|John Hollis' "Zombie Chorus" is a terrific little minimalist chorus unit except for one thing: most builders report an annoying audible tick produced by the LFO (in sync with the LFO speed).|
While the MN3007 BBD-chip the Zombie uses normally requires a handful of additional components (one of them usually a trimpot for tuning) to provide a DC (i.e., steady voltage) bias to the input of the MN3007, John made clever use of the voltage divider circuit that provides the artificial ground (Vref or Vb in the PDF layout file at GEOFEX) to accomplish this so that users would not need to do any trimpot adjusting to have it work. This also requires the use of specific dual op-amps to hold up accurately. There may be others, but the TL062 is dependable for this.
Unfortunately, the LFO circuit also needs a bias or Vref voltage to work properly. It WILL work with the existing one, but since the Vref/Vb is not at the mid-point of the supply voltage (the resistors in the existing divider circuit, R12/13, are 10k and 15k), the LFO swings above and below the Vref asymmetrically. Since it has more voltage headroom (i.e., can swing farther) on one side of the Vref than on the other, it produces an audible tick on the half-cycle with the least headroom. This is why you hear a tick that is in sync with only the peak of the sweep in one direction and not the other.
Local builder Tim Larwill was showing me his excellent Boss CE-1 clone recently and detailed his difficulties in developing it, mentioning that he found asymmetrical LFO swing to be a source of ticking. I tucked that detail away until recently and suddenly realized that perhaps this was the source for the ticking in the Zombie.
Frustrated with the way in which the ticking was detracting from an otherwise nice little pedal, I tried an experiment in which a different (and "true") Vref/Vb was provided for just the LFO chip. Sure enough, it worked like a charm. Thanks to Tim.
The GIF is the mirror image of what RG has in the PDF file, and is exactly what you'll see on the copper side of an already etched Zombie board. It shows where to cut, what to add and where. Essentially the trace linking pins 2 and 5 of U3 (using the layout/schematic that RG Keen graciously drafted and posted) to the existing Vref/Vb is cut. Two 10k resistors are used to provide a new Vref by tying one to pin 8 of U3 (where V+ is), and the other to the nearby ground connection along the edge of the board. Their junction is now soldered to the trace in between pins 2 and 5.
This can probably be best done with 1/8w resistors (the tiny ones) but if you're like me you probably only have 1/4w ones in your bin, so be sure to cover the leads of the resistors with a bit of insulation to make sure their leads don't short out against anything unintenionally.
Thanks to John for the design, to RG for the layout and redraw, and to Tim for the idea. Four heads are better than one.