The zombie chorus - it's a trap!

[lars]

I've finally given up on trying to get a zombie chorus to work well, after trying 3 different layouts over the last 8 years. I have found through trial and error that the only schematic worth going off of for this circuit is the original, hand drawn one that uses a TL064. Other schematics and layouts have many questionable changes to them that tend to make the circuit not work at all, or add a bunch of noise and ticking, which is funny, since those changes are supposed to get rid of noise and ticking.

I did get a functioning circuit with no ticking once I stripped it down to the original specs, but I still had bias problems with the LFO, even after trying a bias trimmer for the LFO. It tended to sound more like a chorus/tremolo mix than just a straight chorus. The rise and fall of the LFO was very deliberate and noticeable and could only be fixed by manually adjusting the bias back and forth as I played (I need three arms!)
The main lesson here is to really stay away from any BBD-based circuit that doesn't have bias trimmers. You will save yourself a lot of troubleshooting.

[JackSkellington]

I built the circuit on veroboard layout from tagobard, I got a light ticking with pot at max or almost. Other settings sound fine enough.

I would like see a schematic of your mod.
Thanks.

[lars]

I built the circuit on veroboard layout from tagobard, I got a light ticking with pot at max or almost. Other settings sound fine enough.

I would like see a schematic of your mod.
Thanks.

I didn't make any kind of mod really, it was just a last ditch effort to try to get the circuit to sound right. I replaced the two bias resistors with pots so I could manually adjust the bias. It never got rid of the wobbly LFO behavior though.

[Scruffie]

The tremolo behavior sounds like it might be the clock topping out, I haven't played with the 4046 in a long time but I think if you diddle with the 100k off pin 12 you can adjust the max clock frequency. If you have a meter or scope that can read clock frequency you can confirm that.

Some low pass filtering off the LFO might help with the overly obvious pitch shift.

[flanagan0718]

I have had 2 success builds off of the 3 I've done. The tag board one from [v]ark's site didn't even fire up for me. I ended up saving the BBD and trashing the rest. The vero from sabrotone worked like a charm, both times!.


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[JackSkellington]

I built the tagboard layout because of the three way mode and the Chorus/Vibe mode. But tagoboard and sabrotone have also some other difference in the schematic and components.
Next time I'll try sabrotone layout, but I would like to know how I can get the same mods of tagoboard in it.

These are the two layouts:
http://www.sabrotone.com/wp-content/uploads/2012/10/ZombieChorus.gif
http://1.bp.blogspot.com/-TkX4gNiMrv4/US54YWBS19I/AAAAAAAAFLA/_P6LItYKcQE/s1600/Zombie+Chorus+with+Chorus-Vibe+switch+2.png

[JackSkellington]

I'm sorry, this is an old thread, but I'm not sure I wanted open a new one.
I want to link with my last post here.
I built some years ago the Zombie Chorus using the layout from tagboard. It works fine, except a bit of ticking at higher setting, but in a normal chorus sound setting it's pretty ok. As we can read in a previous post a Sabrotone layout should be better and "ticking-less".
I am a bit of more expert, now, and I want to look this thing.

http://1.bp.blogspot.com/-TkX4gNiMrv4/US54YWBS19I/AAAAAAAAFLA/_P6LItYKcQE/s1600/Zombie+Chorus+with+Chorus-Vibe+switch+2.png
https://www.sabrotone.com/wp-content/uploads/2012/10/ZombieChorus.gif

1. The Tagboard layout has just one 470uF in the power filter, while in the Sabrotone I see a 100uF and a 47uF (C6 and C12). The bigger cap near the op-amp used for the input and output stage the small one near the CD4046. But I guess it doesn't matter, both the caps goes from 9v to the ground.

2. Sabrotone has in the power supply even a 100nF cap from 9v to the ground. Tagoboard doesn't have it.

3. A Vref resistor, the one on the ground, is different. A 15k for Sabrotone, a 12k for Tagboard. (I have to say that I saw some schematics, maybe updated, with two separated Vref, one for the input stage with the 10k/15k and one for the Speed/Depth stage with 10k/10k resistors).
This Vref resistor in the input stage could be a bit tricky, I saw a schematic with a 4.7k + 10k trimmer, to adjust the voltage around 5.2V, though it was a MN3207 IC schematic version.

4. The filter cap in the Vref of the input stage is 10uF for Tagboard and 100uF for Sabrotone.

5. Sabrotone has a 1N4001 from 9v to the ground. Tagboard doesn't have it. This diode is for the polarity protection.

6. Sabrotone uses two low noise IC TL072, Tagboard uses two low current TL062.

7. Sabrotone doesn't have the Vibe switch, Tagboard yes.

8. The Mode Switch is a three way for Tagboard, and a two way for Sabrotone. So the caps selection is a bit different. Tagboard can choice between lug 6 and 7 of the CD4046 1nF,  500pF and 333pF. Sabrotone has just 1nF and 500pF.
I am a bit confused about this values, But I guess 500pF is the Normal Mode and 1nF is the Clone Mode. The 333pF is the Leslie Mode.

