Audiolab 8000 a bias sm

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I I I I I; I I I I; IIII I I, I I I I I I

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2. BIAS SETTING

2.1 The bias current is set by resistors R809, R811 (L.Ch) and
R810, R812 (R.Ch). The original resistors must be removed from
the PCB before re-biasing. New resistor values are chosen by
measuring the voltage across dual emitter resistors R493 (L.ch)
and R494 (R.Ch). Resistors are selected to give a voltage
reading of 22mv for a bias current of somA.

2.2 Ensure driver heatsinks (Driver heatsink 207/209) securely
fixed with thermally conducting grease (eg. Dow Corning DC340).

2.3 Connect bias test leads by temporarily soldering to the PCB
test pads (TPl-TPS). See drawing CST207-036.

LEFT CHANNEL RIGHT CHANNEL

TPl to +ve DC voltmeter TPZ to +ve DC voltmeter
TP3 to -ve DC voltmeter TP4 to -ve DC voltmeter
TPS to R809, R811 TP6 to R810, R812

TP7 to R809, R811 TPB to R810, R812

Note: Voltmeter inputs must not be grounded, use high input
impedance meter.

2.4 Amplifier baseplate and cover should be in place, fixing
screws need not be fitted. Test leads can be passed out under
rear of cover or through slots in baseplate.

2.5 Select range on DC voltmeter to read 22mV t20%

2.6 Resistors R809/R810 are generally selected from the range
2K0, 2K4, 2K7, 3K3, 3K9, 4K7, 6K8, 12K. R811/R812 are selected

from the range 16K to 150K or O/C. Use good quality metal film
resistors.

2.7 When changing bias resistors do not let the bias current
exceed loomA - a voltmeter reading of 44mV.

2.8 Connect correct voltage mains supply and switch on power.

2.9 After about 1 minute the bias current should be set to
100mA (44mV), after a few minutes it will fall to a steady
value. Select resistors R809/R811 (L.Ch) and RBlO/RBlZ (R.Ch)
to give a bias current of SOmA (Voltage reading 22mV).
Generally the voltage can be set quite close to 22mv, but a
tolerance of 20% is acceptabler Any sudden variations in bias
current caused by changing the resistors must be allowed to
stabilise before choosing the final resistor values.