US3524143A - Amplifier systems for electric guitars and the like - Google Patents

Description

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AMPLIFIER SYSTEMS FOR ELECTRIC GUITARS AND THE LIKE Filed Dec. 15, 1965 +1191: mum

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INVENTORE- final/x i ATTORNEYS mdnh United States Patent 3,524,143 AMPLIFIER SYSTEMS FOR ELECTRIC GUITARS AND THE LIKE Walter Munch, Jr., Covington, Ky., assignor to D. H. Baldwin Company, Cincinnati, Ohio, a corporation of bio Filed Dec. 15, 1965, Ser. No. 513,955

Int. Cl. H03f 3/68 US. Cl. 330-30 5 Claims ABSTRACT OF THE DISCLOSURE The present invention relates generally to electronic musical instruments, and more particularly, to electronic stringed instruments, wherein transducers coupled to the strings are used as tone sources, and wherein the tonal outputs of the strings may be modified in a variety of ways in pre-established degrees by means of a limited number of simple on-off controls, and in such manner as to simulate at will the tonal output of at least some members of a family of stringed instruments.

Many instruments of a family, e.g. the guitar family, are made in various forms, sizes, and tone capabilities. In such cases, musicians may be impelled to own and to transport and have available for playing two or more instruments contained within the family, at considerable cost and inconvenience. Electric guitars are known which transduce the motion of the strings of a guitar to electrical signals, and filter the signals to modify the timbre or quality of the tone prior to electro-acoustical transduction and radiation of the signals. In a copending application in the names of Donald W. Elbrecht and Robert C. Scherer, entitled Tone Control Systems for Electric Guitars and the Like, Ser. No. 514,051, now US. Pat. No.

3,454,702 issued July 8, 1969 assigned to the same assignee as the present application, the filters employed to modify tone quality are specifically designed to produce a wide range of quickly-changeable tone qualities approaching several individuals of a family of instruments. For example, in the case of the guitar, there are the classic guitar, the type used for western music, the rock and roll guitar, the Hawaiian guitar, the bass guitar, the rhythm guitar, and the like.

The modification of timbre or tone quality alone does not exhaust the possibilities of modifying guitar tones electrically. In accordance with a further feature of the invention, a modulator is provided which can provide a range of tremolo frequencies, and a range of modulation depths for such tremolo. Further use may be made of a reverberation facility, in processing guitar tones.

It is, accordingly, a broad object of the invention to provide easily reproducible electrical simulations of, and enchantment of the tone, of several of a family of guitars, while only one member of the family is being played.

According to the invention, two channels are provided each of which provides two inputs. Four instruments can 3,524,143 Patented Aug. 11, 1970 therefore be accommodated at one time. But if only one instrument per channel is utilized, that channel can play at high level or at low level, according to selection of two jacks available in the channel.

One of the channels provides conventional facilities, per se, apart from the provision of double jacks. The other channel contains facilities not heretofore available in electric guitar amplifiers, i.e. push-button selection of tone colors and availability of circuitry for modifying the effect of tone color filters to afford fine gradations of tone.

It is, accordingly, a broad object of the invention to provide an electric guitar system having more sophisticated and expensive facilities than have heretofore been available in such systems.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment therof, especially when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagram showing the correct assembly of the sheets of drawings to provide a complete circuit diagram;

FIG. 2 is a block diagram of circuitry and transduction equipment which may be employed as part of a guitar for providing signals to the electronic circuitry of the invention;

FIGS. 3, 4, 5, 6 taken together constitute a schematic circuit diagram of the electronic circuitry of the invention; and

FIG. 7 is a block diagram of the circuitry of FIGS. 3-6, inclusive.

Referring now to FIG. 2, a string motion transducer 1, called a crystal bridge pick-up, is provided as part of an electric guitar. Such transducers are conventional and involve piezo electric crystals, usually mechanically coupled to the strings of the guitar by being located under or constituting the bridge of the guitar. Additionally, an electric guitar may be provided with an electromagnetic pick-up 2 which responds to the movement of metallic strings without contact therewith. Both facilities are not required, but may be available. A reason exists for having both in that each type of transducer can produce a different character of tonal output.

