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I have read that the preamp boosts the signal so that it is strong enough to send to the power amp section, but it seems like there must be more to it than that.

Two thoughts I have had:

  • The preamp generally has the EQ controls on it, maybe applying EQ to the stronger signal from the power amp would require very expensive components.
  • Maybe there is a need to first amplify the voltage of the signal and then amplify the current of the signal, so you need separate systems to do each of those things.

Why is it difficult to build a guitar amp with a single amplifier section that goes from the very weak signal from the guitar, to the very strong signal that drives the speaker?

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  • \$\begingroup\$ First thought is the correct one : a preamp is better named a control amp; it's there that EQ, effects etc are applied. \$\endgroup\$ – Brian Drummond Dec 13 '20 at 16:06
  • \$\begingroup\$ Actually nothing prevents to construct the amp in digital domain. In the input there's an AD converter. All effects are calculated like in modern effect processors. The final output is made with an DA converter which can drive a speaker. Or someone invents a speaker which is a DA converter. I guess authorities who regulate activities which can cause radio interference would have something to say if one has a modern 1 bit DA converter which pushes say 50 watts to an ordinary 8 Ohm speaker. \$\endgroup\$ – user287001 Dec 13 '20 at 23:50
  • \$\begingroup\$ @user287001, there are amps for sale today which do exactly that. The radio interference issue is solvable. \$\endgroup\$ – Nate S. Dec 14 '20 at 22:31
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Why is it difficult to build a guitar amp with a single amplifier section that goes from the very weak signal from the guitar, to the very strong signal that drives the speaker?

An audio power amplifier uses negative feedback to obtain pretty constant gain and low distortion. Trying to build tone controls into this amplifier will severely affect the negative feedback process and, it might become positive feedback. Then your power amplifier turns into a power oscillator.

That is a certain reason why you don't incorporate tone controls into the power amplifier section.

An audio power amplifier has to produce large currents in order to drive a loudspeaker and, the wiring to the power amplifier (if mixed with the preamplifier) will create large problems; speaker drive currents can leach into the input circuits and will create unwanted problems turning your "amplifier" into an oscillator (again). Look up star-point wiring for amplifiers to see why.

This is another reason why we keep low power signal amplification away from the power amplifier.

A guitar requires a high input impedance amplifier such as from a JFET circuit. This type of circuit needs to be largely free of power supply variations that are caused by driving high currents into a loudspeaker.

This is another reason why the front-end (or pre-amp) is kept separate from the power amplifier. Usually a secondary power supply stage is used that has much more filtering on the power rails.

It then makes sense to split a guitar amplifier into three stages: -

  • High impedance front-end amplifier (specific to normal guitars) that outputs a line-input level signal
  • Tone control section with some amplification (processing regular line input level signals)
  • Power amplifier (line input level to speaker)

But, it's fairly uncontroversial to join the front-end with the tone control section so it usually boils down to two physical sections.

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  • \$\begingroup\$ The "phono amp" also fulfilled that role of a 3rd amplifier: high sensitivity / high impedance, with built-in equalization, preceding the pre-amp, and where possible, isolated. \$\endgroup\$ – P2000 Dec 13 '20 at 19:53
  • \$\begingroup\$ A minor addition: The stability considerations for separating the sensitive input from the power output are not unique to guitar amplifiers and even not to audio amplifiers. It always helps to separate a high-gain amplifier into a multi-stage circuit with manageable gain at each stage. \$\endgroup\$ – fraxinus Dec 14 '20 at 7:39
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    \$\begingroup\$ @P2000 a phono amplifier has only got 47 kohm input impedance; a guitar amplifier has to be circa 1 Mohm upwards to ensure you don't get a dull guitar sound. \$\endgroup\$ – Andy aka Dec 14 '20 at 9:36
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    \$\begingroup\$ Yeah, I didn't mean in lieu of, I meant: as a 3rd amp, beside the guitar amp, there also is the phono amp. That's all. Great answer. \$\endgroup\$ – P2000 Dec 14 '20 at 16:00
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Your thoughts are quite alright. In addition, it makes sense to have two stages since

  • To get both high current and voltage gain, one stage is usually too weak.
  • The input is single-ended, while the output is often push-pull.
  • EQing and tone control are easier to implement single-ended.
  • Pre and power amp use distinct voltage/current regimes, so you'd want to choose different transistor/tube types for each.
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sound effects (e.g. equalizers, delays, overdrives, ...) are usually applied on a line level audio signal. applying them on a weak signal (a signal straight out of guitar pickup or a microphone) may corrupt or weaken the signal further and applying these effects on a high power signal (right before a 100W speaker) needs massive, powerful components. so :

weak_signal ----preamp----> line_level_signal_and_many_effects ----poweramp---->speakers

many guitar amplifiers have both preamp and power amp on board, so for you to apply effects on the line level signal, either they have their own effects or they give you a FX_OUT, FX_IN terminals which are output of preamp, and input of power amp and you can plug your effects chain to this terminals.

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