Boulder 500AE power amplifier Transient Intermodulation Distortion

Sidebar 2: Transient Intermodulation Distortion

TID is due to the active devices being unable to slew the voltages they are asked to handle sufficiently fast to keep up with the demands of the signal. Slew rate, expressed in V/µs, is the speed with which the output voltage of an active device is able to change and is a critical factor in the behavior of a negative feedback circuit: if slewing can't keep up with the changes in input level, the fed-back signal will not be able to properly attenuate the input signal, which will then increase to the point of amplifying-device overload.

The resulting TID causes a particularly obnoxious variety of hashy crud in the sound. Since the overload is triggered only by very steep pulses, the TID products are well beyond the upper limit of audibility to start with, but the severe nonlinearities in the overloaded device generate IM difference components which splatter the distortion down well into the audio range.

TID is most noticeable when reproducing analog discs with a moving-coil cartridge, because the typical MC's HF range—to well beyond 30kHz—is greater than that of any other available signal source. Even though most LPs have little signal content above 15kHz, surface ticks and pops, as well as any groove mistracking, generate HF impulses ranging out to the upper limit of the cartridge, and it is these which put the most stringent demands on the slew rate of a negative-feedback amplifier. If the amp is TID-susceptible, LP ticks and pops will be noticeably exaggerated, and will sound like sharp-edged clicks when they should sound more like raindrops on a sidewalk.—J. Gordon Holt