The difference between acoustic, electric, and bass amps lies in several areas.
1. Speakers
The speakers in acoustic amps are generally designed to have a very broad and clean sound to them. Speakers in electric amps have to be able to compromise between a good clean sound and also good distorted sound. Speakers in bass amps are designed to enhance certain frequencies, and require a good balance between sheer low end power, but also precision of the sound.
2. EQ
Acoustic amps will generally have a much shallower EQ sweep than electric amps will, because the idea is to get an accurate representation of the guitar, not get wild sounds. Electric amps, on the other hand, can have extreme EQ ranges to allow for mid-scooped or mid-boosted sounds. Bass amps focus primarily on the frequencies needed for low bass reproduction.
3. Preamp gain
Acoustic amps need a preamp stage that will allow for a lot of gain before the signal starts to distort (also known as having a lot of headroom). Many electric amps rely on this distortion for their characteristic sound. Bass amps can have something of a balance between them, as some bassists like a semi-distorted sound.
4. Poweramp power
Acoustic amps are typically somewhat lower powered, as they don't need to drive extreme gain to earsplitting levels. Electric amps are in the middle, with power ranging from very low (for practice amps) to rather high (350W on the new Marshall Mode Four amps). Bass amps require much higher power (generally 150-800+ W) just to move such low frequencies with power.
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