SPHINX Project Eight Service Manual
9
7. MEASUREMENTS
To properly service the Project Eight you need
some specific measurement equipment and use a
specific set-up.
General set-up
For the correct measurement set-up and hook-up
please refer to the drawing titled “Figure 3:
Connection Diagram for testing the Project Eight” at
page 14.
Necessary Equipment
q 2x millivolt-meter (Ri >1 MΩ)
q 2x non-inductive load resistor 8 Ω / 250 W
q 1x harmonic distortion analyser
(internal distortion <0.0005%, measured
without filters)
q 1x 2-channel oscilloscope
(minimum bandwidth >60 MHz)
q 1x connection unit / switch box
(to connect resistors and amp outputs to
distortion meter)
Note: See drawing 4. for construction details.
q 2x shorting connector
(for balanced pre-amp inputs)
q 2x balanced cable set
(to connect distortion meter, pre-amp and
power amp)
q 1x special top cover plate (preferably
transparent) with holes for adjustment tools
Note: See drawing 5. for construction details.
q A two channel DC-coupled power amplifier
(e.g. Project Eighteen)
Audio signal distortion measurement
Before adjusting the Project Eight please first
measure the total harmonic distortion at the audio
output. The measured value is an indication of the
current state of the Project Eight.
Set-up
For the correct measurement set-up please refer to
figure 3 at page 14.
q Set the oscillator output level to 0.775 V
(0 dBu).
q Adjust the pre-amp volume control so that
output level equals input level (0 dBu).
q THD should now be 0.1% unweighted (or
0.013% IHF-A).
q Now set volume control to maximum (fully
clockwise).
q Increase the oscillator output of the distortion
analyser to set the pre-amp output just before
clipping.
q THD should now be 0.12% unweighted (or
0.005% IHF-A).
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