EMULATED GUITAR LOUDSPEAKER

Abstract: Circuitry for simulating and emulating the sounds of a loudspeaker driven by a classic rock and roll guitar amplifier in the form of an electrical signal which, when input to a recording console or used to drive high fidelity headphones or a high quality monitor amplifier and monitor loudspeaker, accurately reproduces such sounds. The circuitry is incorporated at the output of a classic tube-type rock and roll guitar amplifier, and consists of three basic modules: a load impedance module (Fig. 3) designed to offer to the tube amplifier output a load impedance as a function of frequency (Fig. 1) which is modeled after and nearly identical (in both real and imaginary components) to the impedance of a classic rock and roll guitar speaker; a highpass filter circuit (24) which emulates the low frequency resonance and rolloff characteristics (Fig. 7) of a classic rock and roll open-back guitar speaker cabinet; and further filtering circuits (26, 30, 32, 34, 36, 40) which emulate the mid and high frequency characteristics (Fig. 7) of a classic rock and roll guitar loudspeaker. Electrical outputs (50) are provided for driving a recording mixer, headphones, a speaker, and an external power amplifier.

Description: 1 2 Emulated Guitar Loudspeaker 3 4 Technical Field 5 g This invention relates to electrical signal η conditioning apparatus for use with electric guitars and g tube type guitar amplifiers to produce an electrical output 9 signal having predetermined tone, timbre and/or distortion τ_o characteristics. 11 12 Background Art 13 14 The sound of a guitar playing through a classic 1960's 5 Fender or Marshall vacuum tube amplifier and speaker, such • j _g as a Celestion 12 inch model G12, became identified with 17 Rock and Roll music, and in essence defined a new form of ■ig musical instrument. Rather than simply a means for 19 providing increased loudness for entertaining large 20 audiences, those "classic" rock and roll guitar vacuum tube 2i amplifiers and associated output transducers provided a 2 reedlike sound when driven into overload which could be 3 finely controlled and shaded into a variety of musical 4 textures by the skilled guitarist. Various attempts have 5 been made over the past twenty years to simulate that 6 highly desirable reedlike sound by means of transistorized 7 amplifiers driving speakers or driving resistors, or by g means of vacuum tube amplifiers driving resistors. 9 0 Laub, in U. S. Patent 3,835,409 teaches a solid state i amplifier which includes means for generating large amounts 2 of crossover distortion in proportion to the amplitude of 3 the signal. Moog, in U. S. Patent 4,180,707, teaches a 4 variety of solid state circuits which can create even or 5 odd order distortion to simulate some of the effects of 6 overdriven vacuum tubes. Smith, in U. S. Patent 4,211,893 teaches an electronic amplifying apparatus which is intended to simulate the distortions of vacuum tubes by the use of either FETs or bipolar transistors. Sondermeyer, in U. ' S. patent 4,405,832 teaches a distortion circuit consisting primarily of a diode clipper and a potentiometer incorporated in an operational amplifier circuit. An article by T. E. Rutt entitled "Vacuum Tube Triode Nonlinearity as part of the Electric Guitar Sound." which was published as Audio Engineering Society Preprint 2141, October 1984 discloses that the distinctive distortion of a tube amplifier is caused by grid current effects which have never been successfully duplicated by semiconductor circuits. Scholtz has sold a commercial product under the brand name Power Soak which resistively loaded a guitar amplifier and provided for an adjustable portion of the electrical output signal- which can then be applied to headphones, a speaker, or a recording console; this allows an amplifier to be driven into overload safely while not having to produce loud sounds in air. All of these prior art devices and inventions have shortcomings which prevent them from providing all of the touch and sound of the classic tube-type guitar - amplifier/speaker/cabinet combination as an electrical signal which can be fed directly into a conventional recording console and/or transduced into musical sounds in air by conventional linear means. Another prior art method which can under certain circumstances provide all of the touch and sound of the classic tube-type guitar amplifier/speaker/cabinet combination as an electrical signal was to place a microphone in front of a conventional loudspeaker contained in a classic small tube type Fender, Vox, or Marshall amp, and then use the microphone to transduce the musical sounds from the loudspeaker into electrical signals which can then • ] _ be further amplified, recorded, and again transduced into 2 sounds in air either at a later time or at a higher 3 amplitude. That technique required that loud performance 4 level sounds be generated in air, was susceptible to undesirable feedback in a real-time reinforcement 6 situation, and was subject to undesirable variations in η sound quality as a function of microphone type, placement, g and the room in which the instrument is placed. It is also 9 susceptible to contamination by outside sounds and noises. 