Audio Output Ramp Circuit for Power-On Pop Noise Suppression
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Solution Overview
Problem
Audio systems experience undesirable 'click and pop' noise during power up and power down due to the charging and discharging of AC coupling capacitors, which produce frequency components within the human audible range.
Innovation Solution
An audio system with a noise reduction circuit that includes a selectable current path and a controllable resistance device, utilizing a field effect transistor (FET) and a ramp signal generator to smoothly apply or remove voltage, ensuring the frequency components of the transitioning voltage lie outside the human audible range, thereby reducing or eliminating click and pop noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the AC coupling capacitor is charged quickly to the DC offset voltage when the audio device is turned on, then the startup time is reduced, but click and pop noise is generated in the audible frequency range
Solution Approach 1:
The patent applies preliminary action by pre-charging the AC coupling capacitor to the DC offset voltage before the audio amplifier is fully activated. This is achieved through a dedicated pre-charge circuit that operates in advance, ensuring the capacitor is ready to accept the full audio signal without causing transient noise when the main amplifier switches on.
Solution Approach 2:
The patent uses an intermediary pre-charge circuit as a mediator between the power supply and the AC coupling capacitor. This intermediary circuit smoothly transfers charge to the capacitor without creating audible transients, and also serves as a buffer during shutdown to prevent discharge noise from reaching the speaker.
2Loss of time
If the AC coupling capacitor is discharged quickly when the audio device is turned off, then the shutdown time is reduced, but click and pop noise is generated in the audible frequency range
Solution Approach 1:
The patent applies preliminary action in reverse during shutdown by maintaining the capacitor charge through the pre-charge circuit until the very end of the shutdown sequence. The circuit ensures the capacitor discharge is controlled and completed just as the amplifier shuts down, preventing audible discharge transients while minimizing overall shutdown time.
Solution Approach 2:
The pre-charge circuit serves as an intermediary that controls the capacitor discharge during shutdown. It provides a controlled discharge path that prevents rapid voltage changes from reaching the speaker, thereby eliminating pop noise while maintaining efficient shutdown timing.
3Device complexity
If a simple direct connection is used from the amplifier output to the speaker, then the device complexity is reduced, but click and pop noise cannot be suppressed during power transitions
Solution Approach 1:
The pre-charge circuit is designed with multi-functionality, serving both to charge the AC coupling capacitor during startup and to control its discharge during shutdown. This universal circuit performs multiple protective functions without requiring separate complex circuits for each operation, thereby suppressing click and pop noise while maintaining reasonable device complexity.
Solution Approach 2:
The patent changes the electrical parameters (voltage and current) of the capacitor charging and discharging processes to eliminate audible transients. By controlling the rate of voltage change across the capacitor and the current through the speaker during power transitions, the circuit suppresses click and pop noise without requiring a completely complex circuit architecture.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces or eliminates click and pop noise during power up and power down operations by smoothing the voltage transition across AC coupling capacitors, preventing noise generation in the speaker.
Implementation Method 1
The selectable current path is adapted to smoothly dissipate charges from the output of the amplifier during a power down operation
Implementation Method 2
the output of an audio device is coupled to a speaker via a capacitor, typically referred to as a direct current (DC) blocking or alternating current (AC) coupling capacitor
Data Source
Figure 1
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Figure 4A~5
AI summary
An audio system that reduces or eliminates click and pop noise during power up and power down operations. In particular, the audio system includes an amplifier with an input adapted to receive an input audio signal and an output adapted to produce an amplified output audio signal for an associated speaker. The audio system further includes a noise reduction circuit adapted to smoothly apply and remove a DC voltage to and from the output of the amplifier in a manner that reduces or eliminates click and pop noise from being generated by the associated speaker. The DC voltage at the output of the amplifier may be derived from a DC reference voltage source and/or from the input audio signal.