pcb’s are ordered! 
red panel picture:
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inputs A,B,C, Inv then? or 1 2 3 - , or ChanA, ChanB, ChanC, ChanI …
Somehow RGB as inputs labels is confusing, at least to my ADHD. But I mean, its just labelling, so doesn’t really matter.
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the panel has just drawings, so you have to learn by using it, not read it.
I did add runic fonts for the jack inputs, that will help with patching
I wanted the columns to be of the same channel, so that is the reason I changed it.
This way the INV pots are correctly placed, and one column is one channel so to speak,.
for clarification, I labeled the inputs on the pcb with letters . for input channels R, G, B
so:
R_A…G_A…B_A
R_B…G_B…B_B
R_C…G_C…B_C
R_inv…G_INV…B_inv
jacks: (inputs left, outputs right)
R G B INV R G B
It is confusing to speak of RGB in this context because normally in a matrix mixer you would see numbers not color channels.
This fooled me for a while too, but I think this is the best way (in this 14HP format)
I have an audio matrix mixer (CGS clone by Falafular) which has the input jacks on the side, that makes it very logical and easy to understand.
It is just a design choice, I think the HP space is important for the DIY people, as most Cadet and Castle modules are very small. I do see this module as a performance unit, so some ergonomic space is vital! This is why I chose to have the jacks under the knobs. (and pots with normal knobs instead of trimmers)
as in panel design , would this be better? I changed this in the order already, as I think this makes it more user friendly. No need to be totally symmetrical in the design.
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I think it’d be cool if the inputs weren’t named R,G,B, but something more general like 1,2,3 or X,Y,Z or whatever (especially since I’m most interested in using a matrix mixer for colorizing three separate, unrelated inputs). But it’s not a dealbreaker for me, especially if you have the outputs like you have them in that detail pic, with boxes around the label to separate them from the inputs. I really like that. (It really drives me crazy when a module doesn’t have a quickly-discernable indication of what’s an input vs. what’s an output.) And in this case It makes having all the jacks below the knobs much less confusing in general.
As I think @joem was getting at, shouldn’t input designations be different from the outputs, so it is easier to grok the coordinates of each pot?
So (if I correctly understand how you have it set up) it would be:
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that is the layout, yes.
update. I changed it to symbols. (like in all my panel designs)
inputs and outputs are clear this way, and the inv_in is standing out.
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ok the pcb’s are in.
I’ll be building and testing next.
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aaaaaaaand it tests OK!
instant gratification with color mixing.
I’ll report back later with feedback patching!
whoooo 
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I want it! Can you send me a few boards tomorrow?
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wow oh wow that looks like it is going to be awfully fun!
I can’t wait to get a built module
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I did some fast feedback patching with 2 loops , 1 direct and 1 with a Differentiator:
there is a lot to play with! just need to wrap my head around the routing 
I have build one with 3x amplification, which can lead to fun keying effects
I have to mod the Inverted input to have the same amp stage, so that this can battle against the brightness (just change 3 resistors to 3.3k instead of 10k)
The Bom is done. I’ll make an Order thread.
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the order thread is live.
note: the 3x3 matrix mixer has changed to a 4x3 mixer, with the option to modify it to a sort of 5x3 .
BOM Matrix Mixer v1 2019:
PCB + Panel v1
Resistors 1% metal film 1/4 watt
10k: r1, r2, r3, r7, r10, r11, r12, r13, r17, r20, r21, r22, r23, r27, r30 (15x)
10k or 3.3k: r4, r14, r24, r34, r35, r36* see below (6x)
499r: r5, r6, r15, r16, r25, r26, r32 (7x)
1k: r9, r19, r29, r33 (4x)
MOD1, MOD2, MOD3 link* see below
Ferrite beads 5mm length
F1, F2
Capacitors
100nf: c1, c2, c3, c4, c5, c7, c8, c9, c10, c11 (10x) ceramic 2.5mm spacing
10uF: c6, c12 (2x) electrolytic 16v
IC
8pin socket: u1, u2, u3, u4, u5 (5x) optional
LM6172: u1, u2, u3, u4, u5 (5x)
Power
10pin shrouder header
10pin to 16pin power cable
Potmeters
b10k 9mm Alpha vertical (12x)
knobs (12x)
Jacks
Thonkiconn (7x)
No MODS:
If you don’t want to do mods, link the pointy side (left) and the middle holes of the MOD headers
If you don’t want 3x amplification, use 10k for r4, r14, r24, r34, r35, r36
The amplification can make it a bit harder to set the gains to precisely 100%, as it goes to 300% in total
Yes MODS:
MOD1, MOD2, MOD3 are header points for the Gain mod.
Solder b10k potmeters here to control the gain for each column. Mount them on a separate panel
The 3k3 resistors can be used as amplification for keying effects
Estimated part costs if you source the parts yourself: 70,- to 80,- euro (without pcb and panel)
This project can also be adapted to an audio matrix mixer. I’ll test this later
This would be cheaper, as you won’t need video opamps.
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how do you get these awesome feedback effects?
If i patch direct feedback with mine it just amplifies the signal, cant get trails or other feedbackfx for now…
I used a Differentiator in the feedback loop
+
a direct loop for extra blurring effects and fadeout or ‘whiteout’
so ch 1 = video input
output 1 = loop1 to ch2
output 2 = loop with differentiator to ch 3
output 3 -> RGB encoder
twist knobs to get all kinds of results!
Note: I do have a 3x amplification on the inputs (see BOM). But I used this feedback routing before. It should also work without that amp stages
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another feedback patch, with nice keying effects. the inv input now also has 3x amplification
this patch uses camera input on ch 1
and VCO input on the INV channel
feedback on ch 2
output 1 to RGB encoder ‘R’
output 2 to feedback loop
output 3 to RGB encoder ‘G’
luma output is multied to a processor and then to the RGB encoder ‘B’
The 3x amp can key out the horizontal lines to mask the image.
The feedback can streak the lines across the luma input.
The brightness and contrast of the video input help wth the colors, as seen at the end of the video
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If I wanted to put a switch on each of the MOD pots to switch the pot out and a fixed resistor in, which size resistor should I use to limit the range to 100% (instead of 300%)?