INTRODUCTION
Some Ultimate Preamp units may have been shipped with the DC offset jumpers in place which bypasses
the coupling capacitors on the Unbalanced and Balanced output boards. For the Unbalanced outputs this
should not present a problem to a proceeding power amp as the offset should be no more than +/- 5-
20mV. However, if any of the channels of the Buffered-Balanced board have their coupling capacitors
bypassed, a common mode voltage of around 1.6 Volts will be present on both pins 2 and 3 of respective
XLR jack. This may cause problems with a power amp connected to the Preamp which is not designed to
handle high common mode voltages on its balanced input.
Although the common mode voltage on the DC coupled output is relatively high, the actual differential
voltage measured between pins 1 and 2 on the XLR jack should be no more than +/- 20 mV and should
not present a problem to any subsequent amplifier which is AC coupled or has a well-designed balanced
input stage which should mitigate this offset. However, if you are having troubles with your power amp
going into protection mode or producing a loud thump when the preamp is switched on or off then it is
recommended to AC couple the outputs of the preamp to the proceeding amplifier. This is facilitated using
a set of jumper pins on the output boards of the Preamp.
BYPASSING THE OUTPUT COUPLING CAPACITORS
Both the Unbalanced and Buffered-Balanced output boards have series output capacitors to filter out any
DC offset on the output of the DAC convertor stages. If it is desired to have a flat response down to DC
then these capacitors can be bypassed but there maybe a residual DC offset which may affect the
operation of the proceeding power amp that it is connected to - if the power amp is DC coupled or does
not have a properly designed balanced input stage which can null out any common mode DC offset
voltages.
As can be see in the following DC measurement the DC offset between balanced output pins 2-3 on the
Channel-1 XLR connector with the DC coupling jumpers in place is of the order of 23 mV.