10 , , Disclosure of the Invention 12 13 It is accordingly an object of the present invention to -,4 overcome the deficiencies of the prior art and to. rovide a • j c signal conditioning circuit which can generate an 16 electrical output signal which when linearly amplified and 17 transduced into sounds in air will emulate the tone and ιa timbre and touch of a classic guitar amplifier such as a , g 1960's Fender amp containing a classic guitar speaker such 20 as the Celestion G12. In accordance with various specific 21 aspects of the invention, such an electrical output signal 22 may be used as input to a recording console or public 23 address reinforcement system, or as a signal for driving 2 headphones. 25 6 I is a further object of another specific aspect of 7 the invention to so arrange the circuitry that a moderately 8 powerful signal is also available to drive a speaker, while 9 still providing the above described electrical output 0 signal for driving a recording console or reinforcement 2_ system. 2 3 The present invention accordingly provides a circuit 4 for loading the output of a classic tube type guitar 5 amplifier of moderate power capacity with the complex 6 impedance of a loudspeaker in a typical cabinet, for emulating the frequency response of a guitar loudspeaker in a cabinet, and for providing an electrical output signal which when linearly transduced into sounds in air properly and suitably emulates the tone, timbre, and touch of a desirable classic 1960's Fender or Marshall guitar amplifier/speaker/cabinet combination, thus allowing a musician to express his artistic will and desires, using his motor skills in conjunction with his perception of the sounds produced by the invention in combination with either headphones or a loudspeaker. Brief Description of Drawings The foregoing and other objects, features, and advantages of the invention will become more readily apparent upon reference to the following detailed description of a presently preferred embodiment, when taken in conjunction with the accompanying drawings in which like numbers refer to like parts, and in which Fig. 1 shows the impedance function, plotted as a generalized Heyser spiral, of the amplifier load module. Fig. 2 is a system block diagram of the filter circuitry which emulates the transfer function of a loudspeaker. Fig. 3 is a schematic diagram of a preferred embodiment of the load circuitry and an initial passive low pass filter section. Fig. 4 is a schematic diagram of a 30 dB pad and 100 Hertz highpass filter. Fig, 5 is a schematic diagram of a five-pole lowpass 4500 Hertz Butterworth filter. 1 Fig. 6 is a schematic diagram of a representative 2 bandpass section. 3 Fig. 7 shows the overall frequency response of the 5 filter circuitry when properly adjusted. 6 7 Fig. 8 is a schematic diagram of the inverting buffer stage which provides final gain adjustment and line and 9 headphone driving capability. 10 11 Best Mode for Carrying Out the Invention 12 - I Particular reference should now be made to Fig. 1 which 14 shows the complex impedance function, plotted as a ■,5 generalized Heyser spiral 10 projected onto three ■ ] _g orthogonal planes 12, 14. 16, of an amplifier load module 17 contemplated by a preferred embodiment of the present ,g invention, and to Fig. 3, which is a schematic diagram of 19 that load circuitry operatively connected to an initial 20 passive low pass filter section. The resistor R3-1 is a 10 2i Ohm. 25 watt unit which can dissipate the full output of ■ 22 the classic tube type transformer coupled guitar amplifier 3 (not shown) which is connected in normal use to the load 4 module's input terminals "+" and "-". Coil L3-1 is 0.9 mH 5 and simulates the voicecoil inductance of a loudspeaker. 6 14 mH Coil L3-3 and 200 uF Capacitor C3-1 constitute a 7 parallel resonant circuit which simulates the fundamental 8 resonance of a conventional "classic" rock and roll 9 electric guitar amplifier loudspeaker having the Q and 0 center frequency which are typically associated with such i loudspeakers. The thus far ^ -described portion of the load 2 circuit is preferably connected through a so-called 3 "normal" jack (not shown) to the remainder of the circuitry 4 so that in the absence of a loudspeaker load plugged into 5 the jack, the circuit properly loads the amplifier to allow 6 it to operate in its usual range of distortion and output level. As will be readily 2 apparent to the skilled artisan, if a loudspeaker having the appropriate voice coil inductance and fundamental . resonance characteristics is connected to the output of the ς guitar amplifier, this portion of the circuitry is no 6 longer required and by virtual of the above described _ construction employing a normal jack or other equivalent - connection means, it may be conveniently disconnected. In Q any event, with either a real or simulated load, the signal 1 - from the output of the