Designed for ETI by Tim Orr, late of EMS and father of some of their range, our new Transcent 2000 is a new concept in DIY synthesizers - a single board design! Apart from the PSU all the circuitry is contained on one easily assembled PCB. Ideal as on-stage machine, the 2000 has plenty to offer the experimenter as well. THE SYSTEM BLOCK DIAGRAM is shown in Fig.1 . The system is prepatehed, but is capable of generating a vast variety of different effects by virtue of its 9 switch functions, 22 pots and 6 input jacks. The VCO is the primary sound source. It produces either a ramp or a square waveform. A ramp waveform has both odd and even harmonics, the square wave has only the odd ones. However, the VCO has a shape modulation circuit which can turn the ramp into a triangle or the square wave into a thin pulse. Thus, a wide range of harmonic structures is available. Also, this shape madulation can be controlled by a sine wave produced by the slow oscillator. By dynamically modulating the shape of this wave- form, it is possible to greatly enrich the sound quality of the VCO (for instance, if the mark space ratio of the squarewave is modulated at about 1 Hz the output can sound like two VCO s.) Pitch It Well The pitch of the VCO can be controlled by several sources. A 'pitch-bend' pot enables notes to be bent up or down by about 1 /2 an octave. A dead band in the centre of the motion enables the turning to be restored. An external input socket with a sensitivity of 1 V/octave allows a sequencer to be connected. A manual tuning pot, (screwdriver adjustment), is provided so that the synthesiser may be tuned to the pitch of other instruments. Vibrato may be added, the speed being that of the slow oscillator. The squarewave also from this oscillator can be used to produce 'two tone' effects. The VCO pitch can be controlled by the ADSR envelope or by random pitches generated by the noise sample and hold circuit. All these controls can produce a wide variety of interesting sounds but the machine really comes alive when it is controlled by the keyboard. This keyboard is a 3 octave, (37 note), C to C device. It is monophonic, that is it only plays one note at a time, this being the highest note selected. It generates two outputs, a pitch signal and a gate voltage. The gate controls the AD and ADSR sections, the pitch, the VCO and the VCF. The pitch voltage is a transitional piece of information which has to be remembered in an analogue memory, a sample and hold device. The droop rate of this S & H is about 15 minutes per semitone. This is quite good. Gliding In A portamento circuit has also been included into the sample and hold so that glides, as opposed to abrupt changes, between notes can be produced. A transpose switch + 2 octaves operates on the VCO. This gives an effective keyboard control range on the VCO of 7 octaves. The keyboard S & H can be controlled by either the keyboard gate or by a pulse from the slow oscillator. This latter mode of operation makes the VCO pitch move in a series of exponentially decreasing steps between the notes played on the keyboard. Noisy Output The output of the VCO is mixed with a noise signal and an external audio signal and fed into the VCF. This is a voltage controlled state variable filter, with both bandpass and lowpass outputs. The resonance is manually controllable from a Q of 1 to infinity, (self oscillation). The resonant frequency may be controlled by either a manual pot, a sweep voltage from the slow oscillator, an external footpedal control, the keyboard voltage or a random voitage or an attack decay envelope. There are very few musical instruments that have any sort of dynamic filtering. The Attack/Decay envelope can be used to produce a rising or falling frequency sweek in the VCF, and by varying the AD time constants, a wide variety of sounds may be generated. The output of the VCF passes through a voltage controlled amplifier to the output socket. This can be on all the time, or it can be controlled by an ADSR envelope. This in turn amplitude modulates the VCF signal so that the output has the envelope of the ADSR voltage. Sustaining Interest The ADSR is a waveform generator, and is initiated by the arrival of a gate voltage. When this arrives it generates a rising RC exponential waveform with a time constant determined by the Attack pot. When it reaches a predetermined level it then begins a RC decay towards a sustain voltage. The 'decay' rate is controlled by the 'Decay' pot and the sustain level is set by the 'Sustain' pot. It sits there until the gate voltage is removed, (when the keyboard is released), whereupon it decays towards ground with a release time constant, this being determined by the 'Release' pot. If at any time the gate is removed the ADSR goes into its release mode. Time constants of 5 mS to 2 S and sustain levels of full on to completely