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For instance, both pedals and amps can produce overdrive / distortion, though perhaps different flavors. Is there something that only an amp can produce? What is it?

Do amp sim pedals approximate it or simply overlap with overdrive / distortion pedals?

  • Aren't you really asking why one should use a guitar amp instead of plugging right into a PA system, with or without pedals? Pedals do not have speakers so clearly you can't hear the output of a pedal unless it's plugged into something that amplifies and drives a speaker. – Todd Wilcox Jul 31 '15 at 12:26
  • I tend to play / record through headphones from my guitar and pedals into my computer. I was wondering what type of difference might exist between me just using pedals or going through a head or a full amp. From what I can immediately tell, overdrive pedals can produce similar sound to what I might hear with a similar effect on an amp. I was mostly wondering whether I haven't experimented enough with it to notice a bigger difference. – user835943 Jul 31 '15 at 16:10
  • Are you running some kind of software on your computer that takes the input from your guitar and pedals and then re-routes it to the headphone outputs? If so, what software? – Todd Wilcox Aug 1 '15 at 16:14
  • You can do this in windows in microphone settings by checking the mirror to speakers box. – user835943 Aug 1 '15 at 19:27
  • The quality of that windows mirror to speaker setup might be poor enough that it's hard to hear the difference between amp versus no amp. – Todd Wilcox Aug 1 '15 at 20:24
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Almost all guitar amps today that are not all tube designs are meant to sound like all tube amps, so I will write about all tube amps and what they add to the sound with the understanding that even non tube amps will be essentially the same.

The part of an all tube guitar amp that has the greatest effect on the signal is actually the speaker. Guitar amp speakers typically only reproduce frequencies between about 150 to 5000 Hz. Most audio systems like headphones, home stereos, PAs, computer inputs and outputs, etc. will at least go from 80 to 16,000 Hz, and many devices go well beyond that. So a guitar amp speaker cuts off a lot of high frequencies and some low frequencies. The frequency response of a guitar amp speaker is also not flat, so some frequencies will emphasized over others. Finally, all speakers can be saturated if driven hard which causes compression and distortion.

Working backward through the signal chain, tube amps require output transformers to couple the power stage to the speaker. Transformers also saturate when driven hard so there's another source of compression and distortion. Transformers are also susceptible to eddy currents and other artifacts that add certain kinds of distortion to the signal. Tranformers are heavy and expensive and not high fidelity, so they are used rarely outside of musical instrument amps and high end studio sound processors.

The rest of a guitar amp can be more closely approximates with a pedal, although the particular sonic character of tube preamps and power amps can't be perfectly imitated by any other technology.

An amp simulator pedal will attempt to recreate all of those aspects of a full guitar amp. They sound very good these days and are used by professionals because they are more affordable, durable and convenient than real tube amps, even though they are not perfect sounding. Distortion pedals alone just add distortion and usually sound pretty terrible if they are not run through a guitar amp, in my opinion.

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