Aliasing clock noise on bucket brigade chorus

Hi all,

Working on a sort of recreation of a Dimension C chorus, but with a custom LFO circuit and some other options.

Running into some noise problems, you can hear the clock signal aliasing on the audio signal when coming out of the bucket brigade chip. I have tried lowering the cutoff frequencies for the anti aliasing filter but it's not sufficient to eliminate the noise.

Does anyone have any ideas which I could try? I have attached a recording of the noise when injecting a 4.5k sinewave (it is quite piercing so beware of volume), and a schematic.

Noise sample: https://drive.google.com/file/d/1Iml_l-Pfv81Bm1PUfkNzTohA6JfgDS7S/view?usp=sharing

What's weird is that the noise seems to disappear when removing one of the clock chips (3102). So there's some interaction going on somewhere, but seems weird to me.

brndvnrdn said:

What's weird is that the noise seems to disappear when removing one of the clock chips (3102). So there's some interaction going on somewhere, but seems weird to me.

Click to expand...

I am still working on my first cup of coffee, but the noise may not be aliasing, but beating between two close clock frequencies.

Answer may involve improved anti-image (post BBD) LPF, or tighten up PS decoupling, or something else?

JR

JohnRoberts said:

......
Answer may involve improved anti-image (post BBD) LPF, or tighten up PS decoupling, or something else?

JR

Click to expand...

"Something else" can be (non-)proper grounding.

Is the circuit on a PCB or are you bread boarding / prototyping?

brndvnrdn said:

Running into some noise problems, you can hear the clock signal aliasing on the audio signal when coming out of the bucket brigade chip. I have tried lowering the cutoff frequencies for the anti aliasing filter but it's not sufficient to eliminate the noise.

Click to expand...

What's the clock frequency?
Could very well be that the clock frequency is too close to the signal's.

Thanks for all your replies.

I'm prototyping on a pcb.

I tried increasing the clock frequency to >100kHz, but the problem still persists, be it at a slightly different frequency.

The weird thing is that when probing both the BBD outputs, one of them has the issue of the aliasing clock noise in the audio and one doesn't. I've tried swapping them, but the problem changes to the other BBD, so its not one of the BBD's.

I thought maybe one of the traces was too close to the clock signal on the pcb, but I probed it out of circuit and the problem remained.

I'll keep looking for solutions, but if anyone has any ideas please chime in. Thanks.

confirm the BBD pins 7 and 8 are connected together as shown on schematic.

JR

JohnRoberts said:

confirm the BBD pins 7 and 8 are connected together as shown on schematic.

JR

Click to expand...

They are definitely connected. I suspect some sort of grounding issue perhaps, will check tomorrow. Thanks for the help.

Hmm, I found a solution. Apparently there was a lot of ultrasonic content in the output of the bbd, and they were aliasing/folding back on each other. By putting a small capacitor on the output and ground pins the problem is gone!

Something else: I am using a lfo/oscillator circuit to modulate the clock chips, borrowed from a ce-2 and duplicated for the dual delay lines. (please note that i am using mn3201 & mn3207 chips, not 3001 as the schematic below).

I would like to have a greater modulation depth/greater frequency range. I've changed c22 but its not that big of a change. Does anyone have any suggestions for a bigger frequency range? Thanks!

brndvnrdn said:

Hmm, I found a solution. Apparently there was a lot of ultrasonic content in the output of the bbd, and they were aliasing/folding back on each other. By putting a small capacitor on the output and ground pins the problem is gone!

Click to expand...

I did a lot of BBD work back in the 70s and into the 80s. The typical BBD has two output pins where the second output is duplicate of the first output delayed 1/2 clock period. Since the BBDs transfer signal during one half of the clock period, without this duplicate output there would be a  clock frequency square wave in the output.

Since these two outputs use similar FET source followers the loading can cause small DC differences between the two. This DC difference expresses as the clock frequency on top of the audio signal. More advanced designs trim out this clock frequency component by loading the two differently.

Something else: I am using a lfo/oscillator circuit to modulate the clock chips, borrowed from a ce-2 and duplicated for the dual delay lines. (please note that i am using mn3201 & mn3207 chips, not 3001 as the schematic below).

I would like to have a greater modulation depth/greater frequency range. I've changed c22 but its not that big of a change. Does anyone have any suggestions for a bigger frequency range? Thanks!

Click to expand...

digital..  ;D 

I expected BBD to be long gone by now.

JR 

> I expected BBD to be long gone by now.

"a really great grainy decay without being too fat."
https://www.diystompboxes.com/smfforum/index.php?topic=123411.msg1167300#msg1167300

PRR said:

> I expected BBD to be long gone by now.

"a really great grainy decay without being too fat."
https://www.diystompboxes.com/smfforum/index.php?topic=123411.msg1167300#msg1167300

Click to expand...

Back in the 80s customers voted with their Benjamins to purchase the then nascent digital delay technology (like delta modulation) instead of BBD based studio efx. Delta mod could get grainy too at low clock frequency, but a different grainy.  :

JR