The output of crystal bridge pick-up 1 is amplified in amplifier 3, since it is normally of low ampltiude, and applied via a conventional volume-tone control circuit 4 to one stationary contact of a switch 5. The output of magnetic pick-up 2 is applied to another stationary contact of switch 5 via a volume and tone control circuit 6. The switch 5 includes two movable arms 7 and 8, cooperating with the stationary contacts, respectively, and these have a common output jack 9. Accordingly, there is available at jack 9, the tonal output of either transducer, or of both, according to the positions of movable contacts 7, 8. As illustrated both transducers provide such output to jack 9.

ADDER (FIG. 3)

In channel 2, a high input jack 10 and a low input jack 11 are provided. The high input jack '10 proceeds via a series RC coupling network 12 to the base of an NPN transistor TR having an emitter load 13. The emitter of TR, is directly connected to the base of NPN transistor T R which in turn has an emitter load 14. TR and TR have a common collector load 15, which is in series with a Voltage source 16 via a voltage dropping resistance 17, the junction of resistances 15 and 17 being lay-passed to ground for alternating currents by capacitor 18. The base of TR is biased by a voltage divider comprising series resistances 20, 20a. A common output lead 21 for the collectors TR and TR is provided, which is in series via capacitor 21b with a potentiometer 21a, acting as a volume control. TR, and TR look, to resistance 15, like a constant current source, so that a replica of the input signal appears across resistance 15, except for a phase reversal.

The low input jack 11 utilizes a circuit similar to that of the high input jack 10, utilizing NPN transistors TR and TR., and identical circuit values associated with these as are associated with TR and TR and the collector load resistance 15 is common to all four transistors.

The jack 11 includes a grounded element 22 and a movable arm 23 normally contacting point 24. The latter is connected to movable arm 25 of jack 10, which includes a grounded element 26 and a contact point 27 which is grounded. When a male element is inserted in either jack the movable arm, as 23 or 25, is raised, and becomes the hot element of the jack. Operation of jack 11 alone then implies disconnection of contacts 24 and 27, so that no signal is applied to TR TR n the other hand, operation of jack 10 alone implies that signal is applied in parallel to TR and TR Collector load 15 then carries double signal current, and its voltage as seen on lead 21 is doubled. At the same time, if both jacks are operated, say in connecting two diverse inputs simultaneously, both inputs will respond at lead 21, but the signal level will be twice that for the single low jack 11, and may be the same as that for the high jack 10 alone.

PRE-AMPLIFIER The section following volume control 21a is a preamplifier including tone control circuitry.

Transistor TR is an NPN transistor having an unbypassed resistance 30 connected between its emitter and ground. A resistance 31 provides a collector load for TR and the collector is connected to the base of transistor TR The latter is provided with an emitter bias circuit 32 and a collector load 33. The bias voltage developed by bias circuit 32 is connected back to the base of TR and serves to bias the latter. A coupling capacitor 34 then couples the collector of TR to tone control circuitry, capable of separately controlling bass and treble.

The tone control circuitry includes a small capacitor C in series between capacitor 34 and output lead 35. A capacitor C of the same size, is connected from capacitor 34- in series with resistance 30. Resistance R is connected to across C and C in series, and a variable slider 36 is connected from the junction of C and C to the resistance R C then supplies feedback signal to resistance 30 while C provides signal to the load. As slider 36 moves up, C is progressively by-passed, which provides bass boost in output lead 35. Since a zero impedance paht 37 is provided from C to R at their ground-adjacent points, moving slider 36 up decreases bass feedback. Thereby, moving the slider up boosts bass by decreasing feed through impedance and decreasing negative feedback; when slider 36 moves down, feedback capacitor C is shunted out of circuit, progressively, while C is cut in. Thereby, series attenuation to lead 35 increases while feedback signal increases.

