Abstract:
An apparatus and method for amplifying low amplitude vibrations from stringed instruments, and in particular guitars while reducing Gaussian and 60 Hz hum noise from the signal. In particular, the invention incorporates the use of one or more bipolar magnets in conjunction with high precision differential amplifiers and a power source. The bipolar magnets may be arranged at varying locations on the guitar as long as they are within range to induce an electrical signal in the strings when they are played. The signal is then fed through the amplifier circuitry whereby noise is eliminated and then played through a standard speaker.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to transducers. In particular, transducers for vibrating strings as those in guitars or other stringed instruments. The present invention enables high-quality amplification of low-amplitude vibrations that originate from metal stringed instruments through the use of one or more bipolar magnets in conjunction with high precision differential amplifiers and a power source. 
   BACKGROUND OF THE INVENTION 
   In stringed instruments, amplification of low amplitude string vibrations is usually accomplished by utilizing an arrangement of wire-wound magnets under the strings of the instrument. To produce sound, the wire-wound magnets sense the vibrations of the strings electronically and a signal cable routes an electronic signal to an amplifier and speaker. The sensing occurs in a magnetic pickup mounted under the strings on the guitar&#39;s body. A bar magnet is typically used. When a vibrating string cuts through the field of the bar magnet in the pickup, a signal is produced in the pickup&#39;s coil. This pickup consists of a bar magnet wrapped with as many as 7,000 turns of fine wire. In the case of an electric guitar, the vibrating steel strings produce a corresponding vibration in the magnet&#39;s magnetic field and therefore a vibrating current in the coil. 
   There are many different types of pickups. For example, some pickups extend a single magnet bar under all six strings. Others have a separate pole piece for each string. Some pickups use screws for pole pieces so that the height of each pole piece can be adjusted. The closer the pole piece is to the string, the stronger the signal. 
   The pickup&#39;s coil sends its signals through a very simple passive component circuit on most guitars. A potentiometer adjusts the tone along with a low-pass filter that eliminates higher frequencies. By adjusting the potentiometer, it is possible to control the frequencies that get cut out. An additional resistor (typically 500 kilo-ohms max) controls the amplitude of the signal that reaches the jack. From the jack, the signal runs via the signal cable to an amplifier, which drives a speaker. 
   The present invention is a device that uses an integrated system of electronics and at least one bipolar magnet that is packaged on a thin and compact plastic tray that can be slipped between a guitar&#39;s strings and the body of the guitar. The device requires no holes be cut into the guitar and includes a volume control, output jack and an off-on battery switch on the tray. The device typically works with batteries that provide direct current and the entire device can be unplugged and removed if the player wants to play the guitar acoustically. The bipolar magnets may be placed anywhere on the tray as long as they are located near, or preferably under the guitar strings. As the magnets may be moved to different locations near the strings, the change in location of the magnets causes the various tones to be available to the player. If the magnets are moved rapidly, tremolo effects can be heard. The magnets may be of virtually any shape, although it is preferred that the magnets remain small and compact in size. 
   The strings are coupled to digital electronic processing for amplifying the string current that is induced from the bipolar magnets while eliminating annoying audio noise, principally Gaussian and 60 Hz hum. By utilizing digital processing, one can selectively boost or cut frequencies and further reduce noise from the string signal that were not possible with analog filters as described above. 
   The amplifiers used in the preferred embodiment are high-precision differential amplifiers that have high common mode rejection of about 90 db or more and low output noise of about 10 nano volt root hertz or less. High CMR is achieved by use of laser trimming of internal resistors in the differential amplifier IC to perfect matching ratio, which provides very high CMR. 
   It is also desirable to utilize super high field strength neodymium magnets of at least grade ND40. By using the high field strength magnets, a high string current is induced and consequently, much of the common noise found with conventional guitar string pickups is rejected. The present invention can also be used with all shapes and sizes of neodymium magnets. Typically, these types of magnets are economical because the manufacturing process simply involves the use of molds filled with neodymium powder and compressed into that mold shape. 
