PRESONUS TUBEPRE User Manual page 16

Can I plug my keyboard/CD player/etc into the TubePRE?
It is not recommended that any LINE LEVEL items be plugged into the
TubePRE. This scenario will not cause damage to the unit. However, it
may cause the signal to distort or sound unpleasant. The reason is
that the signal being fed into the unit is too loud for the TubePRE. This
may cause the unit to distort (In some cases this may be the desired
effect).
Whenever I turn up the tube drive all the way my signal sounds
distorted. Why is this?
The tube drive feeds the signal into the tube. The more signal that is
applied to the tube, the greater the chances of distortion. However, to
some this sounds nice on guitar or other instruments. In certain
applications the tube drive is not needed (for example, while recording
books on tape). In other applications, the tube drive can be mixed with
the gain to create desirable effects (think of the tube drive as extra
gain with added equalization).
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