HM9791 Mk2 - Dual digital oscillator with internal dynamic-depth thru-zero FM

The original small-format digital oscillator returns. Hertz Donut Mark II is a new 16-bit digital audio system delivers high-frequency performance, and the user interface provides direct access to all operational modes with the fewest possible button presses.

Hertz Donut features two independently functioning digital oscillators with wide frequency ranges. An internal bus enables full-bandwidth, high-index modulation of oscillator frequency, wavefolder drive, tracking phase, and amplitude, with the modulation signal appropriately conditioned for each destination. Nearly all voltage control nodes feature "attenuverters" for bipolar modulation of the main parameters by an external voltage. Internal and external frequency modulation is thru-zero.

The wavefolder has three modes depicted by the color of LED on the selector button. The "orange" mode is the XOR-based "fractal" waveshaper known from the original Hertz Donut, improved. The "red" and "green" modes are traditional dynamic-drive nonlinear lookup tables, selected for their wide range of available tones. The oscillator outputs sine, triangle, and sawtooth waves, each offering distinct behavior when routed through each of the wavefolders.

The tracking controller "closes the loop" by instructing the modulating oscillator to follow the frequency of the primary. Two modes accomplish this quickly, with optional offset from the base frequency. Phase modulation of the waveform is possible, whether manual, under voltage control, or through the internal audio-rate modulation bus. A third tracking mode introduces delay and wandering behavior to the tracking, with the time constant under user control. Circular modulation in these modes is the key to the most bizarre, unrepeatable sounds this module is capable of generating.

The center audio output is the exclusive disjunction of the others.


The Hertz Donut MkII consists of two identical digital 16-Bit oscillators, a digital waveshaper and an internal modulation matrix. Both VCOs have an output with three alternate waveforms (sine, triangle, sawtooth) and a pulse output.

Additionally there is a 1V/octave input (0-8V), an exponential CV input (0-5V) and a Thru-Zero linear FM input (bipolar, 10Vpp). The latter two sockets share a common polarizer. Besides this we find for each VCO a sync input and the range button, activating LFO mode.

The upper VCO, the primary oscillator, is followed by a digital waveshaper called Discontinuity, offering three different shaper types. The discontinuity parameter is voltage controllable (0-5V).

Tracking Parameter the modulation oscillator can follow the primary oscillator's pitch and it's possible to add a frequency offset with both the potentiometer and a CV (0-5V). This is a really outstanding feature in combination with the Thru-Zero linear FM as the frequency ratio between both digital VCOs can remain constant this way.

Between both oscillators' outputs a socket is located, carrying a logic OR between both oscillators. Pretty nice for weird noisy square sounds!

Modulation Bus determines how much the modulation oscillator affects certain parameters. Modulation destinations are primary oscillator's pitch (P. FREQ), the discontinuity parameter of the waveshaper, the tracking and the main oscillator's level (AMP); more than one destination can be activated simultaneously. The amount of modulation is set with the potentiometer and a CV.


Frequency range: 11 Hz - 11 kHz
LFO frequency range: LOW - 20 Hz
Digital audio system: 16 bits / 48 kHz
Internal modulation bus with four destinations.
Thru-zero frequency modulation.
Selectable waveform on each oscillator output (three modes of wavefolding available on primary oscillator)
Dedicated square wave output (PWM on primary oscillator)
0-5V control voltage inputs (with attenuverters)

Changes From the Original

  • 24bit/96khz conversion on both FM inputs and waveform outputs - a completely new hardware design.

  • The generated sine wave probably sounds better than the output circuitry is capable of reproducing.

  • Attenuverters on all but one of the CV inputs, with center detent and a musically useful deadband FM inputs are thru-zero nice 3-color illuminated tact switches instead of the big buttons "range" switch for each oscillator, from LFO rate, to useful audio range, and then up to 48khz.

  • Square wave outputs and FM inputs are fixed, no secret button combos necessary.

  • Discontinuity function has clean/dirty waveshaping modes (similar to the 259e), and the "fractal" discontinuity from the original.

  • Tracking generator now has a knob and CV input. The three modes (named by the color of the mode button) are as follows: Orange is the "bad/worse" from the original, with the knob selecting the time constant. Red is an improved version of the "stupid" tracking where the modulator will successfully converge on the destination frequency. Green locks the frequency of the modulator to the primary, with the knob controlling the amount of phase offset. This knob is now also a destination on the modulation bus.

  • Skiff friendlier.


  • 100 mA +12V
  • 35 mA -12V
  • 0 mA 5V
  • 25 mm deep
  • Ø 4.43 (35 Votes) Average Rating

This Module is discontinued.

$475 Price in € submitted Oct 16th 2012, 04:32 by defenestration | last Change Nov 4th 2020, 19:14 by motorhead412