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Old 12th June 2011, 09:44 PM   #1
Arroyo is offline Arroyo  United States
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Default setting bias mosfet amp

Working with a Conrad Johnson SA120 and having to replace outputs on one
channel. Have a circuit prior the drivers that consists of a single transistor
that is positioned between the drivers ( bipolar driving a pair of mosfet drivers)
collector driving gate of one driver emitter gate on other driver. Question I have
is that there is a pot on the base collector of this circuit of this bipolar which
I am aware should be the bias set. Is there any standard practice for setting
this pot ? I assume one is trying to set the appropriate turn on range for the driver and output circuits ( 2sk405's, 2sj 115's).
Any help would be greatly appreciated

Thanks
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Old 12th June 2011, 10:01 PM   #2
AndrewT is online now AndrewT  Scotland
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ask the manufacturer for the service information that should state the bias setting and the method of applying and where to take the measurements.
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regards Andrew T.
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Old 12th June 2011, 10:58 PM   #3
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A general way to do it is when amp is warm:
1/ put your meter across output resistor and put meter on volts setting.
2/ turn bias pot until min volts on meter.
3/ Apply a sine wave to input and monitor output on scope.
4/ Turn bias up slowly until crossover distortion disappears.
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Old 23rd June 2011, 12:18 PM   #4
ceteras is offline ceteras  Romania
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@ nigelwright7557 , do you recommend a certain amplitude of the output signal?
I think the sine wave should be ~1V peak to peak at output, and the scope set accordingly, this way the crossover distortion would be more obvious.
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