DIY synth guru Scott Rider, aka the Old Crow, has announced a new DIY project, inspired by the circuits of the Moog Minimoog and the Yamaha CS-80.
The two main goals for the ‘Crowminius‘ project were:
- to make a new synth in the style of a Minimoog 204D (“model D”) for a good friend, who lost his circa 1974 original Mini to hurricane Katrina; and
- to create a synth that would fit in a compact aluminum attache case.
Here’s an audio demo of the Crowminius in action:
Here’s what Rider has to say about the new synth:
Crowminius is a compact recreation of the 204D, except my Atmel ATMEGA328P/MCP4822-based MIDI interface takes place of the old 44-note Pratt-Read keyboard. Otherwise, every control and jack on the model D is represented here.
The controls are extremely compact as the entire system has to fit on a 8.5? x 11? circuit board and given the inset border needed for mounting the board in the case the actual area available was 7.5? x 10?. Thus far all the subsystems are checking out as shown by this 98% assembled first unit.
An Adafruit LCD monitors the MIDI stream for me; that is a MIDI Note-On code for key 0x34 at velocity 0x5B. I had a spare DAC channel available so one of the MIDI options will be filter modulation by various MIDI sources, such as key velocity.
By request I added a feature the original model D did not have: PWM of the oscillators.
Oscillator #3 was typically used in “control mode” as a low-frequency modulation generator to add vibrato to the first two oscillators or additionally add filter modulation, but there was no routing to provide pulse-width modulation of the primary tone-generating oscillators. To address this (lack of a) feature I devised a pair of simple low-frequency triangle-waveshape oscillators, one for each tone generator.
Separate oscillator PWM is one of the many details that gives the Yamaha CS-80 the rich sonic texture that made it such a great instrument, so I decided to use that concept for Crowminius PWM.
Here’s Rider’s FAQ’s for the Crowminius:
The circuit board is 8.5″ x 11″ (216mm x 280mm), the size of the DuinoKit Essentials PC board the aluminum case of which settled the form factor goal for the Crowminius project.
The green things are some 3D-printed shelf clips I made that serve as makeshift pitch/mod levers. I will print better ones meant for this.
There are three LFO LEDs, two are behind the PWM pots (top middle), the 3rd shows the rate of the control osc. and is over by the pitch/mod wheel pots.
The vactrol/LDR is for MIDI modwheel action, which is kind of experimental at the moment.
The A440 reference is not a Wein bridge sine oscillator, but rather is a DDS table-lookup PWM sine tone generated on the MIDI microcontroller.
USB MIDI is planned.
The power supply will run on a single 12VAC, 250mA wall transformer. The power supply that does not suck mode involves the use of two 12VAC wall transformers so as to provide full-wave rectification.
The DAC is scaled to provide 5 octaves, F1 to F6 over a 4.096-Volt range. The DAC’s 2nd channel will operate the filter cutoff for assignable key velocity or whatever.
I did not use ua726s as they are a pain to get and the thought of exposed parts heated to 80C meant someone would get burned. LM3046s are fine; there is a tempco resistor under the three arrays for the VCOs.
The filter resonance pot is in fact a 50K reverse-log type, but I provided for using a linear pot+resistor to cheeseball it.
Yes I will be offering bare boards. They will be $100.00. I have to correct a few cosmetic issues (mostly reversed knob and switch actions) but every part came from Digikey, Jameco, Mouser or Small Bear.