Capacitor C resistance R resistance R and capacitor C are connected in series from resistance R to ground. Slider 40 slides on R and is connected back to the feedback path, at the junction of R R which are connected between C and resistance 30. As slider 40 moves up treble is cut, because degeneration is progressively added via C while by-pass of treble frequencies to ground is decreased. As the slider moves down, treble is boosted because degeneration is decreased via C and treble by-pass to ground increased via 0,.

4 DRIVER AND POWER AMPLIFIER (FIG. 5)

Signal appearing on lead 35 is coupled to the base of an NPN transistor TR7 having a grounded collector and an emitter load 49 connected back to a negative voltage supply 50. A voltage divider composed of resistances 51, 52 connected back to the same supply 50 furnishes bias for TR is directly connected to the base of a further PNP transistor TR The emitter of the latter is connected to ground through a bias circuit 54 and a feedback resistance 55, a primary winding T of a transformer T being connected in the collector circuit as a load. Across winding T is connected a small resistance 56 and a small capacitor 57, in series, to provide a high-frequency bypass to prevent supersonic oscillations.

Two secondary windings TA and TB are provided in transformer T. These are oppositely poled to provide a push-pull output for power amplifier transistors.

The transistors employed in the main power amplifier are denoted TR,; and TR in parallel and TR and TR in parallel. Bias voltage for the transistors TR to TR is provided from a negative bias voltage source 60. The latter is connected through parallel resistances 61 to the base of TR through windings TA, then via resistances 62 in parallel to the base of TR and thence to terminal 63 of loudspeaker LS, providing a path to ground through the speaker. Point 63 is connected back to resistance 55 through a resistance 64 which is by-passed for high audio frequencies by capacitor 65. Accordingly, the signal appearing across loudspeaker LS is fed back degeneratively to the emitter of TR8.

The collector of PNP transistor TR is connected directly to a negative voltage supply 67, and its emitter is connected via a small fuse 68 directly to the collector of TR the emitter of which is connected via a small fuse 69 directly to the ground. At the same time, the junction of the emitter of TR and the collector of TR are coupled via a large capacitor 70 to terminal 63. Accordingly as TR and TR are driven out of phase from transformer T their relative impedances vary inversely. They act as variable voltage divider for point 63 and thereby serve to drive the loudspeaker LS. Coupling from point 63 to the emitter resistance 55 of TR provides negative feedback to the driver stage.

Transistors TR and TR parallel TR and TR,,, respectively, at all electrodes to enhance power handling capacity and their presence does not modify operation otherwise.

A jack 72 is provided between loudspeaker LS and ground to enable insertion of additional speakers in series with LS.

The system channel above described is channel 2. A further channel 1 is provided, which includes an adder amplifier and input jacks, and amplifier circuitry precisely the same as that above described under the heading ADDER. It is therefore not described in detail. The output of the latter appears on lead 75, which is connected to the input of a pro-amplifier 76 which lacks tone control features, and which has a flat response. The signal on lead 75 is supplied to the base of NPN transistor TR operated as an emitter follower. The emitter of TR is connected to the base of NPN transistor TR which is emitter-grounded and collector-loaded by resistance 77. The collector of TR is connected back as shown to the base of TR to provide considerable negative feeback, and output is provided at lead 78.

The lead 78 proceeds to two normally-closed ganged switches 79 and 80. In the normal position switches 79 and 80 provide channel 1 signal in parallel to a set of tone color filters TCF and to a tone control circuit 81. When switch 80 is opened, the tone color filters T CF are disconnected, but the tone control circuit 81 remains in circuit. In the alternative, when switch 79 is opened, switch 84 remains closed and only the tone color filters TCF remain in circuit. The outputs of tone color filters TCF and of tone control circuit 81 are applied in parallel to a modulator amplifier MA. The signal therefrom is passed along to the power amplifier driver via a reverberator which is described in detail hereinafter. At the power amplifier driver, the signals from channels 1 and 2 are combined.

