Patent Application
20080212818
This specification relates in general to audio reproduction systems that have amplifiers and loudspeakers.
In general, in one aspect, an audio system apparatus includes an audio power amplifier, a transducer electrically connected to the audio power amplifier, an enclosure coupled to the transducer, and a secondary resonant element coupled to the enclosure. An electrical feedback signal representative of the transducer current is negatively fed back to the audio power amplifier to synthesize a positive output impedance.
Implementations may include one or more of the following features. The audio power amplifier may be a switching amplifier. The synthesized positive output impedance may be lossless. The synthesized positive output impedance may be positive over an entire operation range of the transducer. The electrical feedback signal may be produced by a current sensor such as a resistor, a Hall Effect sensor, a closed loop magnetic sensor, a current sensing transformer, or a sensing-field-effect transistor. The electrical feedback signal may be used by the audio power amplifier to reduce the Q of a secondary resonant system that includes the enclosure and the secondary resonant element. The secondary resonant element may include a port. The secondary resonant element may include a drone. The enclosure may be coupled to the first or second side of the transducer. There may also be a second enclosure coupled to the second side of the transducer and a second secondary resonant element coupled to the second enclosure. The synthesized positive output impedance of the audio power amplifier may be used to reduce the drone excursion. The synthesized positive output impedance may be in the range of 0.1 ohm to 100 ohms.
In general, in one aspect, an audio system apparatus includes an audio power amplifier, a transducer electrically connected to the audio power amplifier, and an enclosure comprising a waveguide coupled to the transducer. An electrical feedback signal representative of the transducer current is negatively fed back to the audio power amplifier to synthesize a positive output impedance.
Implementations may include one or more of the following features. The audio power amplifier may be a switching amplifier. The synthesized positive output impedance may be lossless. The enclosure may be coupled to the first or second side of the transducer. The enclosure may be coupled to the second side of the transducer, a second enclosure may be coupled, to the first side of the transducer and a secondary resonant element may be coupled to the second enclosure. The second enclosure may include a waveguide.
In general, in one aspect, a method for reproducing sound includes amplifying an electrical audio signal, applying the amplified electrical signal to a loudspeaker system that includes a transducer, an enclosure and a secondary resonant system, and rising an electrical feedback signal representative of the current flowing through the transducer to synthesize a positive output impedance for the amplifying.
Implementations may include one or more of the following features. The synthesized positive output impedance may be used to reduce drone excursion. The synthesized positive output impedance may be lossless,
In general, in one aspect, an electrical apparatus to sense current through a load includes a first input terminal having a first input voltage relative to a reference, a second input terminal having a second input, voltage relative to the reference, a first load terminal of the load having a first load voltage relative to the reference, a second load terminal of the load having a second load voltage relative to the reference, a first current sensing element connected between the first input terminal and the first load terminal, and a second current sensing element connected between the second input terminal and the second load terminal. A first sense voltage is determined by a relationship between the first input voltage and the second load voltage and a second sense voltage is determined by a relationship between the second input voltage and the first load voltage.
Implementations may include one or more of the following features. The reference may be a circuit common, a circuit ground, or an earth connection. The first current sensing element may be a resistive element. The first current sensing element may have essentially zero resistance. There may he a set of two resistive elements that form a voltage divider and the first sense voltage may be sensed by the voltage divider. The two resistive elements may have approximately equal resistance. The first input voltage and the second input voltage may have a substantially constant common mode voltage. The average of the first input voltage and the second input voltage may be substantially constant over a range of operation. There may be a voltage difference amplifier that senses the difference of the first sense voltage and the second sense voltage. The voltage difference amplifier may have a common mode range smaller than the voltage range of the first input voltage. There may be a bridge amplifier with bridge amplifier outputs where the first and second input voltages are derived from the bridge amplifier outputs. The bridge amplifier outputs may be modified by a filter and coupled to the first input terminal and the second input terminal. The load may include a transducer. There may be an audio amplifier. The audio amplifier may include a switching amplifier. The load may include a transducer coupled to a bass reflex enclosure. The load may include a transducer coupled to a waveguide enclosure. There may be an electrical filter module coupling the first current sensing element to the load.
The Q of a resonant system compares the frequency at which a system oscillates to the rate at which it dissipates its energy. On a spectral graph of the resonant system, the width of the resonant peak is given by the center frequency of resonance (also called resonant frequency) divided by the Q.
