Stripboards

Most of the DIY I do, I do on stripboards. These are fairly cheap, and I don’t need to deal with chemicals for etching PCBs. The stripboard is easy to use, but when the circuit is very complex or when many digital ICs are used, the stripboard layout tend to be messy, complicated, and big. I prefer to use the stripboard that has copper tracks running along the entire length of the stripboard.

First I draw the layout, usually by hand on a checkered paper. These draft layouts are ugly, but they do the work. But sometimes I use a illustration software to make a more easy to read layout… Normally, I see the copper tracks as going horizontally when making the layouts. I always draw all connections, even marking the copper tracks between components (when drawing by hand), that makes it more easy to see all connections and discover errors. Then I mark all cuts/breaks in the copper tracks with an “X”. I usually try to remember to add component values to the layout, as this will make the soldering process much faster. It is also good to remember to double check pins on ICs and size of trimpots and capacitors (etc) to make sure that the layout actually matches the components.

Stripboard layout

Stripboard layout

A more easily read layout...

A more easily read layout…

When cutting the stripboard to the correct size, I use a small hacksaw to cut the board to the right size. I use a small grinding file to trim the edges to remove any leftovers from the sawing that might create shorts between copper tracks.

Cutting and fixing the edges of the stripboard

Cutting and fixing the edges of the stripboard

Then I usually use a pen to make some markups on the board. This will help me when placing the components. A tip is to count the holes (the number of holes in between components) several times (!) before making a mark, to avoid mistakes.

Making marks on the stripboard helps when placing components

Making marks on the stripboard helps when placing components

When some components are soldered in place, I use a 3mm (or something about that size) drill to break/interrupt the copper tracks on the stripboard. I do this by spinning the drill between my fingers, and then I use a sharp knife to cut off any burring. A good idea could be to mark all cuts with a red pen on your layout, then count them on the paper, and when all cuts have been made, count them again to make sure that you’ve made all interrupts.

Cutting the copper tracks under the stripboard

Cutting the copper tracks under the stripboard, use a drill and a sharp knife

Use a magnifying glass to check all interruptions, and while at it check the soldering as well! If you’re careful, you can build any module you want with stripboards. I’ve built anything from VCAs to LFOs, via VCFs and clock pulse dividers.

Diffusing LEDs

Today it’s very cheap to buy LEDs, and one can easily find them in many different colors, from single color LEDs (orange, red, white, blue, green, yellow, violet, and so on) to two or three colored LEDs. However, at least at the component vendor I usually buy from, most of the cheap LEDs are clear and not diffuse that I want. For example you can buy a bag with 25 LEDs for about €2 (or $2.30). That’s perfect for your DIY sequencer! However, most often these cheap LEDs are clear… But, it’s actually easy to make a clear LED into a diffuse LED.

By using a very fine, like P180 or P220, sandpaper/abrasive paper you can easily rough up the surface of the glass on the LED. Make sure to make this fully around the LED, and try to make the grinding smoothly without any deeper parts or leaving any clear parts out. I usually do this by having the sandpaper on the sharp edge of a table, and holding the LED in my fingers moving it back and forth while slowly turning it. Now you have a nice diffuse LED.

Two clear LEDs. The right one has been diffused using a fine sandpaper.

Two clear LEDs. The right one has been diffused using a fine sandpaper.

If the sandpaper is too rough, the somewhat larger sand particles might create a less smooth surface of the LED. There will most probably be deeper strokes in the glass surface of the LED. This in turn will affect how the light emits from the LED. These deep crevices might be clear on the inside and therefore they will affect the direction of the light. But, wait! This might be a cool effect! LEDs with crystal patterns! Maybe, since the LEDs are so cheap, you could try to use a small and sharp grinding file, to create cool light patterns in your modular!

Eurorack modular systems and other modular systems

There are today quite many different modular systems, e.g. eurorack modular systems, Buchla systems, and classical Moog systems. These are mainly compatible with each other, the most obvious difference between them is found in the patching, in the connectors, apart from different sizes of the front panels. This, however, is easily solved with adapters, or cables with banana jack in one end and 3.5 mm phono in the other end, or with a patch/multiple with different connectors. Other differences might be in the signal levels, both when it comes to audio and to CV signals. These differences are also quite easily solved by using attenuators and amplifiers to adjust the signals to the right voltage levels.

Other differences that might be present between different systems and between different analogue synthesisers might be found in the pitch CV and in the Gate/Trigger signals. Volts per octave (V/oct) is the standard pitch CV signal found in Eurorack systems. It was originally created by Bob Moog in the 1960s. V/oct is logarithmic, which mean that when the pitch CV increases with 1V this gives a pitch increase of one octave. The other pitch CV standard is Hertz per volt (Hz/V). This standard is linear wich means that the pitch increases proportional to the pitch CV, where a difference of one V might be the difference of an octave (i.e. between A1 and A2) but higher in the frequencies the difference might be about one tone). These two pitch CV standards are not compatible with each other, however no damage will be done if modules with different standards are connected to each other. There are different gear and modules that convert between V/oct and Hz/V.

