Sequential Circuits Prophet-5

The Sequential Circuits Prophet-5 released in 1978, was the first completely programmable polyphonic analog synthesizer. Dave Smith designed the electronics and coded the firmware.

Overview
The Prophet-5 is five voice subtractive synthesizer. Each voice is an individual synthesizer with two VCOs (voltage controlled oscillators) osc A and osc B, a mixer, a 4-pole resonant low-pass VCF (voltage controlled filter) controlled by an ADSR EG (envelope generator) and a final VCA (voltage controlled amplifier) controlled by another ADSR EG. It also has a single LFO (low frequency oscillator) and pink noise source.

Both oscillators can generate sawtooth wave and square waves (with variable pulse width). Osc B can also generate a triangle wave and be detuned over a semitone. The oscillators can be played in sync, or in poly-mod (polyphonic modulation), where osc B and the filter EG modulates the frequency or pulse width of osc A, or the filter cutoff frequency.

Controlled by the mod-wheel the LFO (with square, saw or triangle wave options) can be mixed with pink noise to modulate the frequency or pulse width of the oscillators or the filter cutoff frequency. The Prophet-5 uses a 4-pole resonant low-pass filter. The filter has a dedicated ADSR envelope and keyboard tracking.

The filter EG modifies the voice timbre and may also serve as a sound source. The VCA EG shapes the voice amplitude. The voices are summed together with a single white noise source. With a keystroke a voice is gated to trigger its two EGs.

The CPU keeps the VCOs in tune, generates most CVs from patch memory or panel knobs and switches, and assigns the voices to the keys being played.

Revisions
The Prophet 5 sustained six revs before Sequential Circuits, Inc., was sold to Yamaha in 1987. Rev 2 was a refinement of the Rev 1 models and largely transparent. Rev 3, however, was a redesign. Introduced were new voltage controlled IC's (CEM), an improved ADC, DAC, and a different control voltage distribution scheme. More sophisticated editing and tuning routines were designed, and to improve serviceability voice trimmers were reduced from 80 to 45. Some have said that Prophet 5's with the early VCO chipset (SSM) have a beefier sound, but those knowledgeable with early and later revs have said the contrary. As far as I know Dave Smith and John Bowen have never commented on this, so for the time being I'll err on the side that there is little, or no sonic difference. What is indisputable, however, is that the majority of the Rev 3 synthesizers are more operationally stable than their Rev 1 and Rev 2 counterparts.

There were three revisions of the Prophet-5 between 1978 and 1984:
 * Revision 1,, instrument code 1000.1,	serial numbers 1-182. The least common. Uses SSM ICs. These were very unreliable. Cannot be retrofitted for MIDI.
 * Revision 2, instrument code 1000.2, serial numbers 184 to 1299. Uses SSM ICs. Is usually considered to be the better-sounding version of the Prophet-5.
 * Revision 3, instrument code 1000.3.0 to 1000.3.3, serial numbers from 1300. The most common. Uses Curtis ICs for increased stability and reliability. Is considered by some to have a thinner sound than the previous revisions. These start at serial number 1300.

Unusual components
The Prophet's eleven oscillators - two per voice and one LFO, are based on the CEM 3340 VCO IC, (before Rev 3 the SSM 2030). The IC is scaled at 1V/octave. This means that an overall CV change of 1V ideally produces a pitch change of exactly one octave. So, a CV change of 1/12V (83.3 mV) changes pitch by one semitone.

The Prophet's five low-pass filters are based on the CEM 3320 VCF, (before Rev 3 the SSM 2040). Its scale matches the VCO scale of 1V/OCT, so it is also adjusted in semitones by 83-mV DAC steps.

The filter and amplifier envelope generators are based on the CEM 3310, (before Rev 3 the SSM 2050 EG). Its output is a positive DC voltage whose level changes over the time periods set by the input timing CV's. When applied to the VCF and the final VCA the transient voltage determines the frequency and amplitude contours of the voice.

The voltage controlled amplifiers are based on the CA3280 transconductance op-amp IC (before Rev 3 the SSM 2020).

The CPU is a Z80.

MIDI
Some of the Prophet 5's that were MIDI'd at the factory or the early Rev 3 retrofits were problematic in that they would lock up randomly, and more often than not, after the units had been on for a while. The reason for this was that the retrofit demanded increased current from the +5 Volt digital supply. The supply design "floats" its 7805 regulator ground via a diode, producing an output voltage of +5.6 volts to allow some headroom (a series diode on the CPU drops this back to 5 volts). In some cases, with component aging and high ambient temperatures, the supply voltage dips below acceptable levels causing the instrument to lock up. The solution to the problem is to add an additional diode to the power supply board, in series with the existing one. This can be done without disassembling the power supply: locate diode IN4002 (D505) at the center of the power supply board just behind the solder pads for the 7805 regulator. De-solder the left (anode) diode lead only and prepare a IN4002 and install on the board with the cathode (banded end) up. Complete by soldering the two diode ends together, keeping the leads no more than 3/8". This procedure isn't a cure-all for Prophet 5 lock-ups as less common causes are power supply, CPU malfunction, battery backup, clock and reset circuits.

Other
Section 11 of the technical manual for Rev 3 describes converting Rev 3.0 or 3.1 Prophets to Rev 3.2.

Section 13 of the technical manual for Rev 3 describes a modification to increase the patch memory from 40 to 120.

Manuals & schematics

 * Synfo.nl, rev.1 service manual, rev. 2 & 3 technical manuals
 * Synthfool.com, Rev 3.3 owners & rev. 3.0/3.1/3.2 technical manual
 * Fdiskc.com, technical manual rev. 2
 * Archive.org, Rev 3.3 owners & rev. 3.0/3.1/3.2 technical manual

Mods

 * Modification for the rev.3 by Juergen Haible
 * Kenton MIDI Retrofit Kits for Prophet 5 Rev 2 and 3.x
 * Electric Druid Noise Generator, replacement noise generator for Sequential Circuits Prophet-5
 * Prophet 5 Technical assistance by Bob Weigel

Prophet-5 like synth modules

 * AMSynths AM8320 Prophet Filter
 * Dave Smith Instruments - DSM01 Curtis Filter
 * Division 6 Filtare SEIII
 * Doepfer A-122 24dB Lowpass (CEM)
 * Synthesis Technology E440 Discrete OTA VCF
 * Tiptop Audio Z2040 Prophet-5 Filter