An analog or analogue synthesizer (or analog synth) is a synthesizer that uses analog circuits and analog computer techniques 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 with a variety of operational amplifier (op-amp) integrated circuits, along with potentiometer (pot, or variable resistor) to adjust the traits of the sound that is produced. 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, eliminating patch cords in favour of integrated signal routing systems.
The earliest synthesizers used a variety of thermionic-valve (vacuum tube) and electro-mechanical technologies. While some electric instruments were produced in bulk, such as the Ondioline, the Hammond organ, and the Trautonium, many of these would not be considered synthesizers by the standards of later instruments. However, some individual studios and instruments achieved a high level of sophistication, such as the Trautonium of Oskar Sala, the Electronium of Raymond Scott, and the ANS synthesizer of Evgeny Murzin  .
Early analog synthesizers used technology derived from electronic analog computers and laboratory test equipment. They were generally "modular" synthesizers, consisting of a number of independent electronic modules connected by patch cables into a patchbay which resembled the jackfields used by 1940s-era telephone operators. Synthesizer modules found in early analog synthesizers included voltage-controlled oscillators, voltage-controlled filters, and voltage-controlled amplifiers. As well, they used envelope generators, low-frequency oscillators, and ring modulators. Some synthesizers also had effects devices such as reverb units, or tools such as sequencers or sound mixers. Because many of these modules took input sound signals and processed them, an analog synthesizer could be used both as a sound-generating and sound-processing system.
Famous modular synthesizer manufacturers included Buchla & Associates, Moog Music, ARP Instruments, Inc., and Electronic Music Studios. Moog established standards recognized worldwide for control interfacing on analog synthesizers, using a logarithmic 1-volt-per-octave pitch control and a separate pulse triggering signal. These control signals were routed using the same types of connectors and cables that were used for routing the synthesized sound signals. A specialized form of analog synthesizer is the analog vocoder, based on equipment developed for speech synthesis. Vocoders are often used to make a sound that resembles a musical instrument talking or singing.
Patch cords were expensive, could be damaged by use (creating hard-to-find intermittent faults), and made complex patches difficult and time-consuming to recreate. Thus, later analog synthesizers used the same building blocks, but integrated them into single units, eliminating patch cords in favour of integrated signal routing systems. The most popular of these was the Minimoog. In 1970, Moog designed an innovative synthesizer with a built-in keyboard and without modular design - the analog circuits were retained, but made interconnectable with switches in a simplified arrangement called "normalization". Though less flexible than a modular design, normalization made the instrument more portable and easier to use. This first pre-patched synthesizer, the Minimoog, became highly popular, with over 12,000 units sold. The Minimoog also influenced the design of nearly all subsequent synthesizers, with integrated keyboard, pitch wheel and modulation wheel, and a VCO->VCF->VCA signal flow. In the 1970s miniaturized solid-state components allowed synthesizers to become self-contained, portable instruments, which soon began to be used in live performances. Electronic synthesizers had quickly become a standard part of the popular-music repertoire. The first movie to use music made with a (Moog) synthesizer was the James Bond film On Her Majesty's Secret Service in 1969. After the release of the film, a large number of movies were made with synthesizer music.
Notable makers of all-in-one analog synthesizers included Moog, ARP, Roland, Korg and Yamaha. Because of the complexity of generating even a single note using analog synthesis, most synthesizers remained monophonic. Polyphonic analog synthesizers featured limited polyphony, typically supporting four voices. Oberheim was a notable manufacturer of analog polyphonic synthesizers. The Polymoog was an attempt to create a truly polyphonic analog synthesizer, with sound generation circuitry for every key on the keyboard. However, its architecture resembled an electronic organ more than a traditional analog synthesizer, and the Polymoog was not widely imitated.
In 1978, the first microprocessor-controlled analog synthesizers were created by Sequential Circuits. These used microprocessors for system control and control voltage generation, including envelope trigger generation, but the main sound generating path remained analog. The MIDI interface standard was developed for these systems. This generation of synthesizers often featured six or eight voice polyphony. Also during this period, a number of analog/digital hybrid synthesizers were introduced, which replaced certain sound-producing functions with digital equivalents, for example the digital oscillators in synthesizers like the Korg DW-8000 (which played back PCM samples of various waveforms) and the Kawai K5 (waveforms constructed via additive synthesis). With the falling cost of microprocessors, this architecture became the standard architecture for high-end analog synthesizers.
Analog synthesizers were mostly replaced by digital synthesizers and samplers over the middle to late 1980s. In the early 1990s however, musicians from the techno, rave and DJ scenes who were interested in producing electronic music, but without the budget for large digital systems, began to take an interest in the then cheap second hand analog equipment available. This led to an increase in demand for analog synthesizers towards the mid-1990s, as larger numbers of musicians also gradually rediscovered their qualities. Thus the sounds associated with analog synths became more prevalent once again. Fundamental differences between analog and digital technology mean some of the characteristics of analog synthesizers cannot be wholly replicated by samplers or digital synthesizers.
This has led to increased demand for used units (such as the 1980 Roland TR-808 drum machine and Roland TB-303 bass synthesizer). Late 1970s-era drum machines used tuned resonance voice circuits for pitched drum sounds and shaped white noise for others. The TR-808 improves on these designs, by using detuned square wave oscillators (for the cow bell and cymbal sounds) and analogue reverberation (for the handclap sound). The demand for the analog synth sound led to the development of a variety of analog modeling synthesizers which emulate analog VCOs and VCFs using samples, software, or specialized digital circuitry, and the construction of new analog keyboard synths such as the Alesis Andromeda, Prophet '08 and Little Phatty, as well as some modular units.
The lapse of patents in recent years, such as for the Moog synthesizer transistor ladder filter, has led to a small resurgence in DIY or kit synthesizer modules, as well as an increase in the number of commercial companies selling analog modules. Reverse engineering has also revealed the secrets of some synthesizer components, such as those from ARP Instruments, Inc. In addition, despite the widespread availability during the 2000s of relatively inexpensive digital synthesizers offering complex synthesis algorithms and envelopes, some musicians have been attracted to the sounds of monophonic analog synths. Another factor considered to have increased use of analog synths since the 1990s is weariness with the complex screen-based navigation systems of digital synths, with the "hands-on", practical controls of analog synths—potentiometer knobs, faders, and other features—offering a strong appeal.