1. Field of the Invention
The low capacitance audio connector for use on audio cables, relates to the transmission of audio information from a source (typically a guitar or musical instrument) to a sink (typically an audio amplifier) with reduced high frequency rolloff attributable to the capacitance of the connector.
2. Description of the Prior Art
An audio connector for use on an audio cable is intended to allow easy, reliable and rapid connection of an audio cable to a musical instrument or amplifier. Audio cables are fitted with connectors at each of two ends. Typical audio connectors have a coaxial construction, with a central single signal conductor surrounded by a tubular shield or ground conductor. Such connectors have capacitance between the signal conductor and shield or ground conductor. These connectors contribute capacitance to the overall capacitance of the cable assembly. For high impedance audio sources, this capacitance acts to degrade the high frequency content of the signal.
Many applications require low capacitance connectors. For example, test and measurement applications are sometimes at risk of fouled measurements due to high capacitance connectors. As a remedy, innovations have been made in the physical design of connectors. Cook (U.S. Pat. No. 7,387,531, Jun. 17, 2008) discloses a universal coaxial connector, and its features: “For many of these and other types of cable, small changes in capacitance/impedance from the connector can often cause significant changes in return loss measurements for the cable. These and other errors are minimized by various aspects of the connector 2, such as the gripping barrel 12, the drain wire 22 and the conductive disk 24, which alone and/or in combination with other features help to reduce stray and/or parasitic capacitance that could otherwise lead to measurement errors.” The advantages stated are a result of optimized physical design of the connector. Such optimizations can reduce the capacitance of a connector only so much.
The telecommunications connector described in Kjeldahl, et al. (U.S. Pat. No. 6,102,730, Aug. 15, 2000) illustrates clearly a problem that is suggested by Cook, above. The problem is that the size of the connector influences greatly the parasitic capacitance of the connector: The smaller the connector, generally the larger the capacitance. Kjeldahl, et al. states, “ . . . it is a desire that the connector be as small as possible, and this, of course, accentuates the capacitive coupling problem because the required small dimensions result in a small distance between the leads of the connector elements and thus a relatively high capacity between these leads.” Separating the stated dependency between the capacitance of a connector and its physical geometry is highly desirable.
The interelectrode capacitance of an audio connector is typically small, on the order of 15 picofarads. This is negligible for some applications. However, in the case where the audio cable itself is operating under a capacitance reduction scheme, such as a driven shield arrangement, the capacitance of a connector at each end of the cable comprises the majority of the capacitance of the entire assembly.
Whereas driven shield arrangements are well known in the prior art as a method of capacitance mitigation, the prior art ignores the capacitance of connectors in audio applications as being negligible. Therefore one finds the prior art devoid of audio connectors specifically designed to participate in driven shield capacitance reduction methods.
Such a driven shield arrangement as applied to a cable requires three conductors, typically in a triaxial configuration. A center conductor carries the signal of interest. A second conductor is arranged as a shield around the center conductor, separated by a first dielectric. An optional semi-conductive layer situated around the outer surface of the first dielectric helps to reduce noise caused by mechanical motion of the cable's components (not shown in the figures). A third conductor is typically arranged as an additional shield, situated around the second conductor shield, separated by a second dielectric as well, though the third conductor could be a single wire insulated from the second conductor shield. The second conductor functions as a driven shield and is connected to the output of a unity gain amplifier, or more generally a transfer function of equal to or less than unity gain, whose input is connected to the center conductor. The ground reference is the third conductor.
(Note that the noninverting unity gain amplifier effectively has it's output and input coupled together through the capacitance in the audio cable. While technically a unity gain amplifier would oscillate under these conditions, in practicality a unity gain amplifier sees a loop gain slightly less than one due to imperfections in the system, such as conductor resistance and a finite amplifier output impedance, so that the system does not oscillate. Please note that while the term “unity gain” is used herein, it should always be understood that the loop gain must be less than one to ensure no oscillations will occur.)
