GB2256539A - Audio signal cable incorporating audio buffer - Google Patents

Description

AUDIO SIGNAL CABLE The present invention relates to cables for use in the transmission of audio signals and particularly, but not exclusively to cables for connecting electronic musical instruments (such as electric guitars) to amplifiers. The present invention also relates to electrical connectors for use in such cables.

In an electric guitar, the transducers which convert the vibration of the stings to an electrical signal typically have a high output impedence. The capacitance of the cable used to connect such an electrical guitar to the input of an audio device (typically an amplifier or an effects pedal or unit), which capacitance is a function of the cable length and type, attenuates the high frequency harmonics produced by the instrument, resulting in a dull tone.

In the past, it has been known to counteract this problem by fitting active electronic circuitry inside the instrument to buffer the signal. Fitting such circuitry however requires that the instruments are modified. Such modifications tend to be costly and, in many cases, undesirable.

In accordance with the present invention there is provided an auoLo signal lead having signal and ground paths and comprising an signal connector and an output signal connector at respective ends of a coaxial cable, wherein the input signal connector includes buffer circuitry in the signal path, the buffer circuitry having a signal input connected to a contact of the input signal connector and an output connected to a conductor of the coaxial cable.

The present invention also provides an electrical connector compris ng first and second electrical contacts, buffer circ cry connected to receive a signal applied to the first contact, rears for connecting an inner conductor of a coaxial cable to the output of the buffer circuitry and means for connecting the outer conductor of the coaxial cable to the second electrical contact.

The present invention also provides an audio system comprising in combination an electronic musical instrument operable to output an electrical signal, an audio device, and an audio signal lead having an input signal connector connected to receive a signal output by the instrument, an output signal connector connected to a signal input of the audio device, and a length of coaxial cable providing signal and ground paths between the input signal and output signal connectors, wherein the input signal connector includes buffer circuitry having an input connected to receive the signal output by the instrument and an output connected to a conductor of the coaxial cable.The audio device to the signal input of which the output signal connector of the cable is connected may be an amplifier, a musical effects unit, or other audio signal processing unit.

The buffer circuitry may consist of active electronic circuitry such as an amplifier of fixed or variable gain or a filter circuit.

The buffer circuitry 'nay be powered by an external source of voltage potential such as a dry cell. Such a source of voltage potential may be contained within the output signal connector or the output signal connector may be provided with means for connection to an external source of voltage potential. The coaxial cable may be a single core cable. The source of voltage potential may be connected to provide a DC voltage between the signal and ground paths. The coaxial cable may be a twin screened cable with the source of voltage potential being connected between the screen and one of conductors, the other of the conductors providing the signal path.

The input signal connector may be configured as a known type of audio signal connector, such as a jack plug.

Preferred embx diments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of a first configuration of audio signal lead embodying the present invention; Figure 2 is a circuit diagram of a second form of audio signal lead embodying the present invention; Figure 3 shows a modified form of the audio signal lead of Fig. 2; and Figure 4 schematically illustrates an audio system embodying the present invention.

In the following description, those features which are comrron to more than one embodiment are indicated by the same reference numeral in the description of each embodiment.

Referring initially to Figure 1, a first configuration of audio signal lead comprises an input signal connector such as a jack plug, a length of twin screened cable, and an output connector, indicated generally in Figure 1 by the letters A, B and C. The input signal connector A has signal and ground contacts 10, 12 between which is connected a resistor R1. Resistcr RI suitably has a value of 1 rnegohm to provide a high input impedence. The signal contact is connected to the gate of an n-chesnnel field effect transistor (FET) T1, such as a type 2N3819 n-channel planar silicon BEST. Input impedence resistor R1 provides protection against static damage to the BET Tl.

The source of FET T1 is connected to one of the conductors 14 of the twin screened cable B. The sheath 16 of the twin screened cable B is connected to the ground contact 12 of the input Signal connector. A second resistor R2 is connected between the source of the BET T1 and the ground contact 12. Resistor R2, having a typical value of 47 kiloohms, lowers the output impedence of BET T1.

The output signal connector C comprises signal and ground contacts 18, 20. The ground contact, 20 is connected to the sheath 16 of the twin screened cable. The signal contact 18 is connected to the first conductor 14 of the twin screened cable through a DC voltage component decoupling capacitor C1, having a typical value of 0.1 micro farads. The output signal connector further comprises a 9 volt dry cell battery 22 having its negative terminal connected to the ground contact 20 and its positive terminal connected to the second conductor 24 of the twin screened cable B.

Where the cable B is several metres or more in length, it is necessary to provide a lower value for resistor R2 to lower the output impedance of FET T1. This however results in an increased supply current demanded of the dry cell battery 22. Accordingly, the value of resistor R2 should be selected to allow the buffer circuitry to drive the cable whilst irtinimising the required supply current. A switch (not shown) may be provided at the output signal connector for disconnection of the dry cell battery 22 when the audio signal lead is not in use. In the alternative, the output signal connector C may be provided with means for connection to an external source of D.C. voltage potential such that increased supply current to the buffer circuitry may be accodated allowing the choice of value for R2 to be dictated only by the recuirenent to lower the output impedance of FET T1. The external D.C. source may be provided from within an audio device to which the output of the audio signal lead is connected or it may be a separate regulated rectified mains source.

