Connecting Arduino Mega to an RGB amplifire then LED strips

Hi

I am using Arduino Mega Rev 3 connected to an RGB amplifier which is then connected to a 5 meters RGB LED strip (12V). I am using a 12V power supply 10A, connected to both the RGB amplifier and the Arduino Mega. I have uploded a simple code on the Arduino that makes the LED strip light with the Green colour.

However, I tried two different ways of connecting the whole thing but none of them worked perfectly, each of them does a part of the wanted output.

The first one, I tried to connect everything like this example (connecting the strip black wire to the arduino 5V):
http://justpaste.it/o18q

When I turn everything on, the LED strips blink (only blink for less than a second) with the green colour I specified in the code. but does not maintain the colour afterwards and it goes off until I turn the power supply off, then when I turn the power supply on again it blink again and goes off. but in this example the only good thing is that the led gives the colour I specified in the Arduino code.

then I tried another way to connect everything like this (the strip black wire to the power supply positive wire):
http://justpaste.it/o192

After connecting it this way, as soon as I turn the power supply on I get a bright white colour straight away. this colour does not change whatever I put in the Arduino code. it only maintains the bright white colour forever but without any change regarding to any code I tried. Moreover, if I remove the pins of the red and green colour from the Arduino (PWM) I get a full blue colour, and if I put only the green pin I get a full green colour. so apparently with this way of wiring things I get tha maximum value of colours whatever the code is.

Do you have any idea what did I do wrong? which way is better? and what shall I edit in order to get a maintainable changeable colour? knowing that I want the whole strip to light with the same colour.
Thanks all for your help.

I have two RBG strips. 5 meters each. They are mounted at the top of the room encircling me.

I control these with 3 pins from the Arduino. Each directly controls an N-channel MOSFET (NTD4815S-35GOS-ND). One resistor, one capacitor. No other transistor(s) needed. The resistor is a 10K pull down on the gate. The capacitor is about 100pf across source to drain. And that's all. NO heatsinc needed as the FETs run cold switching 4 to 6 amps.

Works great. So simple and very bright. If you use transistors, you lose .7v across it and as we know, LED brightness is all "at the top" of the voltage (and current) so losing a little off the top really hurts. I did it with transistors first but found heatsinc was needed and lowered brightness. Switch to FET got rid of those problems.

I get almost no drop across the MOSFET and that means I get top brightness and almost no heat. Also, they are dimmable with PWM. Also quite easy. You can see projects PWM LED brightness all around.

So I like to keep it simple and cheap and I did. Got a little perfboard to mount the FET and resistor. Also jumped the FET with a capacitor. I was hearing the square wave in a cheap computer speaker I have. This quieted it a bit. Probably need a higher value to get rid of it altogether. Or throw the cheap speakers in the trash!

Can post more details if you are interested in going this way. But the circuits for this are all over the web, too.

Mike