Inside the Guitar Amplifier

Weeks 5 through 9 covered every transformation this course teaches that happens before amplification: gain, clipping, filtering, modulation, and delay/reverb. This week shows that the amplifier itself is not one simple multiplier — it is a small signal chain of its own, and it can repeat several of those same processes: it applies gain (more than once), it can filter, and it can distort. By the end of this week you should be able to identify the major stages of a simplified amplifier (preamp, tone stack, power amp); explain the difference between a preamp stage and a power-amp stage; explain gain staging; describe, conceptually, how tube and solid-state amplifying elements differ without claiming one sounds inherently better; explain negative feedback as a general technique; and explain why preamp gain and master level don’t do the same thing.

1. Preamp: gain + clipping→
2. Tone stack: filtering→
3. Power amp: gain + saturation→
4. Master level: output only

Week 5 introduced gain staging as one stage’s headroom. An amplifier with more than one gain stage extends that idea: the same overall amount of amplification can be placed mostly in the preamp, mostly in the power amp, or split between them — and where it’s placed changes which stage actually reaches its own saturation point. This is a genuinely different-sounding choice, not just a louder-or-quieter one.

Both examples end at the same master-level setting, and both put a stage at its saturation ceiling somewhere in the chain — but a different stage. That’s also why master level and preamp gain don’t behave the same way: master level sits after both nonlinear stages and only turns the already-decided result up or down, while preamp gain decides whether, and how hard, the preamp itself distorts.

Historically, guitar amplifiers have been built around two different kinds of amplifying component: tubes (valves), which amplify by controlling a current through a vacuum using a charged grid, and transistors (solid-state devices), which amplify by controlling current through a semiconductor. Both are, at the level this course cares about, doing the same job — turning a small input signal into a larger output signal — using different physical mechanisms to do it. It’s tempting to reach for “tubes sound warm, transistors sound harsh” as a settled fact. Treat that with caution: how any particular amplifying element actually behaves depends heavily on the rest of its circuit (biasing, feedback, gain staging, the tone stack, the power-amp design) and not on the component type alone. This course doesn’t take a side.

Many amplifier stages route a portion of their output back to their input in a way that opposes the original signal — negative feedback. As a general technique (not specific to guitar amplifiers, or even to audio), it can trade away some raw gain in exchange for more predictable, more linear behaviour, and it can change how a stage responds over time as well as in level.

Because the preamp and power amp are separate stages, each with its own gain and its own nonlinearity, a signal can be distorted at one, the other, both, or neither. A preamp typically clips a relatively small, low-power signal, and — as with earlier weeks’ clipping circuits — that clipping is often asymmetric. A power amplifier is driving a much higher-power signal into a speaker’s load, and its own saturation, when it happens, is often comparatively softer. These are two different nonlinear elements in the same chain, not two names for one thing.

These are simplified teaching curves illustrating where each kind of nonlinearity tends to sit, not measured transfer functions of any specific real amplifier circuit.

1. 01

   A simplified amplifier is Input → Preamp → Tone stack → Power amplifier → Speaker — a small signal chain, not a single multiplier.

2. 02

   The preamp mainly provides voltage gain, filtering, and (often asymmetric) nonlinear distortion; the power amp mainly provides enough power to drive the speaker, and may add its own, often softer, nonlinearity.

3. 03

   Gain staging is choosing how the same total amplification is distributed across stages — which stage saturates, and how hard, depends on that distribution.

4. 04

   Tubes and transistors are two different physical ways to build an amplifying element; how an amplifier actually sounds depends on the whole circuit around that element, not the component type alone.

5. 05

   Negative feedback is a general technique that trades gain for more predictable, linear behaviour — not something specific to guitar amplifiers.

6. 06

   Preamp distortion and power-amp distortion are two separate nonlinear events in the same chain; master level, placed after both, changes loudness without changing either.