SPHINX
Myth 11 Service Manual
8
6.
ADJUSTMENT PROCEDURES
The Myth 11 only has three parameters for each
channel (so six 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 parts
of the Myth 11 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!
Bias
With this procedure you set the proper bias level for
the power FETs. This ensures their Class A
operation at low power levels.
Connect the amplifier according to the drawing
“Measurement Set-up” (page 11).
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
•
Set the millivolt-meter to the DC-range.
•
Place the two measuring clips of the meter
across one of the source resistors
Left:
R30L, R31L, R39L, R40L
Right:
R30R, R31R, R39R, R40R
•
The level for each should be 8 mV DC (±2 mV).
If not: adjust potmeter P2L (Left) or potmeter
P2R (Right) until the level is 8 mV.
•
Repeat this procedure after 20 minutes to
finalise the adjustment.
•
Switch the oscillator on and set it to 1 kHz and a
level of 0 dBV (1 V).
•
Check the distortion with a THD analyser: it
should be conform to the specified value
(0.006% IHF-A @ 1 – 20 kHz @ 50 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.
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
“Measurement Set-up” (page 11).
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.
•
Set the millivolt-meter to the DC-range.
•
Place the measurement clips of the meter over
the output terminal.
•
The level should not 1 or -1 mV DC.
If it does: adjust potmeter P1L (Left) or P1R
(Right) until the level is within this range.
•
Repeat this procedure after 20 minutes to
finalise the adjustment.
•
Switch the oscillator on and set it to 1 kHz and a
level of 0 dBV (1 V).
•
Check the distortion with a THD analyser: it
should be conform to the specified value
(0.006% IHF-A @ 1 – 20 kHz @ 50 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.
