• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

A few Adcom GFA 555 questions

mixsit

Member
Forum Donor
Joined
Jul 20, 2019
Messages
75
Likes
23
I've been running this original version of the amp since new -(1985 to 90 perhaps?) and wanted to ask a few questions.
First to it's continued safe use given it's age and concern for some very nice speakers I would hate to see harmed.
Second is simply might there be possible sonic improvements to be had from other amplifiers? I don't hold any particular faults with the amp, but nor have I used anything else in years, plus I wouldn't mind having a bit more headroom occasionally available for the larger of the speaker sets.
Thanks
 

Sal1950

Grand Contributor
The Chicago Crusher
Forum Donor
Joined
Mar 1, 2016
Messages
14,067
Likes
16,597
Location
Central Fl
I'm running 3 GFA545 II's and 2 535 II's in a multich system, none have failed over that last 2 years of heavy use. Internal inspection and cleaning of all revealed no repairs ever done, very minor drift of bias easily tweaked to spec.
Do to age you can be sure there's a chance of failure but I've seen lots of much, much newer amps let the smoke out.
You might look into having it recaped if you want to spend a few bucks.
I doubt you'll ever find anything audibly better in a DBT.
 
OP
M

mixsit

Member
Forum Donor
Joined
Jul 20, 2019
Messages
75
Likes
23
I did the 'bias drift check a few years ago on the output lugs to be ok -per some instructions I found at the time. (I don't recall the number, nor find the source I used at the time.) I assume some components degrade in time correct?
Perhaps one question I should ask; If amps like these do fail, how likely might it damage the speakers?
Thanks
 

Sal1950

Grand Contributor
The Chicago Crusher
Forum Donor
Joined
Mar 1, 2016
Messages
14,067
Likes
16,597
Location
Central Fl
Perhaps one question I should ask; If amps like these do fail, how likely might it damage the speakers?
Very, there's little protection outside the rail fuses.
 

SIY

Grand Contributor
Technical Expert
Joined
Apr 6, 2018
Messages
10,383
Likes
24,749
Location
Alfred, NY
I did the 'bias drift check a few years ago on the output lugs to be ok -per some instructions I found at the time. (I don't recall the number, nor find the source I used at the time.) I assume some components degrade in time correct?
Perhaps one question I should ask; If amps like these do fail, how likely might it damage the speakers?
Thanks

Did you measure the bias by measuring the voltage across the emitter resistors?
 
OP
M

mixsit

Member
Forum Donor
Joined
Jul 20, 2019
Messages
75
Likes
23
Did you measure the bias by measuring the voltage across the emitter resistors?
No, it was mVolts at the speaker terminals with the speakers disconnected.
add.. My recent unsuccessful search to find the source I used I came across some things that were similar and perhaps incorrectly picked up the term 'bias check.
 
Last edited:

Doodski

Grand Contributor
Forum Donor
Joined
Dec 9, 2019
Messages
20,745
Likes
20,756
Location
Canada
No, it was mVolts at the speaker terminals with the speakers disconnected.
Then you measured the, "Idle/offset voltage." That's very different than the, "Bias Adjustment." You need a service manual so you can see where and how to set the bias and the idle voltages.

WARNING: The power supply has some rather large capacitors that can cause damage to yourself in the form of electrocution or even broken bones from the shock. So you need to be veryyy careful you don't touch those caps terminals while you are working on the amp.

Service manual>
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Adcom-GFA-555-Service-Manual.pdf
 
OP
M

mixsit

Member
Forum Donor
Joined
Jul 20, 2019
Messages
75
Likes
23
I wouldn't feel competent enough to do internal tests. Perhaps I should skip to the question; Should the amp, this age, be shipped to someone for refurbishing?
There is one other consideration or option. I have two sets of SP Tech speakers. What I do now for one or the other is swap the speaker line pairs at the amp. I mentioned upping the amp size. I could do that and reserve the Adcom for the smaller set once it's been serviced.
 

Doodski

Grand Contributor
Forum Donor
Joined
Dec 9, 2019
Messages
20,745
Likes
20,756
Location
Canada
I wouldn't feel competent enough to do internal tests. Perhaps I should skip to the question; Should the amp, this age, be shipped to someone for refurbishing?
There is one other consideration or option. I have two sets of SP Tech speakers. What I do now for one or the other is swap the speaker line pairs at the amp. I mentioned upping the amp size. I could do that and reserve the Adcom for the smaller set once it's been serviced.
It is a nice amp. Worthy of a proper recalibration and torture test. I'm not sure what refurbishing would entail but I see big bucks for handfuls of capacitors and labour. It might not even need parts replacement. If you are feeling like a DIY project do it yourself. It's not rocket science. Having a proper sized soldering iron, solder, solder wick and a solder sucker tool from Kester. Just detail all the electrolytic caps that are used, order them in a high quality spec/brand and replace them one at a time. Take a couple days/afternoon and voila you'll have refurb'd it. Otherwise find a decent tech and pay.
 
