Split Output Stage Amplifier for Plop-Free Speaker Switching
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional class AB amplifiers produce an audible 'plop' when switched on due to rapid capacitor charging, which is typically mitigated using bulky relays, unsuitable for portable applications like headphones or earphones.
Innovation Solution
A class AB amplifier design with a primary and secondary output stage, where the secondary stage is gradually connected or disconnected using pulse width modulation, and a DC-offset cancelling unit to prevent audible plop and manage DC current, allowing for 'soft' switching and true ground operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a relay is used to reduce or eliminate the plop, then the audible plop is reduced or eliminated, but the device becomes bulky and heavy
Solution Approach 1:
The patent replaces the mechanical relay system with an electronic solution using a secondary output stage and PWM-controlled switching. Instead of using a physical relay to gradually connect the speaker, the invention uses electronic switching of the secondary stage which is modulated by PWM signals during the transition period, eliminating the need for bulky mechanical components while achieving the same plop-reduction effect.
2Object-affected harmful factors
If a relay is used to reduce or eliminate the plop, then the audible plop is reduced or eliminated, but the device becomes inconvenient for portable applications
Solution Approach 1:
The patent replaces the mechanical relay system with an electronic solution using a secondary output stage and PWM-controlled switching. Instead of using a physical relay to gradually connect the speaker, the invention uses electronic switching of the secondary stage which is modulated by PWM signals during the transition period, eliminating the need for bulky mechanical components while achieving the same plop-reduction effect.
3Speed
If the speaker is rapidly connected upon switch-on, then the connection is immediate, but an audible plop is produced
Solution Approach 1:
The patent applies preliminary action by gradually connecting the speaker through the secondary output stage before the main primary stage is fully activated. During this transition period, PWM signals with variable mark-space ratios are applied to the secondary stage, allowing the capacitor to charge gradually and the speaker to be softly connected, preventing the abrupt voltage change that causes audible plop.
Solution Approach 2:
The patent uses periodic PWM (pulse width modulation) signals applied to the secondary output stage during the transition period. The mark-space ratio of these PWM signals is varied over time, creating a periodic switching action that gradually connects the speaker to the amplifier output, thereby avoiding the sudden connection that produces audible plop.
4Loss of energy
If DC offset is present in true ground operation, then power losses and efficiency losses occur, but eliminating it requires additional control circuits
Solution Approach 1:
The patent merges the DC-offset cancelling function with the existing secondary output stage and its feedback loop. The DC-offset cancelling unit is integrated into the secondary stage's signal path, allowing DC offset detection and cancellation to occur within the same circuit structure that already handles the gradual connection function, thereby reducing overall system complexity.
Solution Approach 2:
The secondary output stage serves multiple functions: it gradually connects the speaker during switch-on, provides PWM control for soft switching, and houses the DC-offset cancelling unit for true ground operation. This multi-functionality reduces the need for separate dedicated circuits, thereby managing device complexity while achieving energy efficiency.
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 eliminates the audible 'plop' during switching and reduces power losses by gradual connection/disconnection and DC-offset cancellation, making the amplifier suitable for portable devices.
Implementation Method 1
The switch may be arranged to be operable by pulse width modulation (PWM) with variable mark-space ratio whereby the secondary stage (23) is connectable gradually upon switch-on to the speaker
Implementation Method 2
the second feedback loop comprises a DC-offset cancelling unit (31). By controlling and cancelling any DC offset, DC current through the speaker, which results in power and efficiency losses and non-ideal operation of the speaker, can be controlled and even eliminated
Data Source
Figure 1~2
Figure 3
AI summary
An amplifier is disclosed, wherein the output stage is split between a primary output stage and a secondary stage. To minimise or eliminate any audible plop when the amplifier is switched on, the primary stage is connected, and the second stage is gradually connected using a switch. The gradual connection can be by means of varying the pulse-density of a pulse wave modulation on the switch, from fully open (0% pulse-density) to fully closed (100%). The inverse process can minimise or eliminate plop during switch-off. Separate feedback loops are switchable, from the primary and secondary stages; in a DC-coupled embodiment, the feedback loop from the secondary stage may include DC-offset cancelling circuitry, to both reduce or eliminate the plop and avoid and DC-offset current through the speaker.