A good amp gives the output voltage of pickups varies between 100 mV rms to over 1 V rms for some of the higher output types. Some high-output pickups achieve this by employing very strong magnets thus creating more flux and thereby more output. These can be detrimental to the final sound because the magnets can tend to attract the strings and damp them thus greatly reducing sustain. Other high-output pickups have more turns of wire to increase the voltage generated by the string's movement