PA Amplifier Input Isolation Without a Large Output Transformer
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Solution Overview
Problem
Existing high-impedance public-address systems require large-sized power amplifiers due to the need for output transformers to prevent ground current during lightning strikes, while maintaining isolation between transmission lines and ground potential.
Innovation Solution
A public-address system design that isolates the power amplifying means from ground potential by using an isolation input circuit, such as a transformer or light-emitting and light-receiving devices, eliminating the need for an output transformer, allowing for a small-sized power amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an output transformer is used to prevent large current flow during lightning strikes, then reliability is improved, but the power amplifier becomes large-sized
Solution Approach 1:
The isolation function is segmented from the output stage and placed at the input stage. The isolation transformer is positioned between the audio signal source and the power amplifying means, separating the grounding reference of the signal source from the isolated reference of the power amplifier. This segmentation allows the power amplifier to be small-sized while still providing ground potential isolation through the input-side transformer.
Solution Approach 2:
An isolation transformer is introduced as an intermediary component at the input stage. This transformer acts as a mediator that transfers the audio signal from the grounded signal source to the isolated power amplifier without direct electrical connection. The transformer's magnetic coupling provides galvanic isolation, preventing large current flow during lightning strikes while allowing the power amplifier to maintain a small size without requiring an output transformer.
2Reliability
If an isolation transformer is placed at the output side, then ground potential isolation is achieved, but the transformer core becomes large due to audio frequency requirements
Solution Approach 1:
The isolation function is performed preliminarily at the input stage before the signal enters the power amplifying means. By placing the isolation transformer at the input side, the grounding isolation is established early in the signal path. This preliminary isolation allows the subsequent power amplifier stage to operate without requiring its own large output transformer, as the isolation requirement has already been satisfied by the input-side transformer.
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 ensures isolation from ground potential without the need for an output transformer, reducing the size of the power amplifier and preventing large current flow during lightning strikes, while maintaining high output voltage and audio signal integrity.
Implementation Method 1
The isolation input means may be a transformer. The transformer has its primary winding connected between the two terminals of the audio signal source, and has its secondary winding connected to the two output-side terminals of the isolation input means.
Implementation Method 2
The isolation input means may include light-emitting means emitting light in accordance with an audio signal from the audio signal source, and light-receiving means providing an electrical signal in response to the reception of light from the light-emitting means.
Data Source
Figure 1
Figure 2~3
AI summary
[Object] To downsize a power amplifier, while keeping isolation between a transmission line and ground potential. [Means to Realize Object] Two terminals (2a, 2b) of an audio signal source (2) are connected to an input of an isolation input circuit (6). An audio signal developed, being isolated from the input-side, between two output-side terminals (6a, 6b) of the isolation input circuit (6) are inputted to a power amplifying stage (12). The power amplifying stage (12) amplifies the audio signal and outputs it at a high voltage from two output terminals (12c, 12d) to a plurality of loudspeakers (22) without using an insulation transformer. Operating power is supplies to the power amplifying stage (12) from two power supply terminals (20c, 20d) of a DC power supply (14). One (6d) of the terminals of the insulation input circuit (6), one (12d) of the output terminals of the power amplifying stage (12) and one (20d) of the power supply terminals of the DC power supply (14) are connected together to provide a common potential point different from the ground potential.