Digisound Voice Card (Part 1)
Detailed notes on the design and construction of a complete synthesiser voice card you can build yourself, by the team that put it all together in the first place.
Connect it to a keyboard and you've a complete monophonic synthesiser, build several modules and you've got a versatile polysynth. Whatever you do, the Voice Card represents value for money.
The voice card design presented in this article provides the complete sound processing circuitry for a monophonic synthesiser voice. On-board design includes two VCOs, a four-pole low pass VCF, two ADSR envelope generators and five VCAs which together comprise a relatively elaborate synthesiser voice. Some of the interconnections between the above circuit blocks are hardwired, but a degree of flexibility has been maintained by the use of electronic analogue switches. The circuit functions are controlled either by manual potentiometers (which are disabled on insertion of a jack plug into the appropriate CV input socket) or by independent potentiometers and external CV input sockets.
A schematic diagram of the voice card circuitry is shown in Figure 1. The design utilises two CEM 3310 VCTGs, two CEM 3340 VCOs, a CEM 3360 dual VCA and a CEM 3372 signal processor. An accurately scalable keyboard control input is provided and may be calibrated to the usual one volt per octave control voltage standard. Inputs are also provided to enable connection to external equipment such as LFOs, sample and hold networks and other control voltage generators. An audio input is also available, allowing an external noise source (or additional oscillators) to be mixed with the audio output of VCO1.
Many constructional options exist, and these include:
a) Use of a single voice card, with associated pots and switches mounted on a suitable control panel
b) Use of several voice cards together in parallel on a simple motherboard with a common control panel and separate input/output sockets
c) Use of multiple voice cards with individual panels
d) For experienced constructors, microprocessor control of one or more voice cards via a suitable interface.
Additionally, owners of existing keyboard equipment may use voice cards as expansion units, always assuming that a one volt per octave keyboard CV and positive-going gate signals are available.
The complete circuit diagram for the voice card is shown in Figure 2. The circuit design is centred around six custom music ICs from Curtis Electromusic Specialties. It's the use of these devices that's enabled a highly compact dual VCO synthesiser voice to be built on a standard 100 x 200mm Eurocard.
The design makes use of all-electronic internal switching and patching (controlled by panel-mounting SPDT sub min toggle switches S1 to S16) and is arranged for maximum flexibility. Connections to and from the PCB are grouped along one long edge of the board and may be effected either by hardwiring or by the use of Molex connectors and a small motherboard. It's the first of these options that we'll discuss here. There are 57 connections in all and these can logically be subdivided into the following groups: control voltage inputs (26), electronic analogue switch control lines (16), audio inputs (1), audio outputs (1), gate inputs (1), power supply connections (5) and those reserved for future expansion (7). All the control inputs provided will accept voltages in the range -5V to +5V (see Table 1), making interfacing to microcomputers a simple matter.
Both VCOs are based on the CEM 3340, and are configured in a similar manner. For this reason detailed analysis of only one of these oscillator blocks is necessary, though we'll mention any differences at the relevant point. The main frequency control input is at pin 15, configured as a summing stage and thus allowing multiple independent frequency control. In this application, resistors R1-8 effect the required voltage control, with two on-board CV inputs and four independent CV inputs being available for connection to external equipment. The on-board CV inputs are:
a) Possweep - ADSR output via VCA and to CEM 3340 via R4
b) Internal exponential modulation - from VC02 via a separate VCA.
The other inputs are:
a) Frequency - via R1 to effect tuning of the oscillator
b) Keyboard CV - via R6 for connection to a musical keyboard (commoned to both VCOs)
c) Pitch bend - allows detuning of VCO1 or connection to joystick controllers and the like.
d) Exponential LFO - for connection to an external LFO whose output will vary frequency in an exponential manner
Pin 14 of the CEM 3340 is used as a scaling factor. Since the current gain of the internal multiplier is set near unity, 100K input resistors and a 1K8 scaling resistor (R16) produce the standard one volt per octave response and about 18mV at the base of Q1 (internal to IC). R8 and RV19, together with an incoming reference voltage (in this case + 15V), set the initial frequency of the oscillator, and have been chosen so that with no external voltage applied, the frequency may be adjusted to 65.406Hz, the frequency of the lowest note on a four-octave keyboard. For greatest accuracy of the internal multiplier, the current from pin 2 should be close to that from pin 1. This balance is achieved by way of RV20.
Pin 13 is available for use as a linear frequency control input with R12 present for connection to an external LFO. R12 produces a 10% change in frequency per volt at this input. Similarly, the inclusion of R10 and its associated electronic switch (IC11D) connects pin 13 to the output of VC02 in the same way (and to the same modulation bus) as the internal exponential modulation input. Note that if the output from a suitable LFO is not capacitively AC coupled, any DC offset present will cause detuning of the oscillators. In addition, a negative current at pin 13 which is in excess of the reference current will gate the oscillator off.
The 3340 is compensated against temperature-induced drift and also included as part of the chip is a method of overcoming high frequency tracking error, an effect resulting in a slight flattening of the frequency at the extreme high end of the audio range. This produces a reliable voltage-to-frequency conversion over the entire audio range.
