Hello! New guy with ADCOM questions..

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MisterDK

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#1
Hey folks, I'm Duane...and I just bought an old Adcom GFA-555.. I think.

Wanted one of these back in the late '80s, but never had the $ to pick it up back then; marriage and 3 kids later.. Recently got a wild hair (hare?) and dug out my old Rega Planar 3 w/Sumiko Blue Point (original one) and set it up with an old thrift store Yamaha CR-800 and some cheap thrift store speakers... It's now a couple weeks later, and I want to get some of the "budget audiophile" stuff that I wanted when I bought that Rega, some 30 years ago.

I have a few questions about components and such, but I want to start off with a question about this Adcom power amp I just bought.. It's marked a GFA-555, but this one looks a little different than others I see.

My first question is, could this just be a very early GFA-555? Serial is only 4 digit, stereo/bridged switch is a toggle not a slide, and faceplate doesn't have any ribs. Is this thing the real deal or is it someone's clone..?
It sounded good to me when demo'd with some other components the seller had and the price seemed fair...so I bought it. (Craigslist)

What did I buy? Is it simply an early version of the 555?
 

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#3
chrisng said:
Yes, it simply an early version of the 555!

https://picclick.com/Adcom-GFA-555-Stereo-Power-Amplifier-Clean-Original-Amp-182526790591.html
Click to expand...

Thanks!

Are these early ones any better or worse than later examples of the 555? Assuming that it may never have been serviced, what might I expect to repair or replace on this old guy?

Another question.. Was the GFP-555 a decent preamp to match up with this GFA, or might I be happier with some other preamp? Might tube preamps sound good with this GFA-555?

I'm not even sure what I want for speakers yet; just have a price range of under $1K in mind for them. Plan on listening to records, and mostly '60s-'70s rock, some '50s-'60s jazz.. Room size is fairly large.

I apologize if these questions really don't belong here, but appreciate any advice and suggestions. 🙂
 
#4
MisterDK said:
Are these early ones any better or worse than later examples of the 555? Assuming that it may never have been serviced, what might I expect to repair or replace on this old guy?

Another question.. Was the GFP-555 a decent preamp to match up with this GFA, or might I be happier with some other preamp? Might tube preamps sound good with this GFA-555?
Click to expand...

It's possible that you might consider replacing the electrolytic caps, but unless
there's a lot of noise, then there's no hurry. Enjoy it for a while.

GFP-555 is decent enough, but there's plenty of preamps out there, and
tubes are a good option. Maybe some older ARC or CJ models would
keep you in the classic motif.
 
#6
Post a few pics of the innards and I could tell you if its the real deal. Seen many of them. This version might be one of the ones Nelson had a hand in. Later versions were in house Adcom mods including dc servos and beefier driver devices...
 
#7
I would consider replacing the input coupling cap and see if you like it any better along with dc block cap in the feedback circuit. These might have audible impacts after ageing. I remember having a new 555 in 1990, it sounded so nice.. but had to sell it. about 15 years later bought another one, it sounded harsh... I surmised the caps probably aged. It had more of a glare and was noisier too..Nelson is right, consider a cap job at somepoint . hope this helps.
 
#8
Yep, an early rack-mount model. Some came with fans and sold as PA amps. It's better than the later ones in some ways. The sheet metal, as you may have noticed, is much thicker. Some of these have annoying circuit boards with no silkscreen on them to label the components. Some are missing the poly cap that bypasses the 47uF cap from the negative side of the LTP input feedback to ground. I simply solder one underneath the board. Also, some early models use older transistor models, I forget exactly what numbers, but I think they are MPSAxx, and have way lower gain and performance than the newer 2SC2363 and 2SC2240 that most 555's use. I replace with Fairchild 2SC1845.

One bad thing about the older rack-mount models, is that they used 80V power supply capacitors in some of them, and the rail voltage is 83V! These caps are getting ancient and should be replaced anyway.

I've generally found that the matching of parts is much better in the early Adcoms. It was a small company back then.
 
#9
Phloodpants said:
Yep, an early rack-mount model. Some came with fans and sold as PA amps. It's better than the later ones in some ways...I've generally found that the matching of parts is much better in the early Adcoms. It was a small company back then.
Click to expand...

Thanks, Chris.

It was sounding good 'till about a week ago. I tried out a different pre-amp, and turned the volume up a bit to "hear how clear" it sounded... From that point, the 555 developed a nice hum. Not sure what I did to it, so went and bought another 555, only a somewhat later one. You're absolutely right about sheet metal thickness on these; that early one seems much heavier duty.

I'm curious; what sort of backlog do you have on repairing & upgrading these?
 
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