   DISCUSSION OF THE PRIOR ART 
   Transducers that are able to detect low amplitude vibrations from stringed instruments are well known in the prior art. The primary limitation of many transducers that utilize electromagnets is that most amplifiers must overcome the elimination of noise from the signal. 
   Other inventions in the prior art have attempted to overcome this problem. For instance, in U.S. Pat. No. 5,723,805 for Lacombe, the patent discloses a transducer for sensing low amplitude vibrations of stringed instruments. However, Lacombe&#39;s invention makes no mention of filtering noise. Additionally, the disclosed invention requires among other things an extensive modification to the guitar including the creation of new holes, and the potential need to reset the neck and utilize an expensive brass nut. Lacombe also utilizes single-ended amplifiers that allow a substantial amount of noise that is highly undesirable. Furthermore, Lacombe does not disclose the shielding of the circuits or cables in order to reduce noise, nor does Lacombe mention the use of ground planes near his circuits. 
   In U.S. Pat. No. 5,484,958 issued to Ogawa, the patent discloses a device for amplifying piezo electric currents flowing in the strings of a stringed instrument. However, this device does not disclose the use of bipolar magnets to act as a transducer. The same can be said of U.S. Pat. No. 5,637,823 issued to Dodge which utilizes a series of transducers that are fixed to the guitar. Specifically, the pickups are fixed to a plastic plate, which requires a massive hole to be cut into the guitar for accepting the plate. This configuration requires that the invention will only work for solid body guitars. 
   The U.S. Patents issued to Benioff (U.S. Pat. No. 2,239,985), Miessner (U.S. Pat. No. 1,915,858) and Vasilach (U.S. Pat. No. 2,293,372) also have limitations including the bulkiness of the elements of the invention; they offer no improvement in sound and often add noise to the output, and will require massive and non-economical changes to the traditional manufacturing of stringed instruments and in particular, guitars. 
   SUMMARY OF THE INVENTION 
   Broadly, it is an object of the present invention to provide a device for amplifying low-amplitude vibrations in stringed instruments; 
   It is a further object of the present invention to provide an amplifier that can be used upon a variety of stringed instruments and in particular, guitars. 
   It is a further object of the present invention to provide a device that can be attached to a guitar that requires no additional change in manufacturing and tooling. 
   It is a further object of the present invention to provide an amplifier that enables a wide tonal range without the use of tone controls; 
   It is a further object of the present invention to provide an amplifier that provides a high common mode rejection thereby creating a substantial reduction in noise contained in the amplified sound signal. 
   It is a further object of the present invention to utilize high-strength bipolar magnets to induce a current in the strings of the stringed instrument. 
   It is a further object of the present invention to utilize a high-precision differential amplifier to amplify the signal from the strings of the stringed instrument. 
   It is a further object of the present invention to include a digital signal processor in the amplification process in order to selectively boost or cut certain frequencies from the electrical signal in the string of the stringed instrument, and further reduce noise by digitally over-sampling the signal. 
   The description of the invention which follows, together with the accompanying drawings should not be construed as limiting the invention to the example shown and described, because those skilled in the art to which this invention appertains will be able to devise other forms thereof within the ambit of the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view of a standard guitar comprising the base plate with bipolar magnetic pickups under the strings; 
       FIG. 2  is a rear view of the base plate showing a basic wiring configuration; 
       FIG. 3  is rear view of the neck and tuning knobs of the guitar showing the wiring contacts connected directly to the strings on the tuning knobs; 
       FIG. 4A  is a front view and more detailed view of the connector cord that is positioned below the guitar strings and is for use in connecting to the plug; 
       FIG. 4B  is detailed view of the wiring used to connect to the strings by way of a plug in  4 A; 
       FIG. 5A  is a perspective view of the front face of the guitar showing the bipolar magnets arranged at the lower portion of the base plate in a vertical configuration; 
       FIG. 5B  is a perspective view of the front face of the guitar showing the bipolar magnets arranged at the lower portion of the base plate in a horizontal configuration; 
       FIG. 5C  is a perspective view of the front face of the guitar showing the bipolar magnets arranged at the middle portion of the base plate in a diagonal configuration; 
       FIG. 5D  is a perspective view of the front face of the guitar showing the bipolar magnets arranged at the upper portion of the base plate in a horizontal configuration; 
       FIG. 6A  is a detailed view of the differential amplifier circuitry; 
       FIG. 6B  is a detailed view of the circuitry forming the batteries and switch used to power the guitar circuitry. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   By way of one example of many to serve as background in understanding the present invention,  FIG. 1  shows a standard electric guitar  100 . The guitar  100  has six strings  110  that are connected from one end at the tuning head  120  to an opposite end of a sting securing mechanism  170  across a fret board  105  and a guitar body  115 . The strings  110  are typical guitar strings made of metal and are wound tight at the tuning knobs  315  so that the stings  110  will be in tune when the strings are picked. The strings  110  rest upon a bridge  150 . The bridge  150  is located slightly above the securing mechanism  170 . 
