HI Westlito and welcome to the forum.
Firstly you really need to be careful when doing this as you can fry the amp or electrocute yourself. I will post the following guide which i found a different forum but i take no responsibility if anything goes wrong:
"Right you have to be DEAD careful doing this, as you could blow your output transistors.
This is one of the things Tom Evans does for the a400gte. he sets it to 200ma
the other is 2 multilayer ceramic coupling capacitors
and the final is a dc servo, ( adjusts the dc in the feedback loop)
Erno Borbely is the inventor of this circuit.
open here up.
You switch it on, play music thro it for 10 mins, get it warm.
no kids or pets when doing this, don't touch anything metal inside, and use rubber gloves if you are ham fisted.
Use a multimeter on the dc volts setting, very low reading, place the leads across the big white resistor close to the ouput stage. and measure the voltage across it.
You ideally want about 150-200 milliamps flowing through this resistor, and the japs run them quite low, maybe 20-50 milliamps.
You get the current by dividing your measured volts by the resistor value. (simple ohms law)
say the resistor is 0.33 ohms, and the volts is ( v ), then this is the dc bias current that keeps the transistors slightly on to prevent crossover distortion.
your current= Volts ( v ) / resistor value.
So work out the new volts for the new current of 150 milliamps, this is
volts= current x resistance.
=150 milliamps x value of white resistor.
You need to tweak the little variable resistor with a philips no 2 screwdrive, insulated handle, stanley very good, VERY VERY carefully indeed, increase a little literally a millimeter at a time, DONT turn it all the way or its goodbye amp. Little minute turns WHILST the voltmeter is clipped on the resistor, and you will see the voltage rise, tweak until it gets to the desired value above, and repeat for the other channel.
You amp will run hotter, more in class A, and the sound may improve. You can do this with any amp with a pot but you need to be sure the pot is for this, and not dc offset, or anything else.
In the a400 its ok.
You must be careful, as if you do it too much, current= fried, and you will set your transistors on fire if you are to heavy handed and turn it all the way.
Also, don't go above 200milliamps, as the heatsinks can't take a large class A current.
Write again if in doubt and be careful, if you are, its easy.
If you go ahead, let me know the value of the white resistor, and the measured volts across it first, to be on the safe side.
And it have to settle a little, you may need to turn the pot, leave a minute, turn a little more, or back, it hovers until it settles.
Its only a 5 minute job and dead easy."