Adjustment Procedures; Bias - SPHINX Project Twelve Service Manual

SPHINX

8. ADJUSTMENT PROCEDURES

The Project Twelve has three adjustable settings:
1. Bias: to set the bias current of the power-FETs
2. Offset: to set the minimal DC voltage for the
output
3. Common Mode: to maximise the common
mode rejection of the balanced input
Re-adjustment of one or more might be necessary
due to ageing or when parts have been replaced or
repaired.
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 -60 V.
Please be very careful during the adjustments!
After removing the top cover plate you will see the
three main sections of the amplifier:
1. The power transformer
2. The main printed circuit board holding the
actual audio amplifier
3. The protection circuit board responsible for
switching the Project Twelve on and off.
Project Twelve Service Manual

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
"conection 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).
•
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 source resistors (R30, R31,
R39, R40, R43 or R45: see schematic at page
15).
•
The level should be 10 mV DC (±2 mV).
If not: adjust potmeter P2 until the level is
10 mV (P2 can be reached via the 'bias'
adjustment hole in the protection circuit board).
Attention:
All source resistors must show the same 10 mV
value. If not this indicates that the FETs are not
accurately matched and their variance is too high.
This will cause offset- and bias-problems which can
be detected with a THD analyser as a very specific
type of distortion.
In that case you should replace the FET sextuplet
with a new one (3 matched ones and their inverse
counterparts): they can be ordered from SPHINX.
•
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.
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