Jake Rothman shows how you can build yourself a 16 step analogue sequencer.

Since the emergence of the microprocessor based sequencer (such as the Roland CSQ series), step-time analogue units like the Korg SQ10 have become progressively less popular, and it is now virtually impossible to buy a ready-built analogue sequencer. This is a pity, as they still have many uses - Kraftwerk for instance have two 64-step units which they use not only to control synthesisers and rhythm units but also to clock lights so that they are in perfect synchronisation with the tempo of a piece. It is true that analogue sequencers are harder to program - the pitch for each note has to be individually set - but this is offset by some of the advantages. The project described here not only offers control for the more traditional functions such as clocking synths and rhythm boxes, but can act as a controller for electronic drum kits such as the Simmons SDS8, light shows and even a waveform generator!

Operational Features

The panel controls include a 16-way switch to determine sequence length prior to repetition, an internal clock with variable tempo control and the set switch which advances a programmed sequence one step when depressed. The set switch allows each of the 16 voltage control pots to be adjusted to the correct pitch, filter setting, etc.

There is an external trigger input which allows the unit to be triggered by a rhythm box, electronic drum kit, contact mic - even another sequencer. The possibilities are limited only by your imagination and finance! The set button, external trigger input and internal master clock are selectable via a 3-way rotary knob while the sequencer program can be made to step up or down by use of switch no. 22.

Each of the 16 channels has a trigger out switch. When this is closed, a trigger pulse is sent to the external trigger output for that channel. When the toggle switch is placed in the open position, no signal is present.

Circuit Operation

The heart of the unit is a 74154 chip. This is used in conjunction with the other TTL chips to provide the Stepped Clocking action.

The 74193 4-bit counter receives its clocking pulse from either the internal clock (the 555 Timer chip, IC1) or an external clock input via jack socket SK1. This is in turn amplified by TR1 and TR2 or switch SW20 to provide the advance function. For setting up the sequencer, this switch is debounced by an RS latch formed by IC7. The Clock Up/Down function is provided by SW21. The 4-bit output of IC5 is fed into the input of IC3. The output of IC3 is a 16-bit walking Low and this has to be inverted (by inverters) to give a stepped high output. This is clocked, allowing the 16 channels of the sequencer.

The channels are indicated by 16 LEDs. These are dimly lit when the output is Low but glow brightly when the output is High. Because of the brightness levels needed, the LEDs are driven via transistors rather than the inverter outputs. These are instead used to feed the potentiometers which set the control output voltage levels. These voltages are then routed via blocking diodes (to prevent interaction between the pots) to the control voltage output sockets. There are three voltage control outputs which can be used for, say, VCO, VCF and VCA functions.

The number of steps before the sequence repeats is set up by SW18 and can range from 1-16 steps. Individual triggering from any of the 16 steps is available from jack sockets SK1 to SK16. The main trigger output is from SK20 and a trigger is obtained when any of SW2-SW18 are closed. If any of the switches are open, no triggering pulse will be sent, so the sequencer will clock through that particular step. LED 17 indicates when a trigger pulse is present.

The final part of the sequencer circuit is the power supply. This is a basic 5v regulated design except for the inclusion of a mains filter and a transient suppressor VDR2. These components are essential to ensure that mains glitches do not disturb the logic. If this occurred, the sequencer might miss a step, which could be disastrous.

Construction

In the prototype the mains logic circuitry was constructed on Vero board. The analogue section was wired directly across the pots on the front panel using busses of 20 SWG tinned copper wire.

The unit's layout is not shown - this project is not for the beginner and it is assumed that the constructor is able to design his own to suit his particular needs. Anyone building this unit will have to decide the number of CV channels required, for instance. I opted for three channels and the prototype (see photos) was housed in a 5U 19" box in line with my policy. However, this is not necessary and the unit will fit into a substantially smaller box if required.

Many of the components are uncritical and surplus parts may be used. The value of the control voltage pots may vary from 5 to 100K ohm. Also, the transistors may be any small signal silicon NPN type, such as BC108. In addition, the diodes may be any type similar to IN4148.

Testing

One problem that may occur is erratic clocking and triggering and this is most likely to be due to insufficient decoupling. It is essential to use a 0.1 mu f ceramic capacitor across the power supply pins of every chip.

Once proper clocking has been achieved, it should be checked that a trigger pulse is obtained when SW8 to SW18 are closed and that none is present when the switches are open. Also, check the CV pots alter the voltage smoothly and give a range of approximately three octaves with 1v per octave synthesisers. Finally, check that the reset button returns the programmed sequencer to LED 1 and make sure that the sequence length switch operates properly.

In Use

Set desired sequence length using SW18. Set SW21 to advance setting and tune the CV pots to each notes required pitch, filter frequency, etc. Next, press advance button SW20 and tune the second channel and so on until your entire sequence is programmed. Where gaps are required, the trigger switches are left open and where a note is required, they are closed. Some very interesting rhythms and pattern variations can be obtained simply by opening and closing channels at random whilst recording.