WELL RESPECTED for his custom modules in the Buchla 200 Series format, Mark Verbos officially entered the Eurorack scene when he unveiled a new line of products at the 2014 NAMM show. While some of his new modules reflect earlier designs, Verbos plans to expand the line while maintaining a focus on what he calls “old-school analog,” which includes the use of discrete transistors and voltage control over every parameter.
One of the Buchla-based designs he carried into Eurorack is based on the ’60s-era Model 148 Harmonic Generator, which offered individual outputs for a fundamental pitch and its first nine harmonics: The Verbos Electronics Harmonic Oscillator ($649) re-imagines the design, adding sliders to set the level of the fundamental and each of the first seven harmonics, CV inputs to control a dedicated VCA for each partial, and a variety of musically useful modulation capabilities that, all told, differentiate this module from all others in the Eurorack format.
The Harmonic Oscillator’s CV inputs allow you to individually address the fundamental frequency and its first seven harmonics as well as scan through them in several ways.Harmonic Convergence The Harmonic Oscillator is a triangle-core oscillator providing individual outputs for square, triangle, and sawtooth waveforms. The fundamental and harmonics from the waveshaping portion of the module are summed to a mix output, but are also available individually at pre-VCA direct outputs. With each fader down, you can play the partials melodically using pulses or CVs: I used the eight outputs of a Doepfer A-149-1 and -2 Quantized/Stored Random Voltages module to address each stage of the Harmonic Generator—instant gratification.
The module’s Harmonic Scan section provides ways to linearly step through the partials. The Center knob selects the partial, around which you can scan the others back-and-forth using a CV that can be altered with the associated inverter knob. The Width control determines how many harmonics are heard at one time, based around the setting of the Center control. When Width is turned fully clockwise, the fundamental and all partials are present. As you dial it down, fewer partials are heard. You can scan outward from the center partials using the Width’s CV input, which has its own bipolar control.
The Spectral Tilt section can be used to favor one end of the frequency spectrum or the other: Turn it fully counter-clockwise to add the fundamental and lower harmonics, fully clockwise to open up the highest harmonics. A CV input (with bipolar knob) is there for modulation. The position of the Spectral Tilt control alters the movement created by the CVs used for harmonic scanning.
And what would a module like this be without frequency modulation capabilities? The Harmonic Oscillator provides both types—a linear FM input with dedicated level control and an exponential FM input (marked CV Input) with a bipolar knob. The 1V/octave CV input controls the fundamental frequency for all audio outputs.
West Coast/East Coast The sum of these features adds up to a module that has a bold sound and a strong personality. The timbres from the waveshaping section (the mix output and discrete outputs of the fundamental and its individual harmonics) are by no means pure sounding. Each is harmonically rich, which provides a wonderful source for aggressive filtering. I especially enjoyed tuning the fundamental frequency to its upper registers while modulating the FM inputs to create heterodyning effects that were inspiring to work with.
Overall, the Harmonic Oscillator is perfect for exploring basic additive synthesis techniques (an important part of the so-called West Coast modular style) while providing filter-like subtractive synth sounds when you need them. To take full advantage of the Harmonic Oscillator, you’ll want plenty of modulation sources, such as LFOs and envelope generators. But whatever you throw at it, this 32HP powerhouse will give you something you didn’t expect in return.