Dynamic distortion (TIM - Transient Intermodulation) - distortion of the signal in the amplifier, manifested by a sharp change in the input signal and due to an insufficient rate of increase of the signal in the amplifier. Subspecies of nonlinear distortions arising in an amplifier covered by deep negative feedback (OOS). Emphasis should be placed on the strong influence of environmental protection, since, in stationary mode, the level of dynamic distortion is not very large, at least significantly less than the level of nonlinear distortion , but in an amplifier covered by deep environmental protection, it is “amplified” by an amount equal to the feedback depth. They are manifested by the so-called "sound-soldering", "smeared sound picture", "sound jamming in the high-frequency spectrum".
Content
Distortion Process
Consider the moment of occurrence of THEM, starting from the time of silence, when the signal is not yet input to the amplifier, the amplifier is “silent”, but the signal received at the amplifier input will be significantly higher than the nominal level, since in normal mode, the input signal is reduced by the output signal weakened by the OOS, and if no measures are taken in the amplifier to increase the overload capacity, the input stage will be overloaded and may go into saturation, the subsequent stages, which operate in the maximum gain mode, are equal to out of looping. After some time, the output signal is highly distorted and enters the OOS circuit, is attenuated by it and is fed to the amplifier input, where by that time that input signal is no longer there, and is amplified again, once again getting to the output in the already inverted form. Thus, the input signal received at the input of the amplifier, after a period of silence, enters the amplifier into a non-standard mode of generating an arbitrary signal that was not in the input signal. If a saturation effect occurs in some cascades, this process can last for quite a long time. This process is estimated time of entry into the stationary mode. Of course, when a periodic signal is received, the distorted output signal will be subtracted from the input signal and the differential signal will be amplified, in some way compensating for distortions arising in the amplifier, though not affecting the “leading edge” or “attack”, which, of course, will be distorted, and if the signal is short, another well-heard generation of a noisy-like signal will appear. The same short-term noise can be heard when the input signal suddenly fades. The duration of this generation is approximately equal to the total delay in the amplifier.
Measurement Methods
Dynamic distortions are clearly visible on an oscilloscope connected to the output of an amplifier loaded with real load, when applied to the input of a square wave amplifier with a frequency equal to or close to the upper operating frequency of the amplifier.
Methods for reducing distortion in an amplifier
One of the methods of suppressing dynamic THEM is to install a low-pass filter in front of an amplifier covered by deep environmental feedback, designed to reduce the rate of rise of the input signal to a level that does not cause a significant increase in THEM. Or a more expensive method - the use of high-frequency elements, allowing to reduce the total delay of the amplifier and increase the slew rate of the output signal. The reduction of IMI is facilitated by the introduction of local feedbacks on all amplification cascades and multiloop EP. Another method, and I must say much more efficient, but much more expensive, is, of course, increasing the linearity of the amplifier without feedback coverage. An important point in this direction is the use of highly linear passive elements (capacitors, resistors, wires, etc.), which is the difference between Hi-End, where the accuracy of sound reproduction is at the forefront, regardless of the cost of its implementation.
Class Amplifier
It is worth noting that due to a variety of factors (nonlinearity and variation of the parameters of elements, interference, etc.) in practice it is not possible to build a “perfect” absolutely non-distorting amplifier without NFB. The pursuit of excellence leads to significant labor and material costs, which are not feasible in the production of mass radio equipment, so there is some compromise in quality and price, assessed by the mass consumer and experts. To assess the quality level of manufactured industrial equipment, it is divided into classes:
- mass equipment, sometimes called Low-End;
- mass equipment that meets the quality indicators Hi-Fi;
- mass equipment level Hi-End, sometimes called Hi-Tech;
- professional equipment of studio quality level.
Literature
- Shkritek P. Reference guide to sound circuitry: Trans. with him.-M. World, 1991.-446 pp., Ill. ISBN 5-03-001603-1
- P. Shkritek. Ways to reduce noise and interference // Sound circuit design guide = Handbuch der Audio-Schaltungstechnik. - M .: Mir, 1991. - p. 244-267. - 446 s.
- Harley Robert, High-End Audio Encyclopedia, 2000. ISBN 5-901186-01-X
- Horowitz P., Hill U. The Art of Circuit Engineering: In 3 volumes: T. 1. Trans. from English. - 4th ed., pererab. and additional - Moscow: Mir, 1993. — 413 s, il. ISBN 5-03-002337-2