Pass Labs pre-amplifier - Manual (Page 9)

9 X0.2 Owner’s Manual
With balanced operation, the common mode rejection of the preamp
reflects the intrinsic common mode rejection of the topology, the
matching of the gain devices, and the matching of the attenuator
channels. In this case we have been able to keep the total mismatch to
about .1%, for a common mode rejection of greater than -60 dB.
The input system of the pre-amplifier will exhibit full common mode
noise rejection with passive balanced sources, where the negative input
is connected to ground at the source through the appropriate source
impedance. This allows adaptation of unbalanced sources to balanced
operation with passive cable connections in a manner that achieves the
noise rejection of active balanced sources.
The use of a micro-controller allows all of the pre-amplifier functions
to be repeatable and accurately controlled. The micro-processor only
controls the operational functions of the pre-amplifier. At no time do
any of the input or output signals come into contact with the digital
control signals. The digital circuits are powered by a power supply that
is completely separate and isolated from the analog supply. Should it
ever be necessary to update the software that controls the functioning
of the preamp, only the socketed micro-processor need be changed.
The analog power supply for the X0.2 consists of a toroidal power
transformer delivering an unregulated +/- 45 volts which is then
actively regulated before feeding passive filtering and powering the
constant current sources which bias the gain stages. Each channel has
its own regulation. The power supply noise reaching the circuit is
on the order of one microvolt, and the little of that which gets through
the circuit is differentially rejected at the output in a balanced system.
The relays and control systems are regulated independently. All digital
circuits receive their power from a separate toroid, which, like its analog
counterpart, is also double regulated, and carefully filtered.
Muting relays, which delay connection during turn-on, additionally
shut off the output when insufficient power supply is available to
maintain regulation, thus guarding the output of the pre-amplifier. The
pre-amplifier is designed to run constantly, and will exhibit optimum
measured performance within about 10 hours of turn-on. For this
reason a power switch is intentionally omitted.
The X0.2 consists of three machined aluminum chassis. Likewise, the
remote control device is ensconsed in its own palm sized aluminum
chassis. All four chassis components are machined from solid aluminum
material on computer controlled vertical milling machines. There are
no sheet metal components is this product. All engraving is done by
laser to make it easy to read and provide a durable and attractive finish.