Now I "know" that an EQ pedal is a pedal that shapes the tone with different knobs.

But how exactly does it differ from the EQ on an amp ? Does it improve the sound ?

How, and more importantly, when, is an EQ pedal needed ?

It's more needed for like big stacks or amps without EQs knobs on them.
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The EQ on your amp is a Shelf (I think that's the term) EQ, it'll boost / cut lots of bands / frequencies at a time. A graphic EQ will have specific frequencies to adjust, and usually more than 3 (typically 7 or 10 in a pedal, and as many as 30 on rack gear). Parametric EQs let you pick a band, set the width (how many adjacent frequencies will be affected) and the boost / cut them.
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An eq pedal just give you more flexibility when it comes to tone shaping.
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Typically, EQ pedals will have more sliders than your amp has EQ-knobs - which means more possibilities for tweaking. On my amp, I have one knob for mids, on my pedal, the (roughly) same range of frequencies can be adjusted with three sliders. Just a lot more tone-shaping there. Now if you use both your amp-eq and pedal for tweaking, you get a lot of tonal options. With cheap EQs being easily available (like the models by Danelectro, Harley Benton, Biyang/Fame, etc.), it's hard NOT to buy one at some point, just for experimentation. It can make as much of a difference as a new amp or new speakers (provided the amp/speakers it's used with aren't crap).

Me, I use a 7-band in my amp's loop for some frequency-correction because I'm running pretty weird speakers which aren't really designed for guitar.
Last edited by TheQuailman at Jun 21, 2011,
the amp's eq typically comes either after the first part of the preamp or right before the power amp.

an eq pedal can go right after your guitar...

or you can put it before or after pedals to shape the sound...
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EQ pedals are also great to use for solo boosts, both for volume and a boost for your mid-range. So you can essentially have one tone for rhythm and one for lead.
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