The long-tailed-pair phase inverter with negative feedback from the output transformer is one of the most ubiquitous constructs in guitar amp design. A signal from a tap in the output transformer secondary drives the feedback input to the phase inverter to counteract the amplified signal. The resulting closed-loop gain, measured from the phase inverter signal input to the speaker, is less than the forward, open-loop gain without feedback. At less than full power, feedback reduces nonlinear distortion and flattens the amplifier's frequency response. More important, however, are the dynamic effects when the power amp is overdriven.
The calculator assumes that the resistance 2(RK+RT) is much greater than RP.
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.
This new approach to guitar amplifier electronics embraces 2018 technology to deliver greater understanding with less math. Computer-based visualization replaces the traditional litany of mathematical formulas. The book's graphing calculators, free on this website, are designed for smartphones and laptops, making them as portable as the book that uses them.
Less Math, Greater Understanding
The book explains the principles of vacuum tube electronics and the design of preamp voltage amplification stages, cathode followers, tone stacks, power amps, phase inverters, negative feedback, and power supplies. An entire chapter is devoted to sculpting the dynamics of overdrive and harmonic distortion.
Copyright © 2018 Amp Books LLC All Rights Reserved