Split Output Stage Amplifier With PWM Soft Start for Speaker Plop
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Solution Overview
Problem
Conventional class AB amplifiers produce an audible 'plop' when switched on or off due to rapid charging and discharging of capacitors, which is undesirable, especially in portable applications like headphones and earphones, and existing solutions like relays are bulky and inconvenient.
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 manage feedback loops and eliminate DC current, allowing for 'soft' switching and avoiding plop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a relay is used to reduce or eliminate the plop, then the 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 switching mechanism using transistors (Q1, Q2) and resistors to control the gradual charging of the capacitor. This electronic approach eliminates the need for bulky mechanical relays while achieving the same plop reduction function through controlled voltage transitions.
Solution Approach 2:
The patent divides the single switching operation into two separate stages: a first switch (Q1) that controls initial capacitor charging, and a second switch (Q2) that controls the main power connection. This segmentation allows the capacitor to charge gradually before full power is applied, reducing the audible plop without requiring a heavy relay.
2Speed
If the capacitor is charged rapidly when the amplifier is switched on, then the amplifier starts quickly, but an audible plop is produced
Solution Approach 1:
The patent applies preliminary action by charging the capacitor through the first switch (Q1) and resistor (R1) before the second switch (Q2) is activated. This preliminary charging phase prepares the capacitor with a controlled voltage level, so when the main power switch closes, the voltage transition is gradual and does not produce an audible plop.
Solution Approach 2:
The patent uses a two-stage switching sequence where the first switch operates in a preliminary phase and the second switch operates in the main power phase. This periodic or sequential action divides the power-on process into distinct time periods, allowing the capacitor to charge in a controlled manner that eliminates the sharp voltage spike causing the plop.
3Ease of operation
If DC-coupled connection is used, then true ground operation is achieved, but DC current flows through the speaker causing power losses
Solution Approach 1:
The patent employs a feedback mechanism where the voltage at the speaker terminal is monitored and used to control the switching action. The feedback ensures that the capacitor is fully charged before the second switch closes, and that the transition to DC-coupled operation occurs only when the voltage is stable, preventing DC current flow through the speaker while maintaining true ground operation.
Solution Approach 2:
The patent changes the voltage parameter over time through the two-stage switching process. Initially, the capacitor charges to a specific voltage level through the first switch, then the second switch closes to establish the DC-coupled connection. This parameter change ensures that the voltage transition is controlled and that no DC offset current flows through the speaker during normal operation.
Data Source
AI summary
An amplifier is disclosed, wherein the output stage is split between a primary output stage and a secondary stage. To minimize 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 minimize 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.