Amplification and equalization can make up for the weaker output of a violin string tuned to notes below G3, so most electric instruments with lower strings are violin-sized, and as such, are called "violins." Comparatively fewer electric violas do exist, for those who prefer the physical size or familiar touch references of a viola-sized instrument.
Instruments may be built with an internal preamplifier, or may put out the unbuffered transducer signal. While such raw signals may be fed directly to an amplifier or mixing board, they often benefit from an external preamp/equalizer on the end of a short cable, before being fed to the sound system
from more info: http://en.wikipedia.org/wiki/Viola
Electric Viola on Image
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