Why do mic preamps sound different? Voltage and current - a refresher
First, a refresher on voltage and current. The good old water tank analogy always works for me. Think of a tank of water with an outlet pipe through which the water gushes. The higher the level of water in the tank, the greater the flow from the outlet, because more water weighs more hence greater pressure. So the weight of water pressing down is like voltage, and the rate of flow is like current. Now consider the pipe... The wider the pipe the more current will flow, even for the same pressure/voltage. The narrower the pipe, the less current. The 'narrowness' of the pipe is analogous to electrical resistance.
So... more voltage leads to more current. Less resistance leads to more current. Simple. (We sometimes also talk about impedance. Resistance and impedance are so closely related that for this text we can regard them as being the same thing.)
Now let's consider why a higher voltage is better than a lower voltage. It's all to do with noise caused by the random motion of electrons in the cables. The lower the signal voltage, the closer it is to the noise voltage. This means a poor signal-to-noise ratio, and the noise is clearly audible behind the signal. Signals that are low in voltage need to be treated with great care. Signals that are higher in voltage are more robust.
Also, let's consider why a higher current is better. Each electron in the signal path contributes its own noise. But if there are many electrons, the randomness in the noise they produce tends to average out. Therefore, a signal that is carried by a strong current, meaning many electrons, is less noisy.
Obviously, I am simplifying a bit, but if you understand voltage, current, resistance and noise in these terms, you won't go far wrong in sound engineering.