In some embodiments, a loudspeaker includes a transducer such as a moving coil or moving magnet transducer, which converts input electrical power into mechanical motion of a diaphragm, and an enclosure to constrain radiation from at least one side of the diaphragm, and at least a first secondary resonant element. Bass reflex loudspeakers utilize the sound from the rear of a transducer diaphragm (in addition to sound from the front of the diaphragm) to increase the efficiency of the system at low frequencies as compared to a closed-box loudspeaker. Bass reflex enclosures incorporate a secondary resonant element such as a drone or port. A drone may be considered a simplified form of a transducer that has some of the moving parts of the transducer but no electrical parts. A drone is also known as a passive radiator. The Q of a bass reflex audio system can be changed by adjusting the amplifier's output impedance. By synthesizing positive output impedance, the amplifier can reduce the Q of the bass reflex system. The secondary resonant element above interacts with the volume of air in the enclosure to form a secondary resonant system. The secondary resonant system has a resonant behavior that is separate from the transducer. The Q of this secondary resonant system can be modified by altering the output impedance of an amplifier that drives the loudspeaker system. Increasing the output impedance of the amplifier can reduce the Q of the secondary system resonance.
In other embodiments, additional resonant elements may be used. For example, an enclosure and port or drone may be coupled to the front or first side of a transducer diaphragm, and a second enclosure and port or drone, separate from the first port or drone, may be coupled to the rear or second side of a transducer diaphragm. In some embodiments, a waveguide enclosure may be coupled to the front, to the rear, or both the front and rear sides of a transducer diaphragm. Waveguide enclosures have multiple resonances at frequencies where standing waves are supported within the waveguide. Other embodiments may use combinations of enclosures, ports or drones, and waveguide enclosures. Increasing the output impedance of an amplifier driving the transducers in these embodiments can reduce the Q of the secondary system resonances and waveguide resonances.
In passive loudspeaker systems, a designer will typically choose parameters for a transducer to achieve desired damping of secondary resonances, to achieve a desired frequency response. By choosing the efficiency, a designer can control the Q of secondary resonances. To lower the Q, a designer needs to reduce the efficiency of the transducer. It was described earlier that increasing the output impedance of the amplifier driving a loudspeaker with secondary resonances can be used to reduce the Q of secondary resonances. Increasing the output impedance of the driving amplifier allows a much more efficient transducer to be used than would be typical. Using a high efficiency transducer in loudspeaker embodiments with secondary resonances would typically result in high Q resonances and non optimal output frequency response. The Q's of the secondary resonances are controlled using negative current feedback to increase amplifier output impedance, so that the Q's can be reduced to desirable levels. This allows high efficiency transducers to be used in systems that would otherwise result in unacceptable frequency responses.
When negative current feedback Is used to synthesize a positive output impedance for an analog (i.e. linear) amplifier, the effect is similar to placing a physical resistor electrically in series with the output of the analog amplifier without feedback. With a resistor in series with the output, there is a voltage divider effect between the resistor and the load (e.g. the loudspeaker). Some of the available amplifier power is dissipated in the resistance, some in the load. When negative current feedback is applied to the analog amplifier, however, rather than dissipating power in a physical resistor, power ends up being dissipated in the amplifier output stage. When a high efficiency transducer is used with an analog amplifier and negative current feedback, the benefit of using a transducer with increased efficiency is offset by the extra power dissipated in the amplifier output stage. The feedback does still provides useful control over system frequency response, and compensates for parameter variation in the system (as explained later), but overall efficiency is not substantially improved.
A further benefit is obtained when negative current feedback is used to synthesize a positive output impedance for a switching type amplifier used to drive a loudspeaker system according to one of the previously described embodiments. The efficiency of the transducer is chosen to be as high as practical. Negative current feedback is used to synthesize a positive output impedance, to provide damping for secondary system resonances. Unlike in the analog amplifier case, however, the power dissipated in the output devices of a switching amplifier does not appreciably change when negative current feedback is applied. The synthesized positive output impedance of the switching amplifier does not effectively dissipate any power, other than through switching and conduction losses in the output devices which do not appreciably change whether or not current feedback is used. We will refer to the synthesized output impedance with the above mentioned characteristic as being lossless, even though there is some finite power dissipated in the output devices. System efficiency is greatly increased because the choice of transducer parameters is decoupled from the need to reduce Q of secondary resonant elements to achieve a desired frequency response. The Q's are reduced to a desired value by use of synthesized positive output impedance that does not dissipate real power, preserving system efficiency while obtaining the desired frequency response.