TREMOLO OSCILLATOR The tremolo oscillator of the present system, which supplies modulation signal to modulator amplifier MA, includes an NPN transistor TR having its collector directly connected to a positive supply source 80, and having a resistive emitter load 81, 82. The emitter of TR is connected directly to the base of an NPN transistor TR The latter includes a collector directly connected to voltage supply 80, an emitter connected to the junction 83 of resistances 81, 82. Junction 83 is coupled via a capacitor 84 across a 1M resistance 85. The latter is connected to the base of TR via resistance 87, so that the coupling is via a T-bridge having capacitor 84 as its shunt arm. From the base of TR to ground is connected, in series, a pair of capacitors 88, 89, from the junction of which a variable resistance 90 proceeds back to junction 83, providing a T-bridge having resistance 90 as its shunt arm.

Feedback from the emitter of TR to the base of TR is positive, but different phase shifts exist in the two feedback paths from terminal 83, one path subsisting through variable resistance 90 and the junction of capacitors 88, 89, and the other through capacitor 84 and the junction of resistances 85,, 87 and together serving to establish oscillatory conditions. Since frequency of an RC oscillator is determined by loop phase shift, the frequency of the oscillator can be adjusted by varying the value of resistance 90. The frequencies selected are in the low sub-audio range, say 4-14 c.p.s. The tremolo oscillator is essentially of a conventional parallel-T type, and therefore its theory of operation is not expanded.

The output of the oscillator appears at terminal 83. That point is coupled via a path 91 to the base of a PNP transistor TR17. The path 91 can be shunted to ground by a foot operated switch 92, and includes an interposed volume control 93, which sets tremolo depth.

The modulator amplifier MA includes a first amplifier NPN transistor TR which is emitter grounded and collector loaded by resistance 94. The collector of TR is directly connected to the base of an NPN transistor TR which includes a small feed-back emitter resistance 95, and a larger collector resistance 96. The collector of TR supplies signal to load 88. The emitter of TR is coupled by a resistance 92 and large capacitor 93 to the emitter of PNP transistor TR and via resistances 92 and 92a to the base of transistor TR for feed-back.

TR17 includes a grounded collector and a base which is biased negative. It acts as a signal-controlled variable resistance in the emitter circuit of TR in parallel to feed-back resistance 95 serving to vary the total feed-back of modulator amplifier MA.

As the resistance of TR17 varies, the feed-back to the base of TR varies. Thereby, the gain of amplifier modulator MA varies and the signal on lead 88 is tremolo modulated. The use of a PNP transistor TR permits a small feed-back resistance 95, since the emitter resistance is small and highly sensitive to base voltage.

The tone control circuit 81 includes a base section 100 and a treble section 101. The treble section 101 consists of a pass filter of conventional configuration, i.e., two series capacitors 102, 103, and a shunt resistance 104 to ground from their junction. Control is effected by a variable resistance 105 in series with the filter, so that the highs may be augmented or decreased. Filter 100 is a low pass filter of conventional configuration, i.e., two series resistances 107 and 108 and a shunt capacitor 109, capable of by-passing highs. A further resistance 110 is connected in series with resistances 108, and the junction of resistances 108, 110 is variable shunted to ground via variable resistance 111.

The combination of filters 100, 101 can provide continuously-variable treble and bass tone control, independently.

TONE COLOR FILTERS (FIG. 4)

Tone color filters TCF constitute a set of components providing resistance, capacitance, and inductance, and which can be selected in various combinations to form any one of five filters in response to five switch assemblies which are mutually exclusive, so that closure of one set opens all others. Such switches, and the mechanical features thereof, are well known and therefore are not described in detail, or illustrated.

Lead 120, connected to switch 80, is a common input lead for the filters, and lead 121 a common output lead.

On closure of ganged switches S S S series capacitor and shunt resistance 151 are connected via lead 152 in series with inductance L and thence to ground via line 153 and S At the same time a circuit is completed from resistance 151 via S to resistance 153 and via S to output lead 121. The lead 121 is thus connected to the junction of capacitor 150 and inductance L and the circuit values are selected to provide a treble filter, having shunt inductance and resistance and series capacity.

