ADCOM GFA5500 bias voltage needed

dballes15 · 7th August 2009, 04:50 AM

Does anyone know the bias voltage for an Adcom GFA5500?

My friends GFA5500 heatsinks get too hot to touch after a few minutes running, even at low listening levels.

Alfatipo · 10th June 2010, 03:24 AM

You get an answer to this anywhere?

I have those figures but I have a little wine in me right now so I'm not too inclined to walk upstairs.

You should be able to keep your hand there for 15-20 seconds before it gets really uncomfortable.

High bias is good but too much will out run the heatsinking and damage stuff.

25 degrees over ambient is about right.

Jef

DCPreamp · 12th June 2010, 01:35 AM

The service manual says:

Let the amp idle for about 5 minutes to let it warm up.
Connect a millivoltmeter across emitter resistor R52 for the left channel (R51 for the right).
Adjust bias pot P2 until the meter reads 50mV +/- 5mV.

My GFA-5500 runs quite hot too. Plus, I've read on this site that the MOSFET outputs, the IRFP240 & IRFP9240, must be passing a moderate amount of current in order to get them into their linear range. This certainly explains the amp's running hot, but also explains their great sound.

Despite my amp running hot, I've never had it go into thermal protect mode even after many hours of listening.

jimangie1973 · 10:13 PM

Thread back from the dead...

I opened up my 5500 only to discover the bias settings were not even close to spec. I suspected this because my unit was never really heating up. One channel was only biased to 15mV.

There are 10 source resistors per channel. After I set the bias correctly, the bias voltages across 8 of the 10 resistors (couldn't access the other 2) for each channel are:

Left.......45.9, 52.6, 53.6, 55.4, -, -, 53.3, 38.7, 52.9, 53.5 -> Avg 50.74 mV
Right.... 48.6, 46.4, 47.2, 58.0, -, -, 45.6, 49.4, 51.2, 59.0 -> Avg 50.68 mV

There's quite a variance to the bias voltages. Poor matching? Now the heatsinks get nice and toasty and the amp sounds great.

Jim

birdyman · 1st October 2010, 02:15 PM

Adjust way up until Jef at #2 post said. That is what Nelson Pass recommends.

7th August 2009, 04:50 AM	#1
dballes15 diyAudio Member Join Date: Aug 2006	ADCOM GFA5500 bias voltage needed Does anyone know the bias voltage for an Adcom GFA5500? My friends GFA5500 heatsinks get too hot to touch after a few minutes running, even at low listening levels.

12th June 2010, 01:35 AM	#3
DCPreamp diyAudio Member Join Date: Jun 2005 Location: Sacramento, CA	The service manual says: Let the amp idle for about 5 minutes to let it warm up. Connect a millivoltmeter across emitter resistor R52 for the left channel (R51 for the right). Adjust bias pot P2 until the meter reads 50mV +/- 5mV. My GFA-5500 runs quite hot too. Plus, I've read on this site that the MOSFET outputs, the IRFP240 & IRFP9240, must be passing a moderate amount of current in order to get them into their linear range. This certainly explains the amp's running hot, but also explains their great sound. Despite my amp running hot, I've never had it go into thermal protect mode even after many hours of listening. __________________ "Believers cling to the myth despite the evidence, reinterpret the myth to suit the evidence, or lie about the evidence to support the myth." "To err is human; to blame errors on external factors is even more so."

30th September 2010, 10:08 PM	#4
jimangie1973 diyAudio Member Join Date: Jan 2007	Thread back from the dead... I opened up my 5500 only to discover the bias settings were not even close to spec. I suspected this because my unit was never really heating up. One channel was only biased to 15mV. There are 10 source resistors per channel. After I set the bias correctly, the bias voltages across 8 of the 10 resistors (couldn't access the other 2) for each channel are: Left.......45.9, 52.6, 53.6, 55.4, -, -, 53.3, 38.7, 52.9, 53.5 -> Avg 50.74 mV Right.... 48.6, 46.4, 47.2, 58.0, -, -, 45.6, 49.4, 51.2, 59.0 -> Avg 50.68 mV There's quite a variance to the bias voltages. Poor matching? Now the heatsinks get nice and toasty and the amp sounds great. Jim Last edited by jimangie1973; 30th September 2010 at 10:13 PM.

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10th June 2010, 03:24 AM	#2
Alfatipo diyAudio Member Join Date: Jun 2010 Location: Portland, OR USA	You get an answer to this anywhere? I have those figures but I have a little wine in me right now so I'm not too inclined to walk upstairs. You should be able to keep your hand there for 15-20 seconds before it gets really uncomfortable. High bias is good but too much will out run the heatsinking and damage stuff. 25 degrees over ambient is about right. Jef

1st October 2010, 02:15 PM	#5
birdyman diyAudio Member Join Date: Jan 2010	Adjust way up until Jef at #2 post said. That is what Nelson Pass recommends.

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