Noise floor marks the greatest difference between the two microphones. The NT-1 produced a noise level of 17 decibels (db). The NT 1-A reduces that level to 5db. Rode advertises the NT-1A as "The World's Quietest Studio Microphone."
Sound pressure level (SPL), the volume at which a signal will distort, varies slightly between the two mics. The NT-1A's headroom stands at 137 db, while the NT-1 distorts at 135 db.
The metallic finish of the NT-1A differs from the duller casing of the original NT-1. Both hearken back to the look of vintage mics. The cosmetic difference between the two microphones does not affect the sound.