When switches S S S S and S are simultaneously closed, a low pass filter section composed of series resistances 156, 157 and shunt capacitor 158 is provided connected to input lead 120, and a further low pass section composed of series resistance 159 and shunt capacitor 160 is connected via 8 to output lead 121. Connecting these sections in series are lead 161, S S and lead 162.

Inductance L is directly connected from the junction of resistance 156 and 157 to resistance 159. The total configuration passes bass and attenuates treble.

Switches S S S provide a circuit which is similar to that provided by S S S except for the inclusion of a resistance in place of 153 and the inclusion of the low-pass filter section composed of series resistance 156, shunt capacitor 158, inductance L shunt capacitor 160 and resistance 159. Accordingly, the mixture switches permit passage of both treble and bass, reducing the mid-tones.

Switches S S S insert a low pass section composed of series resistance 172 and shunt capacitor 173, and resistance 174 in series, between leads 120 and 121. In addition, inductance L is connected in shunt, via S lead 152, L lead 153, S and lead 171. The combination provides a band pass at a mid frequency.

For closure of switches S S S there is inserted series resistance 175, and shunt resistance 176, resistance 177 and low-pass section composed of series resistance 178 and shunt capacitor 179. In addition, L is shunted around resistance 177 via S and S The resultant filter provides a low-pass circuit which accentuates mid-range frequencies.

The output lead 121 deriving from tone color filters TCF proceeds to the base of TRlg, in parallel with the output of tone control circuitry 81, to provide dual control so that the tone color selected can be shaded by the additive tones provided by the tone control circuitry 81.

REVERBERATOR The signal present on lead 88, deriving from modulator amplifier MA proceeds to the base of an NPN transistor TR Which is provided with an emitter load R and a collector load consisting of a rever berator drive coil RDC for the input transducer of a reverberator coil or spring. The latter is shunted by a resistance R so selected that excessive voltage is never developed across RDC due to its increasing impedance as a function of frequency. RDC can respond to about 2000 cps., but carries the entire audio spectrum and is mechanicaly operative at all times during operation of the system.

The signal developed across R is applied to the base of NPN transistor TR which is collector loaded by R and supplies signal to the input lead 190 of TR The pick-up coil of the reverberator spring transducer, RPC, is connected to the base of NPN transistor TR loaded by resistance R at its collector. Reverberated output proceeds via lead 191, a potentiometer 192 which sets depth, and a lead 193 to the base of TR so that both reverberated and unrevenberated signal is presented on lead 190. To remove reverberation the potentiometer 192 is shorted to ground via a foot switch CS Accordingly, the reverberator is always operative, unless CS is closed, but umeverberated signal is always supplied to TR7 so long as signal is supplied to channel 1.

SUMMARY Referring to FIG. 7, two adder amplifiers AA, and AA are provided, constituting channel 1 and channel 2 of the system. Each adder amplifier is provided with two inputs, each for low input, and each of which utilizes a different one half of the adder amplifier, if two inputs are provided. If an input is provided to only input LO, one half the adder amplifier is employed, while if only one input is provided for jack HI, the adders are both utilized in parallel to provide double amplitude output.

Adder AA proceeds, in cascade, to a tone control preamplifier TCP, a driver D, a power amplifier PA, and a loudspeaker LS.

Adder AA proceeds selectively via tone control filters 81 and/or formant filters TCF, to a tremolo modulator TM modulated from tremolo oscillator TO. Tremolo modulator TM drives reverberator circuit RC, and control switch CS indicates that unreverberated signal is provided to driver D, with or/ without reverberated signal.

A wide capability is thus provided. A total of four guitar inputs can be accepted, via the two adder amplifiers AA and AA In the alternative two guitar inputs can be accepted, at high level or low level jacks in each case. One guitar tone may be provided at high level and two additional guitars each at low level. These capabilities do not involve manipulation of gain controls.

Channel 2 involves a wide range level and tone control, having capability of controlling base and treble tone independently, and the output of the control drives a driver and power amplifier which is common to both channels.

