I’ve named all the modules after very local landmarks and history because I want my students to know that this is designed by us in my school rather than being a magical white box created by . I have called it the A Priori synth (my school is called Priory School).
Jimmy Cliffe
The first module is the Jimmy Cliffe - named after the Reggae artist who died the week I was working on this and Cliffe hill which over looks my school. It is based on a dub siren (there are plenty of designs knocking about I based it on this one but sort of deconstructed so that you patch the LFO into the VCO. So with this first modules they get an LFO with variable waveforms and a rough VCO. I figure this will demonstrate the principle of CV control and also provide two useful functions for what comes later.
I made a bunch of silly mistakes when I sent this off to JLPCB the first time. This is my second version. I’m waiting to get the boards back but my protoboard version works nicely. I think this should be good. I’ve used an inverting op amp on the CV control of the VCO so that increasing the voltage increases the pitch, otherwise it works the other way round.
A27 Road Closures
The A27 is the main road that leads to the school. The traffic is bad and is the reason that many people give for being late to school. It is a low pass gate. I am heavily indebted to Shed Synth for this module. I think this is great for my purposes - it does the work of a VCA, envelope generator and a filter, all in a tiny little package. The kids really liked making the Vactrols too. Any horrible noise I put into it seems to come out sounding like a really interesting percussive sound. It is a lovely circuit for tweaking too. With C2 at 100nF you get really short plucky sounds and the effect of the filter is really clear. If you increase C2 to 220nf or 330nf then the decay control does a lot more, you can have a longer range of notes. I’ve not played with the filter capacitors but I would imagine there is a lot of scope there for investigations. I had these boards back and they work great. I made some with my students last night. Oh, and if you don’t plug anything into the input you get a kind of resonant bass note when you drive it with a gate.
Pevensey Levels
This is a basic summing mixer. There is not much to say about it. There are 5 inputs and two outputs. The first output is switched so that if you don’t use it then you mix together all 5 outputs. If you put patch cable in the first output then it works like two separate mixers. It has a +5v output that I thought might be useful to give offsets or to use the mixer as a source of cv voltages. There is also a very basic circuit to allow you to patch a left and right mono into a stereo output. This also uses switched sockets so that if you don’t use the second input then you just get the mono in both ears. This will drive a set of headphones. I’ve had these boards back and they work great.
The next circuits, I’ve made them all on protoboard, they work and I have ordered the boards from JLPCB but they have not arrived yet so these are less verified than the above.
Geo Richardson Scrap Metal Merchant
This is XOR bell/cymbal/drone module based on the hackday “more CMOS cowbell” and the brilliant work of Rich Homes/Analog Output who has turned it into a kosmo module. I’ve done very little to his circuit aside from tweak a few capacitors and use the two spare 4066 analog switches to create an “accent” input.
This module can make a whole lot of sounds. The drone output can go into the low pass filter, the bell and “cymbal” circuits each have a envelope and something like VCA.
Powdermill lane
Powdermill lane is a twin T kick drum. It will self oscillate if adjusted in the right/wrong way.
Princess Royal Maternity Ward
This is a baby 8 sequencer but inspired by a hagiwo module, it has a gate output that can be turned off when the CV is set very low. In Hagiwo’s original he used a PNP transistor and the gate turned off when the CV was high. That didn’t work well with the low pass gate (which is a bit leaky) so a high ringing note still got through the low pass gate even though the gate was off. I change it around so that a low CV is off, the leaky sound is much less audible with the Low pass gate (as the sound that leaks through is much lower pitched).
This one is the most complex. It works nicely on the protoboard but it is a rats nest so I am most nervous about whether this one works when the boards come back.
Saxby Farmer Signal Apparatus
This is a really stripped back version of the hagiwo 6ch sequencer. I’ve modified the code to use a 7 pin SPI OLED display rather than a i2c because I want to be able to use it also as a very basic oscilloscope so that students can see what is going on. I’ve added a switch to switch between the modes.
As I say, this is all a work in progress, it would be great to have a collaborator on it. I’ve yet to put it all together in a case but have been getting some fun sounds out of it all.
Looking over your course book, I had intended making some basic logic modules and a 8 step counter to do basic sequencing. That seems like a straightforward ish kind of module to design. It would certainly be a fun way to learn about logic gates.