guitar amp is preferably led through 0.9 mH coil L3-2 terminated by 22 Ohm resistor R3-2 to provide an initial low pass filter stage which then passes the signal to the filter portion of the present invention, a presently preferred embodiment of that filter section 14 being shown in block diagram form in Fig. 2 and in more , , detailed schematic diagram form in Figs. 4, 5, 6 and 8 and 16 having the frequency response characteristics set forth in 18 Fi »- 7 - 19 Referring now to Figs 2 and 4, it will be seen that the loaded amplifier output is applied, by means of "IN" terminal 18, to a 30db attenuator pad 20 which is of 2 conventional construction and which comprises series 24 resistor R4-1 and shunt resistor R4-2. The attenuator 20 25 serves to reduce the 50 volt peak spikes which can be Δ ».,3 generated in full overdrive conditions to levels which will 27 not clip or distort in even the highest gain stages of the 28 filter stages to follow. The first active filter stage is 29 a conventional 3-pole Butterworth 100 Hz highpass active 30 filter 24 which simulates the low frequency rolloff _. response of the loudspeaker and cabinet (refer also to the _ 2 left hand portion of the overall preferred response curve 33 set out in Fig. 7) and which comprises O.luF capacitors 34 C4-1, C4-2, and C4-3; 15k resistors R4-3, R4-4 and R4-5, 35 and operational amplifier A4-1, so as to similarly rolloff 3 6 the same lower frequencies of the input to isolating operational amplifier A4-2 which is provided between the highpass filter 20 and the lowpass filter 26 to prevent any output impedance 2 interaction between the highpass filter 20 and any 3 subsequent stages. 4 5 Lowpass filter 26 (see also Fig. 5.) is a conventional 6 five pole Butterworth lowpass 4500 Hz filter which η simulates the quick high frequency rolloff of a guitar g loudspeaker (refer also to the right hand portion of the 9 overall preferred response curve set out in Fig. 7). As -,0 shown in Fig. 5, lowpass filter 26 comprises an active 11 first stage 26a including a 36k resistor R5-1, a 6.8k 2 resistor R5-2, and capacitors C5-1, C5-2 and C5-3 having 13 respective values of O.OluF, O.OOluF and O.OOluF (the 14 latter two connected in series), in combination with a -,5 first operational amplifier A5-1, followed by a passive 16 stage 26b comprising a 3.6k resistor and a O.OluF capacitor , η C5-4, which in turn is followed by an active third stage 18 26c comprising a 56 k resistor R5-4, a 2.2k resistor R5-5, 19 a 0.01 capacitor C5-5 and a O.OOluF capacitor C5-6. in 2 0 combination with a second operational amplifier A5-2. 21 22 Butterworth filters are preferred because of their 3 minimal phase error characteristics. Since the sharpness 4 of a Butterworth filter is related to the number of "poles" 5 (reactive elements), a five pole Butterworth is preferred 6 f r lowpass filter 26 since it has been determined that a 7 minimum of five poles provide the desired relatively quick 8 rolloff associated with a "classic" guitar loudspeaker. 9 However, other types of filters having other numbers of 0 poles may be substituted, provided they have highpass and i lowpass rolloff characteristics comparable to the 2 particular loudspeaker/cabinet combination thjat is to be 3 emulated. 4 5 The lowpass filter is then followed by a series of five 6 adjustable bandpass filters 30, 32, 34. 36 and 40 which are preferably adjusted to produce the frequency response of 1 Fig.7, which was originally generated by measuring the 2 frequency response characteristics of a typical "classic" 3 loudspeaker and loudspeaker cabinet originally intended for 4 use with a tube amplified electric guitar, and has been 5 found to simulate very closely the frequency response of 6 such a guitar loudspeaker. 7 8 Reference should now be made to Fig. 6, which shows a 9 typical one of the serially arranged adjustable bandpass 10 filters 30, 32, 34, 36 and 40 and which may each comprise a 11 fixed network of 10k resistors R6-1. R6-4, 100k resistors 2 R6-2 and R6-5 and capacitors C6-1 and C6-2, as well as an 13 adjustable center tapped 500k potentiometer R6-3 and an 3_ operational amplifier A6-1. Capacitors C6-1 and C6-2 may 15 have respective nominal values of O.OluF and 0.05uF for 1 ξ filter 40, and may both be O.OluF for 440Hz filter 30, 17 0.0056uF for 440Hz filter 32, 0.0027uF for filter 34, and. ιg 0.0015uF for filter 36, to provide five respective active ■Lg bandpass filters respectively centered at 220 Hz, 440 Hz, 2o 880 Hz, 1760 Hz, and 3520 Hz which have respective maximum 2i gain (attenuation) of approximately 16db for 220Hz filter 22 40 and of approximately 11 db for the other four filter 23 stages 30, 32, 34 and 36. The individual potentiometers 24 R6-3 of the five individual bandpass filters are preferably 25 set to the provide the respective levels set forth in the 26 central portion of Fig. 7. The bandpass filters are 27 followed by an inverting buffer amplifier 42 (see also Fig. 28 8) comprising a 10k input series resister R8-1 and 29 adjustable gain resistance R8-2 in combination with 30 operational amplifier A8-1 as well as 47 ohm output series 31 resistance 44 and 5uF blocking capacitor 46. This provides 32 a buffered, amplified signal having an adjustable level and 33 appropriate output characteristics for connection to a 34 conventional external passive transducer device (not shown) 35 such as headphones, and/or which may be connected to a 36 cable leading to a external recording console and/or other external equipment , (not shown) such as a high powered linear amplifier and 2 conventional concert hall loudspeakers which may then 3 provide high level playback for a large audience. There 4 may also be provided a fixed or variable termination 5 resistance 48 to provide a user controlled output level. 