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Adcom GFA-5500 troubleshooting

28K views 80 replies 13 participants last post by  KINGKONGG 
#1 · (Edited)
November 2019 Thread Update:

See posts 55, 62, and 63 for updated listening impressions. I executed on the upgrades offered by John Hillig at Musical Designs/Musical Concepts in the form of the LX Elite Mod with Mini-Platinum upgrade.

The results are just stunning.


I'm going to refer to the amp, in its current, highly modified state, as the GFA-"5500 MCEP".




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Original first post:
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I just acquired a used Adcom GFA-5500 two channel amplifier. it is the non-IEC power cord version. I took the liberty of checking bias and DC offset; the bias was quite low at under 15mV on each channel (service manual says 50mV +/-5mV). I adjusted both and it's stabilized at just over 49mV on each channel.

Next, I adjusted DC offset. Right channel was fine at .7mV. Left was way off at 19mV. Now they're both hovering around 1mV after temp has stabilized and some current run through the amp.

My issue: bass, while much improved from my first listen and now very tight and well damped, seems underwhelming.

Using my ir thermometer, there is a running difference between heat sink temps; roughly 117 for the left channel and 110 degrees F for the right channel across the face of the heat sink.

My question: Is this an indicator that I need to start looking at cap replacement? If so, which ones; the smaller low capacity caps on top (there are 8) or the four primary beer-can sized filter caps underneath them?

I checked the fuses and they are all functional (look normal and had continuity according to my fluke meter), but I'm going to replace them anyway as they are a hodge podge of mismatched brands. I know I know....blame the OCD.

Any input is appreciated. If I need service, I'm in the Orlando, FL area and am willing to drive quite a ways to avoid absurd shipping costs. Recs for nearby businesses are welcome (or far away ones with reasonable shipping costs).

Thanks in advance, gents.



Oh, and as a note, I'm coming from an Arcam AVR 300 with a healthy amp section itself. Using it as a pre amp now.
 

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#64 ·
Interesting, one listen speaks a 1000 words. Amp can really make a difference in the sound, particular the sound stage, separation and the 3D.
 
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#65 ·
I heartily agree. The 3D and sense of textural realism are the two most heavily affected aspects. But, there are amps that do indeed sound very similar (that group is growing as amp design improved across the industry).

And, i would qualify this. I’ve heard speakers with labyrinthine crossover networks, using lower tier parts, and mediocre drivers that really swamped and clouded any changes in amplifier even when both amps are VERY different . Swallowed them whole. No apparent difference. So, for people who have less than ideal loudspeaker designs, i really do believe them when they say they don’t hear a difference.

While i won’t say who made the speakers, they weren’t esoteric or terribly expensive. In fact, they were a relatively common brand here on AVS. Just two way stand monitors of easy impedance and relatively flat and linear axial response.

But, other like-priced offerings from competitors put amp differences on display starkly and immediately.

For boxes that don’t show that difference readily, I hesitate to opine what that could mean for the music. Though, the boxes that didn’t let these differences through didn’t immediately sound bad in any real sense. It’s not like they had a huge narrow Q peak or dip at any given frequency, or any blatant uncontrolled resonances.

In fact, a few loudspeakers that were slightly more difficult to drive on paper didn’t have any difficulties laying bare various amps I’ve used.




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#70 ·
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Speakers is the most important part in the system. I consider the job of amp and the cable are to bring out the best of the speakers. So in order to talk about the amp and cable, you have to have define the speakers first.

You read my post, I said with a GIVEN PAIR OF SPEAKERS, amp and speaker cable.


Read the post.
 
#74 ·
I'm getting some random static from my GFA-5500 and was planning to replace all the capacitors. After reading this thread, I'm thinking of sending it off to Musical Design. I have to decide on spending $400 for LX Mod or going all out on the LX Elite + LX Mini-Platinum Upgrade.

My high frequency hearing is very bad, so I'm inclined to just go with the basic mod. The Adcom will be hooked up to an old Proceed AVP and Infinity Kappa 7.1 speakers.

BufordTJustice, what do you think? Is it worth the extra upgrades given my equipment and bad high frequency hearing? My hearing drops off quite a lot at 3k and I can't hear anything from 4k up. However, I do still have perfect hearing below that and am able to appreciate the difference with higher quality audio equipment.

Also, do you have any pictures of the Adcom after the Musical Design upgrades?
 
#75 ·
Let me dig for some pictures. There are no external changes aside from the Cardas rhodium input RCAs and IEC input.

The improvements are broadband for each upgrade step. I am very glad I chose the full boat.

I actually was using it with a Proceed AVP2 before upgrading to dual monoblock Benchmark AHB2 amplifiers (running each in bridged mono mode).

Prior to that, I was able to demo in my space several newer and more expensive amps against the Adcom, which it handily beat up on. It sounded most like a First Watt F5, but it actually has better better bass control and (obviously) a lot more power.

It took dropping $6k on these Benchmark amps to realize a net improvement without giving up ground in any area. And it’s not a huge delta, to be honest. Not an indictment of the AHB2 amps (they’re simply stellar and I’m very happy with them), but just high praise for the GFA-5500, which has some very good bones.