Three simultaneously available waveform outputs are produced by the CEM 3340, namely sawtooth, pulse/square and triangle at pins 8, 4 and 10 respectively. The sawtooth and pulse/square outputs pass via R17 and R18 to CMOS analogue switches (IC11A and IC10A) for later mixing at the input of the VCF. However, the triangle output has a finite resistance and needs buffering to maintain VCO performance.
Pulse width control is achieved by direct injection of a 0V to +5V analogue voltage to pin 5, allowing pulse width to be varied from 0 to 100% values of mark/space ratio, via RV2 or an external CV into J5.
The main difference between the two VCOs is the sync arrangement. In the voice card design, VCO1 becomes the 'slave' oscillator and VC02 the 'master'. The method of sync used (the CEM 3340 permits two types) is soft synchronisation, which on selection causes the triangle upper peak to reverse direction prematurely, with the result that the oscillation period is an integral multiple of the pulse period. The only other difference between the external circuitry of the two VCOs is that VC02 allows less resistive mixing of control voltage inputs.
Power supply to both CEM 3340s is a stable +15V to pin 16 and -5V to pin 3. Note that the use of well regulated supplies is essential to maintain oscillator performance.
The VCF in this design is based on the recently introduced CEM 3372 signal processor. The filter response is of the Butterworth type, with a sharp 24dB/octave roll-off characteristic, almost ideal for electronic music applications. Internal to the CEM 3372 is a VCA to allow overall signal feedback, and hence resonance (or 'Q' value) to be voltage controlled. The passband gain remains constant as the resonance is varied, and this eliminates the drop in volume as resonance increases, often a problem with this type of filter. The CEM 3372 also features low noise, low control feedthrough and temperature-compensated transconductors for cut-off frequency stability.
Also included on the chip is a VCA of the current in, current out type, and this allows ready-mixing of multiple inputs. In this design, the input to the VCA is direct from the output of the VCF so no mixing is required.
The two channels of the voltage controllable input mixer are each fed by three electronically-switched outputs from the CEM 3340s. In addition, though on channel 1 only, there's a noise input to the VCF that allows a noise source to be mixed into the audio bus of the voice card. The signal levels of channels 1 and 2 are controlled by positive-going control voltages (0V to +5V) to pins 5 and 8 respectively. Capacitors C15 and C16 shunt any AC voltage to ground.
Within the CEM 3372 four independent filter stages are hardwired to low pass configuration. Filter capacitors C11 - C14 are chosen such that the filter frequency range covers the entire audio spectrum (less than 20Hz to greater than 20kHz) while control of the cutoff frequency is by means of voltage control at pin 15. In order that multiple voltage sources may simultaneously adjust the filter frequency, resistors R44 to R51 effect a summing stage. The voltage-to-frequency scale is adjustable to precisely one volt per octave by the combination of RV26 and R51: provision has been made for connection to an external LFO via R47. Transposition of filter frequency by potentiometer RV7 is possible via R46, and an initial cutoff frequency may be set up via RV25 and R45 such that with no frequency applied to pin 15, the VCF passes no audible frequencies (individual circumstances will dictate the most useful position of this trimmer). A suitable connection to an external CV jack socket (J13) may be made via R44, which is preceded by a standard sample-and-hold network (IC8B and C19), included for future expansion. Resistor R48 is hardwired to the internal modulation bus which is sourced by VC02, thus implementing an internal VCLFO function. Resistors R49 and R50 carry the ADSR signal from the 'sweep' bus, the former connection being positive-going (hence possweep) and the latter being an inverted version of this. These two control voltages are selected by control of analogue switches IC13B and IC13A. If both of these are selected, no change in filter frequency will occur.
The circuitry allows resonance to be varied from a Q factor of 0.7 up to oscillation. Pin 12 is the signal input to the fixed VCA, the gain control of which is buffered, brought out to pin 13 and hardwired to the output of ADSR1. The output of the fixed VCA is at pin 14 of the CEM 3372 and passes via a low noise BI-FET op amp to J20, the audio output.
The production of suitable ADSR control envelopes is achieved by the use of two CEM 3310 envelope generators. In this design, both ICs are gated simultaneously from a common gate input at pin 4. The gate input jack socket (J16) is wired such that with no plug inserted, the gate input is held at 0V.
Pins 15, 12, 9 and 13 permit voltage control of attack, decay, sustain and release respectively. These three inputs require negative-going control voltages between 0V and -5V, whereas the sustain input requires a CV of between 0V and +5V. In all cases, the greater the deviation from 0V, the larger the A, D, S or R contour produced.
ADSR1 is hardwired to control the final VCA (internal to the CEM 3372), whilst ADSR2 controls the frequencies of VCO1 and the VCF. This attenuable ADSR signal is subsequently buffered by IC7A and split so that two electronic switches allow the signal to alter both the pitch of VCO1 and the cutoff frequency of the VCF. This same signal from IC7A is also inverted by IC7D, providing an inverted sweep to change the VCF cutoff frequency. These three effects are selectable by closure of the relevant analogue switches, whose controls are situated at S3 (possweep VCO), S14 (negsweep VCF) and S13 (possweep VCF).
