JMMP said:In Mesa amps, I believe it affects the negative feedback loop in the power amp.
2:90 Manual said:Derived from the presence shift switches found on our Mark IV amplifier, and the "Red" or Modern Channel of our Dual Rectifier Solo Head
When you decrease the amount of negative feedback two primary things happen. 1) the amp gets louder at lower volume settings because the power amp is not "limiting" itself. 2) The decreased feedback lets the power tubes break up sooner and more gradually - this is the important part.
When there's a lot of negative feedback the power tubes either don't break up at all or they do... sort of like on or off with little grey area in between. This is because the tubes have to be pushed hard before they reach that threshold where the feedback rather suddenly cascades itself out of significance. So you can play power amp clean or power amp distorted with not much wiggle room in between.
When there's little to no feedback (some amps use none, like the more famous Voxes [or our Recto Modern mode]) the power amp can distort gradually. So you can control the dynamics of the power amp much more dynamically by how hard you pick or with the volume control on your guitar. Instead of being a sudden point at the onset of power amp distortion there's a sooner and more gradual onset of power amp distortion which you can interact with through your picking. So the amp feels much more dynamic and responsive.
Tonally, reducing the negative feedback causes a natural filtering off of the very high and very low frequencies, giving a relative boost to the mid-range frequencies out of the power tubes. In fact, the volume increase you hear when reducing negative feedback is actually only in roughly the 70Hz to 11kHz range, with the largest boost happening in roughly the 200Hz to 6kHz range.
To me, the cool thing about the presence knob is that it doesn't interact like the TMB knobs do, so changing the presence doesn't require readjusting the other EQ knobs
Overdriving the power amp causes the output transformer secondary to produce a clipped output signal, which represents a lack of output response to a changing input signal. The output transformer provides the source voltage for negative feedback to the phase inverter, so clipping reduces negative feedback. This creates more closed-loop gain, which drives the amp further into an overdriven state, producing even more clipping. The net result is that negative feedback from the output transformer to the phase inverter accelerates the power amp's transition to an overdriven state.
crane said:According to the schematic, the modern mode on the 2:90 doesn't open the NFB loop as in the Rectos, it just changes its response (probably by lowering the NFB).
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