I read that negative feedback helps prevent amplitude distortion, which sometimes makes a spike in the beginning of a wave, and sometimes changes the highs, mids and lows of a frequency. Do guitar amplifiers use negative feedback to reduce these things, or is amplitude distortion a characteristic in an amp's sound? Does it have anything to do with what class the amplifier is? Thanks.
All solid state made amplifiers have some kind of deep negative feedback, needed to reduce the distortions inheritant for those devices. Negative feedback which is option for push-pull tubes is a must for transistor amplifiers. Solid state transistor or IC made amps will also follow the input slower than tube amp, curving may appear in the output and will employ negative feedback to reduce the distortions.
Joe
Negative feedback works well to reduce steady state distortion -- that is distortions in signals that are continuous and unchanging, e.g., a continuously held tone. And for years, distortion was measured only using continuous tones, so liberal negative feedback was used to reduce measured distortion and control gain, esp in solid state amps, as Joe points out. Maybe 30 years ago (or more) someone realized that negative feedback caused transient distortions (slew rate issues, transient intermodulation distortions for you electronics tweaks). The reason is pretty simple. Negative feedback is error correction method that uses a bit of the output signal added back to the input in proper phase to reduce errors in the signal amplification. Unfortunately, there is a small, but measurable delay through the amplifier and through the feedback path (out to in). Because of this, a signal entering the amp is not really "corrected" until it first traverses the amp chain, then feedback path back to the put. Until that time, the correction signal being applied is actually from whatever signal preceded the current signal. The result is a short, but noticeable distortion -- especially whenever there is a drastic change in the signal, e.g., a transient.
One of the so-called innovations to reduce transient distortions due to negative feedback is/was to localize feedback to each stage (many short feeback loops) instead of have one long feedback path from last stage output to first stage input. This significantly shortens the time of transient distortions.
As far as amplifier class goes, one could use feedback for any class, but if the distortion is primarily even order (falls in octaves), it imparts a warm characteristic to the sound, so why suppress it. Class A tube amps fall into this category. As Joe mentions, push-pull tube power amps (A-B or B depending upon design) may use some negative feeback. This is to reduce what's known as cross-over distortion, which intrinsically happens to contain both odd and even harmonic content and is therefore somewhat nasty to the ear. However, crossover distortion is most obvious at low signal levels (quieter passages, decaying notes). So depending on the type of playing/music, the feedback may be useful. The ideal would be no feedback at high volumes, some feedback at low volumes.
As is the case for many real-world imperfections, even though negative feedback induces some fidelity problems, it also has created a characteristic sound in some guitar amps. That is another reason one will find adjustable negative feedback in some amps these days. Use it if you want it or reduce it for better transient fidelity.
-Greg
-=tension & release=-