Simultaneous control of the three time constants (A, D and R) is made possible by injecting a control voltage at pin 14 of the CEM 3310. These control voltages are derived from the keyboard control voltage (J7) using the two op amps within IC9. On closure of the two electronic switches IC10C and IC10D, feedback resistors R68 and R72 are effectively shorted out, and the keyboard track ADSR functions are therefore disabled. Power to the CEM 3310s is +15V and -5V at pins 11 and 6 respectively.
As already mentioned, the output from ADSR2 is passed through a VCA to allow attenuation of envelope amplitude. The latter is one of a pair of independent VCAs available within the CEM 3360. The two VCAs are of the current in/current out type, with both linear and exponential control of gain over greater than a 100dB range.
The ADSR output is fed via current-converting resistor R62 to pin 6 of IC4 (signal input to VCA1) and is available again at pin 2 (signal output). Control of signal amplitude is made possible by positive-going (0V to +5V) CV to pin 5, which allows access to the internal logarithmic converter. Thus an incoming linear control voltage (in this case potentially divided by resistors R59 and R60) will modify gain in an exponential manner. The output then passes to buffer IC7A and inverter IC7D as previously described.
The second VCA is used for modulation attenuation. Modulation waveforms are produced by VC02 and mixed resistively by R35, R37 and R39 into pin 9 of IC4, the modulation waveshape being selectable by IC12B, IC12C and IC12D.
The voice card has been engineered to be built on a double-sided 100 x 200mm PCB, the two sides being linked by track pins. Because of the high component density, groups of copper tracks run parallel to each other with minimal bare fibre-glass between them, thus providing increased opportunity for solder splashes to bridge two or more tracks. So keep a watchful eye on the amount of solder you use per joint, particularly on the track pins.
Components are best assembled onto the board in the following order:
1 Track pins
3 1C Sockets
4 SIL Resistors
Track pins should be located in all component side holes that terminate in a solder pad: any other holes are for normal components. Push the track pin firmly through the hole and snap off from the strip of pins provided. Solder the top side of all pins first and then solder the underside.
Once you've pinned through the entire board, select and preform the resistors. Resistor leads should be bent so that no excess lead is evident, as on the component side this could give rise to a dry short with a nearby length of track. Component placement is detailed in Figure 3, and should be adhered to for all listed components. After installation of the components, the use of a PCB solvent cleaner is strongly recommended.
To aid construction and connections to panel hardware, a PCB mounting bracket is used. This bracket is secured to the front panel by means of potentiometers RV3, RV6, RV11 & RV15, while the PCB itself is held in place by two PCB slides. The bracket is used in such a manner that the plastic-coated side is face up and parallel to the PCB, providing added insulation in the unlikely event that the PCB and bracket should touch.
Connections from the PCB can be made in one of two ways - using either single-sided terminal pins or standard Molex plug-and-socket arrangements. The former of these options effectively 'hardwires' the PCB to the panel, while the latter facilitates its removal. Connections to the switches and 3.5mm jack sockets are best achieved by passing the appropriate wires through the two holes in the mounting bracket provided for this very purpose. Before the toggle switches S1-S16 are wired up, it's necessary to install a 4k7 pulldown resistor (R89-R104) on each one. Each resistor has one lead connected to 0V and the other to the switch pole, ie. the appropriate 'S' point.
Panel wiring is arranged in such a manner that control potentiometers RV1, RV2, RV3, RV4, RV5, RV6, RV8, RV17 and RV18 are disabled after the insertion of a jack plug into the appropriate socket, thereby providing a facility for either potentiometer or external voltage control. Control of the functions of both envelope generators is by means of potentiometers only (RV9 - RV16), while VCF frequency may be controlled by both RV7 and externally via J13. So the wipers of potentiometers RV7 and RV9 - RV16 go directly to the appropriately marked points on the PCB, while all other connections from the PCB link to jack sockets (J1 - J20) and switches (S1 - S16).
As previously mentioned, the PCB is fixed onto the mounting bracket by two self-adhesive PCB slides. If terminal pins have been fitted, it's recommended that the two slides be attached to the PCB, though not stuck down onto the bracket until the board has been tested. This is because removal would require either the slides to be replaced or all the connections to be unsoldered, which you probably don't need to be told isn't a very happy state of affairs. If, on the other hand, Molex plugs and sockets have been employed, the PCB may be unplugged and slid out for inspection.
Power supplies for a single voice card are a stable +/-15V at 300mA per rail and +/-5V at 100mA per rail. Note that the +5V rail is not connected to the PCB but has to source a number of control potentiometers on the front panel. Other power supply connections are best taken direct to the PCB and from there to panel mounted components.
Part Two next month...
A complete kit for the Voice Card including all parts noted in the components list (except the pots and switches) and a double-sided PCB is available from (Contact Details) for £74.40 inclusive of p&p and VAT.
A set of 18 control pots and 16 control switches is available for £21.05, while a 9" x 9" front panel and PCB mounting kit is also available for £17.20. For further information, write to (Contact Details)
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