When I design modules, I really go all out to set some solid standards that I’ll stick to throughout the process. I make sure to think long and hard about the availability of components, so that building units won’t break the bank. Cost-effectiveness is the name of the game, especially when you’re just getting into SDIY.
Why set standards?
I don’t like the idea that a module utilises rare components. I have countless packs of resistors with strange values which come in quantities of 100, only to be left with 99 spare. I’m aiming to avoid this at all costs.
Unfortunately, avoiding rare components is unavoidable on some occasions. For example, the IC I use in my VCO is the AS3340 – a superb, really stable oscillator chip – which just can’t be beaten. This chip isn’t available from the larger component distributors like Mouser, TME or Farnell, and only really available through synth specific shops like Thonk in the UK and the Spain based Befaco parts section.
Where possible, I’ll make sure that any synth specific components throughout all modules can be bought in the above mentioned shops.
What are the rules?
- Passive components must come from a relatively small pool of values (discussed below in Resistors).
- Passive components should be easily accessible worldwide through suppliers Mouser and TME, and synth specific components from Thonk or Befaco.
So what components do I use?
I’m trying to use as many E12 resistor values as I can. If I need to stray out of this range of resistors, I aim to only dip into the E24 pool of resistor values. That being said, the resistors I’m using are based on the E96 standards, and have a tolerance of 1%. Details on standard resistor values can be found on this useful page.
Resistor sizes are to be small where possible, to allow for a more compact design, without having to start using SMD components. Therefore, a size of Ø1.9×3.7mm is optimal (approximately DIN 0204 size). Using smaller resistors also means that they can be used in other DIY projects or PCBs bought from other designers as they fit in larger footprints too. There are a number of manufacturers I use which work with these sizes, Royal Ohm, Vishay and TT electronics.
Capacitors have a similar set of values to the resistor E12 series, which is a great start as that’s already a small pool of values. The issue with capacitors, however, is that capacitors come in various forms, and for various voltages.
Firstly, voltage. I make sure that all capacitors used have a minimum voltage rating of 35V. In Eurorack the current norm is to have two voltage rails of -12V and +12V, making a range of 24V. 25V capacitors are very close to this value, so I prefer to go for a minimum of 35V to add a little extra headroom.
Electrolytic capacitors are usually polarised, meaning current only flows, and the capacitor only functions, in one direction. There are non-polarised electrolytics, but we’ll ignore those unless they’re totally necessary.
These capacitors are usually of a higher capacitance, and are great for adding between power rails and ground to offer energy storage with a view of maintaining a constant voltage on the rails.
Standard values used for these are 10uF and occasionally 1uF. Each PCB uses a minimum of two 10uF capacitors for the incoming power supply, so these are essential. The rest of the values are for specific use cases, but I try to avoid using them where possible.
I like to make sure that components are pretty low profile in case I decide to sandwhich PCBs. Combine this idea with a pretty standard 2.5mm or 1.5mm lead spacing, a largeish footprint, and you’ve got a multitude of possibilites if something isn’t available.
Ceramic capacitors are pretty much the standard capacitor type used throughout my builds. All ceramic capacitors that I buy have 5mm lead spacing, meaning PCB design and component sourcing becomes easier.
There are various capacitor types available, with varying temperature ranges and stability. X7R is a pretty stable type, and used in most cases in designs.
C0G type capacitors are a lot more stable, and are usually saved for specific functions, like VCO timing capacitors, which stops the pitch drifting as the circuit warms up. My VCO design uses TDK’s FG28C0G1H102JNT06 50V 1nF C0G ceramic capacitors. Datasheet. Availability: TME, Mouser.
One heavily used ceramic capacitor is 100nF. These are used as decoupling capacitors for ICs, so every power supply a chip has, you’ll find one of these nearby. These capacitors sit close to the IC pin and connect the power rail to ground to help reduce any noise coming from the power supply. My go to are TDK’s FG28X7R1H104KNT06 50V 100nF X7R ceramic capacitors. Datasheet. Availability: TME, Mouser.
Integrated circuits (ICs)
There’s only one essential here: the humble TL074, and it’s little.sibling, the TL072. Both of these are iterations of a well established op-amp. The TL074 contains 4, and the TL072 contains 2. Using these op-amps, most functions can be carried out.
Other ICs which are also used, but to a lesser degree are the LM13700 and the AS3340.
The LM13700 is an OTA op-amp and has some very useful functionality in VCAs and filters. Unfortunately the through hole version has been discontinued, so I’ll be using the SMD version in my designs (daunting for a first timer, but manageable).
The AS3340 is a feature rich VCO chip, and is a little more difficult to get hold of, as it’s quite a dedicated chip, and has to come from synth specific suppliers. Although having to be ordered from the likes of Befaco or Thonk, this IC is worth its weight in gold, and it’s cost (including postage) is compensated by the reduction of complexity, and savings made on other components if making one yourself.
For ease, all components mentioned which are available from TME and Mouser above have been included in the below CSV files which can be uploaded to each service, and the relevant components can then be easily sourced. Simply select the components you want, and delete the rest.