May 26, 1970 S. CUTLER 3,514,723 TONE CONTROL CIRCUIT COMPRISING A SINGLE POTENTIOMETER

Filed June 23, 1966 2. Sheets-Sheet l

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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

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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. The numbers except that prime marks () have been added to 20 relative effectivenes of these two filters also may be varied the identifying numbers in FIG. 2. by appropriate changes in the component values of their As will be apparent to those versed in the art of filter respective network branches. Master gain controls may networks, the equivalent circuit of FIG. 2 comprises three be added to either end of the signal transmission channel cascaded L-section low-pass filters. The first L-section in a conventional and well-known manner. comprises resistor 2 and capacitor 5", the second Section As can be seen from the foregoing description the net comprises inductance 4 and capacitor 8, and the third audio response curve may be made to pivot about an cascaded section comprises resistor 11 and capacitor 9. essentially fixed frequency (typically 200 c.p.s.) to give The shunt resistance of potentiometer 6' in this mode of a virtually infinite number of response curves. In a pre operation acts to broaden the selectivity of the reactive ferred construction the frequency roll-off on either side of elements. Resistor 12' is not active as a frequency dis the pivot frequency will be greater than six decibels per criminating element but serves to maintain the “equal octave. The relative position of the pivot frequency will loudness' characteristic of the network. That is, resistor be determined by the actual values chosen for construction 12' in conjunction with resistor 11 divide the output sig of the filter network sections and such selection may foll nal to cause the apparent audible output level in the low classical filter-section design methods well known to “BASS BOOST' position equal to that in the “TREBLE those versed in the art. The sections thus designed are BOOST’ position (to be described more fully hereinafter). combined in the novel manner described above, and which Suffice it to say that resistor 12' is not an essential com incorporates the novel potentiometer arrangement to ef ponent if equal-loudness characteristics are not important. fect transition through the various modes of operation. As will be appreciated by those versed in the art, the cas While the invention has been shown and described in cading of three separate filter sections will result in a 40 terms of a preferred embodiment, it will be readily recog relatively sharp roll-off of the high frequency. nized by those versed in the art that certain omissions, The approximate frequency response curve of a typical substitutions, modifications and additions may be made construction of the circuit of FIG. 2 is given in FIG. 3. without departing from the essential concept of the inven Classical L-section filter design techniques may be em tion. For example, the number of cascaded filter sections ployed to determine the component values for resistors 2 may be augmented or reduced, depending on the desired and 11, inductance 4, and capacitors 5’, 8, and 9'. Typ steepness of the response curves and/or the overall pass ically the turnover point or 3-decibel point, hereinafter band. Thus, it is intended that the invention be limited referred to as the “pivot" frequency, may be set at only by the scope of the appended claims. 200 c.p.s. What is claimed is: 50 1. In an audio signal transmission channel, a network There is shown in FIG. 4 the equivalent circuit cor comprising: responding to the effective portion of the circuit of FIG. an input terminal for receiving an audio signal in said 1 when the arm of the potentiometer 6 is positioned all channel; the way down (as viewed in FIG. 1). Again, like com an output terminal for returning an audio signal to said ponents are similarly identified except for the addition of 5 5 channel; double primes (''). As will be apparent, this operating a ground return terminal common to said input ter mode will result in capacitor 8 being entirely shorted out. minal and said output terminal; The resultant circuit comprises three cascaded L-section a pair of series capacitances connected between said high-pass filters serially connected between terminals 1'.' input terminal and said output terminal; and 3'. The first section comprises resistor 2' and in 60 an inductance and a series resistance connected be ductance 4'. The second high-pass L-section comprises tween said input terminal and said output terminal; capacitor 5' and resistor 6', and the last cascaded sec a capacitor connected from said ground terminal to the tion comprises capacitor 9' and parallel-connected re junction between said inductance and said series re sistors 11’ and 12'. Cascaded sections give a relatively sistance; and sharp roll-off of the low frequencies as indicated in the a single control potentiometer means connected in frequency-response curve 15 in FIG. 5. common with said pair of series capacitances, and When the arm of the potentiometer 6 is set approxi said series resistance and inductance for differentially mately at its mid-position, the resulting network will com controlling the effective insertion loss thereof. bine elements of both high-pass and low-pass filters, and 2. The network defined in claim 1 wherein said po will therefore give a relatively flat response characteristic. O tentiometer means comprises: Specifically, as shown in the equivalent circuit of FIG. 6 a voltage dividing potentiometer having two end ter (wherein the potentiometer 6 of FIG. 1 is represented by minals and a movable arm terminal, one of said end separate resistors 16 and 17) the high-pass filter comprises terminals being connected to said junction between capacitors 5' and 9', and a resistor 16; the low-pass filter Said Series capacitances, the other of said end termi comprises inductance 4'', and capacitor 8', and resistor nals being connected to said junction between said 3,514,723 5 6 inductance and said series resistance, and Said arm a voltage-dividing potentiometer having a first end lead terminal being connected to said ground terminal. connected to the series connection between said sec 3. The network defined in claim 1 including: ond and third series-connected capacitors, a second a resistor connected between said ground return termi- end lead connected to the series connection between nal and said output terminal. 5 said inductance and said second resistance, and an 4. An adjustable network for series insertion in an arm lead connected to said common reference termi audio signal transmission channel comprising: nal; and an input terminal for receiving an audio signal in said a fourth capacitor connected between said common channel; reference terminal and the series connection of said an output terminal for returning the audio signal acted O inductance and said second resistance. upon by said network to said channel; a first series signal path between said input and output References Cited terminals comprising a first resistor and first, second, UNITED STATES PATENTS and third series connected capacitors; a second series signal path extending from the series 15 2,505,254 4/1950 Mesner. connection between said first resistor and said second 2,812,498 11/1957 Hall. series connected capacitor to said output terminal, comprising an inductance, and a second resistance in HERMAN KARL SAALBACH, Primary Examiner series with said inductance; P. L. GENSLER, Assistant Examiner a common reference terminal for said transmission 20 U.S. C1, XR channel; 333-70 Akaa Va. As &AW.K.W. a third resistance connected between said output ter minal and said common reference terminal;