Many engineers and audio people don’t realize just how much signal can come out of a microphone. A hot mic with large signal can output many volts. That a dynamic microphone does this with a 120 ohm impedance is all the more remarkable (Figure 1). My pal, audio guru Steve Williams, recalled an entry in the Ampex Audio Digest bulletin board system (BBS):
“The circa 1968 demo for those who didn’t understand how much level could come out of a microphone (like those who were still trying to do rock-n-roll with Altec 1567 mixers with no input attenuators): Take your Simpson 260. Set it on the 2.5 VAC range, and connect it to the output pins of a Shure vocal mic. Hand the mic to someone with a good set of lungs, and have them do a good rock-n-roll scream. I never saw anyone pin the meter at 2.5 volts, but 1.5 volts was easily achieved.” anon
Figure 1 This plot shows microphone levels of a speaking voice can easily reach 0.1 volts. Shouts, screams, or loud music can be 20 times greater. (Source: Clifton Laboratories)
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