Analog-Precision Ultimate-Preamplifer Manual de usuario
DC/AC coupling and OFFSET VOLTAGE ISSUES
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Copyright © 2018 by Analog-Precision. All Rights Reserved http://www.analog-precision.com
Tuesday, June 05, 2018
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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.
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Tuesday, June 05, 2018
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However, the following two DC measurements show that the common mode voltage measured between
pins 1 & 2 and pins 2 & 3 is of the order of 1.6 Volts which is nearly a hundred times as much as the
differential offset voltage.
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If the DC offset voltages are a problem to the proceeding amplifier then the DC coupling bypass jumpers
should be removed.
INSTALLING OR REMOVING THE DC COUPLING J UMPERS
With the unit switched off and unplugged you should remove the top 8 screws using a 2 mm hex driver.
At the back of the unit is located the Unbalanced and Balanced output boards. As shown below the
jumpers for the Unbalanced board are fairly easy to access without physically removing the board. If you
need to bypass the coupling capacitors then you need to place jumpers or shunts over the exposed pins
for each channel. Likewise, for AC coupling you should remove the jumpers. As each channel has its own
individual jumper you can customize each channel depending on the amplifier it is connected to.
DC/AC coupling and OFFSET VOLTAGE ISSUES
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For the Balanced output board, the procedure is a little bit trickier as the jumpers are buried in a location
underneath the Unbalanced board so the best way to access it is by removing the Unbalanced board first.
To remove the Unbalanced board, first remove all of the IDC connectors connected to both the
Unbalanced and Balanced boards as in the following image.
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Then slide out the board towards the front of the unit without removing the earth straps. Each channel
has a pair of jumpers which represent the hot and cold connections of the balanced output so you need
to either install or remove both jumpers at a time. Do this for each channel that you want to configure.
DC/AC coupling and OFFSET VOLTAGE ISSUES
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Tuesday, June 05, 2018
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Once you have done this the reassembly process is the reverse of the assembly process. Note that the
retaining nuts on the RCA jacks should be hand tightened with the socket without using a bar.
Power up the unit and measure the offset at the output. If the unit is AC coupled (ie jumpers removed)
you should see there is no longer any DC offset at the output of the XLR connector.
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