Video amp (RGBs) to boost SCART to arcade monitor

[arcadeswede]

Hello!

As the title says, I'm looking for an RGBs amp circuit to boost 0.7Vpp video signals to 2-5Vpp (TTL) to connect retro consoles (or similar) to work on arcade monitors.

I was thinking about using an OP amp but I don't know how a circuit like that would look like?
I'm familiar with the etim RGB amp circuits found here: http://members.optusnet.com.au/eviltim/vga2arc/vga2arc.htm#acvid

It uses all different kinds of voltages like +5V, -5V, +10V etc. It would be nice to drive the RGB amp from just 5V or 12V found in many arcade cabinets. Also I think today, that kind of circuit could be smaller in terms of newer components maybe.

Does anyone have any experience in creating RGB amps?

[arfink]

I'd suggest this: https://www.ultimarc.com/arcade-vga-and-video/arcadevga-accessories/arcade-monitor-video-amplifier/

[arcadeswede]

I'd suggest this: https://www.ultimarc.com/arcade-vga-and-video/arcadevga-accessories/arcade-monitor-video-amplifier/

I was looking into that but VGA has DC offset and SCART is AC coupled.

[arfink]

I think you'll find that most arcade monitors care a lot less about signal quality than most.

BTW, I also found this if you're interested: http://www.geocities.ws/podernixie/htpc/cables-en.html#jammascart

EDIT: I should clarify that this document just has an un-amplified scart to arcade setup, which does work, but won't be very bright.

[arcadeswede]

I think you'll find that most arcade monitors care a lot less about signal quality than most.

BTW, I also found this if you're interested: http://www.geocities.ws/podernixie/htpc/cables-en.html#jammascart

Thanks for the link!

Is that so?
That link looked promising but again, I think the circuit at the bottom doesn't take in account the low 0.7Vpp that comes out from retro consoles. It seems strange to be able to just plug in RGB directly to the VGA/Arcade input. A friend of mine tried to connect his SNES (or was it SEGA Genesis?) to his arcade cab but got a very dim picture. And that's the reason I've started to look into boosting RGB and sync signals.

[arfink]

as long as your sync is in range and your signal voltage isn't too high, it should just work.

[arcadeswede]

as long as your sync is in range and your signal voltage isn't too high, it should just work.

What? Doesn't make sense. According to this page, specs for arcade vs scart looks like this:

Arcade video (standard resolution)
Video amplitude: 2-5Vpp
Video DC offset: yes, as much as a volt or two
Video input impedance: 1k-10k
separate horizontal and vertical sync (most of the time)
sync polarity negative (most of the time)

SCART video (RGB SCART)
Video amplitude: 0.7Vpp
Video DC offset: no DC offset, AC coupled
Video input impedance: 75
sync signals stripped from Composite video (CVBS)
sync polarity negative

Maybe sync will be fine but voltages for the SCART signals are way to weak, hence the need for amplification.
Just to be clear, I'm talking about connecting a console with RGB SCART to a arcade monitor, not arcade to TV (aka Supergun).

[arfink]

Right, I understand that. I'm just saying, if you want an image and you don't want to blow up your monitor, just make sure you have the right sync frequency + polarity (usually, composite negative sync, 15khz) and don't exceed 4-5vpp for your video signal. You can amplify up to that point as needed/desired. If you go over on the shmups forums in their RGB modding thread, you can see that people have committed worse abominations in RGB signal land and been fine.

[buttersoft]

Some few arcade monitors will work fin with 0.7Vp-p RGB, most won't be able to boost it and will need an amp. Ultimarc's one will work fine, or if you want to build one a THS7374 is as good, and a THS7314 or 7316 almost as good.

[arcadeswede]

Some few arcade monitors will work fin with 0.7Vp-p RGB, most won't be able to boost it and will need an amp. Ultimarc's one will work fine, or if you want to build one a THS7374 is as good, and a THS7314 or 7316 almost as good.

Ok, good to know!
According to the datasheet of the 7374, it only has a gain of x2 (6db), which equals to 0.7Vpp x 2 = 1.4Vpp. Which is not really enough 
I found a THS7375 with a 15db gain (around x5.6) which equals to 0.7Vpp x 5.6 = 3.92Vpp. That should be good enough, right?

Btw, does these signals need to be AC coupled going into these?

[PL1]

gain of x2 (6db), which equals to 0.7Vpp x 2 = 1.4Vpp.
 . . .
15db gain (around x5.6) which equals to 0.7Vpp x 5.6 = 3.92Vpp.

Sorry, but your math is not correct. You are correct for voltage ratios.  I was thinking about power ratios.   

Add(subtract) logarithmic value ==> multiply(divide) linear value.

