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http://en.wikipedia.org/wiki/Valve_audio_amplifier_-_technical
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http://www.angelfire.com/electronic/funwithtubes/Amp-Phase.html
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http://www.musikding.de/index.php/cat/c138_Sheet-metal-enclosures.html
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https://github.com/audiyofan/jx12-project
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http://www.georgegardner.info/electronics/class-d-avr.html
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https://blog.podkalicki.com/attiny13-8bit-mono-class-d-amplifier