On my Mesa Boogie Mark V 35 the Xtreme mode on channel has most of the negative feedback removed according to the manual. It's louder, fuller sounding and has a much more immediate attack with no compression, the presence control doesn't do much in this mode. I like it and I've read that Walter Trout uses this channel on his Mark V for his live tone.
With the gain dialled back you can get some nice lightly overdriven tones.
What is negative feedback and what does removing it do? Do all amps have it or do some amps have zero negative feedback as well?
Comments
Negative feedback (NFB) is where a small amount of the output of an amplifier (or just a gain stage) is fed back into the input, in reverse - a sort of self-regulation. This reduces the gain, and tends to flatten out the non-linear parts of the response of the amp, so it produces a more even tone and more controlled dynamics. Hence removing it makes the amp more aggressive and spiky.
Not all amps have it - a couple of classic examples that don’t are the Vox AC series and the Fender 5E3 Deluxe. Lack of it tends to be associated with cathode-biased amps, which are more inherently self-limiting since cathode bias is itself a form of NFB, although it’s not usually referred to as such.
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A small amount of the output stage is “inverted” and fed back into the input of the power amp.
Most amps with a Presence control have negative feedback,
The most famous example of an amp without negative feedback is the AC30.
Negative feedback has a pronounced effect on the transition from clean to dirty. Amps without it have a more gradual transition compared to those that do.
Worth mentioning also I think that the triode has inherent NFB when used in its conventional mode (the cathode follower, often used to drive tone stacks has 100% NFB and thus a voltage gain just less than one.
Triodes can be configured as a "cascode" stage which defeats the feedback but the circuit is rarely seen in guitar amplifiers.
Dave.
How’s that for negative feedback?
Degenerate feedback in triodes has a different effect to global negative feedback around the power stage in that it increases output impedance.
Presence controls generally decouple the negative feedback at higher frequencies, thus increasing gain. Of course this method requires that there is negative feedback applied, and explains why the presence control doesn't work in Xtreme mode.
For what it's worth, resonance controls do the same but at low frequencies.
Matchless Lightning Reverb - based on AC15, so no NFB
5E3 clone - no NFB
Micro 5E3 - no NFB
Princetorn Reverb 'Custom '68 RI - some NFB, but less than in the original design
The Revival Drive lets you set the amount of NFB - surprise surprise, I generally prefer it off!