How Do Guitar amps work? |Distortion
Powerful pickups, higher volume settings or both mean an amp can spend a perceptible amount of time at or beyond the onset of clipping, the point where the amp's drive signal exceeds its output stage's dynamic capacity (Fig. 2). This steps up the distortion figure exponentially – and from here on in, an amp's reputation depends on how 'musically' it responds to being overdriven. Now push-pull output stages, whose distortion-cancelling tendency we described in part 2, make little or no use of the important distortion-reducing method applied to hi-fi amps known as negative feedback, or NFB – the technique of returning a small proportion of the output, via a resistor network called the loop, in inverse phase to the driver stage, reducing the available gain… and the THD, by the same amount.
Feedback amplifiers need more stages to achieve the same overall gain, and also high-quality output transformers to avoid instability. Both these factors priced out the use of NFB in early guitar amps, though they provided more than adequate performance if they used class A operation. The eventual need for more power forced the use of class
AB output stages; these need more drive, further loading the driver stage, and have more uneven gain behaviour than class A's, due to the transition into cut-off of one valve during the driven cycle of the other. So NFB can be used to restore their performance to at least class A standards.
So, the matter's settled? Not quite. Check our clipped waveforms diagrams. Fig. 3a shows a class A open-loop amp, Fig. 3b shows a class AB feedback amp. Note the different curvatures caused by balance of 2nd (even) and 3rd (odd) harmonics, and look at Figs. 4a and 4b, a spectrum analyser readout showing this balance. The valves' inherent current-limiting behaviour (see last month) causes the relatively curved waveform in Fig. 3a, but in Fig. 3b, the feedback loop has suppressed the 2nd harmonic, cutting the waveform off more squarely.
An approximately equal amount of the two main types of harmonic sounds best for overdriven guitar work; either on its own is inferior. As each is favoured by different types of circuit, that's an awkward balance to get, but some amps have it at certain output levels, known to us as 'sweet spots'. Either side of the sweet spot, one type or the other is dominant, and experienced players get different sounds by playing around it. Of course, the fixed volume level of your 'sweet spot' can be a problem. Many players have different-sized amps to suit various venues. However, for many this isn’t possible, and another way has to be found.
1. How do Guitar amps work? | Picture Instructions
2. How Do Amps Work?
3. How Do Guitar amps work? |Distortion
4. How Do Guitar amps work? |Transistors
5. How Do Guitar amps work? | Overdrive pedals