Harmonic Oscillators

Buchla made the 148 Harmonic Generator in 1969. It is a sawtooth oscillator core, the same as the 158, with waveshapers creating 9 harmonics above the fundamental. The even harmonics are created with full wave rectifiers doubling lower frequency triangles. The odd harmonics are shaped using a series of diode clippers that mix the triangle with a clipped version to make a higher frequency triangle. The effect is a wavefolder, the precursor to the Timbre circuit in the 259.





I built a modified version of that design 10 years ago. The original has no -15volt rail, so it AC couples the signal in many places. I eliminated all these caps and powered the circuit from +/-15 volts. I never got the quality of the waveforms to where I wanted them. I have since worked on a Buchla 148 and found the waveshapes to be about the same as my clone. They sound a little wavetabley, especially the higher odd harmonics.

Legend tells that Buchla made a version of the Harmonic Generator for the 200 series. It was based on the design of the 148, but had a few extra parts added. It had reversing attenuators on the CV ins and an output mixer with sliders and even/odd outs. I have never seen this module and don't even know the model number.

In 2009 I started designing my take on this forgotten concept. I made up my own waveshapers using distorted CMOS opamps, like the 259 Timbre circuit, instead of diode clippers. This does not sound good. I have since thrown the design out.

Finally, I have my finalized design. The Harmonic Oscillator. The analog oscillator core puts out saw, square, triangle, and spike (208 style). The waveshaper puts out 10 decent quality sine waves. The voltage controlled mixer has sliders and CV ins for all channels as well as scanning circuitry (sort of freq. and bandwidth for the harmonics) and tilt (to favor the high harmonics or the low harmonics from voltage).

Music Easel Oscillator

The oscillator section in the 208 module can be seen as a rest stop on the way between the 258 and the 259. The "timbre" section is introduced as well as a switch to select waveforms. It's the first time Don implemented a dedicated modulation oscillator and trimmed keyboard CV inputs.

Interestingly, the original version of the 208 used a totally different design than the 258 or the 259, with Vactrols in the oscillator core. I don't know how many of these were made, but it was fazed out in favor of the 259-like design you can find schematics for on the web.

The "Timbre" control introduces wave folding on the sine wave, but does not effect the other waveforms at all. There is a separate, unlabeled knob above the waveshape switch that controls the mix between sine and the other shapes. The wave folder's effect disappears at the full CW position of this control.

I built a clone of this oscillator about 6 years ago, using 2 single sided photo etched boards. I modded a few things to make more sense in a dedicated module. There are a few lessons I learned, that I would do differently, if i did it again. It is a nice design and I think it has validity, even alongside other Buchla oscillators.



Going through my box of abandoned old PCBs, I got the idea that maybe this would be a cool module to throw together, considering the hype surrounding the Music Easel. Maybe it's time to revisit this one....