CHAPTER 9 The General Principles of Power Amplifiers simply cured by buffering the VAS from the output stage. Magnitude is essentially constant with fre- quency, though the overall effect in a complete amplifier becomes less as frequency rises and feedback through C dom starts to linearize the VAS. The next three distortion mechanisms are in no way inherent; they may be reduced to unmeasurable levels by simple precautions. They are what might be called topological distortions, in that they depend wholly on the arrangement of wiring and connections, and on the physical layout of the amplifier. Distortion 5: Rail-Decoupling Distortion Nonlinearity caused by large rail-decoupling capacitors feeding the distorted signals on the supply lines into the signal ground. This seems to be the reason that many amplifiers have rising THD at low frequencies. Examining one commercial amplifier kit, I found that rerouting the decoupler ground return reduced the THD at 20 Hz by a factor of 3. Distortion 6: Induction Distortion This is nonlinearity caused by induction of Class-B supply currents into the output, ground, or negative-feedback lines. This was highlighted by Cherry (1981) but seems to remain largely unknown; it is an insidious distortion that is hard to remove, though when you know what to look for on the THD residual it is fairly easy to identify. I suspect that a large number of commercial amplifiers suffer from this to some extent. Distortion 7: NFB Take-Off Distortion This is nonlinearity resulting from taking the NFB feed from slightly the wrong place near where the