Question
How much gain should the mic preamp have to work properly with the SM7B?
Answer
The SM7 (SM7A, SM7B) is a dynamic microphone. A typical dynamic mic has a lower output level than a typical condenser mic. For example, the dynamic SM7 has an output level that is 28 dB (!) less than the condenser KSM44 (cardioid setting).
When selecting a preamp for the SM7 for voice-over work, make certain the preamp has a minimum 60 dB of gain. Here's why:
- With an acoustic input of 84dBSPL - typical speech level at 3 inches, the output level of the SM7 is -69dBV.
- The mic preamp must increase the mic signal level up to about 0dBV - line level.
- -69dBV of mic signal, increased by +60dB of preamp gain, equals an output level of -9 dBV. Not quite to the 0 dBV level, but close enough to work with most line level inputs.
Many modern mic preamps assume the mic is a condenser mic with a "hot" output level. Thus, it is common to find mic preamps, mixers, and interfaces with gains of only 40 to 50 dB. This is not enough gain to bring the SM7 signal up to line level. One example of a mic preamp with sufficient gain: Grace M101; www.gracedesign.com. Or the Radio Design Labs model HR-MP2A; www.rdlnet.com.
Note that the Grace M101 and Radio Design Labs HR-MP2A are preamps, not interfaces. To connect an SM7 to a computer, you will need a digital audio interface with at least 60 dB of gain.
The Shure Motiv
MVX2U audio interface is a great way to connect an SM7B to a Mac or PC. The MVX2U features 60 dB of total gain.
Alternatively, if your interface/preamp does not have enough gain, you can add a phantom powered preamp between the SM7B and your interface. Phantom powered in-line preamps such as the sE Electronics DM1 or DM2, Triton Audio FetHead, or Cloudlifter add 25-28 dB of additional gain.
In 2023, Shure released the
SM7dB, which is an SM7B with a built-in preamp which can provide either 18 or 28 dB of additional clean gain, making an external in-line preamp unnecessary.
