Servo controlled subs are not new technology, there are many different systems that have been used through time, with their own success degree. The range is vary from poor to successful. There are four other system other then the direct servo system, and the first type, accelerometer based servo subwoofer, has been discussed in the previous article. Now let’s talk about the last three servo system, the sensing coil-based, the positive current feedback, and the motor based design.
The sensing coil-based system. It use a sensing coil to detect the cone movement. It was commonly used for the early servo system, it is also the most common theme you’ll find in DIY articles. It involves two feedback path, one from the driver coil or the output of the power amp and the other are from the sensing coil. This method is not a true servo sub, because the frequency response changes with the voice coil resistance. And even worse, some article suggest only using one coil in a dual voice coil driver as the sensing coil.
The positive current feedback system. It was very popular back then. It’s the most misleading servo system with no sensor what so ever. More, when the voice coil resistance changes the frequency response can change even more dramatic than a non servo sub.
The motor based system. It uses DC motors with shaft connected to the cone to control the movement. The motor have few poles. People familiar with turntables will know the force created by those motors are not stable. In turntables the heavy platters are used for smoothing the force. Without it, the warbling on the pick up signal would be high. In subs, this is more serious, the cone movement made by mid bass signal is far less than the low bass signals. So the modulation distortion between a low bass and a mid bass signal is much higher than the non servo subwoofer. They are primarily used in PA systems.