When it comes to Gate/trigger signals there’re mainly two different standards: V-Trigger and S-Trigger. V-Trigger is the standards signal in Eurorack systems. The V-Trigger is a positive going trigger signal, that is when the key is pressed the trigger signal is high, or when the clock pulse comes the signal is high. Doepfer has set the high level to +5V. S-Trigger on the other hand is inverted compared to the V-Trigger, and when the key is not pressed the trigger is high, while it goes low (0V) when the key is pressed. Some old analog synthesisers (Yamaha) has a S-Trigger that goes from 0V when no key is pressed to -V when the key is pressed. It is fairly easy to convert between the different types of triggers, an inverting amplifier with adjustable off set is all that is needed.

For more information about CV/Gate check out this wikipedia page.

According to Doepfer audio signals should be 10V peak to peak, around 0V, in the A-100 system, which I consider to be standard for all Eurorack systems. Audio signals above that are possible, since the power supply gives +/-12V, even if this might provide saturation and distortion.

When it comes to control voltages, e.g. from a LFO or an ADSR, these are from -2.5V to +2.5V (5V peak to peak) for the LFO, and from 0 V to +8 V for the ADSR. It is possible to use other signal levels but this might result in saturation in the signal path, like a filter sweep in a VCF that is way beyond the desired audio range, or that a VCA distorts. By adding attenuators either at the output of the modulation source or at the CV input of the next module solves these problems easily.

Trigger or Gate Signals in the A-100 system, are typically positive going triggers from 0V to 5V. It is most probably possible to use a positive going trigger that exceeds +5V but (!), this might destroy a digital circuit (like a logic gate) at the input. An attenuator or voltage divider will solve this problem. Usually a lower trigger level than +5V will trigger a module, but if a trigger voltage is too low a simple non-inverting amplifier will suffice to get all triggers going.

When DIY, the above standards should be considered to avoid problems. However, for most things it’s not that critical (apart for when it comes to pitch CV). Always add an attenuator in the signal path, either at the output or at the input, to be able to adjust signal levels, and you’re good to go. One thing to consider though is the power supply. The power supply of the typical Eurorack module is a 10 pin bus, that has the following connection in pairs of pins: +12V, GND, GND, GND, -12V. There is also a 16 pin version that also has Gate, CV, and +5V like: Gate, CV, +5V, +12V, GND, GND, GND, -12V. Usually cabinets and power rails comes without the +5V option. This is normally not a problem as most modules that need +5V comes with a voltage regulator built in. Nonetheless, it is easy to add a +5V option to your modular system: +5V option.

There are many excellent designs, schematics, ideas, and modules out there! Some of these follow the Eurorack standard, some don’t. A rule of thumb is, if the module that you’ve found on the net is powered with +/-12V, it’s good to go. If you just add attenuators on the inputs and the outputs (just to be sure) you’ll be fine! However, if the module is powered by +/-15V there might be other problems that you need to consider if you run it on +/-12V. Some modules will work equally well on a lower power supply, others might need to be adjusted, resistor values changed and so on, to work. If the module run on +/-9V or maybe +/-5V, you most probably need attenuators on it’s inputs and an amplifier on it’s output. You also need to check so there are no components that’ll be destroyed by +/-12V, e.g. opamps, logic gates, capacitors. If the module is run by a single supply there’s most probably a virtual GND as well as GND (0V), this might cause problems in a dual supply setup. Furthermore, the signal levels in the single supply module might be less than in a dual supply module, why attenuators at the inputs and an amplifier after the module might be needed. Also, the DC off set might be different compared to a dual supply module.

For more information about the Eurorack standard check Doepfer’s webpage.

Pseudo-random LFO

Sometimes there’s a need to get random voltages, you can do that with a S&H and a noise source (see this previous post), or you could do it with a pseudo-random voltage source. This is a pseudo-random control voltage source that uses four square wave generators, of one with controllable frequency, and mixes these four signals to form one moderately random pulse wave at the output. This illustration shows the mix of four square waves with different frequency.

Summing square waves. From Digital Generation of LFO’s for Modulating Effects. Copyright 2000 R.G. Keen

My design of a pseudo-random LFO is inspired by the design by R.G. Keen as well as a of Ken Stone’s design, and there are probably many other similar designs out there. It has four controls (Coarse rate and fine rate, Range of the pseudo-random CV (1-10V), and Slew (stepped CV or smooth CV), it also has two outputs (pseudo-random CV out and pseudo-random clock out (positive going clock pulses at +5V)).

pseudo-random-lfo

Pseudo-random LFO

The module uses +5V, +12V, GND, -12V, and needs a +5V option installed in the modular system, or have a +5V voltage regulator (e.g. 7805…). It would also be possible to use a HEF40106B Hex inverting Schmitt trigger instead of a 74HC14, and this would remove the need of +5V.

4-channel mixer and multiple

When mounting different modules in one of my cabinets I ended up with a small empty space, 3HP. The reason for this is that I mixed “standard” 8HP modules (DIY and Doepfer) with 5HP and 10HP modules (Eowave). Since the space is too small to add much controls on a front panel, I thought about adding a small blank panel covering the empty space. But, on the other hand, you’ll always need to be able to split and sum signals, why I built a small 4-channel mixer and multiple. The module mixes up to 4 signals without any level controls, and distributes the mix to 4 independent buffered outputs.

mixmulti_schema

4-channel mixer and multiple

The module works fine for both audio and CV signals, and can easily be soldered on stripboard.

4-channel mixer and multiple

4-channel mixer and multiple soldered on two small stripboards