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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https://aes2.org/publications/elibrary-page/?id=15034
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