off are obtainable. On Key The ADSR can be started by the keyboard, or it can be continuously repeated by the slow oscillator, or it can be repeated by the slow oscillator gated by the keyboard, as can the Attack Decay, (AD), circuit. This has two modes of operation: single shot, whereby it attacks to a predetermined level and then decays on its own to ground, or HOLD ON, whereby it only decays upon the removal of the gate signal. Sometimes when playing pieces, it may be necessary to release a key before a new note can be generated. If the piece is particularly fast then errors, in the form of missing notes can occur. However, a device called the New Pitch Detector (NPD), can help eliminate this. When a new pitch is detected, it generates an additional gate signal which is used to reset both the AD and the ADSR. Repeating Both the AD and ADSR circuits can be controlled by the REPEAT function. This is a single piece of electronics to enable repeating envelopes to be generated. The outputs from this circuit then drive the AD and ADSR. With the repeat switch in the ON position, the slow oscillator square wave output continuously gates the AD and ADSR. A DeeEssAhh? The ADSR is similar in operation to the AD circuit except that it has two more parameters to play with. Upon receipt of the keyboard gate the waveform attacks until it reaches a predetermined level. Then it decays to a level known as the sustain level, which is manually controllable. When the keyboard gate is removed the release mode occurs. The A, D, R are all time constants the S is a level. Whenever the keyboard gate is removed the device goes into its re1ease mode. This type of envelope is particularly useful and versatile. With the sustain level at 10 there is no DECAY phase and so an ATTACK, HOLD ON, RELEASE envelope is generated. When the sustain is set at 4, there is an attack and a decay to the sustain level, which is held as long as the keyboard is held down and then a release. Using this setting it is possible to simulate a piano sound, by using a fast attack moderately slow decay and a faster release. The faster release simulates the damping of the strings as the piano keyboard is released. When the sustain level is set at 0 then the unit becomes an attack decay envelope which can be used to produce short sharp plucked sounds. To get a new envelope it is necessary to get a new keyboard gate signal. This either means lifting your finger off of one note before pressing the next, or a new gate can be automatically generated by switching to the NPD mode. Moving On The pre-patched nature of the design is intended to suit stage and other performance applications. The resulting sound from the synthesiser can be quickly and easily modified once the function of the controls and their effect has been mastered. Take a look at the diagram . Another helpful aid to using a synthesiser is a 'program sheet' -- simply a way of recording clearly but instantly a particular set of control settings to allow you to reproduce that sound again at a later date. Such sheets will be available for the 2000 . Despite the high complexity of this project, its construction should pose no electronic problems to the competent hobbyist. As with any synthesiser however fitting the keyboard and its associated mechanics will prove the most onerous task. Since you have to start somewhere, the PCBs are the obvious place. There are five boards all together; power supply, keyboard contact mounting (X3) and main synthesiser. The keyboard we will deal with later. Assembly of the PSU board is very straightforward, but take care fitting the heatsinks to Q1 and Q3. Wire up the board to the transformer, and check that you can obtain the correct voltages at the output: Adjust RV1 until + 12V is obtained on the red output wire. Set to as close + 12V as you can possibly measure. Check that an accurate - 12V is present on the blue output wire. The power supply is now complete. Main Line For the main assembly we're going to assume that you're using the Powertran board. Fitting the components to this is straightforward with the exception of the switches and pots. In order to line up the switches with the front panel and pots, it is necessary to space these from the board - the kit contains suitable spacers for this purpose. Non-kit types have to work out the height of their front panels from the board and act accordingly. The switch toggles must come level with the pot spindles, when cut to take the control knobs. In either event glue the spacer to the board - use some powerful adhesive such as Super-Glue etc. Cut the pot spindles before you fix them to the PCB : it's just too big to handle and too expensive to crack. The terminals should be top soldered onto the board, as should the chiripins used to mate up with the connector. Take care the solder does not run down the pins, else