GFA 5503 Bias adjustment - diyAudio
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Old 26th March 2014, 04:35 AM   #1
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Join Date: Mar 2014
Smile GFA 5503 Bias adjustment

Hey guys and gals, I have a new to me Adcom GFA 5503 that runs quite a bit hotter than my GFA 5500. After an hour or so it heats up to 125-130 degrees F at the cooling fins. Almost as hot as my pure class A Plinius. Anyway I was wondering if a bias adjustment may be necessary or if this is normal operating temperature?

Im a rookie to the diy audio world. My limited experience includes assembling 3 bottlehead kits (Foreplay, Paramount monos and a Crack) and a Welborne Labs Ultrapath pre amp a few years back. I have the basic tools and I'd like to check the bias on this amp myself. I'm not one to ship an amp off to have someone else look at it.

Having said that, Im not exactly sure what emitter resistors legs to measure or exactly where to find the correct pots. Manual says bias pot P2, emitter resistor R52 left and center, R51 right. (Whatever that means) Target is 50mV +/-5mV

I want to learn, Anyone want to walk a rookie through a bias adjustment?

I'm sure it has to be similar to this...

Thanks in advance

Last edited by Race Director; 26th March 2014 at 05:17 AM.
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Old 26th March 2014, 12:58 PM   #2
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Join Date: Apr 2010
Location: Coffs Harbour
If the guy in that video, following Parasound's instructions for their model was more careful with distinguishing volts from millivolts, it might be useful for others like him, who own that model amp.

Adcoms vary in their bias sensitivities and setting methods. The service manual, such as it is, describes the procedure.
You can quickly search the model for a free (with registration) download here: Free Manuals User / Service / Schematics for Download | HiFi Engine
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Old 26th March 2014, 08:14 PM   #3
Mihkus is offline Mihkus  Estonia
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For me that video was full of dump, all you need is the amps circuit.
You need to find bias pot or fixed resistor... Whats the point of setting your bias to higher point? On most amps its optimised .
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