6 _ As indicated by the dashed boxes 22, 28 and 38 of 8 Fig.2. the filter portion of the invention may conveniently q be constructed in three individual modules, the first , n module 22 comprising the highpass filter 24 and the lowpass 11 filter 26, the second module 28 comprising the four upper 12 bandpass filters 30, 32, 34 and 36, and the third module 13 comprising the lower bandpass filter 40, the inverting 14 buffer 42 and the associated output termination impedance ς network comprising resistance 44 and capacitor 46. 16 Thus in accordance with the teachings of the present 17 invention there may be provided an electrical signal 18 faithfully representative of the desired sound of a classic 19 rock and roll guitar amplifier and loudspeaker, without 20 21 requiring any critical microphone placement, or presenting 22 any possibility of unwanted acoustic feedback. 23 . . It should be appreciated that the preceding 2J - specification describes a presently preferred embodiment of ,. c the invention as an illustration and not a limitation 27 thereof. Equivalent modifications and variations of the 2g invention will doubtless occur to those skilled in the 2 9 art. Accordingly, the scope of the invention as set forth 30 with greater particularity in the appended claims should be 31 interpreted to encompass all equivalents to which the 32 invention is fairly and legally entitled. 33 34 5 36

Claims: What is claimed is: 1. A circuit for generating a conditioned electrical signal representative of the sound of an electrical guitar comprising input means for receiving the output from a guitar amplifier, conditioning means for conditioning said output in accordance with a desired sound characteristic and output means for producing an electrical signal from the conditioned output, characterized in that said desired sound characteristic emulates the sound of a predetermined type of loudspeaker in a predetermined type of cabinet driven by a conventional tube-type guitar amplifier. said conditioning means filters said output in accordance with the frequency response of said predetermined loudspeaker in said predetermined cabinet. and said conditioning means comprises a highpass filter for emulating the low frequency rolloff characteristics of said predetermined loudspeaker in said predetermined cabinet, a lowpass filter for emulating the high frequency rolloff characteristics of said predetermined loudspeaker in said predetermined cabinet, and a plurality of adjustable bandpass filters for emulating the intermediate frequency response of said predetermined loudspeaker in said predetermined cabinet. 2. A circuit as set forth in Claim 1 further characterized in that said input means further comprises a load impedance portion which simulates the complex impedance of said predetermined loudspeaker in said predetermined cabinet and provides said tube-type amplifier with a simulated load having said complex impedance. • ] _ 3. A circuit as set forth in Claim 2 further characterized 2 in that said load impedance portion further comprises a 3 parallel resonant circuit for simulating the fundamental 4 resonance of said predetermined loudspeaker, and a coil for 5 simulating the voice coil inductance of said predetermined 6 loudspeaker. 7 o 4. A circuit as set forth in Claim 2 further characterized a in that said load impedance portion may be disconnected 10 from the output of said tube type amplifier and replaced ,-, with a loudspeaker. 12 5. A circuit as set forth in Claim 2 further characterized 13 14 in that said output means is connected to a recording mixer 15 6. A circuit as set forth in Claim 2 further characterized 16 , 7 in that said output means is connected to a linear ,g amplifier. 19 7. A circuit as set forth in Claim 2 further characterized 20 2 i in that said output means is connected to headphones. 22 23 8. A circuit as set forth in Claim 1 further characterized 4 in that said highpass filter is ' a Butterworth filter having 5 a relatively slow rolloff characteristic and said lowpass filter is a Butterworth filter comprising at least five 7 poles and having a relatively quick rolloff characteristic. 8 9 0 1 2 3 4 5 6

Inventors: BUCK MARSHALL D, PITTMAN R ASPEN, BUCK, MARSHALL, D., PITTMAN, R., ASPEN
Applicants: GT ELECTRONICS
Application year: 1986
Main classification: H03G5
IPC codes: G10H1, G10H3, H03F1, H04R3