Some not very happy experience with my used GFA-555 II AMP - diyAudio
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Old 19th August 2011, 01:49 PM   #1
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Default Some not very happy experience with my used GFA-555 II AMP

I am a new member, thank you very much if you can help me!

I finally got a USED GFA-555 II from AudioGon, In the hope to improve my Monitor 10B's sound quality, unfortunately it doesn't. Here is the story:

I hook it up with Harman Kardon HK-3490's preout

I find the sound from the GFA-555II lacks some clarity compare the sound directly from the HK3490 receiver's speaker out. It's not very easy to describe the difference. it likes a very mild noise in the high end. something like Sibilance, I have to listen to it carefully, then I can notice it.

In term of volume, the volume from GFA-555II even slightly smaller than the volume from the HK3490's speaker out. I expect GFA should and can produce bigger or much bigger volume.

Finally, when I turn off the power in GFA, it still sounds for a few seconds, first the left channel fade out, and then is the right, is this means anything wrong.(is this normal?)

I think I used a very good Monster cable, I also tried Luna 6 cable to connect the HK3490 preout and GFA , I got the same thing.

If listen this GFA-555II AMP alone, it seems it works fine, but just by comparing with the output from the HK3490 receiver. I feel this AMP does not fully work but I am not sure.

Please help me to sort out the situation,
Thanks a lot in advance!
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Old 20th August 2011, 01:31 AM   #2
djk is offline djk
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If you replace the 30 year old electrolytics caps in the Adcom it will sound quite a bit better.

Gain and power have little to do with each other. Change the gain if you want more gain.
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Old 21st August 2011, 01:04 PM   #3
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This is why I stopped buying anything from Audiogon. Sooner or later,you'll get a lemon,& there really is not much you can do about it..
Unless you are really attached to that Adcom,I'd try something else,& just sell it for parts.I would certainly not put any more money into it..
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Old 21st August 2011, 01:28 PM   #4
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How long does it take to remove the cover? 5 min tops?

It takes about the same amount of time to discharge the filter caps remove the bad and replace with new. You have to remember when you purchase something used that has some age to it there are bound to be potential problems. While your at it you might as well replace the small caps(electrolytic) in the signal path as well. As mentioned by DJK its got thirty years on it. You cannot expect new performance on any piece regardless of where it was purchased.

Quote:
Finally, when I turn off the power in GFA, it still sounds for a few seconds, first the left channel fade out, and then is the right, is this means anything wrong.(is this normal?)

Yes, any amp without a relay on the speaker output will continue to put audio out until the power supply voltage goes down.

I would't put a lot of value on the audio from the HK piece its not high quality. There are much better pieces out there.

Last edited by Original Burnedfingers; 21st August 2011 at 01:35 PM.
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Old 21st August 2011, 01:37 PM   #5
wahab is offline wahab  Algeria
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It s not a matter of supply caps..
Does anyone think that the sibilance is due to bad supply filtering.???.

More likely, this amp is not well designed stability wise , as corroborated
by the numerous threads about its flaws...
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Old 21st August 2011, 06:21 PM   #6
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quote:
It s not a matter of supply caps..
Does anyone think that the sibilance is due to bad supply filtering.???.

Just pointing out the amplifier has age and therefore the caps should be changed. I sincerely doubt the amplifier has any problems. The flaw probably lies in the fact the HK piece sounds very colored and I'm being nice. In other words garbage in garbage out. I'd like the opinion of the poster if he mated the amplifier up to a decent preamp.

More likely, this amp is not well designed stability wise , as corroborated
by the numerous threads about its flaws...

Everything has some type of flaws. The Adcom has had a fairly decent track record. I base this on the first 5 or so years after it came out. One cannot expect amplifier to perform the same as it did day one.

Last edited by Original Burnedfingers; 21st August 2011 at 06:26 PM.
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Old 21st August 2011, 06:48 PM   #7
infinia is offline infinia  United States
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Quote:
More likely, this amp is not well designed stability wise , as corroborated
by the numerous threads about its flaws...
yer still on some personal crusade against this amp. Stability problem based what? on some hobbyist's flawed transistor models and simulations. There lot's of repair threads because there are lots of these amps sold with some folks wanting to learn to fix and keep them up and running. Yes sadly some people break them, they are not bullet proof to stupidity.
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Old 21st August 2011, 10:00 PM   #8
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Thank you all very much for all the valuable inputs, I have some updates...

