Fixing an Adcom GFA-5500: -20db bass attenenuation on both channels - Page 2 - diyAudio
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Old 21st September 2012, 06:17 PM   #11
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Location: jacksonville FL
Does anyone know if this was fixed.and what it ended up being the problem???
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Old 21st September 2012, 10:39 PM   #12
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Location: jacksonville FL
I have a adcom gfa 5500...with very simular issues...weak bass responce, other than this it seems ok, wanted to kno what would fix this issue?
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Old 25th November 2012, 01:41 AM   #13
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Hmm...still no word on this?? well when I get to the bottom of my simular issue...I will post how it was resolved, incase someone else has simular issues, they will have an idea
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Old 27th April 2013, 04:11 PM   #14
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Originally Posted by jacobmj View Post

I recently purchased a used Adcom GFA-5500 and would appreciate some assistance diagnosing an issue with it. The low frequency response is extremely attenuated. This isn't a matter of splitting hairs: It is easily measured by SPL meter anywhere under
500hz, reaching more than 20db down by 30-40hz. The difference in sound is obvious, as is the difference in movement of the woofer by eye.

I've compared the response of this amp to that of my Denon receiver, using a cheap calibration microphone and laptop soundcard. This doesn't yield a flat response, but is perfectly reasonable for comparison, especially since the difference is so great. The attached plot shows two connectivities:

purple - SPDIF into the receiver, out to a Sound Dynamics 300ti.
teal - SPDIF into the receiver, pre-amp to the Adcom, same speaker

I have not moved the microphone or speaker between the two tests. Though not shown, I have also tested using my Hagerman Chime DAC connected directly to the Adcom. I do not believe there is any difference between the sources. There is effectively no low frequency in either case. Interestingly, all frequencies sound fine and have no notable distortion. Music can sound okay if my ear is placed next to a woofer.

I'm unsure what could cause this sort of problem, so I have made a few spot checks of easy and obvious measurements. I do notice that the DC rails have voltages rather greater than those printed on the silkscreen. On the right supply (left is about the same), for the "60VDC" rail, I see 77.6V and -77.6V. For "73VDC", it's 89.4V and -90.2V. They are very flat on my oscilloscope.

Voltage offsets at the speaker terminals are 4mV (right) and -8.9mV (left). Bias on left is around 40mV and close to 58mV on right. I haven't adjusted any of these.

I don't have a schematic for this amp, but have looked at the audio channel schematic for a GFA-5800. It doesn't cover the power supply though.

That the problem occurs with both channels suggests a common failure mode, or the power supply. I suppose I also can't be sure the amp has not been modified, but it does not appear so. I also can't imagine why this would be a beneficial modification.

I'd appreciate any assistance. Thanks! ~Jacob

Have you ever resolved this problem?
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Old 17th June 2013, 11:50 PM   #15
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I just picked up a GFA-5500 with the exact same problem. Turned out to be 3 bad 50v caps on each channels circuit boards. Replaced (not by me) and repaired.
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Old 18th June 2013, 01:21 AM   #16
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Location: West Australia, near to the beach, natural ambient sounds mostly.
Default Big Birthday.....

A while back I renovated a 32 channels of amplifiers powering an Omnimax planetarium theatre installation.
These amps have been powered continually for the past 20 years or so.
All channels had response (no bass) and sensitivity problems.
I replaced all signal and bypass caps and this restored FR and sensitivity to all channels to within 0.1dB.
Notable is that pretty much all caps with cans 10mm diameter or less measured completely open, or very close to.
So, for your 20 years old amp, just replace all the signal sized electro caps with new, decent quality caps and you should be good to go.....I used Hitano EXR low esr caps that I have consistently found to be good quality, and good sounding.
It can be worth replacing series coupling caps with bipolar types...less distortion.
The main supply caps are likely perfectly fine.


Last edited by Max Headroom; 18th June 2013 at 01:23 AM.
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Old 5th August 2013, 04:32 PM   #17
DanTana is offline DanTana  United States
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Location: Illinois, USA
I just purchased a GFA-5500 and noticed most the small plastic caps from the top 8 capacitors were loose inside the amp. It appears those capacitors had swollen and pushed the the small platic tops off. Would it be prudent to replace those caps and what would be good replacement caps? I noticed ostripper posted replacements for the 4 main capacitors earlier.

Thanks in advance,
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Old 29th November 2013, 02:33 PM   #18
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Location: jacksonville FL
mine ended up being the 8 caps that were bad...and needed a bias adjustment, now plenty of bass
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Old 19th April 2014, 03:10 PM   #19
srinath is offline srinath  United States
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Bringing this thread back form the dead ...

I just picked up an all original 5500, and its working fine to my ears on these little headphone sized Minimii 7's.

Is there any preventive fix I could do ? I will get to recapping it in a few months. Till then, something quick and easy to hold me over ? I'll run 8 ohm speakers etc etc. Usual precautions, dont pull the speaker wires with amp powered up etc etc.

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Old 20th April 2014, 12:54 PM   #20
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I checked the bias (just curious ) to see what it was( ran a lil warmer on one side....found it was consistently moving around......mine played loud and had weak bass output, I didn't use mine in fear of damaging it until repaired
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