This is an updated version of my first synth module in Kosmo format, Mikrokosmos, inspired by Music Thing Modular’s Mikrophonie. Like that module, Mikrokosmos II has a piezo pickup attached behind the front panel, and the circuit amplifies the piezo signal and sends it to the output. There is a tactile scratch area on the front panel. You can use your fingernail or a guitar pick to scrape across it and generate output. It will also pick up sounds when you tap or thump on the synth case, plug or unplug cables, and so on. People have attached springs to the front panel, even a coil spring type doorstop. There are lots of possibilities.
There also is a jack for plugging in an external piezo pickup attached to a cable.
The first version of Mikrokosmos was a Kosmo conversion of the Mikrophonie circuit. Mikrokosmos II replaces that circuit with one based on a design by composer, performer, and instrument maker Godfried-Willem Raes. This preamp is designed specifically for piezo pickups and gives better frequency response than a typical constant impedance amplifier. Quoting from that site:
Piezo disks are used almost universally by experimenting musicians all over the world, not in the last place because they are cheap. They have been around since the mid seventies of last century… Generally people connect them to unbalanced microphone inputs of power amplifiers. The resulting sound is more often than not, harsh and metallic. Here we propose a much better solution, based on a good understanding of the nature of piezoelectric transducers. A piezo disk basically is and behaves as a capacitor, generating a voltage when exposed to vibration. As a consequence, the impedance is a function of excitation frequency and can be considered infinite when no signal is generated. Such a generator makes a very bad source for a regular microphone input with constant input impedance.
Here is a much better solution… The opamp in this circuit is configured as a current amplifier. Basically the piezo disk connects to the inverting input and thus sees a zero impedance. It is virtually shorted. The current produced by the disk is amplified by a factor … [which] is inherently independent of frequency here, thus we avoid the sharp and metallic sound obtained when using a non-inverting opamp configuration…
There also are a couple of Schottky diodes to provide soft limiting of high amplitude signals. In my breadboard tests I found the results clearly preferable to the first Mikrokosmos version.
About the name
Mikrophonie’s name comes from that of a composition by Karlheinz Stockhausen. “Mikrokosmos” is an obvious portmanteau of “Mikrophonie” and “Kosmo”, and pays homage to a collection of piano pieces with that name by Béla Bartók. Apparently it also is the name of a song by BTS, but that has nothing to do with this module.
Photos
(Yes, I managed to forget to have the order number omitted from the panel.)
Documentation
- Schematic
- PCB layout: front, back
- BOM
- Build notes