   Directly under the strings  110  and below the fret board  105  sits the base plate  130 . The base plate  130  is typically made from thin, rigid, abrasion resistant plastic designed to fit underneath the stings of most guitars without modification. Attached to the base plate are the volume knob  140 , the power switch  135 , and an output jack  210 . A plastic magnet tray  180  is also shown and positioned on the plate  130  directly below the strings  110  between the fret board  105  and the bridge  150 . The plastic magnet tray  180  is used to hold at least one, but preferably a series of strong bipolar magnets  190 . The bipolar magnets  190  must be positioned below the strings  110  in the preferred embodiment. 
     FIG. 2  shows the details of the bottom the base plate  130 . The bottom of the base pate comprises a thin metal plate  205 , which is of the same general shape as that of the base plate  130 . Attached to the base plate are the volume knob  140 , the power switch  135 , and an output jack  210 , the bottom details of which are shown in the drawings. An electronic circuit board  200  with a ground place containing the electronic circuitry that further comprises a battery  220  and the electronic circuitry as described in  FIGS. 6A and 6B . A shield cable  197  for the strings  110  is wired to a connector jack  210   a  that mates to a connector plug  335  from the string cable, as shown in  FIG. 4A . The base plate  130  is removable from guitar body  115 . If a performer wishes to remove the base plate  130 , he or she can unplug the connector jack  210   a  and remove the base plate  130 . Because there is a metal plate  205  located on the bottom of the plate  130 , the high-powered magnets  190  remain secure and stationary on the magnet tray  180 , which sits directly on top of the plate  130 . The metal plate  205  also acts as a ground plate for the electronic circuitry further reducing noise. 
   As shown in  FIG. 3 , a shielded cable  300 , which contains six conductors  310  must be electrically coupled to the six strings  110  at the metal tuning knobs  315  on the guitar peg head  120 . The cable  300  must be long enough so as to extend across the entire length of the guitar  100 . Most all guitars have a backplate or headplate that is glued to the peg head  120 . Prior to gluing the backplate or headplate, the electrical connection to the tuning knobs  315  will have been made, and then securely covered by the backplate or headplate. Most all of today&#39;s guitars have a slot milled into the neck into which a steel torsion bar is place before the nut  106  and fret board  105  is glued together, as shown in  FIG. 1 . The shielded cable  300  can be run down this slot to the body of the guitar  100  where it will terminate at connector plug  335 . The strings  110  at the bridge  150  must be electrically isolated from each other in a fashion as shown at point  155  in  FIG. 4A . 
     FIG. 4A  shows the strings  110  electrically isolated at the bridge  150  and continuing on through to the securing mechanism  170  at the bottom of the guitar  100 .  FIG. 4B  shows the strings  110  terminating at an individual securing hook  340  for each string. A second set of conducting wires  330  are coupled to the securing hooks  340  which are in turn connected to the strings  110 . The wires  330  are shielded and terminate at the connector plug  335 . 
