Negative feedback extends bandwidth, lowers Harmonic and Intermodulation Distortions and lowers output impedance.
Unfortunately, if not used wisely, is increasing TIM - Transient Intermodulation. In time domain it will show as just small overshoot on fast changing signal like square wave. In frequency domain it shows as exaggerated odd harmonics that our ears are very sensitive to (especially higher order - responsible for perception of loudness). In really bad case it can momentary saturate output transistors that will stop responding for a short time since charge is trapped at the output transistor junctions. Our brain fills small gaps like that but it will make us tired. Whole thing (overshoot) happens because of limited bandwidth that is causing delay thru the amp. Delayed signal when summed (in opposite phase) with input signal that is changing rapidly is coming too late and amp for a moment has much higher gain. Class A amps don't require a lot of global feedback and gain (without feedback) is often as low as 200 but class AB has gains reaching 4000.
How amp should be designed? I would pick the most linear transistors I can find. I would use a lot of local feedbacks. I would measure bandwidth without global feedback and would limit bandwidth of the input stage to that bandwidth (necessary condition). Harmonic distortion would be probably 5-10%. I would use just enough feedback to get distortion below 1%. That would be great sounding amp that nobody would buy because of poor spects (distortion, bandwidth).
No feedback (or low feedback) design might sound more alive because distortion gives this effect (like distorted vs clean guitar) but mostly it would sound pleasant and not tiring instead of sounding brightly Hi-Fiish.
TIM was discovered in 70s. Before that designers went crazy with negative feedback - still claiming that it has to be sounding better than tubes. Logic says that if you see numbers like THD=0.000001% something else has to give. I believe that spects are pretty much useless since amp with greatest spects might sound the worse. People often use amps exact power doubling with 4 ohm load vs. 8 ohm load as a sign of great amplifier. I'm not so sure. It will show that power supply is strong but it will also show that a lot of negative feedback is used (since power supply is most likely unregulated).
Unfortunately, if not used wisely, is increasing TIM - Transient Intermodulation. In time domain it will show as just small overshoot on fast changing signal like square wave. In frequency domain it shows as exaggerated odd harmonics that our ears are very sensitive to (especially higher order - responsible for perception of loudness). In really bad case it can momentary saturate output transistors that will stop responding for a short time since charge is trapped at the output transistor junctions. Our brain fills small gaps like that but it will make us tired. Whole thing (overshoot) happens because of limited bandwidth that is causing delay thru the amp. Delayed signal when summed (in opposite phase) with input signal that is changing rapidly is coming too late and amp for a moment has much higher gain. Class A amps don't require a lot of global feedback and gain (without feedback) is often as low as 200 but class AB has gains reaching 4000.
How amp should be designed? I would pick the most linear transistors I can find. I would use a lot of local feedbacks. I would measure bandwidth without global feedback and would limit bandwidth of the input stage to that bandwidth (necessary condition). Harmonic distortion would be probably 5-10%. I would use just enough feedback to get distortion below 1%. That would be great sounding amp that nobody would buy because of poor spects (distortion, bandwidth).
No feedback (or low feedback) design might sound more alive because distortion gives this effect (like distorted vs clean guitar) but mostly it would sound pleasant and not tiring instead of sounding brightly Hi-Fiish.
TIM was discovered in 70s. Before that designers went crazy with negative feedback - still claiming that it has to be sounding better than tubes. Logic says that if you see numbers like THD=0.000001% something else has to give. I believe that spects are pretty much useless since amp with greatest spects might sound the worse. People often use amps exact power doubling with 4 ohm load vs. 8 ohm load as a sign of great amplifier. I'm not so sure. It will show that power supply is strong but it will also show that a lot of negative feedback is used (since power supply is most likely unregulated).