SPHINX
Project Ten Service Manual
10
8. ADJUSTMENT PROCEDURES
The Project Ten is an integrated amplifier, meaning
the pre-amp and power amp sections are combined
into one cabinet.
The Project Ten only has two parameters for each
channel (so four in total) that might need
adjustment:
-
Bias
: to set the bias current and bias voltage of
the amplifier for normal use.
-
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 Ten have been replaced.
Attention:
When re-adjusting any setting please ensure that
there is no loudspeaker connected to the output!
Otherwise the loudspeaker may be seriously
damaged.
Attention:
The amplifier is able to generate high output
voltages of over + or -40 V.
Please be very careful during the adjustments!
The Project Ten is a two channel amplifier.
After removing the top cover plate you will see the
Left and Right Channels clearly marked.
The power transistors are mounted underneath the
heatsinks.
Bias
With this procedure you set the proper bias level for
the power transistors. This ensures their Class A
operation at low power levels.
Connect the amplifier according to the drawing
“Connection Diagram Bias Adjustment” (page 14).
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 two measuring clips of the meter
across one of the emitter resistors
Left:
R267, R271, R273, R277
Right:
R467, R471, R473, R477
(refer to schematic in Pj10main.PDF).
These resistors are positioned to the left of the
power supply condensers.
•
The level for each should be 40 mV DC
(±2 mV).
If not: adjust potmeter P202 (Left) or potmeter
P402 (Right) until the level is 40 mV.
•
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 @ 80 W into 8 ohm).
•
If this is correct the procedure is finished.
•
You may now switch off the amplifier or continue
with another adjustment procedure.
