments. tioning of the apparatus of FIG. 1 in its high-pass filter One shortcoming is that it is difficult and inconvenient mode of operation; to rapidly manipulate two separate tone controls, such as FIG. 5 is a graphical plot illustrating the frequency independent bass and treble controls, which must neces response of the circuit of FIG. 4 wherein the relative sarily interact to some degree with respect to the re gain is plotted along the axis of the ordinate and the sulting frequency response curve, while simultaneously frequency is plotted along the axis of the abscissa; playing the instrument. Single tone controls of the prior FIG. 6 is an equivalent circuit useful in describing art have been found to be ineffective in providing the 50 the functioning of the circuit of FIG. 1 when adjusted desired range of frequency response adjustment over the to its frequency-flat mode of operation; entire audio spectrum with particular regard to sharp FIG. 7 is a graphical plot illustrating the frequency cut-off in the attenuative band. response of the circuit of FIG. 6. There is provided by the present invention a novel Referring to FIG. 1, there is shown a four-terminal and improved single-control circuit which combines cer 55 LRC network constructed in accordance with the inven tain advantages of prior multiple-control circuits and tion. The input signal is applied to a terminal 1 after which additionally provides certain benefits when used which it passes through the series combination of a re in conjunction with electric or electronic musical in sistor 2 which may be the internal source impedance of struments. More specifically the invention is particular the driving stage and an isolating capacitor 3 for blocking ly useful in connection with bass guitars. The audio fre 60 any DC from entering the network. The input signal is quency spectrum of a note played on a bass guitar, for referenced to a ground terminal 7. The common con example, consists of a very low frequency fundamental nection between an inductance 4 and a capacitor 5 is con waveform and a Series of higher harmonic waveforms. nected to capacitor 3. A voltage-dividing potentiometer 6 If, in the process of amplification and reproduction the comprises the single adjustment or tone-control element fundamental tone is emphasized, and the harmonics are 65 of the network. The arm of potentiometer 6 is returned attenuated, the resultant sound may be described as to ground terminal 7. A capacitor 8 is shunt-connected round and smooth with a booming quality not unlike a across the arm and one end of potentiometer 6. The fre bass drum. If, on the other hand, the fundamental is quency response of the circuit may be manually adjusted attenuated and the harmonics are accentuated, the re by appropriately setting the position of the arm of poten sultant sound will have an entirely different quality 70 tiometer 6, as will appear hereinafter. A capacitor 9 and which more closely resembles a standard or lead guitar. a resistor 11 have a common connection which is tied 3,514,723 3 4. to shunt a resistor 12 and also an output terminal 13. The 11'. The high-pass and the low-pass filters are connected output signal is referenced to ground terminal 7. in parallel with each comprising a separate branch path In a typical application, the above-described network from the input terminal 1' to the output terminal 13'. will be inserted in series between amplification stages of The combination of these two filters, plus resistors 2' an audio frequency signal transmission channel, Such and 12', and series capacitor 3', may be considered as channel having sufficient gain to overcome net insertion 5 a bridged-T network. The overall response is relatively loss of the tone-control network when set to its frequency flat as indicated by the frequency response curve of flat position. FIG. 7. The operation of the circuit may best be understood by Ideally, the pivot frequency of the high-pass filter is considering equivalent circuits corresponding to the active O made to coincide with the pivot frequency of the low-pass portions of the principal circuit under given operating filter so that any dip in the central portion of the response conditions. The circuit of FIG. 2 corresponds to the ef curve at the mid-position of the single tone control will fective portion of the circuit of FIG. 1 if it is assumed that be minimized. At all other positions of the tone control 6 the arm of potentiometer 6 is moved upward (as viewed intermediate of the end position and/or the mid-position, in FIG. 1) to the junction 14 between capacitors 5 and there will be corresponding contribution of the high-pass 9. This condition corresponds to the bass-boost or low and low-pass filter sections. In effect, the adjusted single pass filter mode of operation. The components in the net control potentiometer operates as a means for selectively work of FIG. 1 having like components in the equivalent and differentially varying the insertion loss of either one circuit of FIG. 2 are identified with similar identifying or both of the hybrid high-pass and low-pass filters.
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