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Old 3rd February 2007, 11:54 PM   #1
John Irvine is offline John Irvine  United States
diyAudio Member
Join Date: Nov 2006
Default Need Bias Help

This is my first thread ever so bear with me. I have several Soundcraftsmen PCR800 Amps, these amps orginally came with four mosfets output transistors per channel. The board and heatsinks were set up for six.
I have added two more mosfets per channel, as per soundcraftsmen schematic. However the schematic doesn't tell how to set the Bias so I need help here.
I also have read that these mosfets should be matched, the problem is they are no longer available, so how critical is this match?
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Old 4th February 2007, 11:26 AM   #2
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
the larger the source resistor value the less critical the matching.

With 0r22 and three pairs I think there will be problems getting the stages to run with similar temps in each.

With 0r47 you might be in the ballpark, but 0r68 would probably be better.
What did Soundcraftsmen fit?

I expect the Bias to be set by adjusting the voltage on the gates to effectively control Vgs. When you add an extra parallel FET it also sees the same Vgs and will draw it's own quiescent current.
Effectively the bias will have correctly increased by 50% and dissipation will also be +50%. It's going to run hotter unless you have increased the sinks to dissipate the extra heat.
regards Andrew T.
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