Zombie Chorus Pedal question about the mode switch on the CD4046

[bushidov]

Hi Guys,

I think I am understanding this correctly, but I just wanted to make sure I didn't get this backwards. On the Zombie Chorus schematics I find all over the place, I see the timing capacitor(s) that go between pins 6 and 7 of the CD4046, which set the "mode". Some call it 1024-stage and 512-stage, even though it's just a delay time setting, not a stage setting. I also know that when I go between 1nF and 2nF for my timing capacitance, the delay changes. When using 1nF, my outputs on the CD4046 to the BBD become a wider waveform, which means the delay in frequency is slower. When using 2nF, my outputs on the CD4046 to the BBD become a narrower waveform, which means the delay in frequency is higher coming out of the CD4046. This also appears to be true with the output of the BBD chip's pin 7 and 8.

My question is which of those two "settings" is supposed to be the "Clone" mode (512-stage emulated)? Is it the 1nF or the 2nF? I also hear that Mark Hammer uses third setting to get into Flanger delay-time territory. What is the total capacitance he uses between pins 6 and 7 of the CD4046 to accomplish that?

Sorry for all the technical questions.

Bushido-V

[Mark Hammer]

Hang on a sec, I'll ask him.  "Hey!!  MARK!!  Get your sorry ass out of the fridge and come over here for a sec.  What cap values do you use for chorus modes?" 

I wouldn't refer to it as a "mode", but more as a range.   The cap value sets the clock frequency that steps the BBD through its paces.  Smaller cap values raise the clock frequency, which shortens the delay times produced, and larger values lengthen them.  Note that such changes shift the entire range up or down, rather than simply extending the range more in a given direction.

In general, time-modulated BBD-based effects will assume some range of clock frequencies used, and incorporate lowpass filtering designed around what needs (or doesn't need) to be filtered out of the wet signal-path, trying to keep as much bandwidth as is audibly "safe".  As a result, it is generally unwise to raise the value of the clock cap too much, since that may lower the clock rate low enough to be heard, without implementing changes to the filtering.

At the opposite end, Panasonic/Matsushita BBDs are penalized by having capacitance on their clock input pins that corrupts the clock pulses coming in at higher clock frequencies unless there is suitable buffering.  The sound quality of such chips depends on a smooth handoff from one member of the bucket brigade to the next, and the clock pin capacitance makes that handoff clumsier once it tries to go too fast.  So, unless the circuit includes that buffering to keep the handoff crisp and (virtually) seamless, it is unwise to reduce the clock cap value too much.

The stock Zombie circuit places two 1000pf (1nf) caps in series for a 500pf effective value.  Bridging either one of those caps makes the effective value 1000pf.  I just popped down to the basement to bring my own Zombie up and pop the back off.  I see a pair of 510pf and a 560pf where the two 1000pf are supposed to be.  I wish to heck I could tell you how they are connected and switched, but there's a gob of hot glue in there, and I can't see how the caps are connected.  Plus, it's been a long time since I built it so I can't rely on memory. 

That doesn't really answer your question.  I will suggest the following, however.  If you have a third cap of 560pf, in series with the two existing 1000pf, that will give an effective capacitance of around 260pf when all three caps are in circuit and in series, 500pf when the two 1000pf are in use, and 1000pf when the 560pf and one of the 1000pf are bridged.  For clarity's sake we'll say the caps are wired/installed as follows:

1000pf -> 1000pf -> 560pf

Wire up a SPDT on-off-on toggle with the switch common (middle lug) at the "far" side of the 560pf.  One of the toggle's outside lugs goes to the junction of the two 1000pf caps, and the other outside lug goes to the junction of the 560pf and the 1000pf.  In the toggle's middle position, you get the smallest effective capacitance, highest clock frequency range and shortest delays (in the zone between flanging and chorus), and the two outside positions provide the two "modes" (ranges) of the stock Zombie.

[bushidov]

Thanks for the quick reply! I see where I made the rookie mistake. For some dumb reason I was thinking capacitors in series add and parallel they add inversely, but that's resistors and I should have known better. Yeah, BBD's are fun things. I've made a Zombie Chorus clone called a Lich King Chorus that has at its base a Zombie Chorus, but I have the switch mode mods, the vibrato mod, added a bias trimmer, and I added jumpers so that one can set it to work with the PMOS or NMOS 3x07 BBD chip, so that I can do a shoot out on the chip running on the same board, same trace lengths, and same dial settings.

By the way, Mark, just gotta say it: you 'da man!

[Mark Hammer]

I'm just here to learn and to help, bro.  But pleased to turn on the lights for ya.  Let us know how you like the added 3rd mode.

[bushidov]

That makes a lot of sense as well, using lower value caps. I also built a clone of the John Hollis' Ultra Flanger with some success and noted that it had a lot of similarities to the Zombie Chorus. The CD4046 in this case had smaller values for the R1 and R2 resistors (pin 11 and pin 12) and the timing capacitor was a tiny 22pF. It seems to work pretty well as a Flanger, and this above explanation kind of helps me understand why.