8
NEGATIVE FEEDBACK
Negative feedback is a technique used in the output or power stage of some amplifiers. The
inclusion or absence of negative feedback, and the degree to which it is used, have a
massive effect on the sound of amplifier, its transition from clean to dirty and – most
crucially – how it feels to play.
In simple terms, negative feedback involves taking some of the signal from the power
amp’s output and feeding it back to the power amp’s input, where it is compared with the
original input signal coming from the preamp stage. If there are any discrepancies between
the two signals, the amplifier will attempt to correct them. This helps extend and even out
the frequency response, makes the power amp behave in a more stable and linear way, and
generally contributes to a cleaner, more accurate sound.
That’s all very good until the signal rises enough to push the power amp into distortion.
Faced with an amplifier now behaving in a decidedly non-linear way, the negative feedback
loop is no longer able to self-correct. The shackles are off and the power amp is free to
distort.
Because of this, lots of negative feedback will mean the amp stays cleaner for longer as
volume increases (it has more “clean headroom”), but once you pass a certain threshold, it
will suddenly distort a lot. With no negative feedback, gain is increased and the amp will
distort much sooner, but the transition will be smooth and gradual with no hard border
between clean and dirty. Some people prefer amps that don’t use negative feedback, often
praising their “natural” or “organic” overdrive, while others prefer the more precise feel
that negative feedback provides, with the ability to go from very clean to very dirty simply
by varying picking dynamics.
Once again, the RevivalDRIVE lets you freely experiment with different amounts of
negative feedback using the MORE side of the two-way MORE|PRES control. At 12
o’clock, the negative feedback applied to the RevivalDRIVE’s output stage is at its
maximum extent – akin to a Class-AB Marshall with the “Presence” knob turned right
down. Turn fully counter-clockwise and negative feedback is removed altogether – as on
the Vox AC30, Tweed Fender Deluxe, Sampson-era Matchless amps and many other “hot-
biased” designs.
We recommend that you experiment with this control while varying your picking dynamics
to find the right combination of headroom, break-up and dynamic sensitivity for your
particular pickups and playing style.
Turned the other way (PRES), the MORE|PRES control releases the higher frequencies
from the negative feedback loop, boosting the top end just like the classic Marshall
“Presence” control.