OP
M

mixsit

Member
Forum Donor
Joined
Jul 20, 2019
Messages
75
Likes
23
There is a fellow named Jim Williams of Audio Upgrades that has been doing services for various pro audio gear including some Adcom amp upgrades. He may be closer to retirement these days(?) but I believe I'll give him a call and ask.
I appreciate all the folks here helping me out by the way. :>)
 

Sal1950

Grand Contributor
The Chicago Crusher
Forum Donor
Joined
Mar 1, 2016
Messages
14,067
Likes
16,597
Location
Central Fl

Kw6

Active Member
Joined
Dec 23, 2019
Messages
143
Likes
44
I have one from the same era ('85). All it's ever done is work. You can find modern amps that have better measurements, but they won't sound any different.

I'm on a budget and wondering if the new GFA 555SE can sound like the high end amps like from Pass and Sim audio? Thanks
 

Sal1950

Grand Contributor
The Chicago Crusher
Forum Donor
Joined
Mar 1, 2016
Messages
14,067
Likes
16,597
Location
Central Fl
I'm on a budget and wondering if the new GFA 555SE can sound like the high end amps like from Pass and Sim audio? Thanks
Humm, that's a hard question to answer. I'm not sure how closely the design follows the original Nelson Pass 555's design, etc. Of course everything you can read about the new ones is marketing hype, etc. MHO of modern Pass amps is that he has attempted to voice them in various ways, where in the older designs he leaned more towards accuracy. All other things being equal, personally I believe I'd prefer the older designs if anything.
If budgeting is tight, I just looked around and The Music Room has one for sale on ebay, $829 + 60 shipping. I've dealt with them before, good people. A lot of amp for the money
Spoiler-Bias Alert, I run 5 Adcom GFA amps in my multich rig, I'm addicted to them. ;)
 

Kw6

Active Member
Joined
Dec 23, 2019
Messages
143
Likes
44
Humm, that's a hard question to answer. I'm not sure how closely the design follows the original Nelson Pass 555's design, etc. Of course everything you can read about the new ones is marketing hype, etc. MHO of modern Pass amps is that he has attempted to voice them in various ways, where in the older designs he leaned more towards accuracy. All other things being equal, personally I believe I'd prefer the older designs if anything.
If budgeting is tight, I just looked around and The Music Room has one for sale on ebay, $829 + 60 shipping. I've dealt with them before, good people. A lot of amp for the money
Spoiler-Bias Alert, I run 5 Adcom GFA amps in my multich rig, I'm addicted to them. ;)
Ok thanks! My top two choices right now are Adcom gfa-565se or Van Alstine Set 400!
 

Bruce Morgen

Addicted to Fun and Learning
Forum Donor
Joined
Aug 3, 2019
Messages
864
Likes
1,237
I owned an original GFA-535 -- the subsequent versions were devised without the participation of the venerated Mr. Pass -- that I used with an SAE 2900 preamp. I positively swore by that setup for many years -- but, to be perfectly honest, the (several) sub-$50(+PSU) Aiyima A07s I now have sound every bit as good driving the same vintage JBLs. Adcom power amps were a great value back in the day, but times have changed and IMO there's no longer anything special about their performance -- but the bigger ones sure look imposing! :cool:

Unfortunately, they're notoriously hard to work on when they malfunction. :(

First look at a failed Adcom amp by a veteran tech
 

Sal1950

Grand Contributor
The Chicago Crusher
Forum Donor
Joined
Mar 1, 2016
Messages
14,067
Likes
16,597
Location
Central Fl
Unfortunately, they're notoriously hard to work on when they malfunction.
Don't know about that but I will say I'm a bit squeamish on the later lines of Adcom amps. I can't say exactly why, but there were some issues like the failed power supply caps on the 565s IIRC, etc I stick with the 535-545-555 lines since they seem to last forever and measure great..
Ok thanks! My top two choices right now are Adcom gfa-565se or Van Alstine Set 400!
OK, another amp you might want to consider in that range is the Parasound A23+, strong factory backing when bought new with high reliability factor.
 

Sal1950

Grand Contributor
The Chicago Crusher
Forum Donor
Joined
Mar 1, 2016
Messages
14,067
Likes
16,597
Location
Central Fl
I'm on a budget
You kind of threw me a curve when you started talking about a "budget, and low cost Adcoms" but now we're in the $2k range. Not to be forgotten at this level is the new NAD C 298. Reviewed by our friend @Kal Rubinson and measured by John Atkinson at Stereophile, this looks to be an amp to beat at this price level. A Class D Bruno Putzey / Purifi based amp, JA measured as delivering 275w into 8 ohms, both channels driven at 1% distortion.!
John closes the review with this,
"Going back to the days when NAD's amplifiers were designed by the late Björn Erik Edvardsen, I have always been impressed by the company's conservative and competent engineering. The NAD C 298 continues that tradition but, with its "Eigentakt" class-D output modules, sets a new standard for combining very high power with supremely low distortion.—John Atkinson"
https://www.stereophile.com/content/nad-c-298-power-amplifier-measurements

Don't overlook this one,
Sal
 
Top Bottom