WO2011003148A1

WO2011003148A1 - Preamplifier system for stringed musical instruments

Info

Publication number: WO2011003148A1
Application number: PCT/AU2010/000874
Authority: WO
Inventors: Bradley Roy Clark
Original assignee: Australian Native Musical Instruments Pty Ltd
Priority date: 2009-07-10
Filing date: 2010-07-09
Publication date: 2011-01-13

Abstract

A preamplifier system (10) for a stringed musical instrument such as an acoustic guitar. The preamplifier system includes a first input for receiving a signal from an under saddle sensor (14) attached to a bridge portion of the instrument and a second input for receiving a signal from a microphone (32) associated with the instrument and positioned to detect sound waves within, or emanating from, the instrument. A filter and mixing circuit (34, 36, 40) combines the signals from the inputs to form an output signal (42) comprising low frequencies from the under saddle sensor (14) and higher frequencies from the microphone (32). In a preferred embodiment, the preamplifier system (10) includes an additional input for receiving a signal from a sensor (22) attached to a body portion of the instrument. In this embodiment, the filter and mixing circuit combines all three signals with mid-band frequencies from the body sensor.

Description

PREAMPLIFIER SYSTEM FOR STRINGED MUSICAL INSTRUMENTS

FIELD OF THE INVENTION

The present invention relates generally to electronic amplification of acoustic musical instruments, and in particular to the amplification of stringed instruments such as acoustic guitars. It should be understood however that the invention is not restricted to this example application and is intended for broader application and use.

BACKGROUND OF THE INVENTION

An acoustic guitar includes a body or resonant cavity having (i) a front panel, often referred to as the face or soundboard, (ii) a back panel and (iii) side walls extending between the soundboard and back panel. A neck, carrying a finger board, projects from the body, and strings are stretched between a "nut" at the head end of the finger board and a "saddle" supported by a bridge attached to the soundboard of the instrument. The strings oscillate, when plucked or strummed, between the nut and the saddle.

In an acoustic guitar, these oscillations are transmitted mechanically as vibrations to the soundboard of the instrument, and hence to the resonant cavity, including the back panel and side walls. These vibrations are then transmitted to the surrounding air, predominately by the soundboard of the instrument but also by the back panel and side walls, and to some extent also by the strings directly. Air vibrating within the resonant cavity, forming sound waves which reflect internally within the cavity, is also projected from the sound hole of the instrument.. The sound hole is typically, though not always, located in the soundboard below the strings.

The tonal qualities of an acoustic guitar are thus determined by a combination of all of these factors. Amplification or reproduction of an acoustic guitar therefore presents particular difficulties because of the complex interactions between the various factors.

For a live performance, acoustic guitars have generally been amplified using piezoelectric sensors situated between the bridge and the saddle of the instrument, i.e. immediately under the strings. Such sensors will be referred to herein as "under saddle sensors", with systems using these sensors being referred to as "under saddle systems". The response achieved by under saddle systems is predominately the reproduction of vibrations of the strings according to how they are stretched between the nut and the saddle of the instrument, and of course the performance or playing of the instrument. The overall structure of the instrument affects the manner in which the strings vibrate and therefore the sound produced. Similar considerations thus apply to conventional "electrified" acoustic guitars as apply to "electric" guitars, which may be solid and may not have a resonant cavity.

For an acoustic guitar, however, it is the soundboard or face of the instrument, the back and then the sides that are predominantly heard "acoustically", i.e. without piezoelectric sensors or without power applied to them. These components of the resonant cavity vibrate in sympathy with the strings and, in turn, cause vibrations, in the form of sound waves, in the air within and surrounding the body of the instrument. Sound waves from within the resonant cavity also emanate from the sound hole of the instrument and add to the "texture" or timbre of the sound actually heard acoustically. The overall sound or "character" of the instrument thus arises as a consequence of the construction of the instrument.

Whilst most musicians and recording engineers agree that systems using under saddle piezoelectric sensors deliver a palatable result, it is a very different sound to that produced by the guitar when heard acoustically.

Using a microphone to amplify an acoustic guitar produces a more natural or realistic sound than is possible using an under saddle system, but this method of reproduction also has inherent limitations, particularly in a real performance space. Sources of sound other than the intended source, being the guitar, are also detected by the microphone, and may also include the reproduced sound of the guitar from the sound reinforcement system. This effect often causes feedback during live performances such that microphones cannot generally be used in this situation.

