THS7374 verse LT6557

[VajSkids Consoles]

Can someone give me a super technical reason the THS7374 is found to be the best amp for RGB mods and bypasses?

I have being playing with the LT6557 and maybe it’s my imagination .. but I find it improves the non one chip SNES and super famicoms RGB output and it’s absolutely gorgeous on the megadrive.

I know there’s full on tech experts on here that can shed some light on this

[Opatus]

The THS7374 is not the best but it's probably the cheapest. It's easy to use, it has an optional low pass filter, reasonable output swing, clamping, slightly lower gain error. But the LT6557 has a really good slew rate and fast settling time, but you most likely cannot use it with 3.3V.

I believe it is only your imagination on the 3-chip SNES. The analog signal that comes out of the graphics chip is already blurry. You have to use the digital signals from the graphics chip if you want sharp video out of it.

Also as a note I've checked your board on OSHPArk. It seems to be missing the resistor on pin 16. Without that your output signal will not be linear under 0.8V. It seems that you have upto 4V on the output, meaning that the video signal will not be linear under around 0.14V at the reciever. It's probably not a huge problem for the Megadrive. You could set the input voltage bias with pin 16 to at least 0.5V, attenuate your input video signal to 0.7V, then you can remove the 22 Ohm completely and your signal should remain linear.

[VajSkids Consoles]

Thanks for the feedback, oshpark really screwed me on that batch of PCB's. My computer shows everything is correct - but it came with broken traces to the divider (blue channel) which should probably be more around 25 ohms as I'm sitting just under .7vpp at the moment. I will draw up a quick schematic in paint for more feedback- but it seems perfect as it is (other than adding a botch wire).

I have actually connected a pot to that pin to swing it around and find an optimal input bias - I don't know. You could take a look shortly and give us your feedback. I've actually killed 2 model segas by switching them on and off 5000 times in a row but my model one i prototyped survived. I am having to jump start my model 2's to get them to go .... lol (ok- it started working normally again)

[VajSkids Consoles]

https://imgur.com/a/0wxbqfF

[VajSkids Consoles]

and for reference this is my PVM with all settings neutral/ centred



be easy on feedback , im a comms tech not an electronic engineer (I started with 25 ohm, then 22 then 20, but i think 25 is closer to .7vpp this is 20ohm pictured)

[VajSkids Consoles]

Also pin 16 open sits at around .5vpp? wouldn't this mean the input bias is around that without a resistor to ground?

[Opatus]

If you leave Pin16 open, then the input bias is disabled. You should set this resistor to 400-700 Ohm (for 1.1-0.7V input voltage).
Forget the 25 Ohm on the output, it can disturb your signal integrity. Instead drop the voltage of the incoming video signal to 0.7V, right now you divide it to around 2V. To do that you should resize the 1.3k and 2k voltage divider. What cable do you use? Because normally a Megadrive cable has a series 75 Ohm and 220 uF, meaning you don't need any extra resistors or capacitors on your board.

Also just an advice, when you are routing a board you should try to route the most important signals on one layer if possible. Your input signal changes layers 3 times.
You can also add a ground plane for the bottom layer.

[VajSkids Consoles]

When I went any larger on the 2k I would get crazy interference and jailbars. It was awful. So I used trial and error but I found a lower input was giving me interference. I ended up settling with having the output far too bright as initially it was too dark or too much interference. I know there are 75r to ground in TV sets and it should be .7 in / 1.4 out then back to .7 at the TV with the terminating resistor and in series resistor.

Anyway I couldn’t figure out how to get this attenuated as per “proper standards” so I sort of cheated to divide the output down to .7vpp and haven’t had an issue on any televisions I’ve tried it on including 3 RGB hacked ones - a Loewe and a 14 and 20inch PVM through my little cat5 adapter with BNC screw down terminals.

I use a $5 cable,
I removed the components in the scart head.

Hell if someone wants to re-attenuate this that’s cool but I’m moving onto other projects - I need a master system RGB board now. Arcade TV shared his in 2014 I think it was - it’s very component heavy compared to other sites boards I’ve seen. Pretty sure they only passed the inputs through one resistor and used the in cable components on the output.

[VajSkids Consoles]

Also schematic is slightly off, I have the pull downs on the output on the other side of cap / boards output
Which would be negative side

And yeah I made this without a schematic so I couldn’t choose the fill/ flood it with ground.

[VajSkids Consoles]

Finally, I removed the 75ohm resistor on the composite out and jumped across to the front of its footprint with a 470ohm resistor in heatshrink off the signal side of the 2k2 c sync pull-up to send C sync out on pin 4 to use a regular cable

** also if you could name a game where I’ll notice visually the ill effects of the circuit would be great, I am just a tinker type - and when I refer to the output I mean under load connected to the set