Differentiator VC Feedback Project

#1

So, I want to make a Differentiator pcb with additional voltage controlled feedback.
I normally feedback it with a summing mixer, but I think CV control will make it even more interesting.
The effect of the feedback is that the edges repeat themselves several times until chaos ensues.

I paired the Cadet Multiplier schematic with the Differentiator and changed some stuff
The design is not finished yet and must be tested.
Right now I’m thinking 1 pcb for pots and jacks, 1 pcb for the main circuit and a panel pcb.
All sandwiched together to decrease depth. 8HP if it fits

  • feedback path switch (normal / inverted) - be to tested
  • 4 edge filter inputs (instead of 6) - to save space. If it fits, this can be changed to 5 or 6.
  • switches for feedback routing
  • not added yet, but interesting: diode clipping distortion or more filtering in the feedback path?

example of feedback edges: (in a patch, no cv yet)
The white dots = edge of the green shape routed though a Differentiator and feedbacked with a modified summing mixer.
(red & blue channel = camera feedback)

Remark: this project is directed towards DIYers and will be open source.
—>> All input and help is welcome!

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#2

Current schematic:

This will be my first time to do a pcb project with sandwiched pcb’s.

note: I left the 4 quadrant switch in. this has cool effects

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#3

testing with separate modules. fast screensots
The dots appear when the CV in modulated with a VCO.
The signal needs some boosting to get the repeats, so I have to add this in the schematic (or change some values / make a gain control

The feedback path routing adds nice features, so must be implemented on the front panel; with a rotary encoder or multiple positions switch (4x1)

When several inputs are used, the image get more interesting details (picture 2)

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#4

Complex CV possibilities.
pic1: minimal setting
pic2: half way
full on: chaos (not shown)

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#5

a 100k pot (VR3) in the amp stage of the differentiator works pretty good.
it needs a range limiter (r38) for the low end, which value I have to determine yet.
The Chaos tipping point is tricky. but will be solvable with the right resistors.

I’m getting a lot of cool images already :slight_smile:

I wonder if r41 & r42 (or r6) can be skipped. they were final outputs, but not anymore
the same with r44, r47, r45, r46.

and is r3 still necessary?

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#6

the differentiator is one of my favorites I think of it like a little utility mixer basically

a similar idea to the nearness module but for video

this module seems like a neat furthering of that concept. I’m always up for more feedback I’ll happily be following along but have no technical knowledge to lend

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#7

This is magnificent. I can’t wait to see how this ends up. Sign me up for one when it’s ready!

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#8

Following with intense interest!

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#9

Looks great! As far as skipping some parts, I would leave those resistors there to provide some isaolation from the contacts of the mechanical switches. (Are you using a rotary switch here?)

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#10

The one in the middle is a toggleswitch,
The 4x1 will be either a rotary or a 4position slide switch.

What about r3?

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#11

Only sort of half understanding what all is going on here, admittedly, but I wonder if there is a way to introduce variable time/delay to the feedback path for thicker & thinner edge feedback effects (if edge feedback is indeed what is happening here)?

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#12

@reverselandfill R3 keeps the input of U3A from floating while the switch is momentarily in a no contact state (which will prevent ringing or other problems you want to avoid.)

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#13

that makes sense, I had not thought about it that way.

Do you see any other points that can use improvement?

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#14

R7 and R8 should be ferrite beads and not resistors.
It would be ideal if the switches were replaced by analog multiplexer (74hc4053, etc) but then you need to add a +5V power supply and some input protection to the lines. For a DIY design I would not worry about it. For a production design, it’d be worth the fix.

What is the intention of the SW and SW2 pads and the switch S1? My suggestion would be to turn the LT1251 into a 4 quadrant multiplier – basically crossfade between the inverted and non-inverted signals. U3A and U4B could be eliminated, as you would send the differentiator buffer amp (U4A) to the non-inverting input on one half of the LT1251 and to the inverting input on the other half of LT1251 in parallel.

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#15

If you are looking for more ideas for functions, one would be to use a rectifier to split the positive and negative sides of the differentiator output and make them selectable (left edge only, right edge only, etc.)

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#16

r7 & r8 are already ferrites, I don’t have footprints for ferrites :slight_smile:

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