RGB matrix mixer [DRAFT]

NOISEBOB · Joined: Oct 13, 2019 Posts: 33 Location: NOISE

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fresh idea.. matrix mixer for red, green and blue.

would this work?

edit: fixed some bad errors, original schem had pwr and gnd short circuiting and used NPN where they were supposed to be PNP. if you want to see the wrong one it is here: http://shitcore.org/imgbrd/db/157143040955.png

inspired by this: https://web.archive.org/web/20060112082440/http://www.anatekcorp.com/driving.htm

PHOBoS · Posted: Sat Oct 26, 2019 11:07 am Post subject:

I hardly have any experience with controlling RGB signals but I do see some flaws.

I think the input impedance should indeed be 75 ohms, although I am not sure how important this is, but it could form a voltage
divider with the output to get the correct voltage. In your schematic you have 3x 75 ohms in parallel resulting in 25 ohms.

If a pot is at 0 ohms the corresponding transistor is connected directly between +5V and GND and it probably will result in a hot
but unhappy trannie. If a pot is above 0 ohms but still at a pretty low value it might develop a smoking habid and although
smoking pot can be fun that is usually a different kind of pot.

The transistors might need a current limiting resistor on the base but there could be some current limiting on RGB outputs
that is already enough. Also because of the way they are configured it might not be needed at all.

What you have now looks more of a current mixer btw and you probably want a voltage mixer. Then again this current will be
converted to a voltage when you plug it into something.

NOISEBOB · Joined: Oct 13, 2019 Posts: 33 Location: NOISE

PHOBoS · Posted: Sat Oct 26, 2019 4:08 pm Post subject:

Had a look over at AAC. Yeah you basically want 3 of those mixers, but I don't know the frequency of the RGB signals. I'd assume that for a
single color it would actually just be DC so you don't want any AC coupling (no caps on the inputs).

The voltage level for RGB signals is only 0.7V which could become a bit of a problem. It might not be a bad idea to first boost the signals and
attenuate them again after mixing. You probably also need to limit them because if you're summing the signals you could get a max voltage
of 2.1V, I'm not sure if that could damage anything.

PHOBoS · Posted: Tue Jan 19, 2021 12:09 pm Post subject:

Here's one I came up with. It's not perfect but does the job for what I need it for (video feedback).

RGB mixer basic.gif

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