The Spectrum Synthesiser (Part 1)
The Spectrum is a monophonic two oscillator switch-linked synthesiser featuring advanced specification, constructional simplicity and low cost. Modulation, timbre control, and interface facilities not found on any comparable synthesiser make it extremely powerful and versatile for keyboard playing, sound effects and many other home, stage, or studio applications. Construction is simplified by the use of integrated circuits that each perform major synthesiser functions with few external components. Error-prone control wiring is eliminated by the use of a single PCB mounted behind the panel and holding the pots (PCB mounting) and switches. No glueing of contact blocks or bending of gold wires is needed to assemble the keyboard contacts — a new contact system only requires soldering of the contacts and drilling of the chassis to mount the contact PCB. This also contributes to the low cost — the Spectrum can be built for around £200 including metalwork and PCB's, but not including the case.
Figure 1 shows a block diagram of the synthesiser and the front panel legending is reproduced below. Modulation routing is accomplished by source and function switches and depth controls, rather than the usual method of providing each source with its own depth for each controlled function found on some small synthesisers. Switching is most suitable for a large number of sources as here, and allows fast selection of source and selection of modulation effects with preset depths, in favour of simultaneous modulation of one parameter by more than two signals. Six modulation signals are available: keyboard, controller, low frequency oscillator (LFO), noise generator and external. The keyboard is of the highest note priority type and has a glide which always completes even after the key is released — this makes the keyboard much more useful as a controller for effects sounds. The joystick controller routes a voltage dependent on the side-to-side position of the stick to various voltage controlled circuits, allowing it to be used to control the pitch (pitch bend), timbre, or both (see later). The external voltage fed into the controller jack can override or add to the joystick voltage for control by additional synthesiser equipment, or a pedal can be plugged in and used for control by attenuating a fixed joystick voltage.
The low frequency oscillator generates random and regular sample and hold effects in addition to the four common waveforms. The regulator S/H option allows rising and falling scales, rising and falling repeating groups of two, three or more notes, and other sequencer-like effects, with the pattern controlled by the LFO rate. An LED displays the LFO cycle and the joystick's vertical position determines the amplitude at the LFO manual output. The envelope generator is of the exponential ADSR type and like the LFO has + and - outputs that can be separately selected for each controlled parameter. The envelope generator shares its gate signal with the envelope shaper, which determines the loudness contour of each note. 'Single' on the gate selector switch causes gating each time a first key is depressed; 'Multiple' retriggers when any new note is played, allowing fast runs without 'missed' notes. 'Hold' keeps the gate high for continuous effects, and 'LFO' causes gating on each LFO cycle. In the 'Repeat' position the envelope generator retriggers at the end of the decay period, acting as an additional LFO with variable symmetry. This allows complex rhythmic effects when used with the LFO, and gives great scope for 'backdrop' sounds based around complex S/H patterns with periodic timbre sweeping effects derived from the EG. 'Key Repeat' brings in the repeat only when a key is held, allowing key-synchronised repeating notes and delayed modulation (the delay determined by the attack time). An LED indicates the EG's attack segment.
The voltage controlled oscillators (VCO's) each have five switched octave ranges and five waveforms. The sub-octave output is a pulse wave with a square wave added an octave below, making the sound fuller and richer. The tuning LED detects the beats between the oscillators, and indicates when the pitches are in simple musical intervals, useful for tuning without sounding a note (e.g. on stage). The pulse width of VCO 1 is variable, and VCO 2 has a tune control with a ± one fifth range.
The VCO's can be used together to provide a vast range of sounds not possible with basic synthesisers having only waveform, shape, VCF cutoff and VCF resonance as the controls affecting basic timbre. This is done by frequency modulation and synchronisation — special features of this design. FM uses the triangle output of VCO 1 to modulate the frequency of VCO 2 up to ±100% giving a whole range of non-harmonic tones for bell, gong, chime sounds etc. Synchronisation gives various waveforms from VCO 2 (see Figure 2) which have particular bands of harmonics emphasised for strong, voice-box-like sounds. This is achieved by resetting the output of VCO 2 upon each cycle of VCO 1, so the tones generated are always harmonic. Two modes of sync, are provided: Sync. I is that normally found on ramp wave oscillators, the VCO 2 waveform beginning in the same way after each reset; Sync. II is something totally new — the triangle output is set to mid way each time but then carries on in the same direction in the new cycle. VCO 2 locks on to VCO 1 harmonics with the change from one harmonic to the next emphasised by a sharp change in tone. This enables automatic arpeggiation and incredible tone sweeps to be obtained since VCO 2 now is effectively a voltage controlled waveform generator/frequency multiplier. The sync. control attenuates the pulses fed to VCO 2 so that it only resets if the wave form is above a certain threshold, resulting in the oscillators being locked together in musical intervals (3rds, 5ths etc.). Simultaneous sync. I and FM produces harmonic tones with the shape of FM-ed waveforms within each cycle.