The preset inventor has previously recognised that the sound produced using under saddle systems is not consistent with the "natural sound" of an acoustic guitar (as heard acoustically) because an under saddle sensor is not in intimate contact with the other resonant components of the instrument. In his earlier patent application, published as WO2005/00181 1 (and granted in the United States as Patent No US 7,271 ,331 ) and incorporated herein by cross reference, the inventor proposed a system in which a combination of sensors was used to detect different parts of the audio frequency spectrum from different parts of an instrument.

Put briefly, in the inventor's earlier system, lower frequencies were taken from an under saddle sensor and higher frequencies were taken from a sensor attached to the soundboard of the instrument. This was achieved by filtering, or

"rolling off", the higher frequencies from the under saddle sensor and, conversely, filtering the lower frequencies from the soundboard sensor so that only the "top end", or frequencies not produced by the under saddle sensor, were transmitted.

A crossover circuit was employed to mix the two desirable frequency ranges to produce a substantially uniform combined frequency response. Thus, the under saddle sensor reproduced only the lower, or "bottom end", frequencies whilst the soundboard sensor reproduced only the higher, or "top end", frequencies.

This system provided a substantial improvement over the state of the art and produced a sound which was the most realistic to an instrument's "natural" or "acoustic performance" available to date.

Nevertheless, the inventor felt that the amplified sound could be further improved, as it still could not match a microphone to fully capturing the true character of an acoustic instrument. The use of high quality microphones thus remains the performance standard in recording studios, where extraneous sources of sound can be controlled, but has not to date been feasible in a live performance situation where substantial volumes are often required. Feedback in public address (PA) systems has been an inhibiting factor.

Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia or any other country on or before the priority date of the claims herein.

SUMMARY OF THE INVENTION

The present invention is based in part on a realisation that the highest, "top end" frequencies generated by an acoustic instrument, such as a guitar, convey important aspects of the character of the instrument. This realisation lead the inventor to think that a microphone must still be used to capture these frequencies. But then feedback becomes a serious problem in a live performance space.

In the latter regard, the inventor recognised that some mid-band and lower end frequencies are substantially more prone to feedback than higher, top end frequencies. This recognition lead the inventor to the proposition that a microphone should be used only for top end frequencies, together with an under saddle sensor, and/or a sensor attached to the soundboard (or some other body part of the instrument), for lower and mid-band frequencies.

In one embodiment, low frequencies are detected by an under saddle sensor and higher frequencies are detected by a microphone placed in or near the instrument. Appropriate filter and mixing circuits are used to produce a combined output signal.

In another embodiment, low frequencies are detected by an under saddle sensor, mid-band frequencies are detected by a sensor attached to the soundboard of the instrument and the highest frequencies are detected by a microphone placed in or near the instrument. Once again, appropriate filter and mixing circuits are used to produce a combined output signal.

More specifically, one aspect of the present invention provides a preamplifier system for a stringed musical instrument, the system comprising: a first input for receiving a signal from an under saddle sensor attached to a bridge portion of the instrument;

a second input for receiving a signal from a microphone associated with the instrument and positioned to detect sound waves within, or emanating from, the instrument; and

a filter and mixing circuit for combining the signals from the inputs to form an output signal comprising low frequencies from the under saddle sensor and higher frequencies from the microphone.

According to another aspect of the present invention there is provided a preamplifier system for a stringed musical instrument, the system comprising: a first input for receiving a signal from an under saddle sensor attached to a bridge portion of the instrument; a second input for receiving a signal from a sensor attached to a body portion of the instrument;

a third input for receiving a signal from a microphone associated with the instrument and positioned to detect sound waves within, or emanating from, the instrument; and

a filter and mixing circuit for combining the signals from the three inputs to form an output signal comprising low frequencies from the under saddle sensor, mid-band frequencies from the body sensor and high frequencies from the microphone.

According to a further aspect of the present invention there is provided a preamplifier system for a stringed musical instrument, the system comprising: a first input for receiving a signal from a sensor attached to the instrument; a second input for receiving a signal from a microphone associated with the instrument and positioned to detect sound waves within, or emanating from, the instrument;

a low pass filter for passing signal components from the first sensor below a first frequency;

a high pass filter for passing signal components from the microphone above a second frequency; and

a mixing circuit for combining the signals passed by the low pass filter and the high pass filter to form an output signal.