The connectors on a reduced capacitance cable, which uses the driven shield technique, are in the prior art either a) two conductor connectors, which do not participate in the driven shield capacitance reduction happening along the length of the cable, thus adding some parasitic capacitance of their own, or b) three-conductor connectors which carry the driven shield through the connector, having their capacitance reduced, but exposing the driven shield signal to the outside world.
As an example of the former, Dunseath, Jr. (U.S. Pat. No. 4,751,471, Jun. 14, 1988) discloses a driven shield cable with a connector: “As shown in
An example of the latter three-conductor device is a standard triaxial cable connector, such as that marketed by Pomona Electronics as the model 5056 male connector. This connector has three conductors and can participate in the driven shield capacitance reduction technique. However, the driven shield signal is exposed to the outside world, and the connector is not designed to be connected to a mating two-conductor connector.
Adding a third shielding conductor to each connector on a cable allows the driven shield electronics to eliminate the capacitance of the connectors as well, reducing the capacitance of the entire cable assembly to just a few picofarads. Hiding the driven shield conductors at the mating surfaces of one or both connectors permits connection to preexisting two-conductor equipment while gaining the benefits of low capacitance cabling and connectors. This technique is not taught in the prior art. Additionally, applying the driven shield technique to connector as well as cable eliminates the interdependency of capacitance and connector geometry.
Several objects and advantages of the low capacitance audio connector are:
The low capacitance audio connector is structured to participate in driven shield audio cable systems to reduce the capacitance of the overall cable assembly to the minimum practically attainable value.
Driven shield arrangements require three conductors in a cable, typically a triaxial cable with a center conductor and two shields, or a center conductor and one shield and a ground return conductor, as described above. With this arrangement, a unity gain amplifier samples the signal on the center conductor and drives that signal into the second, or driven shield.
The cable wiring terminals are implemented by metallic terminal 102, serving as a solder lug or screw terminal for a cable's shield (in the case of a low level audio cable) or ground conductor, and second metallic terminal 103, serving as a solder lug or screw terminal for a cable's signal conductor. Terminal 103 is attached by physical attachment retainer 107 to the hidden shaft of signal contact 104 by means of a swaged end, or by soldering, or by brazing, or by other techniques practiced in the art. An insulating wafer 108 insulates terminals 102 and 103 from each other. Terminal 102 comes into direct contact with the metal body 100 and is the same electrical conductor for the purposes of the electrical connection.
Metal part 100 has screw threads 101 to receive screw-on backshell 106 that protects the wired connector from damage. The backshell 106 has a circular hole in the rightmost end (as pictured) for exit of the cable from the wiring area occupied by terminals 102 and 103.
If connectors of this type are used on a driven-shield reduced capacitance audio cable, the capacitance of the entire assembly is approximately twice the capacitance of one of the connectors since the capacitance of the cable is forced to near zero. The driven shield arrangement is of no benefit in reducing the capacitance of standard connectors.
From the figure it is apparent that there is a capacitor structure formed by the shaft of signal contact 104 and metal body 100. This capacitance reduces the high frequency response of signals passing through the connector.
The specific dimensions of signal conductor 104, tubular conductor 112, and the insulators 109 and 113 are not critical from an electrical perspective because the driven shield technique reduces the capacitance of the connector irrespective of those dimensions. Therefore, an advantage of this low capacitance audio connector is that the physical dimensions of the components of the connector may be chosen to optimize manufacturability, cost, durability, or other factors, without regard to the natural capacitance of the structure.
This structure is, to the right of the threads 101 in the figure, similar to the structure of a typical three-conductor (or stereo, or tip-ring-sleeve) audio plug. However, the structure near the tip of the connector and the insulator 105 is different, with there being no ‘ring’ contact exposed to the outside world. The assembly technique for the present low capacitance audio connector is very similar to that of a typical three-conductor audio plug, once the parts are machined to the proper shape, allowing present assembly equipment to be used to construct the new low capacitance connector.
Note that the critical innovation here is the addition of the driven shield conductor 112 between the signal conductor 104 and the return or ground conductor 100 (the metal connector body). The specific methods for the machining, assembly and retention of the parts are numerous in the prior art. The overall structure of the connector may be quite varied, as long as the driven shield conductor 112 is interposed between the signal conductor 104 and the return or ground conductor 100 (the metal connector body). This innovation is applicable to any male audio connector of this general shape, whether with a standard 6.35 mm, 3.5 mm, or 2.5 mm barrel diameter, or some other dimension.
Applications
The center conductor 120 carries the signal within the cable 133, and is connected to a wiring terminal 103 on the connector 130 at each end of the cable. Similarly, the outer shield 123 is connected to a wiring terminal 102 on the connector 130 at each end of the cable 133. The inner shield 121 is connected to a wiring terminal 110 on the connector 130 at each end of the cable. The capacitance reduction is provided by amplifier 126, which is typically a unity gain buffer serving as a low impedance driver, driving a one-to-one replica of the signal on the center conductor, but this amplifier could be another transfer function to accomplish a desired frequency response of the connector and cable assembly.
The amplifier 126 is shown mounted near the center of the cable assembly, but it can be mounted at any position along the cable, or even within either connector backshell. Not shown are shielding around the amplifier 126, powering of the amplifier 126, or the physical mounting means of the amplifier 126, which are immaterial to the low capacitance audio connector.
The reduction to zero of the AC voltage between the inner shield 121 and center conductor 120 results in zero AC current flowing between the center conductor 120 and the outer shield conductor 123, just the condition that would occur if the capacitance were zero. This benefit is carried through to the tip of each connector due to the additional shield 112 (shown in
Note that
Note that the standard three-conductor connector 131 is not suitable for use as a low capacitance audio connector on a cable with an integral shield driver amplifier because the shield driver signal would be exposed on conductor 134 and thus susceptible to shorting or loading. Thus the construction of the low capacitance audio connector 130 hides and protects the driven shield conductor from such exposure at the mating connector interface. That interface is the surface of the low capacitance audio connector that mates with a female connector, specifically the surfaces of the signal conductor 104 and the metal body 100 which are visible with backshell 106 installed.
Note also that the configuration shown in the drawings can just as well be applied to right-angle plug connectors, and other physical variations, which are equivalent electrically.
Marketing by applicant of an audio cable featuring the low capacitance audio connector, after the filing of U.S. Provisional Application No. 61/135,974, has resulted in comments from professional musicians praising the enhanced tonal range that it provides, after they have purchased and used an embodiment of the cable.
The specific configuration of the embodiments discussed should not be construed to limit implementation of the low capacitance audio connector to those embodiments only. The techniques outlined are applicable to embodiments in other physical formats, using various power sources, and using various electronic amplifier and transfer function topologies. The low capacitance audio connector is functional with the broad range of instruments used by musicians, which convey sound signals from instrument to an amplifier and loudspeaker, or processing equipment. The amplifier or transfer function can be built into a musical instrument amplifier, musical instrument, or mounted on the audio cable itself or its attached connectors. These techniques, structures and methods find applicability outside the realm of musical instruments and related amplification, including but not limited to industrial electronics applications. Therefore, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.
This application claims priority through U.S. Provisional Application No. 61/135,974 filed by Henry B. Wallace on Jul. 25, 2008 for “Low Capacitance Audio Cable.”
Number | Name | Date | Kind |
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4751471 | Dunseath, Jr. | Jun 1988 | A |
5759069 | Kitatani et al. | Jun 1998 | A |
6102730 | Kjeldahl et al. | Aug 2000 | A |
7387531 | Cook | Jun 2008 | B2 |
Number | Date | Country | |
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61135974 | Jul 2008 | US |