Sunday, 27 October 2013

The Soundlab Diaries - Part 1 - General Design

I have always wanted to build a modular synthesizer, but components were expensive, racks were the size of a large bookcase and it would have taken years to build even the simplest system, so like many other things, I never got round to it.
I had almost given up on the idea when in 2007 I stumbled across a website showing a range of modules built using the new Eurorack, format. These were much smaller, simpler, and used readily available components if I bought one from completed modules it would me more than I would normally pay for a car, including tax and insurance. But there were also kits, and better still some manufacturers would sell you just the blank PCB.
I spent weeks researching the various options and eventually came up with a clear idea of how I wanted my system.

1 - Eurorack panel size format

Ironically Eurorack panel sizes were originally defined in imperial measurements and are all 5.25" (3U) high with widths in multiples of 0.2" This is small enough to be easily made by hand, and smaller modules can even be made from lengths of aluminum strip. Best of all I already had enough parts to make two double height 19" racks.

This skelton rack from 4ms wuld be enough to get started, but you will soon run out of space and need something bigger. The bus here is made on a ribbon cable, but most use a PCB fixed across the back of the rack.

 2 - Eurorack bus

Eurorack bus connector
All eurorack modules use a standard 2x8 connector, as shown above. The pin spacing is 0.1" so it is easy to use on pin board and veroboard.
The only thing I don't like is that it doesn't require a keyed connector so its easy to plug in round the wrong way, potentially destroying your module.
The IDC ribbon connectors do have a key block on one side so I will be using keyed connectors on the other half, just to make sure.
Bus cable being used to connect a module to the back plane. Note the back plane connectors do not have a key so the convention is colored stripe down, but it would be easy to wire a connector up wrong and blow up your module.

3 - Connectors

4mm Banana plugs. Note this type has a hole so that plugs can be stacked up so you don't need "multiple" modules.

Between modules in the same case I am going to use 4mm Banana plugs. This may seem like and odd choice as most Eurorack modules use 3.5mm jacks, but I might want to use this live and my experiences of these have not be good.
4mm bananas are almost bullet proof and if the worse came to the worse you can always strip some bell wire and stuff it in the sockets.
Between cases I will be using standard 1/4" jacks, they have their own earth, so are shielded, and are robust enough not to worry about. I use these for audio connections so already have a good supply.

4 - Build standard

I might want to use this live, so it has to be reliable and be able to take a few knocks. I used to work in the Avionics industry where even prototypes were built with an eye to military specification, and anything less now seems like sloppy work.
For my music gear this means
  • All standard PCB components are soldered through the board and secured on the other side with solder, so any strain is pulling the track towards the board and not lifting it. (Except SMD)
  • If I am adding a component and there is no spare hole than I will fit a vero pin and solder to that. This might mean lifting out an existing component from a hole, fitting a pin, then both components are wrapped round it.
  • Flying leads on a PCB will be as short as possible and either soldered to a pin, or if under the board soldered onto a pad with a component lead, so again any movement of vibration can't lift it.
  • No components in flying leads. Any vibration or movement in the wire could cause the component lead to repeatedly bend where it enters the body, leading to stress and potential failure.
  • Panel wiring will be neatly laid out and fixed down. Laying things out neatly makes it easier to check your progress and find any faults that might occur later. Fixing the wires down, reduces movement at the solder joints and thus stress, making it far more reliable.