Ian Fritz, January 2009
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.
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.
A really cool, inexpensive through-zero VCO! The Bridechamber panel is nice quality. Its quirky layout with the offset pots and jacks is non-conformist, in keeping with the module itself!
Construction comments
I got the panel, PC board, and a kit of some special parts from Bridechamber. High quality pots are used throughout. As shown in the photo, I was able to mount the PC board to a Stooge 4-pot short bracket I had lying around. The PC board was not designed for the bracket. I could use only four of the six mounting holes in the board and had to drill holes in the bracket to match. There was just enough room. The power connector sits on the back of the board, cantilevered over the end of the panel. I wish Mr. Fritz had provided more ground lands on the PC board; there is only one included in each of the two header-style connectors. I used twisted pair for all the IO wiring and glommed all the grounds together onto a solid wire soldered into the one hole.
Circuit comments
I did the 15 volt power supply modification, as described in the instructions, using zener diodes in place of ferrite beads. Five separate internal power busses are used! Prior to soldering in the integrated circuits I connected power and verified all the supply voltages were as expected. Good thing, because I had installed one zener diode backwards! That was easy to fix. As recommended, I had carefully matched the zener diodes D12 and D13 to within 0.01 volt. Despite this, the triangle wave had a glitch that could not be trimmed out. I modified the circuit to add a small resistor and got an improved waveform. Still there is a small glitch that is audible as a harmonic. You can hear this in the MP3 below.
Performance comments
This VCO tracks very well over at least six octaves. I didn’t notice any problems. I compared the thru-zero FM with the Cyndustries ZerOscillator (ZO). The two VCOs do FM quite differently. The Teezer has a simple-to-understand Initial Frequency pot that is a manual input to the linear FM. In the mid position, the frequency is theoretically zero. To the right is positive frequency (and a rising sawtooth) and to the left is negative frequency (falling sawtooth). The Coarse Tune has an extremely wide range with audio frequencies being to the right of center. Very low frequencies are possible for LFO use.
Through Zero shootout
I found that comparable effects are obtained from the ZO with the Range in HIGH and Bias switch in the LOW position. For this test recording, I used a MOTM-300 sine wave as the modulation source. The ZO and Teezer were set to the same initial frequency and the triangle outputs of each VCO were sent to a mixer. You first hear the ZO unmodulated triangle, followed by a manual twisting of the LIN FM pot up and back. This fades out and fade in to the unmodulated Teezer. Here you notice more harmonics on the triangle than the ZO. The LIN FM pot is twisted up and back. This is a log pot on the Teezer (vs a linear pot on the ZO), so my jerky fingers are more noticeable. You hear a slight beat also, which could be tuned out with the fine tuning. I haven’t yet understood how to make the best use of the relationship between the initial and coarse frequency pots.
Thank You for Such a Wonderful and Fair Comparison.
Congratulations on what looks like a fine new module, and
Great Post!
Thank you for taking the time to write such a nice review.
The Tri wave is derived from the Saw, and so will always have a bit of an anomaly. To see an example of a unit with the Tri trimmed to what I consider an acceptably small glitch please see David Brown’s scope shots:
http://modularsynthesis.com/bridechamber/tzfm/tzfm.htm
and very importantly his assembly note #6. This should help if you need better symmetry.
Thanks again!
Ian, thanks for the comments! And the link to Dave Brown’s Teezer page, which I had not seen before, even though I frequent Dave’s site. I will have to ponder his circuit changes for 15 volt supplies, although my Teezer works fine with only your recommended zener diode modifications.
Ian, I remember reading your articles in Electronotes in the late 1970’s. I have a complete hard copy of EN.
Richard — I worked with David some in developing his 15V mods and I think they are fine. I prefer the series-zener solution somewhat, because it provides extra PS decoupling between the different parts of the circuit. Oh, those were exciting times, back in the 70’s.