Some pedals are based on the circuit of the EP-3 and additionally prevail a booster, others are exact replicas of the EP-3 preamp – including the higher operating voltage.
Reasons for its popularity
Actually, the preamp pushes the volume of the signal only marginally.
Pure booster pedals can do this much better.
But as an EQ the preamp becomes pretty interesting: it cuts the low end a little bit and boosts the upper mids. Thereby its sound reminds of a treble booster which cuts the signal in front of an overdriven amp slightly to represent the overdriven sound in a quite differentiated way.
At the days Jimmy Page made the EP-3 pretty famous by constantly using it as a tone enhancer in his signal chain, there were no or only very limited equalizer for guitarists available.
Although some critics doubt that phase shifting is the decisive factor in terms of the unique sound of the EP-3 preamp, I definitively agree with this theory.
The phase of the EP-3 output varies depending on the frequencies.
Of course, there are many other devices that also shift the phases. But in their cases, it is mostly impossible to perceive the phase shifting because it occurs in a range under or above the frequencies the human hearing is able to notice.
The phase shifting of the EP-3 preamp occurs in the frequency range of the guitar.
The vibrating string of a guitar not only produces a sinus curve but also harmonic overtones.
The preamps reproduction of these tones with a slight phase shifting are pretty pleasing to the human ear.
An equalizer also produces phase shifting but to those the phase distortion is more characteristic.
This is the reason why it makes sense to use an EP-3 preamp as a tone enhancer although it almost lacks ability to boost and an EQ might create more extreme (but finally, different) sound colorations.
Especially in combination with a delay and a slightly overdriven amplifier, the tone enhancement becomes significant.
And those who are skeptical about the modest alteration of the sound, will at least perceive the effect after they turned the preamp off.