Global Negative Feeback

[Bluzsteel]

I have a Dr Z Maz 38 that Im thinking about putting a negative feedback loop in for some more clean headroom. I have always heard it will clean it up. I read on Z's page that the Mazerati he added global feedback to give it alittle more clean headroom. it is my understanding the first Maz 38 had feedback. anyone ever done this to one of their amps that had no feedback, and after the fact how did it sound?

 

[Wakarusa]

Aiken has a nice explanation of global NFB on his site. The overall effect really depends alot on how much NFB you add and where you inject the NFB signal back into the preamp (or PI as the case may be).

If you think of a graph where vertical is output power and horizontal is input signal, the the general output of an amp is a diagonal line with positive slope (as input rises, so does output). Feedback changes the slope of the line. Positive feedback rotates the line ccw (more positive slope), nfb rotates the line cw (less positive slope)

So the "clean it up" bit has to do with NFB's effect of reducing the change in output for a given change in input. As you'd suspect, this also has an effect on dynamic range. As it turns out, it also has a pretty strong effect on when and how abruptly the amp goes into clipping distortion.

 

[Bluzsteel]

Thanks for the reply, yeah Ive read the Aiken report, Im not real tech minded , just know the tones I like , I notice you put a switchable negiative in your amps.

 

[n8b]

Originally posted by Bluzsteel
I read on Z's page that the Mazerati he added global feedback to give it alittle more clean headroom. it is my understanding the first Maz 38 had feedback.

He uses true global nfb on the Mazerati, as opposed to injecting the nfb into the PI as in Fender and Marshall style amps. An amp without nfb is carefully designed so that things like blocking distortion don't occur. If you do add nfb, it will change the response and feel of the amp, not just the amount of headroom.

Something about the idea of changing a Z freaks me out.

 

[Wakarusa]

Originally posted by Bluzsteel
...I notice you put a switchable negative in your amps.

Yes on the little Champ clone, but most of my custom designs are without global NFB. Some of them include a small amount of local NFB in the gain stage just prior to the PI (either to set the gain on the stage, or to stabilize it as a mixing stage), but in general I try to avoid using it -- I prefer the dynamics and smooth onset of clipping distortion you get with minimal or no NFB.