According to a still further aspect of the present invention there is provided a preamplifier system for a stringed musical instrument, the system comprising: a first input for receiving a signal from an under saddle sensor attached to a bridge portion of the instrument;

a second input for receiving a signal from a microphone associated with the instrument and positioned to detect sound waves within, or emanating from, the instrument;

a variable low pass filter for passing signal components from the under saddle sensor below a first frequency and for variably attenuating remaining signal components above the first frequency;

a high pass filter for passing signal components from the microphone above a second frequency; a control circuit for enabling a user to simultaneously vary the level of the signal components above the first frequency which are passed by the low pass filter and vary the level of the signal passed by the high pass filter; and

a second input for receiving a signal from a sensor attached a body portion of the instrument

a third input for receiving a signal from a microphone associated with the instrument and positioned to detect sound waves within, or emanating from, the instrument;

a first variable low pass filter for passing signal components from the under saddle sensor below a first frequency and for variably attenuating remaining signal components above the first frequency;

a first high pass filter for passing signal components from the body sensor above a second frequency;

a first control circuit for enabling a user to simultaneously vary the level of signal components above the first frequency which are passed by the first low pass filter and vary the level of the signal passed by the first high pass filter ; a first mixing circuit for combining the signals passed by the first low pass filter and the first high pass filter to form an intermediate signal;

a second variable low pass filter for passing signal components of the intermediate signal below a third frequency and for variably attenuating remaining signal components of the intermediate signal above the third frequency;

a second high pass filter for passing signal components from the microphone above a fourth frequency;

a second control circuit for enabling a user to simultaneously vary the level of signal components above the third frequency which are passed by the second low pass filter and vary the level of the signal passed by the second high pass filter; and a second mixing circuit for combining the signals passed by the second low pass filter and the second high pass filter to form an output signal.

Further preferred features of the preamplifier system may be as defined in dependent claims 3, 7, 8 and 1 1 to 18, these claims being incorporated into this disclosure by cross reference.

Finally, the present invention also provides an acoustic guitar including a preamplifier system as described above.

Comprises/comprising and grammatical variations thereof when used in this specification are to be taken to specify the presence of stated features, integers, steps or components or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

To assist the further understanding of the invention, reference is now made to the accompanying drawings which illustrate preferred embodiments. It is to be appreciated, however, that these embodiments are given by way of illustration only and the invention is not to be limited by this illustration.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Figure 1 A shows a graphic illustration of a frequency response curve for a preamplifier having a conventional under saddle sensor;

Figure 1 B shows a graphic illustration of frequency response curves for a mixing preamplifier system as described in WO2005/00181 1 and having two sensors;

Figure 1 C shows a graphic illustration of frequency response curves for a three-way preamplifier system in accordance with a preferred embodiment of the present invention;

Figure 1 D shows a graphic illustration of frequency response curves for an alternative two-way preamplifier system in accordance with another embodiment of the invention;

Figure 2 shows a functional block diagram of a three-way preamplifier system in accordance with a preferred embodiment of the invention; Figures 3A to 3H show a number of charts displaying frequency response curves representing actual measurements for a prototype of the preamplifier system shown in Figure 2; and

Figure 4 shows a functional block diagram of a two-way preamplifier system in accordance with an alternative embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring initially to Figures 1 A to 1 D of the accompanying drawings, there are shown a number of graphic illustrations of frequency response curves for two prior art preamplifier systems and two embodiments of systems made in accordance with the present invention. Whilst these curves are somewhat idealised, they help to illustrate the fundamental principles of the present invention.

In particular, Figure 1 A illustrates a prior art preamplifier output in which the entire output 1 is take from an under saddle sensor, such as a piezoelectric sensor. Figure 1 B illustrates the output of a preamplifier system in accordance with the inventor's previous design as described in WO2005/00181 1 . In this system, lower frequencies 2 (below about 350 Hz) are taken from an under saddle sensor and higher frequencies 3 are taken from a sensor attached to the soundboard of the instrument. The soundboard sensor may be a "Face Brace Sensor" as described in the inventor's earlier patent publication number WO2007/025330, or a sensor of any other suitable type.

Figure 1 C graphically illustrates the output of a three-way preamplifier system made in accordance with a preferred embodiment of the present invention. In this system, low frequencies 4 are taken from an under saddle sensor, mid band frequencies 5 are taken from a face brace sensor attached to the soundboard of the instrument, and high frequencies 6 are taken from a microphone mounted in or on the instrument. It can be seen that, with appropriate filtering, mixing and gain control for each sensor/microphone, a substantially uniform overall response is achieved.