the plug may not fit at all. Use insulated wire to link the dual gang pot RV30 and the waveform switch to the board. The PCB cannot be mounted into place until the alignment proceedures have been carried out, so there is no excuse for not checking the assembly very carefully indeed especially the IC orientation and soldering quality. This is a BIG board which means there is more space to be careless: - check it! Powerfull Mount Following the rear panel wiring diagram, fit the hardware onto the case, taking care to mount the trans- former as low down - away from the main PCB - as possible. This will lessen the chance of hum being induced into the circuit. Insulate the mains wiring wherever possible, and take careful note of the earth wiring arrangements - lest the demon hum return to plague thee! Anything with mains voltage on should have a rubber sleeve over it. Keyed Up? Now for the tricky bit. The keyboard. This has to be mounted in the casing first. For this the front panel should be in position. Fix the brackets to the ends of the keyboard assembly, and lower it into place. Follow the diagram below to adjust the spacing at either end of the keyboard. If the gap is more than 0.1 ", the woodwork underneath the fixing screws will need countersinking. Set the gap between the black notes and the front panel as shown. Once the alignment is eorrect, screw the brackets into the woodwork. Onto the contacts. Fit and solder the 27 R4 resistors to the three PCBs as shown on the diagram above. One will be full with 12 resistors and the other two have eleven each, one missing at the right end on one PCB, and one missing at the left end of the other. (There are two resistors left over at this stage.) Close Contacts Of The Key Kind Solder in the contact assemblies, but make very sure that right angles exist between the block and the PCB. This is imponant. Graph paper may help in lining up. Leave out a contact block where the resistors are omitted the diagram may hel p. All three PCBs are wired together as shown in the keyboard wiring diagram taking care to place the 'gaps' correctly. Get the spacing correct by lining up the eontact blocks with the keyboard plungers. Note the positioning of those two spare resistors. Sand down one side of the contact strip, and lay some contact adhesive all over it, and the same with the soldered-in contact blocks. Make very sure that before you affix the strip onto the blocks that you have lined it up properly, as once the glue gets hold you've had it. The contact blocks are very delicate, so handle them carefully, and don't touch the wires with your fingers. If you do you'll leave a deposit behind which may well cause malfunction. When satisfied that the assembly is O.K. position it over the plungers, and screw it down to the metalwork. Check that every plunger operates a contact and that both contacts in each block operate when the key is depressed. Keyboard completed - wasn't that bad after all (was it?) The contact assembly for the keyboard should be the last thing you fit into place before wiring up all the boards as per the interconnection and wiring diagrams. With this in place fit the base plate. Alignment is best carried out with the front panel removed, and the PCBs fixed in. Before commencing align ment though, check everything very carefully. When attaching the front panel to the machine, check that it does not foul the keyboard, and that the gap between it and the woodwork is the same at either end. Alignment This will be dealt with in sections. To aid setting up and alignment procedures, test point waveforms are given for important nodes throughout the design. VCO Alignment There are several pitch controls for the VCO. All control voltages are injected via large resistors and are thus suitably attenuated. The pitch bend pot uses a couple of diodes to produce a dead zone in the middle of its motion, This control voltage is then fed in via a 180k resistor and mixed with all the other control voltages. Pitch Spread The keyboard sample and hold produces 830mV/octave. This has to be attenuated to 18mV to produce octaves. To do this a resistor of 46k is required. R31 and RV3 constitutes a variable resistor (39k2 to 49k2) that should enable the keyboard pitch spread to be aligned. Turn RV6, 7, 8, 9 fully anticlockwise. Put the transpose switch in central position. Set RV5, 4 to their central position. Play the top note on the keyboard and measure its frequency, using a scope or a frequency meter, or maybe if you are a musician just listen to it! Now play a note one octave below it and adjust RV3 until the interval is one octave below it and adjust RV3 until the interval is one octave. Recheck the top note and then try the tracking for two or three octaves down, making any necessary adjustments to RV3. Note that the top note on the keyboard is not affected by RV3 