We recommend the use of the balanced output mode whenever possible. It offers
less distortion, less noise, more gain, and more voltage swing, all without
compromising the sound.
Additional gain and voltage swing are available beyond the settings with which the
preamp is shipped. In “high gain” mode, the preamp can give an additional 6 dB of
voltage gain and nearly twice as much maximum output swing, with a penalty of
proportionately higher noise and distortion.
If you find that you must have more gain and/or voltage swing, high gain mode can
be enabled by removing the four “gain” jumpers on the interior main board of the
preamp. These are located near the output connectors.
Please avoid electrical shock by adhering to the following advice:
As removing these jumpers requires opening the preamp, it is very important that
this task be performed by someone with some skill. AC power to the preamp must
be removed and insured by removing the power cord from the preamp. Allow at
least one minute from removal of AC power before opening the top, so that the high
voltages inside have a chance to bleed down to safe levels. When you remove the
gain jumpers, we advise that you “park” them on one of the two pins so as to not
lose them.
The X-2 uses power Mosfets exclusively for its gain stage. These Mosfets were
chosen because they have an excellent transfer curve for an asymmetric Class A
design. The gain Mosfets are rated at 32 watts each and peak currents in excess of
5 amps. Needless to say, they do not work very hard when sourcing 30 milliamps
into a load. The use of such devices does provide very high transconductance and
charge surface area over small gain devices, and this shows in the excellent
linearity obtained with only one device operated without feedback.
Mosfets provide the widest bandwidth of solid state power devices, however they
were not chosen for this reason. The design of the X-2 does not seek to maximize
the preamplifier bandwidth as such. The capacitances of the Mosfets provide a
natural rolloff in conjunction with the resistive impedances found in the circuit, and
the simplicity of the circuit allows for what is largely a single pole rolloff
characteristic. Nevertheless, the bandwidth of the circuit will typically extend to
about 200 kHz (-3dB).
There is no such thing as a slew rate for this circuit, as it will retain the linear RC
characteristic for any input signal.
The common mode rejection of the preamp reflects the constant current source
biasing, the matching of the gain devices, and the matching of the output loading. In
this case we have been able to deliver a total match of about .1%, for a common
mode rejection of approximately -60 dB.
The input system of the preamplifier will exhibit full common mode noise rejection
with passive balanced sources, where the negative input is connected to ground at
