SPHINX
Project Two Mk2 Service Manual
10
8. ADJUSTMENT PROCEDURES
The Project Two Mk2 only has two parameters that
might need adjustment:
-
Bias
: to set the bias current and bias voltage of
the amplifier.
-
Offset
: to set the DC-offset voltage of the
output.
These adjustments might be necessary when the
amplifier has been used for a period of time (and
settings have changed due to ageing) or when a
part of the Project Two Mk2 have been replaced.
After opening the Project Two Mk2 (by removing the
6 screws in the bottom), the following sections will
be visible:
- The Mains Transformer.
- The Main Print Board with inputs, outputs, the
dual power supply and both amps.
- The Display Print Board.
Bias and Offset
Both adjustment procedures use the same controls:
potmeters P1 & P2 (Left Channel) and potmeters
P3 & P4 (Right Channel).
At first both Bias parameters should be set (re. the
bias current and bias voltage of output transistors).
The positive and negative bias are adjusted
separately.
This may result in an output DC-offset voltage
which has to be adjusted afterwards.
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 Two
Mk2” at page 13).
By connecting a DC-coupled power amp to the
Project Two Mk 2 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.
Initial Set-up
•
Connect a low distortion signal generator
(<0.002% internal distortion) to an input of the
pre-amp.
•
Connect the distortion analyser input, the DC
mV-meter and the oscilloscope to an output of
the pre-amp.
Bias / Offset Adjustment
•
Short-circuit the input.
•
Measure the voltage across resistor R102 (Left)
and R110 (Right). Both should read 9 V (+/- 500
mV).
Adjust the value with potmeter P1 (Left) and
potmeter P3 (Right),
see file Pj2Adjust.PDF.
This will set the bias for one side of the
differential amp.
The Offset is not important.
•
Measure the voltage across resistor R117
(Right) and R109 (Left). Both should read 9 V
(+/- 500 mV).
Adjust the value with potmeter P4 (Right) and
potmeter P2 (Left),
see file Pj2Adjust.PDF.
This will set the bias for the other side of the
differential amp.
The Offset should now also be correct (<1 mV).
Although due to small variances in components
it may be higher so you have to re-adjust it.
•
Turn the volume control to MAX.
The mV-meter should show an Offset of less
than 1 mV. If this is not the case please re-
adjust the value with potmeters P1 and P2 (Left)
and/or P3 and P4 (Right).
•
Select a 1V / 1 kHz signal at the function
generator.
•
Check the output for the correct THD (<0.005%)
and clip-level (>10 V).
•
If all values are correct this will indicate the
Project Two Mk2 is properly adjusted and the
procedure is now completed.
