Stupidly Simple Zombie Chorus Alternative (3207 & No Tick or Special Clock Chip)

[Scruffie]

Cheap, Dirty & Simple Analog Chorus & Vibrato Pedal.

I wanted something that could be built without having to order any special parts like clocks (other than having a BBD around) and would be small enough to fit in to a 1590B without any issue.
I also wanted to use a 3207 BBD (most 9V choruses using a 3007 which in my opinion is a waste of a good BBD) and to not have the tick issues of something like the zombie chorus or issues of PT2399s locking up but keep it as a project that could be built by a begginner with relative ease.

For me it was mainly an exercise in seeing a clock & LFO set up work with just transistors, but it happened to coincide with the competition on F r e e stompboxes so I thought I would share it here too.

It's nothing new, it borrows from a few sources of other DIYers work glued together with the odd tweak but it sounds good and could be built for just a few dollars, the only pricey component being the BBD.

I left it with a fixed depth at the point I thought it sounded good, you could of course add a 1M to 4M7 value pot in series with a 470k resistor at R5 or switch between two/three values with a toggle, it can also be a fixed chorus or vibrato if you prefer.

Not a massive range of speed but definitley useable, if I was to expand the design I would also use the ol' 8V2 zener trick but for the simplicity this works fine, D1 being utilised to drop pin 4 to 14/15th as reccomended by the datasheet, although this part is not entirely necessary.

If finding 15pF caps is an issue, 22pFs would be fine or two 33pFs in series. A Tantalum for C5 would be good, personally I used some miniature Electrolytics I had (they're absolutely tiny!). Any dual opamp would be fine, I just used the NE5532 for it's low noise/sound quality.



As you can (probably) see the audio path is much the same as the Zombie chorus, I couldn't get it much better for the parts count, I just added in a V.Ref & Bias Trimmer to avoid those tick problems, I think the few extra parts and trimmer tweaking out weight the tick issues many people suffer.

Any way, quite a nice chorus for its size (although personally, with the Lower-Fi of the audio path, I actually prefer this in the Vibrato mode, it adds a retro quality) and no 4046 to find in your parts box.

Then we have a Phase Shift Oscillator / Multivibrator combo for the Clock & LFO all done with transistors, perhaps not the best solution but we all have some NPNs and it was nice to see it work like this and work it does.

Enjoy!

[deadastronaut]

nice one scruffie, you know what i'm going to ask though .....clips?..

[Scruffie]

nice one scruffie, you know what i'm going to ask though .....clips?..

You know what i'm gunna tell you? 

[deadastronaut]

yep!...

[frequencycentral]

Nice work - that's not many more parts than a LA!

[Renegadrian]

nice...waiting for some some sound samples!!! 

[Scruffie]

nice...waiting for some some sound samples!!!  

Well you're gunna be waiting untill you or someone else build it  

It's not on the breadboard at the moment and i'm too busy on the run up to christmas to mess about with that sorta stuff.

It sounds like a chorus! It's not too complicated to just give it a whirl. If you like chorus and want a simple one... build it.

Nice work - that's not many more parts than a LA!

Cheers Rick!

Not quite as simple but getting there! It would have less parts with a CMOS Clock chip but that'd be bigger in size and less transistory.

[Slade]

Looks very nice, Scruffie, thanks!

[Thomeeque]

 Hi!

Few notes:

If the right U1 half ("U1b") stage should work as usual LPF, right leg of C10 should lead to U1b output (pin 7) not to inverting input (pin 6). Or just replace R17 by jumper (as Zombie does).

R16 could be replaced by jumper too (reason why it is in Zombie is that it forms network for +3.5dB boost in the input stage - does not your chorus suffer from slight volume drop btw.?).

You don't need R19, there already is DC at usable bias level at the BBD outputs.

T.

[Scruffie]

Hi!

Few notes:

If the right U1 half ("U1b") stage should work as usual LPF, right leg of C10 should lead to U1b output (pin 7) not to inverting input (pin 6). Or just replace R17 by jumper (as Zombie does).

R16 could be replaced by jumper too (reason why it is in Zombie is that in forms network for +3.5dB boost in the input stage - does not your chorus suffer from slight volume drop btw.?).

You don't need R19, there already is DC at usable bias level at the BBD outputs.

T.

I may not need R19 but it did have an effect on the tone, yes it can be dropped though if you wanted to spare another part.

R17 was changed to 100k because of the slight volume drop, but yes you're right, C10 should go to pin 7, my mistake on the drawing.

R16 could be a jumper but once again I decided to keep it in.

[Thomeeque]

I may not need R19 but it did have an effect on the tone, yes it can be dropped though if you wanted to spare another part.

Well, it could have set better dry/wet ratio actually (but it lowers slightly LPF effectivity too).

R17 was changed to 100k because of the slight volume drop, but yes you're right, C10 should go to pin 7, my mistake on the drawing.

R17 value should not affect volume of this stage - if it does, it does it in undetermined way and it can act on different op-amp differently.

R16 could be a jumper but once again I decided to keep it in.

OK..

[Scruffie]

I may not need R19 but it did have an effect on the tone, yes it can be dropped though if you wanted to spare another part.

Well, it could have set better dry/wet ratio actually (but it lowers slightly LPF effectivity too).

R17 was changed to 100k because of the slight volume drop, but yes you're right, C10 should go to pin 7, my mistake on the drawing.

R17 value should not affect volume of this stage - if it does, it does it in undetermined way and it can act on different op-amp differently.

R16 could be a jumper but once again I decided to keep it in.

OK..

Hm, well if it effected the dry/wet, it was fine for me, without it, it seemed a little 'flat'.

What if I had C10 connected as in the drawing, it's been a while since I did this but I swear that R17 altered the volume... if not fair enough, this was a 1 hour breadboard job.

R16, yeah, I decided to keep it in because I was too lazy to cut a new jumper wire to test it without after breadboading the usual Zombie  it's not doing any harm.

[culturejam]

I'm going to try to breadboard this one as soon as possible. I'll try to do a demo clip for you for the contest.

[Scruffie]

I'm going to try to breadboard this one as soon as possible. I'll try to do a demo clip for you for the contest.

Awesome, hope you like it!

Cheers CJ, that'd be much appreciated!

[culturejam]

I got it on the board, but can't get it to work. I get clean signal but no modulation. 

I went over all the connections a few times, so I'm not sure what the issue might be. Most likely "user error", I'm sure. 

[Scruffie]

I got it on the board, but can't get it to work. I get clean signal but no modulation. 

I went over all the connections a few times, so I'm not sure what the issue might be. Most likely "user error", I'm sure. 

Should work, unless my breadboard is magic, voltages?

[culturejam]

I'm sure it works, Scruffie. I must have made a critical error somewhere in my mass of jumpers. 

[allesz]

A necro bump two years later.....

I am starting to experiment driving a bbd (a coolaudio 3205) chip with bjt (tried the multivibrator and a negistor too), right now I got almost nothing but a faint chorused like warble and a lot of ticking, loud and proud btw 

Someone is working on something like this?
The search function showed very little (and this old tread was the most interesting), but maybe the problem is that I am not doing the right question...