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Old 14th July 2008, 12:40 PM   #1
Bushulo is offline Bushulo  India
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Default How To Adjust Bias Current?

Im working on Quasi Nmos500W amp almost finished just waiting to fire up. I need your help to adjust Bias. I think amplifier settings depend on different technique. To set VR1 and VR2.Multimeter I own Fluke 80 Series, I need some clarification, how to adjust Bias current. Can someone please put in detail the procedure? Selection of Multimeter range? How to place multimeter Red probe and Black probe on the circuit point while setting Bias current.

Power amp under development

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Old 14th July 2008, 02:11 PM   #2
Corax is offline Corax  Germany
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That's quite simple:

First leave the amp turned off for now:
Turn the idle current pot to the position where the base of the transistor (which is connected to the idle current pot) is connected to the most positive voltage level (closer to the collector potential).
Then hook up your test leads (red/black or +/-, polarity actually doesn't really matter, the relative dc voltage drop is to know), at one of the legs of the output FET pairs, right between the FET and the low-R (0.47 ohm resistors). This way you'll measure the voltage across the two in series connected low-R resistors. In between the output to the speaker.
If a total idle current of 100mA is desired and 3 FET pairs (legs) are provided - resulting in a ~33mA current through each leg - the voltage you should finally measure is approximately 31mV dc.
Now turn on the amp and - if no smoke comes out of it - let it run for a few minutes to heat up slightly.
Now adjust the idle current pot to the aforementioned voltage level (desired idle current). Let the amp run at least 15 to 30 minutes and observe the idle current during that time. In case of any rapid rise turn the amp off immediately and check for any issues that might have lead to this behaviour (i.e. output FETs are not firmly connected to the heatsink, etc.). After 30 minutes of additional warm up, with an appropiate idle current applied, it will be usually necessary to re-adjust the idle current (voltage across the low-R resistors) to the desired voltage drop. Since MOSFETs have a positive temperature coefficient it's more likely that the idle current has dropped a little bit after 30 minutes.
Now you can turn your amp off and remove the testleads/testclips and your amp should be ready to rumble
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