RGB video amp

[boardjunkie]

Ok, so as far as homebrew video amps....anyone have a circuit that makes MAME look decent on RGB arcade monitors? I've not had to fuss with video stuff in the past since I worked with all regular arcade stuff. I'm using an ATI card (via Soft15khz) with a WG K4600 monitor that looks great with arcade boards, but lackluster with the PC video output. Its hard to get good color saturation with acceptable black level. I threw together a quick video amp based on a single transistor (2SC1815) and it seemed to help some, but still doesn't quite get it. I can get the games to look ok, but returning to the desktop, it looks like ass. Seems like it may be a loading issue, so I might build  an interface based on the LM310 voltage follower and see if that does any better.

[MonMotha]

Most designs are based on high bandwidth op-amps wired up for whatever the necessary voltage gain.  TI, National, and Maxim make good options.  I've used the TO OPA3692 with good results, but I'm not sure if that part is available in through-hole.

Check out the TI OPA350 (you'll need 3 - one for each channel).  You can just wire it up as a standard non-inverting amp with whatever gain you select.  TI will give you samples.  As this is a fairly high-speed circuit, it's a very good idea to place quality bypass capacitors near the power supplies for each chip.  There should be (at minimum) a 0.1uF ceramic very close to each chip's power supply pins, a 1uF ceramic somewhere on the board near the chips, and a largeish (10-100uF) bulk cap near the power supply into to the board.

A gain of 2-2.5 is about right if you have a relatively high impedance input (~1k ohm recommended) from your PC (which results in ~1.4Vpp signals).  If you properly terminate the inputs from the PC into 75 ohms (recommended for best quality, but probably not really necessary with 15kHz timings), you'd need a gain of double that (so about 4-5).

If the amplifier oscillates, try placing a small resistor (~10-100 ohms) between the output of the amplifier and the wire running up to your monitor.  Also check your bypass capacitors or add more (smallest ones closest to the pins then getting larger as you move away).

Building this on a solderless breadboard may give mixed results.  The parasitics on them are pretty bad.  A solder based perfboard is recommended.

[boardjunkie]

Ok, cool. I'm gonna build a more sophisticated circuit that is buffered on the outputs and see how that does before going to an op amp based design. I build everything on "stripboard" IE protoboard so its all soldered. I just don't trust anything thats not soldered.....

A Donkey Kong monitor video amp set to non-inverting would prob'ly work ok, but all my game stuff is in storage and I can't get to it right now. Just trying to get this last detail worked out before I go dragging one of my cabs back here. I think I'm gonna use my blue w/red t-molding "Grand Products" drawer style cab. That should be ideal for this kind of thing.

[Gray_Area]

Or, for about $25, you could buy one from Ultimarc.

[boardjunkie]

Meh....why buy when you have the ability to build? More fun that way.....

[boardjunkie]

Ok....got where I needed to be. I built this circuit and it did work a bit better, but contrast was still overblown although the colors were stronger.

http://members.optusnet.com.au/eviltim/vga2arc/vga2arc.htm

What I did was modify the bias network to be variable. On the left side R14 and R15 are paralelled. I replaced that with a 500 ohm trimpot so I could vary the overall bias. This was the ticket. Much improved and very tweakable to get the best possible image. I may add individual color amp bias trims in the future if I feel the need to....