adjustments. Now put the transpose switch to + 2 octaves and adjust RV12 for a 2 octave increase. Then switch to - 2 octaves and adjust RV13 for a 2 octave decrease. VCO Shape Modulation IC14, 13, 15 is the VCO shape modulation circuitry. IC14 is a half wave rectifier, and is used to sum together the manual shape voltage (RV14), and the sine wave voltage from the slow oscillator. The output from this circuit is limited to a range of 0 V to about - 10 V. As the manual shape pot is rotated clockwise the waveform at the junction of R57, R59 will change from a ramp into a triangle, this being due to the full wave recti-fication. With RV14 fully clockwise and RV15 anticlockwise adjust RV15 so that the waveform is a symm-etrical triangle. The last shape generator is a fast comparator. The ramp waveform plus the modulation voltage are fed into the comparator input. The modulation voltage shifts the DC level of the ramp and in doing so the comparator levels change resulting in a varying markspace ratio output, IC15 pin 6. The diodes limit the voltage excusion to about + 0.5 V. Set the VCO to + 2 octaves, tune the keyboard high anr! play the highest note. Now set RV10 anticlockwise, RV14 clockwise and monitor the squarewave output, IC13 pin 6. Adjust RV11 until a very thin pulse is generated. Rotate RV14 anticlockwise and the markspace ratio will revert to 1 to 1 . Now set RV14 to 5 on the dial and slowly rotate RV15. The markspace ratio will be modulated at the speed of the slow oscillator. There may be some problems with control breakthrough in the VCA but this can be minimised with a present adjustment, RV22. Turn the VCO and noise levels to 0. Make sure that the filter is not oscillating. Put the ADSR on a fast repeat with fast attack and decay and no sustain level. Set the BY-PASS switch to ADSR and look at the synthesiser output. There will probably be some control break through caused by the ADSR, which will sound like a series of thumps. By adjusting RV22 a minimum in the thump level will be found. Just like the 3080s in the VCF, best performance can be obtained by carefully selecting IC22. The VCF pitch spread should be set up as follows. Turn off RV16 and RV33. Switch the filter 'CONTROL' to KB, the 'RESONANCE' to 'OSC' and the 'AD SWEEP' to 0. Play the top note on the keyboard and adjust the 'FREQUENCY' pot to give a 1kHz sinewave output. Now play a note, one octave below the top note and adjust the present RV20 for a one octave decrease. Check the lower octaves making any necessary adjust- ments to RV20. Turn the RESONANCE pot anticlock wise until the filter stops oscillating. Turn up the VCO level and insert a ramp waveform at a frequency of about 100 Hz. Now switch the VCF 'Control' to RANDOM. The tone of the filtered signal should now vary randomly. Problems? Any problems in the VCF circuitry are likely to emanate from IC16 or IC19. If there are any large input offset voltages or current mirror imbalances or output leakage currents, then these will degrade the VCF performance. What will probably occur is that there will be a large DC offset voltage on the outputs that varies as the resonant frequency varies. This may cause severe signal dipping at certain frequencies and will only be cured by replacing the errant 3080. The filter has two outputs, a bandpass and a lowpass. The signal volume will generally be less from the band- pass output because this output attenuates all but the harmonics that lie close to its own resonant frequency, whereas the lowpass output has a flat response area which extends from somewhere just below resonance down towards low frequencies, and harmonics in this region are not affected. Sweeping Statement The 'synthesiser sound' is generated by sweeping the VCF resonant frequency with an AD waveform. This sweep voltage is variable in both depth and direction. The sweep pot is a dual pot; on one of its tracks there is an AD waveform at one end and the inverse at the other. Thus the wiper will pan from a sweep going upwards to one going downwards. Two diodes provide a dead zone in the middle so that a pot position of No Sweep can be easily found. The second track on the AD sweep pot is used to provide a compensating DC level shift so that the frequency pot doesn't need to be retuned when the AD sweep depth is altered. ADSR Alignment Set up the VCO and VCF so that a ramp waveform at 500 Hz is presented to the VCA. Turn the RELEASE pot fully clockwise and put the BYPASS switch in the ADSR position. Listen to the VCA output and adjust RV28 so that the signal only just disappears. Now you can play the keyboard and experiment with different sounding envelopes. This completes the alignment procedures for the design, and the front panel can now be fitted over the main PCB and the control knobs added. Take care with the panel and keyboard positioning. See the diagrams where applicable.