Amp/Preamp Asylum

Posted by morricab (D) on March 07, 2025 at 13:07:45

In Reply to: RE: more gaslighting. posted by Ralph on March 07, 2025 at 08:27:44

Doug Self lists the 8 types of distortion mechanisms when he is discussing his "blameless amplifier"

TABLE 1.
SOURCE ORDER SLOPE ORIGIN
1 Input stage (balanced) 3rd-order 18dB/oct Inherent
Input stage (unbalanced) 2nd-order 12dB/oct Inherent
2 Voltage Amplifier Stage 2nd-order 6dB/oct Inherent
3a Output stage LSN (4 Ohm) 3rd-order 6dB/oct Inherent
3b Output stage crossover Complex 6dB/oct Inherent
3c Output stage switchoff Complex Variable Inherent
4 Non-linear VAS loading Complex Flat Inherent
5 Decouple return in ground Even Flat Topological
6 Rail induction Even 6dB/oct Topological
7 Wrong feedback point Even Flat Topological
8 Capacitor non-linearity Odd 12dB/oct Inherent

The distortions we have been discussing are distortions 3a, 3b and 3c, which he lists as "inherent", meaning they are fundamental to transistor function.

He then states: "In a field like Audio where consensus of any sort is rare, it is widely acknowledged that crossover distortion is the worst problem afflicting Class-B power amplifiers. The pernicious nature of crossover distortion is that it occurs over a small part of the transfer characteristic, and so generates high-order harmonics. Worse still, this range is around the zero-crossing, so it is present at all levels, the THD percentage potentially increasing as output level falls, threatening very poor linearity at low powers."

Furthermore: "To further get the measure of the problem, Fig 23 shows how HF distortion is greatly reduced by increasing the load resistance, providing further confirmation that almost all the 8 Ohm distortion originates as crossover in the output stage."

So, a typical loudspeaker load makes the situation worse and particularly at high frequencies. The lower the impedance, the worse it gets for a Class B amp.


Finally: "This leaves Distortion 3, in its three components, as the only distortion that is in any sense unavoidable, as Class-B stages free from crossover artifacts are so far beyond us."


THat sums it up. He found a way to mostly eliminate all the distortions on his list EXCEPT distortion 3, which are inherent traits of transistors when running Class B.

Doug Self, himself, has declared that avoiding zero crossing distortion in Class B transistor amps is IMPOSSIBLE with current semiconductor technology.