I have conversed with Nelson Pass himself via email and he hinted at the fact that the GFA-5500 was designed as a very close relative to the legendary Aleph 0 (Pass Labs first commercial product). Same topology. Same number of gain stages. Same input devices and design. Same output devices. Similar power supply section. Etc etc. I would call the ‘5500 a disproportionate share of the performance you’ll get from a pair of Aleph 0 amps. And that makes it a stunning deal.

So, I would humbly (especially in light of current corona virus events) suggest and recommend getting the full boat at Musical Concepts. The sweetness in the midrange, increased texture detail and even more holographic imaging are worth it in my humble opinion.


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#77 ·
Alan, I'd describe the sound like a faint record scratch sort of sound. It would happen with the RCA cable hooked up or not. Switching to a receiver to serve as an amp and using the same preamp, the noise was gone.

I called John at Musical Concepts today and he said the sound I described was not something he had ever encountered. His suggestion was the same as your's, Alan. I started with the intent of pulling some terminals off posts and putting them back on, and looking for bad caps but noticed a ton of dust under the cover. So, maybe the dust and a little humidity could cause some conductance somewhere it shouldn't be.. grasping as straws here. So, I decided I need to vacuum off all the boards and to do that, I had to tear both sides off.

I didn't see any obvious signs of cap failure. Didn't notice any dodgy crimps. But I DID notice an awful cold solder joint on R38. See pics below. :eek:

So I fed that joint some new solder, finished dusting, and put it back together. I've had it on for a couple of hours so far and no scratching record anymore. I'll leave it on over night and see what's happening with it tomorrow. I have a good feeling the issue has been fixed. Listening to it makes me appreciate the jump in sound quality from the old Harmon/Kardon receiver I was using in it's place!

So, from talking to John, it seems their mods mainly occur on the two amplifier boards and not the power supply board. I was thinking that all those caps would be replaced with their service but that's not the case. Is it advisable to go ahead and replace those 8 caps on the PS board before sending off for Mods? John indicated they don't often need to replace them.
 

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#78 · (Edited)
Oh, you open the amp!! I don't dare to suggest you to do that before. But since you have it open, have the amp on, use a wood stick ( chop stick for me!!) and tap the components and move the wires inside the amp and listen to any noise.

Yes, I don't think it's the cap at all, but I don't troubleshoot on too many amps. I design amps, all new components, so failure is something I don't see that often. But it just doesn't sound like it's leaky caps. My bet is loose connection. You touch up with solder, disconnect the wires and reconnect them again, there is a good chance you clear up the dirt on the contacts of the connectors.

It's helpful that you touch up the solder. Do the wooden stick test, jiggle the wires and report back.




I hope you did not remove the big transistors from the heatsink, you have to be very careful to grease the insulator to put it back. It's best not to tough them.
 
#80 · (Edited)
Since you guys are really getting into the circuit, I have something I want to mention. Now, don't do it unless you are going to take FULL RESPONSIBILITY if anything goes wrong or you injure yourself. This is NOT a technical forum, I don't know what knowledge level you guys have.

I attached the amplifier schematic that I circled the few Zener diode in question. I have experience with zener diodes that they have popping noise when I look at it with the oscilloscope. This is very common, this will definitely give you a popping sound. I usually put a cap across it to tame the noise in my design. I am not going to advice you to do that as I don't want to be responsible. But I will definitely look deeper particular Z4 and Z6 as shown.

I suspect one of those zener gets old and get noiser. What you can do is to heat them up with a hair dryer ( now don't stick onto the pcb and heat for a while, it can get really hot). Just a few seconds to make it a little warm will do. See whether it make it worst or better if it pops.


Or, you can freeze it and see. I use the canister duster, turn it up side down and spray, the freon will come out and freeze anything it spray on. Be careful don't spray on you skin, that can really hurt and do damage to your skin.


Bad filter caps usually don't give noise like this. You might get more hum or the lower frequency sound degrade some( questionable). I have experience with old filter caps that already looked old according to the oscilloscope. I wasted $120 buying new caps and put it in myself. It did NOT make a damn of a difference in the sound. To me, if the top is not swollen up or oozing things out, I won't change it.

I would adjust the bias of the output stage according to spec. ( I use hand test, don't follow me, but this is what I do. I adjust the bias up, wait 15 minutes and feel the heatsink with my hand. If I can grab onto it without feeling uncomfortably hot, I am good, but don't follow me, this is me).



Look for lose connection, tap the components with the wood stick, most likely you can find the problem. Then the Zener diodes. We talk more if it comes to that.


Again, don't do any of these unless you are willing to take full responsibility if anything goes wrong or you hurt yourself. What I said is very common practice of service technicians. You might have fix the problem by touching up the solder.
 

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#81 ·
Awesome thread! I just acquired the 5503 and 5500 to power my theater speakers. I opened them up to clean and I noticed something rattling around. I was trying to find what these copper bars came from and from the photos here they fell off and were acting as small heat sinks. Also I noticed the heat shrink sleeving slipped down exposing the tops. I haven't powered these on to test if I get any sound yet. I'm note quite familiar with electronics. What so you use to clean up the pcbs and parts?

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