     FIGS. 5A through 5D  show perspective views of the guitar  100  with the magnets  190  secured on top of a magnet plate  180  in various positions.  FIG. 5A  shows the magnets  190  at the lower portion of the plate  130  with the magnets  190  in the vertical position. In  FIG. 5C , the magnets  190  are at the lower portion of the plate  130  with the magnets  190  in the horizontal position. In  FIG. 5B , the magnets  190  are at the middle portion of the plate  130  with the magnets  190  in the diagonal position, and finally, in  FIG. 5D , the magnets  190  are in the top portion of the plate  130  with the magnets  190  in the horizontal position. It is understood that any shape or sized magnet can be positioned anywhere on the plate  130  as long as they are within a mutual induction range with the metal strings  110 . It is preferred that the magnets  190  be made of Neodymium (NdFeB) at least grade ND40 magnets. These magnets are made from a powder which is compressed into many shapes, i.e. donut, cylinder, bar, sphere, triangle, etc. These different shapes when placed on the plate  130  allow for many unusual tones when the guitar strings  110  are plucked. If the magnets are moved rapidly back and forth, a tremolo effect is produced. When the strings  110  are plucked and there is current running through the metal vibrating strings  110 , induced by the magnets  190 . That signal in the strings  110  is then fed into the circuitry as described in  FIGS. 6A and 6B . 
   As shown in  FIGS. 6A and 6B , each of the strings  110  of the guitar  100  are connected to a plug  335  via a shielded cable  300  and  330 . The plug  335  mates to a jack  210   a  that is connected to the difference amplifier  500 . The amplifier  500  has the desired characteristics of a high common mode rejection, low power, low noise, and good gain in an economical integrated circuit. Preferably, there should be one difference amplifier for each string  110 . Their differential amplifier outputs are electrically summed into a low noise amplifier  510 . The resistor  513  is matched to create even volume for each string  110 . This is necessary since the lower strings  110  vibrate at a lower frequency. Consequently, their signal is less so the resistors  513  get smaller in resistance for the lower strings  110 . In the preferred embodiment, the average value for resistor  513  is 100 ohms. Resistor  511  of summing amplifier  510  is selected to provide more gain to the signals in the strings  110 . A typical value for resistor  511  is 10K ohms. 
   The audio codec  520  is connected to the output of the summing amplifier  510 . The codec  520  functions to anti-alias the string signal, over sample the string signal and convert it from an analog to a digital signal. The codec  520  then sends the digital signal to a digital signal processor  550 . In the preferred embodiment, the codec  520  has at least a signal to noise ratio of 83 db, which is considered very low noise. Its sampling rate is at least 44.1 KHz, which is well faster than any frequency obtained from a vibrating string. The codec  520  will have at least a 16-bit analog to digital conversion of each analog signal sample. The codec  520  will have at least two times over-sampling of the analog signal. 
   The digital signal processor  550  is typically programed to be able to modify selected frequencies by use of an interface  540 . By doing so, many usual and pleasing frequency modifications can be achieved by the digital signal processor  550 . The codec will pass the string frequencies between 20 Hz and 20 kHz, which adequately encompasses guitar frequencies. With the signal to noise ratio at about 80 db, the digital signal processor  550  will be programmed to enhance those frequencies, when amplified, are deemed enjoyable to listen to. Once the digital signal processor  550  has been programmed, its program is stored in permanent memory  530 . The digital signal processor  550  receives the digital signal from the codec  520 . When the signal is received, the digital signal processor  550  processes the digital signal and returns it to the codec  520 . The codec then converts the digital signal back into an analog signal and sends it to the volume control amplifier  560 . The volume control amplifier  560  then feeds its signal to a variable resistor  514 , which serves as the volume control knob  140 . The volume control amplifier  560  is connected as a unity gain-follower amplifier  570 , which provides a very low impedance output to the output jack  210  on the base plate  130 . The low impedance output serves to minimize the noise in the guitar cord that carries the audio signal from the output jack  210  to the typical guitar power amplifier. An on-off power switch  135  with battery  220  and battery filter capacitors  220   a  provides power to the circuit board as shown in  FIG. 6B . Battery voltages will typically be between 1.5 and 5 volts. 
   While the inventive apparatus, as well as a method of cooling ambient air as described and claimed herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiment of the invention and that no limitations are intended to the detail of construction or design herein shown other than as defined in the appended claims. 
   Although the invention has been described in detail with reference to one or more particular preferred embodiments, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow.