Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Reexamination Certificate
1999-02-02
2001-03-13
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
C361S058000, C361S091100
Reexamination Certificate
active
06201680
ABSTRACT:
BACKGROUND
1. Field of Invention
This invention relates to the protection of audio loudspeakers that may be damaged by high-speed audio electrical transients, power overload, or direct current, including any combination thereof, and an electronic circuit used to protect against these types of damages.
BACKGROUND
2. Prior Art
Prior art loudspeaker protection circuits were attempted by various passive methods. Light bulbs, fuses, relays, and special thermistors were used as elements for these types of protection circuits. All of these approaches have limitations and deficiencies, as discussed below.
The earliest method of loudspeaker protection involved the use of a light bulb. This method was rather rudimentary and provided little protection to the loudspeakers. This method uses a light bulb with a specific voltage that would be placed in series with a high-frequency tweeter or compression driver. Unfortunately, this method is limited to protecting the loudspeakers only against power overload. A light bulbs response is too slow for adequate transient protection. Light bulbs also have the disadvantage of eventually burning out, thereby halting sound output until the audio system is serviced. U.S. Pat. No. 4,122,507 to Queen (1978) demonstrates that a light bulb is susceptible to bum out. A bulb's voltage rating can be easily surpassed by an audio signal with large amplitude. Light bulbs have the additional disadvantage of adversely affecting audio quality. As the filament heats up and cools down, sound compression occurs. A further disadvantage of using a light bulb is that its filament is sensitive to damage by vibration. This can occur in an environment such as a loudspeaker cabinet. Because of the inherent problems mentioned in using light bulbs, this element is unreliable.
Fuses are the most common method currently used in protecting loudspeakers. They are used to reduce fire hazards. Fuses, like light bulbs, do not provide a fast enough response to protect loudspeakers from transient damage. Admittedly, fuses protect loudspeakers from power overload, assuming that the proper amperage rating is selected for the circuit. However, an obvious drawback is that a fuse must be replaced once it is blown. U.S. Pat. No. 3,925,708 to Picciochi (1975) demonstrates secondary fuse protection, if his circuit fails. But, if the fuse blows in Picciochi's circuit, the entire audio system is rendered inoperative. Fuse replacement becomes a nuisance and is very costly in audio loudspeakers that are mounted in remote or inaccessible areas.
A relay can easily burn out when a fault surpasses the circuit's trigger point. There is no known prior art available which would avoid this issue. Other prior art involving the use of relays have employed reed-type relays, Queen (1978). However, the contacts in these types of relays have low current-handling capability. This leads to high resistance when high current is passed through the contacts. As a result, voltage drops across the contacts (which reduces power transfer to the loudspeaker), thereby disrupting tonal characteristics. The problem is exacerbated when the audio signal passing through the contacts has high frequency characteristics. Modern power amplifiers easily surpass two kilowatts of output power capability. Also, prior art has not shown any reliable form of contact-arcing suppression. During disconnection, arc suppression allows a clean break. Lack of arc suppression causes an audible “ripping” effect when driven by audio program material. Furthermore, coil heating characteristics are of concern when using relays. As a relay coil's temperature rises, the coil resistance rises, thereby reducing magnetic field intensity. When this occurs, disconnection speed is reduced. Prior patents have not produced a solution to this issue.
Recently, speaker protection has been attempted with a special thermistor commonly called a “positive thermal conductor” (hereinafter referred to as a “PTC”). PTC's are made from a conductive polymer material. PTC's are used for over-current faults in general electronic circuits. The PTC is a device that is like a resistor which changes to a high resistance when its current rating is surpassed. However, this type of thermistor requires approximately one hour to cool down and restabilize to its initial resistance. The PTC cannot protect a loudspeaker from transient damage. PTCs have poor audio characteristics and effect audio quality. U.S. Pat. No. 4,093,822 to Steinle (1978) demonstrates a typical audio protection circuit using a PTC. PTC's are slow in reacting to overload conditions due to a thermistor's inherent operating characteristics. In practice, these devices offer the least overall protection. Thermistors also have amperage limitations, and have not been commercially manufactured for high-power applications.
Other Deficiencies Common to Prior Art
All of the foregoing prior art mentioned, fail to provide broad adjustability of protection ranges for differing audio system power requirements. This author believes a good power protection range should be about 25-1400 continuous watts at 8 ohms, (14-106 volts). Several protection circuits that were found in prior art, would self destruct when approaching higher power levels.
Most prior art found, use filter capacitors as an inherent part of the triggering detection circuitry. With this method, audio program material is rectified into direct current, and then filtered using a capacitor. When filtered in this manner, the circuit creates time delay with response to any fault in an audio system. This method is inadequate to handle fast transient quashing and is further affected by frequency. The affect of frequency on mentioned circuitry, also create differences in capacitor charge time, resulting in disconnection lag time. U.S. Pat. No. 4,122,507 to Queen (1978) expands on the fact that optimum performance was only attained at certain frequencies. This type of circuitry also draws high idle current during normal operation that has been found unacceptable to audio system installers by this authors personal experience.
Other prior art require external wires to be connected to both the loudspeaker and power amplifier. This type of circuit makes installation within a loudspeaker cabinet impractical and expensive. This is especially true in professional or commercial audio systems. U.S. Pat. No. 3,959,735 to Grosjean (1976) demonstrates such a circuit. Still other prior art requires a power supply. An example of this is U.S. Pat. No. 4,330,686 (Roe).
Objects and Advantages
My circuit is a two-stage loudspeaker protection system. The advantages of this circuit are numerous. The first stage of my circuit contains an adjustable threshold detection feature that does not utilize capacitors. In addition, trigger response time can be as fast as 65 nanoseconds. This is much faster than the time in which a fuse could blow open. No prior art found has this transient quashing speed. Furthermore, the threshold detection point can be adjusted from 14 to 106 volts without modification of any components or values. Further broadening of power adjustment ranges for future situations can easily be attained by changing component values. My circuit's voltage threshold trigger point is adjusted by calculating the square root of the speaker's power rating, multiplied by its impedance. Audio transducers with a specific power rating are protected beyond the calculated maximum power handling figure. Triggering occurs irrespective of an audio signal's frequency, pulse width, quadrant, or rise time.
The first stage also contains high-speed transient quashing capability. This feature protects relay contacts from damage by suppressing arcs during the circuit's second stage of operation. This allows higher-flowing currents to be disconnected, extending relay contact life. Relay contacts are especially susceptible to pitting damage when high frequencies pass through them. The quashing circuitry allows for a clean disconnection between the amp
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