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Simple Amplifier for Single Frequency Subwoofer
Frequency mapped amplifier driving single frequency subwoofer is an inexpensive way to add the missing low notes to small satellite loudspeakers. The whole amplifier system consists of a band-pass filter (typically 20-120 Hz), an envelope detector, a single frequency (typically 50-60 Hz) constant amplitude generator, a mixer (which multiplies output from the envelope detector and the single frequency generator) and a conventional power amplifier. The amplifier proposed in this paper unites the functions of the mixer, the generator and the amplifier in a single unit and does not need a DC power supply, but it runs on 50/60 Hz AC power supply, without a rectifier and without big voltage-smoothing capacitors. With an appropriate MOSFETs the proposed amplifier can run directly on the 120V/60Hz mains supply line - without a power transformer, but in that case, it needs a loudspeaker with a sufficiently high impedance on the frequency of 60 Hz in order not to stress output transistors of the amplifier.
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