Analog sequencer

An analog sequencer sequentially steps through a series of adjustable CV/Gate signals to control synthesizer modules, e.g. oscillators from the control voltage and envelope shapers from the gate signal. Typically it is used to produce repeating 4 to 16 note tonal patterns or arpeggios.

Description
The simplest form of analogue sequencer succesively progresses through a number of steps. The active step is indicated by an illuminated LED and a potentiometer or slide-fader at each step determines the output voltage. An internal or external clock generator causes the sequencer to step from one pot to the next, and from the final step back to the first. If the CV output is between 0V and +2V and is patched into the pitch CV input of a 1V/octave oscillator, the pitch can be set anywhere in a two-octave range. This makes it possible to construct simple melodies that repeat continuously while the clock is running. The configuration of the pots or faders on the sequencer exactly mimics the notes as they would appear on a musical staff.

A more complex sequencer will offer further features e.g. multiple rows of sequences, multiple outputs, individual variable clock inputs for the rows, CVs for the clocks themselves, range controls for the CVs, note-skip capabilities, variable sequence lengths, and others. However the underlying concept remains the same.

Voltage Quantisation
The CV pots are capable of producing any voltage within their range, including values that lie between the notes in a conventional musical scale. For precise tuning a Voltage quantiser, or simply a quantiser is required. This device rounds every voltage passing through it up or down to that which produces an exact semitone. No matter what the input voltage may be, the quantiser output is the nearest 1/12th Volt defined by the well-tempered scale for a 1V/Octave synthesizer. This makes it much easier to create recognisable melodies or bass lines.

Circuitry
Traditionally most analog synths sequencers relied upon CMOS counter chips, working like simple shift registers which just pass a value along. More recent CMOS analog sequencers such as the Klee or CGS13 use actual shift register ICs instead. With these since the bits are accessible, logic, switching, and other functions can be applied within the sequence.

Programmable analog sequencers use a microprocessor or microcontroller to set the count. These are capable of more modes than simple logic chips.

Compared to MIDI
Analog sequencers have no audible difference to MIDI. Neither generate audio both pass data but the data is encoded differently, CV/Gate vs. encoded 0s and 1s. Analog sequencers will never be as precise nor as algorithmically versatile as digital sequencers. However they're said to be more intuitive, better for hands-on interaction or live performance and feature different functions such as:
 * Controlling modules or devices with CV/Gate interfaces.
 * Not just oscillators but sequencing other modules such as filters, LFOs or envelopes.
 * The ability to vary tones without regard to a standard musical scale.
 * To control parameters such as the tempo of the sequence from the output of other modules, to create interacting sequences.
 * Triggering the sequencer at audio rates thus using the sequencer to create unique waveforms.

DIY analog sequencers

 * ARP 1601 Sequencer
 * Klee sequencer
 * Ryk M185
 * Velleman MK107 Sequencer

Schematics

 * 16 step sequencer schematic by Armen
 * Arp 1601 Sequencer clone, Muff Wiggler forum, December 2015
 * The Baby 10 Sequencer
 * Chrutil Modular MU Sequencer
 * Cloned Analog Gear, various from Elektor, PE, ETI and Thomas Henry
 * Digisound 80 80-22 Patcher, "can function as a simple 4 channel, 8 step/note sequencer"
 * DIY Sequencer circuits, Hyperreal.org. Including Superseque by Thomas Henry
 * ETI International 603 Sequencer, Aug. 1977 dual 8 step or 16 step, 5V control voltage, external sync mod
 * Fonik 8-Step Sequencer with voltage controllable pattern (VCPS)
 * foniks baby 10 sequencer
 * Hackme Vectr
 * Ken Stone's CGS modules, a few here and some links PCBs to kits
 * Klee Sequencer, electro-music.com
 * M185 sequencer Project - Software Update !!, electro-music.com. Based around a PIC.
 * MFOS 16 Step Sequencer by Ray Wilson
 * MFOS Ten Step Analog Sequencer by Ray Wilson
 * Mikado Analog Sequencer
 * PAiA 4780 Sequencer Assembly and Users Manual with Schematics
 * Practical Electronics 16 Note Sequencer by Linsay Robinson, Part 1 March 1979, Part 2 April 1979. Corrections in PE May 1979 p. 45, September 1979 p. 32
 * SEKU Analog Sequencer
 * Sequencer Schematics, E-Music DIY Archive. ETI 603, Oberheim Mini-Sequencer and PE Sequencer
 * Simple Sequencer, Instructables
 * sympleSEQ, hexinverter.net
 * Synthrotek sequencers, 8, 10 and 16 step
 * SynthR 960 Clone
 * The Design, Construction, and Operation of an Electronic Music Synthesizer by Joe Paradiso, 1977, pp. 88-95, pp. 124-128, includes modified twelve stage PAIA sequencer.
 * Turing Machine

PCBs and kits
Not that many around as of writing this (early 2016), and a listing here won't stay up to date. It'd be better to search through PCB and kit suppliers to see what's currently available.

Music theory

 * Time signature, Wikipedia