The ring modulator uses triangle and square VCO waveforms to provide further complex tones. Its output is mixed with the noise signal and fed into a special voltage controlled amplifier (VCA). This can be controlled by the LFO or EG, and gives the signals their own loudness contours. Hence noise 'chiffs' can be added to notes, or ring modulation set to swell in as a note decays.
The VCA output is fed to the voltage controlled filter (VCF) mixed with the VCO outputs. The VCF offers the two most useful responses, low pass and band pass, plus an intermediate response for bright sounds that remain strong in lower harmonics. Cutoff frequency and resonance controls perform their normal functions and a keyboard follow control determines how the cutoff frequency varies over the keyboard range.
After envelope shaping, the signal is fed to the voltage controlled pan circuit which can modulate the location of the sound in the stereo field by the LFO or EG signals. The stereo outputs can also be used for voltage control of the depth of external effects such as reverb, phase, and echo, by routing one signal via the effects unit and one direct to the amplifier. A mono output is also provided, and the pan VCA can also be used for additional amplitude modulation with the LFO as source (for tremolo and other effects).
The interface jacks allow connection to external devices such as sequencers, additional VCO banks, waveform processors etc, and designs for these will be published in the future. The Spectrum Synthesiser and future equipment will use the 1V/octave CV standard, and can be interfaced to any other exponential CV synthesiser.
The Spectrum uses a unique key contact system which is cheaper, more reliable, and easier to construct than alternatives using gold-plated wire and contact blocks. A single moving contact is used for each key with all contacts and their associated divider chain resistors held on a PCB (in two parts) fixed to the keyboard chassis.
The moving contacts are silver plated springs, each fixed at one end and moved at the other by the plunger of the respective key such that the spring makes contact with two palladium bars when the key is depressed (Figure 3). The first bar is connected to the sample and hold circuit which stores the voltage representing the last key depressed, and the second to a circuit which generates a gate signal for the S/H and the envelope generators. The moving contacts connect to the divider chain (see Figure 4). These functions are usually carried out by separate contact pairs, where unless the contacts are precisely set up, note-jumping will occur when the envelope is gated before the S/H receives the new key voltage. The system used here is immune from this since the construction ensures the correct sequence of operation, and no initial setting up is required. The keyboard recommended in the parts list has removable key plungers so that cleaning the contacts is much easier too. Unclipping a plunger allows access to the sides of the bars and springs that meet.
Use the printed circuit board as a template to mark the fixing holes on the underside of the keyboard chassis. Mark them such that the edge of the board holding the bars will be about 5mm from the plungers and then drill for 6BA clearance. Fit the 48 divider resistors on the component side of the board along with the 12 veropins and solder in place. Cut the palladium bars to length and fit them to the track side using small loops of wire passed over the bar, through the mounting holes and twisted on the component side. Make sure each bar is well seated before soldering at each loop position on both sides. Cut each plunger to length, leaving the nearest slot to the key end for the contact. Tin 5mm of both ends of the contact springs and fit each one by passing the thin end through the detached plunger and soldering it to the pad on the PCB. If you've marked the PCB mounting holes correctly then for proper operation the end of the spring should be about 2mm from the far edge of the pad. The positioning of the PCB and the springs on the PCB is not critical as long as when the PCB is mounted and the plungers clipped on, the springs are under slight tension to ensure positive contact. Mount the PCB to the chassis using 6BA bolts, ½" spacers and nuts, and washers to separate them further. The keys opposite the mounting positions will have to be temporarily removed to fit the bolts, and this should be done before drilling if a hand-held drill is used, to avoid the possibility of damage to the keys. Again, the spacing is not critical so long as all the contacts normally clear both bars and make contact with both when their keys are depressed.
