United States Patent 3182129
US Patent References:
Sound reproducing system
- - 2285769

Audio frequency amplifier
- - 2887532

Transistor hearing aid
- - 3014995

Electronic stethoscope
- - 3132208


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Primary Class:
381/67
Other Classes:
381/109, 381/173, D24/134, 381/98
Foreign References:
GB656851A
GB871595A