SPHINX
Project Ten Service Manual
11
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 Offset Adjustment” (page 15).
The input of the amplifier must be shorted (by way
of the MUTE function of the oscillator)..
•
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 5 or -5 mV DC.
If not: adjust potmeter P201 (Left) or P401
(Right) until the level is within this range.
•
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 (0.01%
IHF-A @ 1 - 20 kHz).
•
If this is correct the procedure is finished.
•
You may now switch off the amplifier or continue
with another adjustment procedure.
Attention
:
Do not connect speakers on the Project 10 when
adjusting the offset.
Please be careful during the adjustments!