Four easy-to-remember logarithmic to linear conversions:
  +3dB (3dB gain) = linear value x 2
  -3dB (3dB loss) = linear value / 2
+10dB (10dB gain) = linear value x 10
-10dB (10dB loss) = linear value / 10

You can get to any whole number dB value by adding/subtracting 3's and 10's.
---------------
Various amounts of amplification applied to a 0.7Vpp signal:

+4dB (+10dB -3dB -3dB) amplification = voltage x 2.5 (10 / 2²) ==> 0.7Vpp x 2.5 = 1.75Vpp

+5dB (+3dB +3dB +3dB +3dB +3dB -10dB) amplification = voltage x 3.2 (2⁵ / 10) ==> 0.7Vpp x 3.2 = 2.24Vpp

+6dB (+3dB +3dB) amplification = voltage x 4 (2²) ==> 0.7Vpp x 4 = 2.8Vpp

+7dB (+10dB -3dB) amplification = voltage x 5 (10 / 2) ==> 0.7Vpp x 5 = 3.5Vpp

+8dB (+3dB +3dB +3dB +3dB +3dB +3dB -10dB) amplification  = voltage x 6.4 (2⁶ / 10) ==> 0.7Vpp x 6.4 = 4.48Vpp

+9dB (+3dB +3dB +3dB) amplification = voltage x 8 (2³) ==> 0.7Vpp x 8 = 5.6Vpp

+15dB (+3dB +3dB +3dB +3dB +3dB) amplification = voltage x 32 (2⁵) ==> 0.7Vpp x 32 = 22.4Vpp

Arcade video (standard resolution)
Video amplitude: 2-5Vpp
. . .
SCART video (RGB SCART)
Video amplitude: 0.7Vpp

You'll need +5dB to +8dB of gain to amplify the RGB SCART signal level to the range used for arcade video.   


Scott

[arcadeswede]

Sorry, but your math is not correct.

You'll need +5dB to +8dB of gain to amplify the RGB SCART signal level to the range used for arcade video.   

Thank you, Scott!
So to understand your calculations above. If I want 11dB gain, it would look something like this:

+11dB (+3dB +3dB +3dB +3dB +3dB +3dB +3dB -10dB) amplification = voltage x 12.8 (2x2x2x2x2x2x2 / 10) ==> 0.7Vpp x 12.8 = 8.96Vpp ?

[PL1]

If I want 11dB gain, it would look something like this:

+11dB (+3dB +3dB +3dB +3dB +3dB +3dB +3dB -10dB) amplification = voltage x 12.8 (2x2x2x2x2x2x2 / 10) ==> 0.7Vpp x 12.8 = 8.96Vpp ?

Exactly right.   
Sorry about posting the wrong ratios.   


Scott

[PL1]

Oops.  You had it right originally.   

Just noticed that I was using the approximate conversion ratios for power (watts), not voltage.   

The handy calculator here converts dB to voltage and power ratios or convert one of those ratios to dB.


Scott
EDIT: Found a better calculator.

[arcadeswede]

Oops.  You had it right originally.   

Just noticed that I was using the approximate conversion ratios for power (watts), not voltage.   

The handy calculator here converts dB to voltage and power ratios or convert one of those ratios to dB.


Scott
EDIT: Found a better calculator.

No worries, man!
I used this in my first post about x2 gain:
http://www.sengpielaudio.com/calculator-amplification.htm

So, do I then take 15dB which is 5.56 voltage gain and multiply that with the 0.7Vpp voltage like so, 0.7 x 5.56 = 3.92Vpp ?
Is this the maximum amplification I can do with this THS7375 chip?

[PL1]

So, do I then take 15dB which is 5.56 voltage gain and multiply that with the 0.7Vpp voltage like so, 0.7 x 5.56 = 3.92Vpp ?
Is this the maximum amplification I can do with this THS7375 chip?

The datasheet indicates "5.6-V/VGain(14.95dB)" so it looks like you're on the right track.   


Scott

[buttersoft]

The THS7374 will work fine and is about as good quality as you can get, but a lot of other things will work ok. The THS7375 you refernce looks about the same as the THS7374 save the higher gain, which isn't needed (and i've be careful feeding anything over about 4Vp-p into a video amp as it might top out at 5V if there are IC's involved). I can't find one for sale at any of the usual places though, so if you do find any they might be worth a packet

[arcadeswede]

The THS7374 will work fine and is about as good quality as you can get, but a lot of other things will work ok. The THS7375 you refernce looks about the same as the THS7374 save the higher gain, which isn't needed (and i've be careful feeding anything over about 4Vp-p into a video amp as it might top out at 5V if there are IC's involved). I can't find one for sale at any of the usual places though, so if you do find any they might be worth a packet

Mouser have them in stock!
I'll maybe order some one day and try to figure out a schematic.