Finally, Figure 1 D shows a slightly simplified form of preamplifier system in accordance with another embodiment the present invention in which an under saddle sensor is used for the low frequencies 7 and a microphone is used for mid-band and high frequencies 8. Once again, these two inputs are filtered and mixed to produce a substantially uniform output, with the crossover frequency between the under saddle sensor and the microphone being chosen to achieve the most natural overall sound possible from this combination.

Referring now to Figure 2, there is shown a three-way preamplifier system 10 for amplifying (or recording) a stringed musical instrument, such as an acoustic guitar 12 in accordance with a preferred embodiment of the present invention. The system includes a first input for an under saddle sensor, or "bridge" sensor, 14 (which may be of a conventional type) connected to a buffer and pre-emphasis circuit 16. The circuit 16 also provides, in this embodiment, power gain and low frequency filtering below about 60 Hz, and some roll off above 10-15 kHz. The signal is then fed to a first variable low pass filter 18, the cut off frequency of which is variable between a lower limit fi of about 350 Hz and an upper limit of about 20 kHz. However, this frequency range may be different, depending upon the overall output required and the slope of the filter employed. In this embodiment, the filter 18 passes signal components from the bridge sensor 14 below the 350 Hz lowest cut off frequency fi and variably attenuates remaining signal components above that frequency. In effect, the level of the signal beyond 350 Hz has adjustable attenuation controlled by the first gang of a dual gang potentiometer POT-1 of the control circuit 28 described below. The output of the low pass filter 18 is then fed to a mixing circuit 20.

The preamplifier system 10 also includes a second input for a soundboard sensor, or "face" sensor, 22 connected to a first high pass filter 24. In this embodiment, the cut off frequency f₂ of the high pass filter 24 is also set at about 350Hz, corresponding to the lowest cut off frequency U of the first low pass filter 18. The high pass filter 24 passes signal components from the face sensor 22 above the 350 Hz cut off frequency. A passive treble roll-off filter 26 set at about 10-15 kHz may also be provided to restrict the highest frequency passed to the mixing circuit 20.

A first control circuit 28 including the dual gang potentiometer POT-1 is provided to control the blend of signals from the bridge sensor 14 and the face sensor 22. The potentiometer may be of a rotary type or of a linear slider type, as is often used for electric guitar tone controls. In this instance, the dual gang potentiometer POT-1 includes two elements 28' and 28" to enable a user to simultaneously vary, in the opposite sense, the level of signal components above 350 Hz which are passed by the first low pass filter 18 and vary the level of the signal passed from the first high pass filter 24. These signals, namely the low frequency signals from the bridge sensor 14 and the higher frequency signals from the face sensor 22, are then combined in the first mixing circuit 20 to form an intermediate signal 30.

The preamplifier system 10 also includes a third input for receiving a signal from a microphone 32 associated with the guitar 12 and positioned to detect sound waves within, or emanating from, a body portion of the instrument. For example, the microphone 32 may be positioned within the sound hole of the guitar 12 or within its resonant cavity. The microphone 32 is connected to a second high pass filter 34 which, in this embodiment, has a cut off frequency U of 2.5kHz. This cut off frequency is selected to be near to the 2 kHz lower cut off frequency f₃ of the second low pass filter 36 described below. However, the selection of an appropriate frequency would depend on the type of microphone used (for example, an electret FET microphone in this embodiment) and the slope of the high pass filter 34. As with the other inputs, the high pass filter 34 may also include some roll off above 10-15 kHz.

A second low pass filter 36 is connected to the output of the first mixing circuit 20 so as to receive the intermediate signal 30 representing the face plus bridge signals. This low pass filter 36 passes signal components of the intermediate signal 30 below a cut off frequency which is variable between a lower limit f₃ of about 2 kHz and an upper limit of about 20 kHz. In effect, the second low pass filter 36 variably attenuates remaining signal components above 2 kHz. Thus, the variable operation of the second low pass filter 36 is similar to that of the first low pass filter 18. The output of the second low pass filter 36 accordingly includes low frequency components from the bridge sensor 14 and mid band frequency components from the face sensor 22. Frequencies above 2 kHz are variably attenuated by the second low pass filter 36.

