I'm using a PWM shield (http://www.practicalmaker.com/products/arduino-shields/pwm-shield-assembled) which allows to extend the number of PWM pins for an Arduino.
In my project I'm using NPN TIP120 transistors to control more than 5 motors.
So when I try my circuit without the shield, everything works fine, I connect the Arduino pwm pins to control de transistor (external power source with 20 A)
Then If I try the PWM shield directly powering a simple led it works fine as well, the problem is the PWM as I understand is very led-oriented and has a negative and positive pin for each PWM terminal which Arduino doesn't have. And my problem is where I connect the negative pin of the PWM shield? I tried just connecting it to the ground pin in Arduino but the motors just run, not responding to the different speeds I'm trying to achieve.
Also, I'm using a midi library, which I hope doesn't interfere on this since as I said, the PWM shield works fine when using a led directly on negative-positive pins on the shield.