Loudspeader overload protection

• US 5,729,611 A
• Filed: 01/24/1997
• Issued: 03/17/1998
• Est. Priority Date: 02/02/1996
• Status: Expired due to Fees

• Current Assignee: Paradigm Electronics Inc.
• Original Assignee: Paradigm Electronics Inc.
• Inventors: Bonneville, Marc Etienne
• Primary Examiner(s): Isen, Forester W.
• Application Number:
  US08/788,580
• Time in Patent Office: 417 Days
• Field of Search: 381/55, 381/59, 381/96
• US Class Current: 381/55
• CPC Class Codes: H03F 1/52 Circuit arrangements for protecting such amplifiers monitoring arrangements G01R31/28; increasing reliability in communication systems, e.g. using redundancy H04B1/74
• CPC Class Codes: H03F 2200/426 the amplifier comprising circuitry for protection against overload

Abstract

In a loudspeaker of the kind in which negative feedback derived by monitoring actual displacement of the loudspeaker'"'"'s electroacoustic transducer or cone is used to compensate for transducer non-linearities, overload protection is provided by two variable gain amplifiers between the audio input and the power amplifier. One variable gain amplifier is within the feedback loop and the other variable gain amplifier precedes the feedback loop. A detection circuit has two thresholds close to but below the level at which clipping will begin. A first threshold level is about 1.0 dB below and the second threshold level is about 0.5 dB below. A first control signal to reduce the gain of the variable gain amplifier preceding the loop is generated when the first, lower threshold level is exceeded and causes the gain of the amplifier to decrease at a relatively slow rate. A second control signal to reduce the gain of the amplifier within the loop is generated when the higher threshold level is exceeded and reduces the gain of the amplifier within the loop very rapidly so that clipping is avoided. The first and second control signals may each be derived by a threshold amplifier and a charging circuit, the first charging circuit having a longer time constant than the time constant of the second charging circuit. Once the overload situation is under control, i.e. the signal is below the second threshold level, the second control signal increases the gain of the amplifier inside the loop, restoring full feedback. The feedback path may derive a first feedback signal component substantially proportional to the position of the electroacoustic transducer, a second feedback signal component substantially proportional to the velocity of the electroacoustic transducer and a third feedback signal component substantially proportional to acceleration of the electroacoustic transducer.

Claims

1. A loudspeaker unit comprising an electroacoustic transducer, input means for an audio signal to be reproduced by the electroacoustic transducer, a power amplifier connected between the input means and the electroacoustic transducer, sensor means for providing a motion signal in dependence upon motion of the electroacoustic transducer, feedback means for providing a feedback signal in dependence upon the motion signal, and subtracting means connected between the input means and the power amplifier for subtracting the feedback signal from the audio signal and supplying the resulting difference signal to the power amplifier, the loudspeaker unit comprising control means for providing a first gain control signal and a second gain control signal in response to the audio signal amplitude exceeding first and second predetermined levels, respectively, the first predetermined threshold level being lower than the second predetermined threshold level, first variable gain amplifier means connected between the input means and the subtracting means and responsive to the first gain control signal to decrease amplification of the audio signal and second variable gain amplifier means connected within the feedback loop comprising the subtracting means, power amplifier and feedback path, the second variable gain amplifier being responsive to the second gain control signal to decrease gain within the feedback loop, the arrangement being such that the rate of change of gain of the first variable gain amplifier means in response to the audio signal exceeding the first predetermined level is significantly slower than the rate of change of the gain of the second variable gain amplifier means in response to the audio signal exceeding the second predetermined level.

2. A loudspeaker unit as claimed in claim 1, wherein the means for providing the first and second gain control signals comprises means for deriving a signal proportional to audio signal amplitude, first deriving means for providing said first gain control signal in proportion to the difference between the proportional signal and a first threshold, and the second deriving means for providing said second gain control signal in proportion to the difference between the proportional signal and a second threshold.

3. A loudspeaker unit as claimed in claim 1, wherein each of the first and second deriving means comprises a threshold comparison circuit and a charging circuit connected to the output of the threshold comparison circuit, the charging circuit of the second deriving means having a charging time constant that is shorter than the charging time constant of the charging circuit of the first deriving means and also significantly less than a period of the audio signal.

4. A loudspeaker unit as claimed in claim 3, wherein the second charging circuit has a discharging time constant significantly shorter than that of the first charging circuit.