A second control circuit 38 and second mixing circuit 40 are then used to blend the signal from the microphone 32 with the intermediate signal 30. Similar to the first control circuit 28, the second control circuit 38 includes a dual gang potentiometer POT-2. Again, this potentiometer includes two elements 38'and 38" to enable the user to simultaneously vary, in the opposite sense, the level of signal components above 2 kHz passed by the second low pass filter 36 (from the bridge and face) and vary the level of the signal passed from the second high pass filter 34 (from the microphone). The mixing circuit 40 combines the signals to form an output signal 42 which includes low frequencies from the bridge sensor 14, mid band frequencies from the face sensor 22 and high frequencies from the microphone 32.

It will be appreciated that the cut off frequencies fi to f₄ of all of the filters may be selected to produce a substantially uniform overall response in the output signal 42. The selection would need to take into account the type of microphone and sensors used, and the slopes of the various high pass and low pass filters.

The system shown in Figure 2 also includes tone controls 44 including base, mid and treble controls, however these tone controls are optional. Similarly, a gain control 46 is shown in Figure 1 but, once again, this control is optional. The output signal is then provided at an output jack 48 which would be typically provided in a side wall of the guitar 12.

Details of the variable low pass filters 18 and 36 and the high pass filters 24 and 34 have been omitted from this description because it is considered to be well within the ability of any reasonably skilled person in the art to construct these filters using conventional techniques based on the information provided in Figure 2 and the frequency response curves provided in Figures 3A to 3H described below. Similarly, it is considered well within the ability of any reasonably skilled person in the art to construct suitable mixing circuits 20 and 40 for combining the signals as required. Thus, these circuits do not need to be described herein in detail.

Turning now to Figures 3A to 3H, there are shown a number of frequency response curves for varying blends of signals from each of the three inputs. Figures 3A to 3D show the effect of varying the position of POT-1 of the first control circuit 28 whilst Figures 3E to 3H show the effect of varying the position of POT-2 of the second control circuit 38.

More specifically, referring to Figure 3A to 3D, a first curve 50 represents the signal from the bridge sensor 14 at the output of the first low pass filter 18. A second curve 52 represents a signal derived from the face sensor 22 following the high pass filter 24 and attenuation via the element 28" of the potentiometer POT-1 . A third curve 30 represents the intermediate signal at the output of the first mixing circuit 20. .

It can be seen that as the potentiometer POT-1 is panned from maximum bridge (Figure 3A) to maximum face (Figure 3D) the level of the bridge signal 50 above a frequency of about 350 Hz gradually reduces, whilst the overall level of the face signal 52 increases. These actions occur simultaneously such that the intermediate signal 30 at the output of the first mixing circuit 20 remains at a substantially constant level throughout the frequency range of interest (approximately 60 Hz to 20 kHz). Throughout Figures 3A to 3D the position of POT-2 of the second mixing circuit has been kept constant.

Referring now to Figures 3E to 3H, a first curve 54 represents a signal derived from the microphone 32 following the high pass filter 34 and attenuation via element 38" of POT-2 in the second mixing circuit 38. A second curve 56 represents the signal appearing at the output of the second low pass filter 36 which is derived from the intermediate signal 30 appearing at the output of the first mixing circuit 20. A third curve 42 represents the combined output signal following the second mixing circuit 40.

It can be seen that as POT-2 is panned from maximum "bridge plus face" (Figure 3E) to maximum microphone (Figure 3H) the level of signal 56 above a frequency of about 2kHz gradually reduces, whilst the overall level of the signal 54 derived from the microphone gradually increases. These actions occur simultaneously such that the output signal 42 from the second mixing circuit 40 is maintained at a substantially constant level throughout the frequency range.

Through appropriate manipulation of the two potentiometers, POT-1 and

POT-2, a musician can select any desired blend of signals from the bridge sensor, the face sensor or the microphone. Overall, it will be appreciated that the output signal produced by the preamplifier system shown in Figure 2 reflects a mix of the input signals approximately as shown in Figure 1 C.

Figure 4 shows an alternative, somewhat simplified embodiment of the present invention in which a bridge sensor 14 is combined with a microphone 32. Similar reference numerals are used in this Figure to denote equivalent components shown in Figure 2. Thus, it is unnecessary to describe Figure 4 in detail. It will be appreciated that the signal components from the bridge sensor 14 and microphone 32 are combined in a manner similar to that shown in Figure 1 D.

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention. Such variations are considered to be within the scope of the appended claims.

Claims

1. A preamplifier system for a stringed musical instrument, the system comprising:

a first input for receiving a signal from an under saddle sensor attached to a bridge portion of the instrument;

2. A preamplifier system for a stringed musical instrument, the system comprising:

a second input for receiving a signal from a sensor attached to a body portion of the instrument;

3. A preamplifier system according to claim 1 or claim 2 wherein the frequency ranges are selected to provide a substantially uniform overall response in the output signal.