9. I didn't saw any schematic with a diode or a resistor in line to the 9v, except the MN3207 version, it has a 100R just for the supply of the op-amp for the Speed/Depth stage.

Who wants to add something else?

[chromesphere]

Erik spent quite some time re-engineering the zombie chorus due to the ticking problem and came up with the lich king chorus, which is a really nice sounding chorus pedal and totally tick free.  What was the secret?  He told me a while ago but i cant recall (placement and grounding possibly).  Anyway, the resultant PCB is here, have a look in the build doc for the schematic and the PCB layout for placement.  Ive asked Erik for comment and will post here once he responds:

http://www.diyguitarpedals.com.au/shop/index.php?main_page=product_info&cPath=35&products_id=601

Sound Demo (note: no ticking )
https://www.youtube.com/watch?v=dIDLT0_Tmdo

Eriks response:

1. Ground Plane (due the fact the zombie chorus needs a ground plane, vero may not be the best option for this project)

2. Made sure the Op-Amps were as far away from the CD4046 as possible, because the CD4046 releases a bit of electronic noise.

3. The resistors to the CD4046 for the R1 and R2 are very close to the CD4046 and are grounded to a ground plane. Those two resistors seem to be the "antennas" to a lot of the ticking.

4. Having a variable trim pot for the bias is extremely helpful. As is true from the old Boss CE-2, the MN3xxx BBD's are more sensitive to BIAS drifting than other chips, and so it needs more fine tuning.

[bushidov]

Hi there. I am Erik, who works with Chromesphere (dbe/diygp) and these are the main things I applied to this pedal to make the noise go away.
1. I used a ground plane. The CD4046 is a noisy chip (ask folks who designed 8-Bit computers and used them) and it needs "thorough" grounding.
2. I made sure the op-amp for the input and output buffer, as well as the op-amp for the square and triangle wave creator were as far away as possible to that CD4046 because, again, that chip is noisy.
3. The noise tends to be loudest at the resistors that come off pins 11 and 12 (aka R1 and R2) of the CD4046. I kept those resistors on the furthest side of the board (which in turn will be close to the walls of the enclosure, a make-shift faraday cage). They are also kept close to the CD4046 and then grounded close via the ground plane mentioned in list item 1.
4. I made sure the bias had a trim pot. Like the designers of the Boss CE-2 before me, it is well understood that BBD chips are more sensitive to BIAS drifting than other chips and discrete components, so careful fine tuning will help with the performance dramatically.

I am not saying that's a "slam dunk" to fixing the issues, but they were concerns I used to make sure the board I designed didn't have the ticking issues, and it seems to have removed all of them.

[JackSkellington]

Excellent!
I can build just on veroboard, and my tagboard version, as I told early, is not so bad, it start to tick just at higher setting, when depth and speed are really extreme. I heard that the Sabrotone was better, maybe are not the components (ICs or filter caps) to make difference, maybe it's just a more lucky layout, though reading the document attached by chromesphere, the bias setting is very important.
Just a pair of question:
1. Is there some reason to use 1uF electrolytic and not a non polarized? I'd use a non polarized.
2. Is there some reason to use TL062 and not TL072?

[bushidov]

1. Is there some reason to use 1uF electrolytic and not a non polarized? I'd use a non polarized.

Just space and cost, at least in this case. There isn't much on this circuit that you are doing that would require that 1uF to be able to switch faster than what an electrolytic can handle.

2. Is there some reason to use TL062 and not TL072?

TL062 uses less current than the TL072, but at the cost of being slightly more "noisy". If you made a version that never was going to use a battery and always would be using a wall power supply, then TL072 all the way. If you were using 9V batteries only, TL062 might squeeze a little more lifespan out of that battery.

[JackSkellington]

I never use batteries for my pedals, so I try to use just TL072, but I read sometime a less current IC like TL062 or TL022 cause less noise.

[Scruffie]

Use a TL072 for the audio (less noise) and a TL062/TL022 for the LFO (less current draw, reduced chance of tick).

Common audio op amps make lousy LFO op amps and vice versa.

[thomasha]

You can roud the square wave up to avoid the ticks, boss uses it too.
Take a look at fig. 3 here>
http://www.moosapotamus.net/files/stompboxology-mo-tremlo.pdf

[JackSkellington]

Use a TL072 for the audio (less noise) and a TL062/TL022 for the LFO (less current draw, reduced chance of tick).
[...]

Yes, someone else explain me this.

You can roud the square wave up to avoid the ticks, boss uses it too.
Take a look at fig. 3 here>
http://www.moosapotamus.net/files/stompboxology-mo-tremlo.pdf

Thanks!