Biasing a Adcom GFA 585

Tre' · 2nd September 2006, 05:41 AM

I talked to Adcom and was told 24mv between test points. That puts 36ma. of current through each output transistor. With a +/-80VDC rail there will be 2.88 watts dissipation per output. There are 20 output transistors total. This thing is going to get hot. Does this sound right to you?

Thanks...Tre'

EchoWars · 2nd September 2006, 06:32 AM

Lukewarm.

But...depends on the heatsink. I'd think that if it got hot just idling, that the heatsink would be seriously undersized.

Tre' · 3rd September 2006, 12:38 PM

The heatsinks are huge. It doesn't get hot, just warm. I'm used to cold SS amplifiers.

http://gon5.audiogon.com/i/c/f/1156611494.jpg

This is a pic of the 555 but the heatsinks are the same.

Thanks

anatech · 3rd September 2006, 01:17 PM

Hi Tre',
That amp runs warm, but not very warm.

Did you follow the heat soak procedure before setting the bias?

-Chris

Tre' · 3rd September 2006, 05:32 PM

"heat soak procedure"?

Thanks

anatech · 3rd September 2006, 06:34 PM

Hi Tre',
Yes. Run the amp at about 1/3 power into 8 R dummy loads for 20 min or so. The heatsinks will be very warm. Allow the amp to cool and the bias to settle down and stabilise. If you make a correction, allow 1/2 hour and check again. The top cover should remain on except for when taking measurements.

-Chris

Tre' · 4th September 2006, 08:28 AM

Chris, thank you for that explanation and all your help.

Tre'

2nd September 2006, 05:41 AM	#1
Tre' diyAudio Member Join Date: Feb 2006 Location: Ojai, Ca.	Biasing a Adcom GFA 585 I talked to Adcom and was told 24mv between test points. That puts 36ma. of current through each output transistor. With a +/-80VDC rail there will be 2.88 watts dissipation per output. There are 20 output transistors total. This thing is going to get hot. Does this sound right to you? Thanks...Tre'

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2nd September 2006, 06:32 AM	#2
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	Lukewarm. But...depends on the heatsink. I'd think that if it got hot just idling, that the heatsink would be seriously undersized.

3rd September 2006, 12:38 PM	#3
Tre' diyAudio Member Join Date: Feb 2006 Location: Ojai, Ca.	The heatsinks are huge. It doesn't get hot, just warm. I'm used to cold SS amplifiers. http://gon5.audiogon.com/i/c/f/1156611494.jpg This is a pic of the 555 but the heatsinks are the same. Thanks

3rd September 2006, 01:17 PM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Tre', That amp runs warm, but not very warm. Did you follow the heat soak procedure before setting the bias? -Chris

3rd September 2006, 05:32 PM	#5
Tre' diyAudio Member Join Date: Feb 2006 Location: Ojai, Ca.	"heat soak procedure"? Thanks

3rd September 2006, 06:34 PM	#6
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Tre', Yes. Run the amp at about 1/3 power into 8 R dummy loads for 20 min or so. The heatsinks will be very warm. Allow the amp to cool and the bias to settle down and stabilise. If you make a correction, allow 1/2 hour and check again. The top cover should remain on except for when taking measurements. -Chris

4th September 2006, 08:28 AM	#7
Tre' diyAudio Member Join Date: Feb 2006 Location: Ojai, Ca.	Chris, thank you for that explanation and all your help. Tre'

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