4. A preamplifier system according to any one of the preceding claims, further comprising a control circuit for enabling a user to vary the levels and/or frequency ranges of each input signal appearing in the output signal.

5. A preamplifier system for a stringed musical instrument, the system comprising:

a first input for receiving a signal from a sensor attached to the instrument; a second input for receiving a signal from a microphone associated with the instrument and positioned to detect sound waves within, or emanating from, the instrument;

6. A preamplifier system according to claim 5 wherein the first sensor is an under saddle sensor.

7. A preamplifier system according to claim 5 or claim 6 wherein the first and second frequencies are selected to provide a substantially uniform overall response in the output signal.

8. A preamplifier system according to any one of claims 5 to 7, further comprising a control circuit for enabling a user to vary the levels and/or the frequency ranges of the signal components passed by the low pass filter and the high pass filter.

9. A preamplifier system for a stringed musical instrument, the system comprising:

a second input for receiving a signal from a microphone associated with the instrument and positioned to detect sound waves within, or emanating from, the instrument; a variable low pass filter for passing signal components from the under saddle sensor below a first frequency and for variably attenuating remaining signal components above the first frequency;

a high pass filter for passing signal components from the microphone above a second frequency;

a control circuit for enabling a user to simultaneously vary the level of the signal components above the first frequency which are passed by the low pass filter and vary the level of the signal passed by the high pass filter; and

10. A preamplifier system for a stringed musical instrument, the system comprising:

a first high pass filter) for passing signal components from the body sensor above a second frequency;

a first control circuit ( for enabling a user to simultaneously vary the level of signal components above the first frequency which are passed by the first low pass filter and vary the level of the signal passed by the first high pass filter; a first mixing circuit for combining the signals passed by the first low pass filter and the first high pass filter to form an intermediate signal;

a second variable low pass filter for passing signal components of the intermediate signal below a third frequency and for variably attenuating remaining signal components of the intermediate signal above the third frequency; a second high pass filter for passing signal components from the microphone above a fourth frequency;

a second control circuit for enabling a user to simultaneously vary the level of signal components above the third frequency which are passed by the second low pass filter and vary the level of the signal passed by the second high pass filter; and

a second mixing circuit for combining the signals passed by the second low pass filter and the second high pass filter to form an output signal.

1 1 . A preamplifier system according to claim 10 wherein:

the first frequency defines a lowest cut off frequency of the first variable low pass filter;

the second frequency defines a cut off frequency of the first high pass filter;

the third frequency defining a lowest cut off frequency of the second variable low pass filter; and

the fourth frequency defines a cut off frequency of the second high pass filter.

12. A preamplifier system according to claim 10 or claim 11 wherein the first to fourth frequencies are selected to provide a substantially uniform overall response in the output signal.

13. A preamplifier system according to any one of claims 10 to 12 wherein: the first control circuit includes a first attenuator for varying the level of the body sensor signal passed from the first high pass filter to the first mixing circuit; and

the second control circuit includes a second attenuator for varying the level of the microphone signal passed from the second high pass filter to the second mixing circuit.

14. A preamplifier system according to any one of claims 10 to 13 wherein the first frequency is within the range of 200 Hz to 3kHz, and is preferably about 350Hz.

15. A preamplifier system according to any one of claims 10 to 14 wherein the second frequency is within the range of 200 Hz to 3kHz and is preferably about 350Hz.

16. A system according to any one of claims 10 to 15 wherein the third frequency is within the range of 1 kHz to 7 kHz, and is preferably about 2.0 kHz.

17. A preamplifier system according to any one of claims 10 to 16 wherein the fourth frequency is within the range of 1 kHz to 7 kHz, and is preferably about 2.5 kHz.

18. A preamplifier system according to any one of claims 10 to 17, further comprising an under saddle sensor, a second sensor configured to be attached to an internal surface of a soundboard of the instrument, and a microphone configured for mounting within or on the instrument.

19. An acoustic guitar comprising:

an under saddle sensor;

a second sensor attached to a soundboard of the guitar;

a microphone mounted within the guitar and positioned to detect sound waves within, or emanating from, the guitar; and

a preamplifier system as defined in any one of the preceding claims wherein the under saddle sensor is connected to the first input, the soundboard sensor is connected to the second input, and the microphone is connected to the third input.