Hello, synth people.
So i have this dream of an instrument that can replicate low frequencies (cello range at least) but is also small, compact and self contained like a violin. It doesn’t require batteries, or a wall plug. And you can just play it in the porch or at the park with your friends.
Physics naturally says no no to that. Because to produce big wavelengths you need big things. You could try to circumvent it with a wind instrment made from thin tubing that coils around a lot. But i’ve no experience with these types of builds so i’ll just default to a synth solution.
So my idea for a synth that attends these requirements is something like this.
CAD is somewhat to scale (with that speaker having ~98 mm in diameter).
You’d play it with a leather double strap like an accordion. Right hand for cranking, left hand for everything else. This concept also has a crude way of doing polyphony, where each column of buttons corresponds to a monosynth that shares its controls with the other columns (with the ADSR circuit needing to have 4-ganged potentiometers). That means a 4 voice semi-polyphonic synth. The synthesis method could be subtractive, though additive would be just as interesting.
But now, let’s come back to the material world and talk about the elephant in the room. Power budget. Cheap hand crank generators that i can find online have an absolute power output of 20 Watts. Not much… With those 20 W being delivered as a 24 volt DC signal for this specific one i bought. Assume we use a 20 W speaker at a 10 W limit, that leaves 10 W for the synthesis circuit (minus conversion and heat losses). And to be honest, i’ve no idea if 10 W, when converted to acoustic power, would be enough to compete with acoustic guitars, violins and accordions.
The model i bought was this one (minus the electronics) https://youtu.be/cmxhTwZAx5Q?si=x3AR1MkNot83TCpe
Apparently this DC motor has already a RPM converter of 1:32 (1 turn of the crank spins the motor 32 times). And it would also be possible to mount gears on the outside of the case to double or triple that ratio.
But apparently you can make better generators using stepper motors. https://youtu.be/cJ_vDA7xsGs?si=0o00QbCJ0-hYHd9W
My greatest concern at the moment is how to make this board that will handle the power input of the crank, delieve part of it to the synthesis circuit and deliever the rest to the speaker. Ofc, the fastest you crank, the higher is the energy available for the speaker. So crank speed has to be proportional to volume. And idealy, the circuit would have a constant voltage output to power it.
I assume i’d need a voltage regulator at the circuit’s input voltage. But for the speaker. I’m not sure. Should i buy an amplifier online? Does it need to have voltage control? Where would i get this voltage?
There’s also the possibility of using huge batteries or supercapacitors to make the available charge enough to play 1 song. But then i’d need to crank for twice as much (or more) to refil the charge for the next song. So i believe the best time constant/charge storage for this thing would be like 5 seconds of cranking to reach max voltage. And you’d have to constantly crank to play the thing
One of the great remaining questions is if i can make 4 monosynth circuits draw less than 10 W. Or in this case draw the least ammount of power possible. Maybe those integrated solutions for oscilators and filters (CEM chips, i think). Could be the best option.
There are some other lesser design questions that i can think of. Like, how to best place the controls for pitch bend and modulation and what type of control to use. What type of button to use in the keyboard (i’m thinking about mechanical computer keyboard switches with 3D printed round caps).
Any thoughts, ideas? What do you think?
If only the power efficiency of loud speakers wasn’t as low as 0.1%…