PreSonus TubePRE - Manual (Page 12)

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3.1 OPERATION
3.2 Dynamic Microphones
Lower output levels characterize dynamic microphones, so more gain
is needed to achieve sufficient operating levels. Occasionally it is
necessary to add the -20dB pad to the microphone to avoid distortion
(e.g. when recording percussion). Do not use phantom power when
using dynamic microphones.
3.3 Phantom Powered Microphones
Phantom powered microphones such as condenser and some ribbon
microphones require external power to preamplify the microphone.
These microphones typically have much higher output than dynamic
microphones. Therefore the -20dB pad is almost always necessary
when close mic-ing to avoid clipping (overdriving) the preamplifier.
3.4 Instrument Input
The instrument input is designed to handle 1/4” plugs from
instruments such as guitars and basses. This instrument input is an
ultra high impedance amplifier designed to allow the full audio
potential of an acoustic or electric instrument pickup. Plugging a
preamplified signal (line level) into the TubePRE is not recommended.
3.5 Some Thoughts on Vacuum Tubes
The TubePRE comes supplied with a 12AX7 vacuum tube that meets or
exceeds the stated performance criteria for the unit. We expect some
owners of the TubePRE will try different tubes to investigate the
various performance possibilities they might provide. Remember, tube
life and performance are affected by how often a tube is used and by