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Old 5th February 2006, 12:19 PM   #1
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Default Mosfet matching compensation

Hi,

I am biasing an Adcom GFA-5802 amplifier and discovered that the transistor matching in the left channel was not done accurately. To get a nominal 33 mV across the .33 ohm source resistors of the IRF244 Mosfets one of the resistors measures 60 mV, way above the maximum 43 mV I should see. Since these transistors are no longer available would it make sense to install a larger value source resistor for this Mosfet in an effort to match it?

Thanks,
Will
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Old 5th February 2006, 12:28 PM   #2
lineup is offline lineup  Sweden
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Yes.
But do not make them too high value.
Maybe 0.47 Ohm.

Only (and best) alternative to this is to buy yourself a number of IRF244.
They should not be imposible to find.

Or maybe you could use some other replacement for those IRF244.
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Old 5th February 2006, 04:46 PM   #3
anatech is offline anatech  Canada
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Hi Will,
When I was doing warranty service for Adcom, we had to match the mosfets ourselves. There was a requirement and a procedure for this. I believe those are still available aren't they?

Buy many from the same lot number and match them. I disagree completely with changing the emitter resistor value. Fix the problem, don't create more.

Your amplifier must have been repaired before, as dealer stock or a return to the dealer if you got it originally and didn't have it serviced yourself.

-Chris
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