Channel 1 includes facility for passing tone signals either via pre-set tone color filter TCF, or via a tone control circuit TCC, or both. The outputs of the filters are applied to a tremolo modulator. The output of the latter can, selectively, be reverberated (in the reverberator), or not reverberated, and the thus processed signal is then amplified and electro-acoustically transduced.

The tone control amplifier TCP is essentially a high fidelity pre-amplifier, so that channel 2 can be used not only for guitars, but also for microphone input, tape recorder input, and the like. The tone control circuitry TCC and TCP are designed for forming guitar tones of a wide variety of characters. The tone control circuits TCF provides five distinct tones, available by operation of push button switches, labeled A, B, C, D, E for purposes of identification. Each filter is selected by operating one push button, the push buttons being interrelated mechanically so that operation of any one disables all others. Switches A (see FIG. 4) provide treble tone only, switches E pr0- vide bass tones only, switches B provide a mixture of bass and treble, while switches C and D provide emphasis of difierent mid-range frequencies. For many purposes this set of filters suflices in reproducing guitar music, and the arrangement has the advantage of being reproducible and obviating knob-controlled adjustments which are diflicult to reset.

The tone control filters TCC provide a bass section and a treble section essentially, either of which can be largely disabled, as by inserting full series resistance 105 (FIG. 6) or full shunting at the slider of potentiometer 111.

The filter TCC may therefore add bass or treble or both to the output of the system, over and above that provided by filters TCP, and therefore modify the characteristics provided by each individual filter of TCP. This modification can subsist in the addition of treble, or bass, by selectively cutting in or out the filter sections and 101, individually or together. The filters TCC have a null or near-null attenuation point in the mid-band, which is not substantially afiected by the adjustments of the filters.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. An amplifier, including a first amplifier section,

a second amplifier section,

a common load for said first and second amplifier sections,

a first input jack including a first movable contact and a first stationary contact for said first amplifier section, said first contacts being normally closed,

a second input jack including a second movable contact and a second stationary contact for said second amplifier section, said second contacts being normally closed,

a lead interconnecting said movable contacts,

a ground connected to said second stationary contact,

said jacks being responsive only to insertion of a plug in either of said jacks for separating the movable contact from the stationary contact of that jack, and

means connecting said amplifier sections respectively to said movable contacts.

2. An amplifier system, comprising a first amplifier section,

a second amplifier section,

a common resistive load arranged for adding the outputs of said first and second amplifier sections,

a first input jack for said first amplifier section,

a second input jack for said second amplifier section,

said first jack including a first movable signal input contact and a first stationary contact normally contacting said second movable contact,

said second movable jack including a second movable signal input contact and a second stationary contact normally contacting said second movable contact,

said first amplifier section having an input terminal, coupled to said first movable contact, said second amplifier section having a second input terminal coupled to said second movable terminal,

a lead coupling said first movable contact to said second movable contact, and a ground point permanently connected only to said second stationary contact.

3. The combination according to claim 2, wherein each of said amplifier sections includes first and second cascaded NPN transistors, each of said transistors having a collector, an emitter and a base, the first of said transistors being solely emitter loaded, the second of said transistors being emitter loaded and collector loaded, the collector load consisting of a resistance and being common to said amplifier sections and a lead of negligible resistance connecting the collectors of said transistors.

4. An amplifier system including a second amplifier,

means coupling said amplifiers to a common load,

a first signal input device connected to said first amplifier including a first normally closed switch having first and second contacts,

a second input device connected to said second amp1i- 9 10 fier including a second normally closed switch having References Cited third and fourth contacts, said first amplifier having an input terminal connected UNITED STTES PATENTS to first ontact Bulls said second amplifier having an input terminal connected to said third contact, 6 ROY LAKE Pnmary Exammer a lead connecting said second contact to said third LLDAHL, A i tant E i r contact, and a ground connection for said fourth contact. US. Cl. X.R. 5. The combination according to claim 3, wherein are m 84--1.16 provided separate signal input devices selectively couplable at will with said first and third contacts, respectively.

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