this is from the manual of the 33h
Note
Before making balanced connections, remove the U-shaped
shorting-strap between pins 1 and 3 on the Nº33H XLR input
and save it for possible future use. This shorting-strap is
installed when the Nº33H is shipped.
4. single-ended input
Receives unbalanced audio input from a preamplifier with single-
ended outputs. These signals are converted to balanced signals
upon entering the chassis and processed as balanced signals
thereafter. One RCA connector is available.
If the associated preamplifier does not support balanced
connections, connect the RCA output on the preamplifier to
the RCA input on the Nº33H. To reduce the chance of noise at
the (otherwise unterminated) inverting XLR input, insert the
supplied U-shaped shorting strap between pins 1 and 3. (This
shorting strap is installed when the Nº33H is shipped.)
Note
Before making balanced connections, remove the U-shaped
shorting-strap between pins 1 and 3 on the Nº33H XLR input
and save it for possible future use. This shorting-strap is
installed when the Nº33H is shipped.
4. single-ended input
Receives unbalanced audio input from a preamplifier with single-
ended outputs. These signals are converted to balanced signals
upon entering the chassis and processed as balanced signals
thereafter. One RCA connector is available.
If the associated preamplifier does not support balanced
connections, connect the RCA output on the preamplifier to
the RCA input on the Nº33H. To reduce the chance of noise at
the (otherwise unterminated) inverting XLR input, insert the
supplied U-shaped shorting strap between pins 1 and 3. (This
shorting strap is installed when the Nº33H is shipped.)