Building An Analog Sequencer

Analog Sequencer

Analog Sequencer

 

My first knowledge of analog sequencers was through a paperback book with basic electronic music schematic diagrams in it. At the back was a device simple called an analog sequencer. I had just built an analog sound generator that could vary its pitch by voltage control and the fact that the sequencer could kick out sequences of control voltages seemed a pretty big reason to try and build one.

It was a pretty basic device and even I could understand how it worked. A 555 timer with a variable frequency clocked a CMOS 4017 with variable resistors on the outputs. It worked but not correctly! Instead of going from 1 to 8, it ran through its own sequence! A few wiring errors corrected and it was working properly. If you’ve ever played about with an analog sequencer you were probably like me and fell in love with them. I quickly thought of the possibilities of controlling other things instead of just the pitch, and what if there were more than 8 steps? I had read that Kraftwerk had one built for them that was 32 steps!

Yep I was pretty sold on the idea of analog sequencers. I built a few and played about with various ideas before coming up with my current one. Given the flexibility of modular synthesisers over hard wired ones I wanted my sequencer to be the same. After some experimentation a came up with the idea of having two separate sequencers, one just for control voltages and one for triggers. The idea behind this was that by using the traditional method you need both variable resistors and switches, costing more and taking up more panel space. Splitting the two meant that the cost was only slightly more because the actual electronics is the cheapest part of the components. Each of them has sixteen steps and both are clocked with trigger inputs.

The Pulse And Voltage Analog Seqencer

The Pulse And Voltage Analog Seqencer

Starting from scratch it seemed the easiest way to make a sixteen step analog sequencer was to use 74LS154 4 line to 16 line decoder and a 74LS193 4 bit up/down counter. This would produce the sixteen steps easy enough and the 74LS193 had various inputs that could be used to extend and link sequencers together in other clever ways in the future. Particularly the up/down and add carry options!

 

The "Heart" of the Analog Sequencer, the 74LS154

The “Heart” of the Analog Sequencer, the 74LS154

I also used a couple of transistors to protect the inputs and a 555 Timer was used in monstable mode to debounce, just something simple to make sure a clean pulse made it to the 74LS193!

The Sequencer Stripboard

The Sequencer Stripboard

The outputs drove 74LS04 hex inverters and I used another transistor to drive the LED’s and variable resistors. The voltage outputs produced went into a 741 Op Amp with variable gain and shift controls to add a bit more flexibility when driving voltage controlled modules.

74LS04 Hex Inverters

74LS04 Hex Inverters

The “pulse” version substitutes locking push buttons instead of variable resistors and has a divide circuit to make it run at half speed. I decided to build four of each giving me eight individual sequencers that could be interlinked in various ways! Two inputs and two outputs are included on the both versions, a numbers of steps rotary switch with the “control voltage” ones also having a manual step button to aid programing the things!

If you’ve ever built anything like this before you’ll quickly discover that it’s easier to wire up to the components that are panel mounted, directly to each other. The chip stuff was just built on strip board.

Analog Sequencer Internals

Analog Sequencer Internals

When it comes to using this machine there are a lot of possibilities when using it in conjunction with analog synthesisers, especially if you have individual modules where you have access to all the modulation possibilities

 

Sequencer Board and Panel

Sequencer Board and Panel

In its most basic use a sequencer will be connected to an analog module with the trigger or gate output connecting to synthesisers trigger or gate input which will operate the envelope generators and the sequencers control voltage output will be connected to the control voltage input where it will operate the pitch of the oscillators. This is the configuration that most people will be familiar with and no doubt it will have given you many hours of happy synthesis!

If your analog synthesiser gives you a filter control voltage input, you will have no doubt plugged a sequencer into that as well to give you the sounds that, well, only an analog sequencer driving the control voltage of a filter can give you!

With a few more sequencing modules you can do a few more adventurous things (just as you can with modular synthesisers) if you have a voltage control resonance input on your filter try running another sequencer controlling that as well as one modulating the frequency. That effect is one that’s not heard as often!

If your envelope generator provides voltage control of its attack, that’s also another source of experimentation that’s works especially well in conjunction with another sequencer controlling the decay. When using with an oscillator try sequencing the pulse width. Use two oscillators and synchronise them together and use a sequencer to alter the locked one, and then use another on its pulse width!

My Analog Sequencer Without Patch Leads!

My Analog Sequencer Without Patch Leads!

Another way of patching that has become one of my favourites over the years is to set two different patches as two complete synths and split the output of the control voltage of a sequencer to the two separate control voltages of the individual synths or modules then use two separate sequencers and connect the trigger outputs to each envelope generator, as you program different rhythm patterns on the rhythm sequencers you can get the melody that’s programmed on the control voltage sequence to bounce between the two modules and overlap where you trigger them both simultaneously. This becomes even more complex when you add a third rhythm sequence and module!

I think your probably starting to get an idea of what you can do but believe me once you start you will stumble across so many others; you won’t know where the time has gone! I must have wasted years of my life! No wonder my musical output is so sparse