SPHINX
Project Eight Service Manual
10
8. OUTPUT DC-OFFSET ADJUSTMENT
Proper DC-offset adjustment is very important.
A simple trick will greatly increase the accuracy of
this critical adjustment (see also the drawing titled
“Connection Diagram for testing the Project Eight”
at page 14 ).
By connecting a DC-coupled power amp to the
Project Eight outputs and measuring the offset at
the speaker outputs, the offset is boosted to around
30 times the original value!
Note
: We strongly recommend that you use a
power amp with a very low DC-offset, such as the
Project Eighteen.
Note
: The pre-amp and the power amp should be
switched on at least 1 hour before performing this
adjustment so they can reach optimum working
temperatures.
Set-up
q
Connect a distortion analyser input and a DC
mV-meter to a pre-amp output (or to the power
ampifier). The correct pre-amp output will be
mentioned later.
q
Connect the distortion analyser output to a pre-
amp input, the correct input will be mentioned
later.
q
Switch off the oscillator of the analyser, this will
short-circuit the pre-amp input.
q
Remove the top cover of the Project Eight and
replace it with the special cover plate (with
adjustment holes, see figure 5 at page 16).
Adjustment
For the actual adjustment you may refer to Figure
20 (
Main PCB drawing
) at page 29 and Figure 7
(
Schematic overview of all relevant potentiometers
)
at page 17.
In Figure 6
(Schematic operational diagram of
project Eight
) at page 16 you will find the placement
of all amps:
Connect the (muted)distortion analyser output to an
unbalanced input of the project eight and select the
correct input at the front panel.
Connect the power amplifier (or, if no power
amplifier is used, the distortion analyser and mV-
meter) to the unbalanced output of the project
eight.
The unbalanced signal will only be processed by
disamp A1 & A3 (left channel) and disamp B1 & B3
(right channel). The remaining disamps have no
effect on an unbalanced signal.
q
Turn the volume of the project eight to ‘off’. The
offset currently visible on the mV-meter is the
offset of disamp A3(left channel) and/or disamp
B3(right channel).
q
This offset can be adjusted by potentiometer
P1 & P2 of disamp A3 for the left channel, and
by potmeter P1 & P2 of disamp B3 for the right
channel.
q
Turn the volume of the project eight fully open.
The offset currently visible on the mV-meter is
the offset of disamp A1(left channel) and/or
disamp B1(right channel)
q
This offset can be adjusted by potentiometer
P1 & P2 of disamp A1 for the left channel, and
by potmeter P1 & P2 of disamp B1 for the right
channel.
The distortion analyser output must now be
connected to a balanced input of the project eight.
The correct input must also be selected at the front
panel.
The power amplifier (or, if no power amplifier is
used, the distortion analyser and mV-meter ) must
now be connected to a balanced output of the
project eight.
The total offset of all disamps is now visible on the
mV-meter, but since the offset of disamp A1 &
A3(left channel) and disamp B1 & B3(right channel)
has already been adjusted, the offset visible is the
offset of the remaining disamps(A2&A4 and
B2&B4).
Notice: do not commence with the following
procedures before the procedures mentioned
above have been completed.
q
Turn the volume of the project eight to ‘off’. The
offset currently visible on the mV-meter is the
offset of disamp A4(left channel) and/or disamp
B4(right channel).
q
This offset can be adjusted by potentiometer
P1 & P2 of disamp A4 for the left channel, and
by potmeter P1 & P2 of disamp B4 for the right
channel.
q
Turn the volume of the project eight fully open.
The offset currently visible on the mV-meter is
the offset of disamp A2(left channel) and/or
disamp B2(right channel)
q
This offset can be adjusted by potentiometer
P1 & P2 of disamp A2 for the left channel, and
by potmeter P1 & P2 of disamp B2 for the right
channel.
Repeat the offset adjustment proceduces a few
times for a perfect adjustment. When this is done,
the offset adjustments are completed
