Mosfet matching compensation

willcycle · 5th February 2006, 12:19 PM

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

lineup · 5th February 2006, 12:28 PM

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.

anatech · 5th February 2006, 04:46 PM

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

5th February 2006, 12:19 PM	#1
willcycle diyAudio Member Join Date: Oct 2003 Location: MD	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

5th February 2006, 12:28 PM	#2
lineup diyAudio Member Join Date: Dec 2005 Location: the north	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. __________________ lineup

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
does MOSFET need thermal compensation ?	djdamix	Solid State	25	11th February 2005 03:46 PM
Matching Mosfet's	Jena	Pass Labs	4	5th April 2004 06:23 AM
Mosfet Matching Question	vinay	Pass Labs	1	20th October 2002 08:24 PM
Matching Power Mosfet	Forte	Pass Labs	4	30th October 2001 09:58 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

5th February 2006, 04:46 PM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)