An analog (or analogue) synthesizer is a synthesizer that uses analog circuits and analog signals to generate sound electronically. The earliest analog synthesizers in the 1920s and 1930s, such as the Trautonium, were built with a variety of vacuum-tube (thermionic valve) and electro-mechanical technologies. After the 1960s, analog synthesizers were built using operational amplifier (op-amp) integrated circuits, and used potentiometers (pots, or variable resistors) to adjust the sound parameters. Analog synthesizers also use low-pass filters and high-pass filters to modify the sound. While 1960s-era analog synthesizers such as the Moog used a number of independent electronic modules connected by patch cables, later analog synthesizers such as the Minimoog integrated them into single units,