Offset; Common Mode - SPHINX Project Twelve Service Manual

SPHINX

Offset

The Offset adjustment procedure minimises the DC
offset value of the amplifier output. This DC offset is
important when capacitive loads are used, such as
electrostatic loudspeakers. These loudspeakers
often use a very low-impedance step-up
transformer. The amplifier 'sees' this load as a
short for the DC voltage.
Connect the amplifier according to the drawing
"Connection diagram for testing the project 12"
(page 13).
The input of the amplifier must be shorted (by way
of the MUTE function of the oscillator).
Attention:
Be careful not to trip the offset protection mode. It
will activate when the output DC offset exceeds +/-
350 mV.
This mode can only be reset by switching off the
amplifier with the Mains switch (6.) and switching it
on again after a waiting period of at least 30
seconds.
Please be careful during the adjustments!
•
Switch the amplifier ON and wait until it has
reached the proper working temperature (this
takes an hour).
•
Set the millivolt-meter to the DC-range.
•
Place the measurement clips of the meter over
the output terminal.
•
The level should not exceed +5 or -5 mV DC.
If not: adjust potmeter P1 until the level is within
this range (P1 can be reached via the 'offset'
adjustment hole in the protection circuit board).
•
Switch the oscillator on and set it to 1 kHz and a
level of 0 dBu.
•
Check the distortion with a THD analyser: it
should be conform the specified values (0.006%
IHF-A @ 1 kHz).
•
If this is correct the procedure is finished.
•
You may now switch off the amplifier or
continue with another adjustment procedure.
Project Twelve Service Manual

Common mode

The Common mode adjustment procedure
minimises the amplification error of the (internal)
differential amplifier.
If the balanced input amplifier receives an identical
signal at the normal (+) and inverted (-) input the
output signal will be zero. This helps to reduce the
effect of external noise signals while these will be
induced at the same level in both signal
conductors.
The Common Mode adjustment is optimally set
during manufacturing (the error is as low as
possible).
Connect the amplifier according to the drawing
"Connection diagram for testing the project 12"
(page 13, except with both + and – input connected
to eachother, and use the special input connector.
This connector supplies both the plus (+) and
minus (-) input pins of the XLR with the same
signal.
The 'balanced/unbalanced'-switch on the rear panel
must be set to 'balanced'.
If there is no signal analyser available you may use
an oscilloscope at the output to view the waveform.
•
Switch the amplifier ON and wait until it has
reached the proper working temperature (this
takes an hour).
•
Switch the oscillator on and set it to 1 kHz and a
level of 0 dBu.
•
Adjust potmeter P3 until the minimum level is
set (P3 can be reached via the 'bal.' adjustment
hole in the protection circuit board).
When using a phase analyser the minimum
point is reached when the output phase
reverses 180° re. the input.
•
Also check the setting at 10 Hz, 100 Hz and
10 kHz. Readjust when necessary.
•
If the common mode is at minimum level at all
frequencies the adjustment is completed.
•
You may now switch off the amplifier or
continue with another adjustment procedure.
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