Negative Feedback?

[bluevibeamps]

What is the point of negative feedback(eg. Trainwreck)

Just to clairify, it looks like it goes from the secondary of the OT back to the pre-amp.

Anything helps, even if you know a book.

[Deadbeat2]

From my (very) limited understanding the negative feedback loop injects some of the output voltage back (as a negative voltage)into the PI to make the amp sound "cleaner" and it takes away some of the rawness of the amp. Even the Fender Champs has a NFL. Bruce J.

[markh]

In general, negative feedback (NFB) is when a portion of the output signal of a linear amplifier is fed back out of phase to the input. For example, every unbypassed cathode resistor provides some NFB to that stage. NFB is usually used to linearize (reduce distortion), control gain, and thus broaden freqency response. It is also sometimes used to directly affect frequency response. In an audio amplifier, NFB also tends to increase speaker damping, which results in a tighter, more controlled sound.

The usual guitar amp NFB reference is when a signal is fed back from a tap on the output transformer to the shared cathode resistor of a Schmitt PI a la Marshall/Fender. These circuit usually include a capacitor to ground as part of the loop (sometimes through a presence control). This acts as a treble boost (with the -3 dB freqency controlled by the cap value and the impedance of the NFB network). How can a cap to ground boost the treble? By shunting higher freqencies to ground in the NFB loop, it reduces the amount of treble NFB, which means more treble gain overall.

--mark

[zaphod_phil]

There is a further twist. As noted, the intention of NFB is to give you a cleaner-sounding amp, and was common in many tube hi-fi amps. It also gives better low-end definition, as it increases what's known as the damping factor. Now here's the twist to the tale. When you drive an amp to the point where it starts to clip, the NFB circuit basically 'gives up' and actually increases the distortion. Up to that point the NFB circuit is fighting hard to keep the power amp and PI circuits running very clean. But when they're driven into clipping, then the dam breaks and you get lots of crazy distortion. This is used to great effect by the bigger Marshall amps, and their derivatives, although you will therefore tend to get a rather sudden transition between clean and distortion sounds.