Referring to PIG. 1, there is shown a drawing of a speaker module 100. Speaker module 100 produces sound from an amplified electrical audio signal. Speaker module 100 includes enclosure 102, drones 104 and 110, transducer 106, and amplifier 108. Only one drone 104 is visible in
Referring to
Audio system 200 accepts audio input signal 201 and couples it to one input of summer 202. The output of summer 202 is coupled to the input of amplifier 204. The output of amplifier 204 is coupled to transducer 208. Transducer 208 produces sound vibrations that reach the ears of the listener. Transducer 208 also causes air pressure variations within enclosure 206. These internal pressure variations cause motion in drones 209 so that drones 209 help provide the desired output from the audio system 200. An electrical feedback signal representing the current in transducer 208 is sensed by current sensor 210. The output of current sensor 210 is amplified by amplifier 212, filtered by filter 214, and is differentially coupled to a second input of summing module 202. By differentially coupling the current signal to summer 202, negative current feedback is applied to amplifier 204.
Power amplifier 204 applies gain to the input signal 201. In some embodiments, power amplifier 204 may be an analog amplifier. In some embodiments, power amplifier 204 may be a switching amplifier In some embodiments, amplifier 204 may be of any known amplifier class, such as class A, AB, B, C, AD, BD, D, G, or T. Amplifiers may have unipolar or bipolar power supply voltages.
The system of
Referring to
The circuit of
In the implementation shown in
In some embodiments, one of the sense resistors 310 and 311 may be eliminated (reduced in value to 0 ohms). Eliminating one sense resistor does not significantly effect the common mode voltage stability. In such an embodiment, the benefit of small common mode swing is maintained with the burden of only one sense resistor rather than two and the change in gain can be compensated for by re-scaling other circuit values. The combined audio and sensed current signals from the output of amplifier 326 are fed through filter 214 which applies low pass filtering to reduce the feedback gain, and aid with loop stability, and then to power amplifier 324, effecting negative feedback of the current signal. This configuration will create an essentially non-power dissipating synthesized output impedance for power amplifier 204 of 2*Rs*K1*K2, where Rs is the resistance of resistors 310 and 311 in ohms, K1 is the gain of power amplifier 324, and K2 is the gain of amplifier 326. This simplified synthesized output impedance equation assumes the gain of filters 214 and 308 are essentially unity, which is typically true over the effective frequency range of operation of the speaker module 100. Although it varies with audio frequency, the synthesized output impedance is always positive.
Current measurement may be made by resistor, Hall Effect, closed loop magnetic sensor, current sensing transformer, sensing Field Effect Transistor (senseFET). These alternative current sense devices may take the place of element 210 in
In addition to filter 308, in some embodiments there may be filters added between the sensing resistors 310, 311 and the transducer 208.
The synthesized output impedance of audio system 200 is defined as the impedance measured across the two points where the transducer 208 connects to the electronic circuit. The synthesized output impedance may be in the range of 0.1 ohm to 100 ohms. In the implementation shown in
Referring to FIG 4, there is shown a graph of audio output in dB SPL vs. frequency in Hz for one implementation of audio system 200 at 1 meter distance from transducer 208. The curves in
Curve 400 has a high Q peak in the 39 Hz area corresponding to drone resonance. This peak can shift significantly with manufacturing variations in the acoustical and mechanical components of the speaker module. The peak is also likely to shift over the life of the speaker module. In order to achieve the desired frequency response with conventional equalization, each unit's amplifier would have to be custom equalized. These response variations make it impractical to use equalization processing to achieve the desired frequency response. By using current feedback, the system can compensate for variation in components and achieve a flattened response as speaker module parameters vary.
Referring to
The behavior of a loudspeaker system depends on the parameters of the transducer selected, and the parameters of the secondary resonant system. A designer may wish to develop a system with high efficiency and small size. To achieve this, a designer may select a transducer having a high motor force. When such a transducer is used in a system with a small enclosure, the result may be a peaked SPL output in the frequency range of the secondary system resonance. The secondary resonant system may have a high Q.
For systems where a secondary resonant system has high Q and a drone used as the secondary resonant element, the drone may be more easily overdriven at the secondary resonant frequency than at other frequencies. Overdriving occurs when the displacement of the drone's moving parts exceed the maximum intended displacement and the materials of the drone are deformed beyond their design targets. When overdriven, the drone may produce undesirable noises or it may be damaged. Systems with high Q's are easily overdriven. In order to avoid this overdriving condition, audio system 200 may increase its synthesized positive impedance so that the Q of the secondary resonant system is reduced. Increasing the output impedance by using negative current feedback can flatten the frequency response of the sound pressure output of the loudspeaker system around the secondary resonance, and also reduce displacement of the drone, improving reliability. When positive output impedance is synthesized for an embodiment where the amplifier is of a switching type, the frequency response is improved without affecting the system efficiency because no real power is dissipated in a physical impedance. Using the current-controlled synthesized output impedance technique allows drone damping control without the use of a power dissipating element. Because this is accomplished using a feedback system, the frequency response improvement is obtained if parameters of the transducer and secondary resonant system vary, either due to production tolerances or aging over time.
Other implementations are also within the scope of the following claims.