Quote:
Originally Posted by 6BG6GA View Post
quote:
It s not a matter of supply caps..
Does anyone think that the sibilance is due to bad supply filtering.???.

Just pointing out the amplifier has age and therefore the caps should be changed. I sincerely doubt the amplifier has any problems. The flaw probably lies in the fact the HK piece sounds very colored and I'm being nice. In other words garbage in garbage out. I'd like the opinion of the poster if he mated the amplifier up to a decent preamp.

More likely, this amp is not well designed stability wise , as corroborated
by the numerous threads about its flaws...

Everything has some type of flaws. The Adcom has had a fairly decent track record. I base this on the first 5 or so years after it came out. One cannot expect amplifier to perform the same as it did day one.

I hope HK3490 is the problem but unlikely as the external preout is also used internally by the HK 3490 power amplifier portion and it ends up a better sound. If the preout of HK-3490 is noisy, the sound of its speaker output should be noisy too. This is my logic so far.

I means 'Better' just by comparing the sound from GFA-555II, not means the sound from HK-3490 is perfect, it can considered colored or something else.

I have sent the AMP to a nearby repair store to check as they are in promotion period for free check.

They have used Oscilloscope to check the wave form to detect distortion, they did found distortion, they are in the process to further diagnosis which component cause the distortion, I hope the problem is caused by some dry out caps rather than the bad design of Adcom itself!

Thank you all and have a very nice weekend.
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Old 21st August 2011, 10:57 PM   #9
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Quote:
They have used Oscilloscope to check the wave form to detect distortion, they did found distortion,

Good for you. Since your repair shop found the amplifier incapable of reproducing a sine wave correctly I am hopeful they have the expertise to repair it. It would have been interesting to run the amplifier into a distortion analyzer for a true reading of how much distortion it was making. Like I said you cannot expect an amplifier of that age to reproduce like it was day one. I am hopeful that you repair this amplifier and beg or borrow a decent preamp so that you will be able to experience what a good preamp coupled with a good power amplifier do sound like.

Someone here on this forum seems to feel that the Adcom 555 II is plagued with problems.

Quote:
More likely, this amp is not well designed stability wise , as corroborated
by the numerous threads about its flaws...

I was unable to dig up any information or numerous threads to this effect. I will repeat the amplifier is around 30 yrs old and therefore will not and cannot be expected to perform like day one. This is why you can find all kinds of used amplifiers on different sites for sale on the internet. If its got a problem they put it up for sale and unfortunately you got one of them. To be perfectly honest here I would repair the 555 and send the HK down the road. The HK is a cost effective piece for someone wanting to do home theater on a budget.


Quote:
I hope HK3490 is the problem but unlikely as the external preout is also used internally by the HK 3490 power amplifier portion and it ends up a better sound. If the preout of HK-3490 is noisy, the sound of its speaker output should be noisy too. This is my logic so far.


Well I don't agree 100% with your logic. I once owned a Dunlap Clarke Fet 10 preamp and matching power amplifier. Together they sounded good but when I mated the pre with a different power amp it sounded shrill with the sound of breaking glass in the highs. There was nothing I had that I could mate with the preamp and come up with a winning combination it sounded best with its mate piece. My experience of owning probably 100 different preamps and power amps is that this does come up rarely. In your situation you did hear something out of the ordinary (the distortion) the 555 was making. My suggestion is to repair the 555 so it reproduces signal to factory specifications and a competent shop should be able to do that. When that is done you can replace the caps a mentioned and have something that will provide years of enjoyment.

Last edited by Original Burnedfingers; 21st August 2011 at 11:12 PM.
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Old 21st August 2011, 11:12 PM   #10
wahab is offline wahab  Algeria
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Quote:
Originally Posted by 6BG6GA View Post

Someone here on this forum seems to feel that the Adcom 555 II is plagued with problems.

Quote:
More likely, this amp is not well designed stability wise , as corroborated
by the numerous threads about its flaws...

I was unable to dig up any information or numerous threads to this effect. I will repeat the amplifier is around 30 yrs old and therefore will not and cannot be expected to perform like day one. This is why you can find all kinds of used amplifiers on different sites for sale on the internet. If its got a problem they put it up for sale and unfortunately you got one of them. To be perfectly honest here I would repair the 555 and send the HK down the road. The HK is a cost effective piece for someone wanting to do home theater on a budget.
You should check better , as regularly there s a GFA 555 owner that
pop up to ask for some help.

One can suppose that this is some kind of lemon that goes from one
hand to another until it find someone willing to fix it definitly.
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