Music Amplifier with Instrument Tuning Device (German Patent)

German patent application for a music amplifier with a built-in electronic tuning device for instruments. The amplifier includes two preamplifiers with instrument inputs, an oscillator generating a reference frequency (329.6 Hz, corresponding to the note E), and a frequency comparison circuit with an indicator lamp that pulses when instrument and reference frequencies differ and steadies when they match. A switching system allows the musician to hear the tuning tone through the speaker while tuning. US priority February 19, 1969 (US 800425). Filed January 14, 1970; published July 1, 1971.