Ian Fritz Teezer Thru-Zero VCO (Black Magpie)

Special gratitude to the original inventor of the thru-zero vco for authorizing and contributing to this project!

Euro panel by Magpie Modular, 18hp

The thru-hole synthCube kit includes everything you need- all parts individually bagged and labelled. 

This module is a sawtooth-based VCO capable of frequency modulation (FM) extending past zero frequency

into the negative-frequency regime. Thru-zero FM provides a much wider and richer variety of sounds than

"ordinary" (positive frequency only) FM. The negative-frequency version of a waveform is simply a time-reversed replica of the original waveform.

When a VCO is modulated through zero frequency, the waveform slows down to a stop and then speeds back up

in the reverse direction -- just like the wagon wheels in the old Western movies. Here is a picture of the core

sawtooth wave being modulated to negative frequencies. On the left side of the picture the wave is a down

ramp, and on the right side it's an up ramp, with the zero-frequency point being at the broad dip.

The Teezer's output waveforms also include triangle and sine waves, for producing sounds with fewer high

harmonics, as typically used for bell sounds, train whistles, and so on. The unit also features a variable

synchronization control that can be adjusted over a range of settings from hard sync to a fairly loose soft sync.

The module can also serve as a highly accurate and stable "ordinary" VCO (i.e., without the deep FM), with

upramp, downramp, triangle and sine output waveforms.

Note: With LM311P and LM311C chips there is sometimes a problem with chaotic behavior. The trouble seems to be with the positive feedback path. As used previously, this overvoltages the input and the response depends on the chip flavor. The "N" flavor seems not to care.

 The issue is resolved by adding 100pF caps from the (+) inputs (pin 2) to ground. Hopefully, if you stick with the "N" variety there will not be problems. The extra caps can easily be soldered directly to the board.