Great little article from nearly a decade ago about Robert Moog’s filter that shaped the sound of synthesizer music. Oddly enough, while the article mentions some of the Moog module’s prices, it doesn’t actually mention the price of the ladder filter. Having been researching lately exactly what Tangerine Dream would have been working with at any given moment on Phaedra, I happen to have the 1969 Moog price list (PDF) in front of me – $730 for the full-fledged 904 Voltage-Controlled Filter. That’s about $4,900 dollars today, nearly ten times as much as Moog’s current ladder filter. Bonus link: Robert Moog’s patent number 3,475,623: Electronic High-pass and Low-pass filters employing the base to emitter diode resistance of bi-polar transistors (PDF).