Note that the ordinary transformer provides DC isolation between the input and output windings. In many applications this is an important feature. However, when DC isolation is not required then an autoformer will often outperform an ordinary transformer. This is because virtually all electronic devices exhibit some imperfections; they are not ideal. But compared to an ordinary transformer, an autoformer can often be made with fewer (or smaller) imperfections. The behavior of an autoformer is often closer to ideal.



You might have noticed that the autoformer is a simple three-terminal device with a common ground. This makes it a direct replacement for resistive voltage dividers like potentiometers or stepped attenuators.



Q - Why use an autoformer volume control? I've been happy with a good quality potentiometer.



A - The simple answer is that most people find that the autoformer volume control sounds significantly better than even the best potentiometer or stepped attenuator. And it's not a subtle difference; most people report that the autoformer has such an open, effortless sound that they would not consider going back to a resistive device.



Exactly why they sound better is still under debate, but one theory is that it's because an autoformer doesn't attenuate by wasting energy. A fairly good analogy can be made to the transmission in a car. If you need to drive at a slow steady speed which method would you choose: (1) leave the transmission in high gear and apply the brakes to keep from going too fast, or (2) downshift into a lower gear that will allow the car to go the desired speed with minimum effort?



An autoformer is essentially an electronic gearbox that operates without wasting significant energy. Potentiometers and stepped attenuators adjust the signal level by literally turning the excess signal into heat. On the other hand, when an autoformer is adjusted for low volume level it actually makes things easier for the source, much like a low gear makes things easy for your car engine.