Pass Labs X1 - Manual (Page 4)

4 X1 Owner’s Manual
If your signal source is unbalanced, input will always occur through
the RCA input connector. Inputs 1 and 2 give you a choice or either
single ended or balanced inputs. When single-ended inputs are used
on input 1 and input 2, a shorting plug, which is provided, should
be inserted between XLR pins 1 and 3 of the associated connector.
This plug shorts the negative input to ground thus optimizing the
input connection. You can run the X-1 without this shorting plug,
but performance will not be ideal. (NOTE: inputs 3 & 4 are single
ended only and do not have XLR’s associated with them.) You are
free to use any combination of balanced and unbalanced connections
with the X1. The unbalanced input impedance of the preamplifier is
33 kOhms. In balanced mode, the input impedance is higher, with a
differential impedance of at least 66 kOhms.
Next to the inputs on the rear panel, the preamplifier offers tape
inputs through both XLR and RCA connectors. This output is a
direct connection to inputs 1, 2, 3, and 4 when they are selected from
the front panel. Input 5 (TAPE IN) is deliberately not available
through the tape output connection. Input 5 is designated for use
with a tape recorder if you have one, and we have arranged that it
will not place its output on the tape out, which will prevent you from
accidentally creating a feedback connection with your tape machine.
The main outputs are located at the left-hand side (viewed from the
rear) of the rear panel, two male XLR connectors, and two RCA
connectors, “R” (right) and “L” (left). The RCA connectors ground
is in parallel with pin 1 of the XLR outputs and the RCA hot is fed
from an independent summing junction that maximizes the X circuit
benefits. The RCA, single-ended, outputs are buffered from the
balanced outputs. You may use both the single ended and balanced
outputs at the same time. On the XLR, pin 1 is ground, pin 2 is
positive, and pin 3 is negative.
The X1 takes maximal advantage of four identical gain paths (two
identical paths per channel) very much unlike the typical two gain
path (one signal path per channel) pre-amplifier you may be familiar
with. Almost without exception traditional audio pre-amplifiers,
unlike their science and industry counterparts, have maintained single
signal path per channel (two identical paths in a stereo pair).
In order to manage balanced signals these two gain path pre-
amplifiers must utilize phase combining networks to process
incoming signals and subsequently drive outputs via phase splitting
circuits to re-establish a quasi-balanced signal. This manipulation of
signal typically adds additional gain stages and degrades the inherent