At the input signal connector A, the second conductor 24 of the twin screened cable B is connected, via a ferrite bead LI, to the drain of FET T1. The drain of BET T1 is also connected, through a 100 picofarad capacitor C2 to the ground contact 12. Ferrite bead L1 and capacitor C2 act to ground any short wave radio frequency signals picked up by the second conductor 24.

A second form of audio signal lead, shown in Figure 2, uses a technique known as phantom powering in which a single screened cable B' connects the input and output signal connectors A', C'.

The voltage potential from the dry cell battery 22 is supplied to the buffer circuitry in the input signal connector A', through a load resistor R3 in the output signal connector C', as a DC voltage component on the signal line 26.

In the input signal connector A' the connection of FET T1 and resistors R1, R2 to the signal 10 and ground 12 contacts is as for the lead of Figure 1 although a lower value for R2 such as 15 kiloohms is used. Similarly, the connection of ferrite bead L1 and capacitor C2 to the drain of FET T1 is as for the lead of Figure 1.

The value of C2 is chosen to bypass to ground very high frequency signals, outside the audible range, picked up by the cable. The other side of ferrite bead L1 is connected to the conductor 26 of the single screened cable B'.

The gain of the buffer circuitry in the input signal connector ' is given approximately by R3/R2. Hence, if R2=15 kiloohms and R3=33 kiloohms, the buffer will have a gain of approximately tvo.

A modification of the phantom powered audio signal lead of Figure 2, providing variable gain, is shown in Figure 3. The input signal connector A' and single screened cable B' are as for the lead of Figure 2, and the output signal connector C" is substantially identical save in that fixed resistor R3 is replaced by a potentiometer R3V. The output signal contact 18 is connected, via DC decoupling capacitor C1, to the wiper contact of the potentiometer R3V and the track contacts are respectively connected to the positive terminal of the dry cell battery 22 and the conductor 26 of the single screened cable B'.

An audio system, comprising an electronic musical instrument 30 (such as an electric guitar) operable to output an electrical signal, an audio device 32 (such as an amplifier) and an audio signal lead A, B, C having a buffered input signal connector A, is shown schematically in Figure 4. An arrow 34 indicates the direction of travel of the signal. The audio signal lead A, B, C, which may be configured as one of the audio signal leads described above with reference to Figures 1 to 3, has the power supply 36 for the input signal connector buffer circuitry 38 located in the output signal (amplifier) connector C in order to reduce the size and weight of the input signal connector A.

Claims (17)

1. An audio signal lead having signal and ground paths and comprising an input signal connector and an output signal connector at respective ends of a coaxial cable, wherein the input signal connector includes buffer circuitry in the signal path, the buffer circuitry having a signal input connected to a contact of the input signal connector and an output connected to a conductor of the coaxial cable.

2. Apparatus according to claim 1, in which the output signal connector includes a source of voltage potential and the buffer circuitry in the input signal connector is powered by the said source.

3. Apparatus according to claim 2, in which the source of voltage potential is connected to provide a d.c voltage potential between the signal and ground paths.

4. Apparatus according to claim 2, in which the coaxial cable is a twin screened cable and in which the source of voltage potential is connected to provide a d. c. voltage potential between the screen and one of the conductors, the other of the conductors providing the signal path.

5. Apparatus according to claim 1, in which the output signal connector includes means for connection to a source of d.c. voltage potential to provide a d. c. voltage potential between the signal and ground paths, and the buffer circuitry in the input signal connector is powered by the said voltage potential.

6. Apparatus according to claim 1, in which the coaxial cable is a twin screened cable and in which the output signal connector includes means for connection to a source of d. c. voltage potential to provide a d. c. voltage potential between the screen and one of the conductors, the other of the conductors providing the signal path, and the buffer circuitry in the input signal connector is powered by the said voltage potential.

7. Apparatus according to any of claims 1 to 6, in which the input signal connector is a jack plug.

8. An electrical connector comprising first and second electrical contacts, buffer circuitry connected to receive a signal applied to the first contact, means for connecting an inner conductor of a coaxial cable to the output of the buffer circuitry and means for connecting the outer conductor of the coaxial cable to the second electrical contact.

9. Apparatus according to claim 8, in which the electrical connector is a jack plug.

10. Apparatus according to any of claims 1 to 7, in which the input signal connector is a connector according to claim 8.

11. An audio system comprising in combination an electronic musical instrument operable to output an electrical signal, an audio device, and an audio signal lead having an input signal connector connected to receive a signal output by the instrument, an output signal connector connected to a signal input of the audio device, and a length of coaxial cable providing signal and gradnd paths between the input signal and output signal connectors, wherein the input signal connector includes buffer circuitry having an input connected to receive the signal output by the instrument and an output connected to a conductor of the coaxial cable.

12. An audio system according to claim 11, in which the audio device is an amplifier.

13. An audio system according to claim 11, which the audio device is a music effects unit.

14. An audio system according to any of claims 11 to 13, in which the audio signal lead is an audio signal lead according to any of claims 1 to 7.

15. An audio signal lead substantially as hereinbefore described with reference to any of Figures 1 to 3 of the accompanying drawings.

16. An electrical connector substantially as hereinbefore described with reference to any of Figures 1 to 3 of the accompanying drawings.

17. An audio system substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.