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Old 20th June 2009, 09:49 PM   #1
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Default Adcom GFA-5800 question

Hello, I'm wondering if anyone may no the answer to my delemma: I have enjoyed my Adcom GFA-5800 for almost 15 years now but have noticed that it has always been a bit louder in the right channel regardless of what speakers I have had hooked up to it over the years. I tested it to be sure it was the amplifier and not the input signal to it. When no audio input is present the "hiss" is louder as measured with a dB meter by 2-3dB. I know some hiss is normal but now that I have it hooked to more efficient speakers it is somewhat annoying in between music! I was told that it may need a bias adjustment or a DC offset adjustment and wondered if anyone had a procedure on how to do this task? If I could just reduce the hiss to the level of the left channel, it would be great!
Thanks, Pete
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Old 21st June 2009, 01:41 AM   #2
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Post GFA-5800 Schematics

Hi Pete,

Nice amp. I like Adcom stuff.

Attached are the setup/bias/DC offset page, and the schematics.

The bias and DC offset adjustments may cause additional hiss, but I doubt it would affect the gain.

By looking at the schematics, the gain is a pretty basic Rf/Rin and has no gain at DC.

However, given the differential input, be sure to check the BAL(enced) switch for proper function, and be sure to check your input connections. If you are running a differential signal, depending on your source, interconnects, etc., there could be a ground/signal reason for a gain imbalance. Just my two-cents.

Now for the blah-blah. . . The schematics are for reference only - don't mass-produce this amp or infringe any copyrights, etc.
Attached Files
File Type: pdf gfa-5800 setup.pdf (75.5 KB, 264 views)
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Old 21st June 2009, 01:49 AM   #3
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Default Adcom GFA-5800 Schematic

Schematic 1/2
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File Type: pdf gfa-5800 schem-a.pdf (36.6 KB, 171 views)
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Old 21st June 2009, 01:59 AM   #4
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Default GFA-5800 Schematics

Schematics 2/2
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File Type: pdf gfa-5800 schem-b.pdf (42.1 KB, 154 views)
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Old 21st June 2009, 08:46 AM   #5
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Default Adcom GFA-5800

Thank you so much for the fast reply, I truely appreciate it! I will study the schematics you attached along with the procedure. I purchased a signal/function generator and I have a Fluke DVM and O-scope. Hopefully I can resolve this issue so I can enjoy great music again. I had originally purchased this along with an Audio Research LS-3B preamp and they have been making sweet music together for 15+ years on a variety of speakers including my Klipschorns, VMPS Tower IIs, and many of my own designs. I'll keep you posted as to my findings. Have a great weekend, Pete
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Old 21st June 2009, 04:02 PM   #6
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I remember that. It's one of mine.

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Old 22nd June 2009, 05:30 AM   #7
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Beautiful. Fascinating amp. I love the separate power supplies.

I wonder (out loud) if Adcom would mind if we DIY'ed a GFA 5800?

How old is this design? Early 90's? What's the acceptable age of equipment before we are okay to DIY it here?

..Todd
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Old 25th June 2009, 06:31 AM   #8
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Quote:
Originally posted by Nelson Pass
I remember that. It's one of mine.


Nice work. Well designed amp, not overly complex nor stripped down. It just does its job well and sounds very nice.
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Old 29th June 2009, 01:45 AM   #9
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Default Adcom GFA-5800

Yes, the GFA-5800 is one of the nicest sounding amps I have ever heard --it has provided hours of the most pleasurable listening. I hope to find the source of the pronounced hiss in the right channel and get it back to like new again. I noted a response from its designer Nelson Pass [the main reason I considered the amp in the first place!] Perhaps he could shed some light on my issue--I would be truely honored! Thanks again for all responses to my original posting, hopefully I will get enough input to resolve my problem.
Pete
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Old 29th June 2009, 10:22 AM   #10
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Default i am

i am pretty sure that the legend Pappa Nelson did his very best while design this amp ....

i am not really sure what adcom did while producing this amplifier ..... at the time some of the boards where produced in taiwan but the final assembly was done in us .....

some of the pcs come in to my service ...with alsmost weird problems .... most of them related to quality control ....

beyond that .... a wonderfull amplifier
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