A ½" spacer and one nut were found to be about right, though washers could be used if a high or low action to the keys is preferred. Connect the two halves of the board together using short wire links across the veropin pairs. This completes the keyboard construction.
The assembly can be tested on its own by connecting a multimeter set on the low resistance range across pins 1 and 3. Depressing the bottom key should give zero resistance, and the top key about 2.4K. Check that all the other keys give intermediate readings and repeat for the other bar with the meter across pins 1 and 4.
The proper operation of synthesiser circuits requires a stable, noise-free supply, so it is important that the Power Supply Unit (PSU) is well-regulated and has current in reserve. The Spectrum PSU uses monolithic regulators and low temperature coefficient components in a dual design to provide ± 15V at 270 mA maximum.
The Power Supply Unit consists of two identical circuits providing the positive and negative supplies, driven by a dual secondary transformer. Each secondary produces about 21V when the AC signal is rectified and smoothed, and is fused for protection in the event of a power supply fault. Regulation is carried out by the well-known uA723 regulator IC which is used with an external power transistor in series pass mode to provide the required current. This current limits at 270 mA when the voltage across series resistor R1 (R2 in the -ve side) reaches 0.6V. RV1 (RV2) allows the rail voltage to be adjusted to exactly 15V, and D1 (D2) protects against reverse polarity, again in the event of a fault. The +15V regulated output of the side based around IC2 is connected to 0V of the IC1 side, giving the -15, 0, +15V supply rails.
The prototype PSU was mounted on an aluminium panel along with the mains cable, switch, and fuse. This was then fixed by screws to the wooden back of the synthesiser allowing the PSU to be separately assembled and easily removed if necessary (see photograph).
All components except the transformer fit on the PCB. Assemble the components onto the printed circuit board, starting with the resistors, diodes, polystyrene capacitors and IC sockets. These can all be inserted and soldered in together, before the large components are fitted. Note the orientation of the diodes, particularly in the bridge rectifiers (D1-8).
Use 4BA bolts and nuts to fix the chassis fuse holders, and connect them using veropins through the tag holes. Insert and solder in the cermet presets and electrolytic capacitors, followed by the two power transistors. The leads leave the transistors quite close together and should be bent apart about 1/8" from the package before putting them in place. Check that you have put them in the right way round, with the metal sides facing the nearest board edge. Solder in the eight remaining veropins and check the orientation of the electrolytic capacitors diodes, and transistors. Fix the PSU board to the back panel, or whatever else you are using, using 6BA bolts with spacers. Fit the mains switch, fuseholder, and transformer to the panel, using 4BA bolts for the latter and including two solder tags on one side for the earth connections. Strip back a length of the mains cable and cut off some of each core for connecting the switch, fuse, transformer and PCB. Connect these as shown in Figure 7, and then connect the mains cable and secure it with the grommet. The fuse is wired on the supply side of the switch, so that the switch neon will go out if the fuse blows. Fit a mains plug with a 3amp fuse and then check the wiring before going on to the next stage.
Before inserting FS2, 3 or IC1, 2 first insert FS1, plug in, switch on, and measure the voltage across each secondary. This should be around 15V RMS. Now fit FS2 and 3 with the power off, switch on, and measure the voltage across C1 and 2, each of which should be about 21V. Switch off again, insert IC1 and 2, switch on, and measure the output voltages (across points 6,5 and 7,6). If you are confident in your constructional abilities, you can leave these checks and try the PSU with all the fuses and IC's in first time. Either way, the rail voltages should be measured and RV1 (+ve) and RV2 (-ve) adjusted so that they are exactly 15V. An oscilloscope will probably be more accurate for this than a cheap multimeter, though use a digital multimeter or a good mechanical meter if one is available.
Digisound Ltd. will be offering a full set of the CEM IC's used in this project at the special reduced price of £29.00 inc. VAT and postage. Ready-made metalwork and PCB's will be obtainable from Maplin Electronic Supplies. A cassette demonstrating the Spectrum's facilities will be made available from E&MM.
Feature by Chris Jordan
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