Differance between Adcom GFA 555 and 555 ll

Dexter

Planning on purchasing GFA 555 and notice there's a 555 ll also. Can some one please explain the difference between them.

Thank you

venomclan

Where is Ben? He is our Adcom expert.
V

zombie boy 2000

As would be our very own Heiney9.

SLOCOOKN

I have the 555 II. What I read was that the 555 II sound was less grainy. They are both well built amps for the money.

Ricardo

Maybe I'm wrong, but I think I heard that the 555 II was not common ground??? This would be important only if you plan to use it with SDA's.

Edit: I see slocookn has it with a 3.1TL, so I must be wrong.

heiney9

Series II had coupling capacitors for the input to output rather than being direct coupled (coupling caps can get weak and go bad over time)

Series II had larger heat sinks as well a bit more complicated Darlington output stage whose only advantage was allowing it to be a bit more stable driving extremely low impedence loads (less than 2 ohms) which is not typical of traditional speakers (think ribbons or electrostatic).

Series II also had a thermal protection indicator on the front panel. The iput terminals were farther apart and used nicer Cardas type connectors.

I also recall reading the series II had a slightly beefier transformer.

IMO, the biggest weakness to the Series II as a used unit today (some 15-20 years later) are the coupling caps. They were electrolytic and by now may be weak and not functioning to spec.

I also believe (not 100%) sure the Series II has dc servo's for correcting dc offset. Series I mayhave had those as well, but I can't remember of hand.

IMO, I prefer the series I amps, but I have no real scientific basis it's just my preference.

The later 5xxx series are completely different as they use Mosfet instead of bi polar out put tranny's and from the 5500 up are non-common ground so can pose problems with some SDA's. I believe the neg speaker terminals can be connected together to make them common ground amps.

Use the search feature as I believe Darqueknight and others has had some posts as well.

H9

heiney9

HTrookie wrote: »

Maybe I'm wrong, but I think I heard that the 555 II was not common ground??? This would be important only if you plan to use it with SDA's.

Edit: I see slocookn has it with a 3.1TL, so I must be wrong.

555's are common ground

Ricardo

Hey...don't quote me until I'm finished with spell check.....:)

heiney9

Also search the Stereophile site (archives) and follow the links. They did a review on the original 555 and followed up with a review on the 555 II when it came out. Sort of did a compare and contrast of the 2 units.

H9

heiney9

HTrookie wrote: »

Hey...don't quote me until I'm finished with spell check.....:)

Is that better ?

Ricardo

Thanks!

heiney9

For the lazy people

John Atkinson wrote about the Adcom GFA-555 Mk.II in October 1990 (Vol.13 No.10):

Introduced in 1985, the original GFA-555 was the subject of a rave review in Vol.8 No.4 from the patrician Anthony H. Cordesman, who felt that its natural presentation of upper-octave detail and its "extraordinarily holographic" imaging might well make it a "legitimate rival" to the Krells and Audio Researches of this world. Three issues later, in Vol.8 No.7, the venerable JGH pretty much agreed that the '555 was something special, rivaling his then reference, the Electron Kinetics Eagle 2.

Personally, I felt the '555's high frequencies to be somewhat emphasized, perhaps even grainy, which for me somewhat offset its superbly defined, massively weighty low frequencies, its excellent dynamics, and its wide, deep, and well-focused soundstage. It would also mean that the '555 required rather more care in the choice of matching source components and loudspeakers than AHC's original review would suggest. Nevertheless, the 200Wpc Adcom, priced at an eminently affordable $600, was superb value and went on to become one of the best-selling amplifiers of all time. Two Stereophile reviewers, TJN and RH, used '555s for much of their listening; and GL was also impressed by its virtues when he reviewed an early 1989 sample in Vol.12 No.12.

At the 1990 SCES, Adcom finally announced a revised version of the '555 to sell for $800, the GFA-555 II. Before continuing the amplifier's inclusion in Stereophile's "Recommended Components," therefore, I thought it a good idea to compare the new with the old (in this case, the same sample we had purchased for GL's 1989 review).

Superficially, the new '555 appears identical to the old. A closer look, however, reveals a greater number of cooling vents in the case and a cooling fan lying just under the top plate. (This fan, a $100 option, is triggered by heatsink temperature and didn't turn on during my auditioning, where average output levels were generally below 8V RMS into the 6 ohm Avalons—just under 11W.) There is now a red LED on the front panel labeled "Thermal Overload," a greater number of fuses between the rear-mounted, curved-edge heatsinks, and the input sockets are now much higher quality and use Teflon insulation. The power transfomer has been increased in size, potted to improve heat transfer, and offers greater regulation. The output circuitry has also been changed, and the LF signal path has been stripped of polarized (electrolytic) capacitors. A new DC-servo circuit minimizes output DC offset.

Both amplifiers are polarity-correct, but the Mk.II version has slightly higher gain—27.75dB vs 26.9dB at 1kHz when loaded by the Avalon speakers. The 0.85dB difference was compensated for during the auditioning. Each amplifier was fed from a Mod Squad Line Drive Deluxe AGT via 1m of AudioQuest Lapis interconnect. Source was exclusively CD from a Meridian 208 CD player/preamp, connected to the Line Drive via 10' of Audio Research interconnect. Loudspeakers were Avalon Eclipses, bi-wired via 8' lengths of Mission spaced-conductor cable. (The fact that the Adcom's output binding posts are placed between the heatsink fins meant that there wasn't sufficient clearance to use the bi-wired sets of AudioQuest Clear that I used for the other follow-up reviews I carried out for this issue.)

The first difference wasn't hard to spot. After three hours' warmup, the heatsinks of the '555 II were appreciably hotter than that of the Mk.I, suggesting a higher degree of output-device bias current. The first track I played was Drew Minter singing Handel's "Va Tacito," from the Harmonia Mundi USA CD Arias for Senesino (HMC 905183). While I felt the older amplifier's high frequencies to be still a little grainy, it drove the Avalon speakers with a degree of authority that was musically satisfying. The voice was a little smaller-sounding than via the Audio Research Classic 60, which has taken pride of place in my system. Nevertheless, the soundstage was well-defined,the low frequencies weighty, and the music communicated well. Switching to the Mk.II '555 revealed an even deeper soundstage—the accompanying French horn could be heard to be set further behind Mr. Minter—while the voice was more robust in its midrange tonality.

Turning to my recording of Anna-Maria Stanczyk playing Chopin on the Stereophile Test CD, the same differences manifest themselves. Via the original '555, the sound of the Steinway was a little lightweight, even "tinkly" in its upper registers, while the Mk.II '555 presented it with what I feel to be a more natural tonal quality. This is not to say the new amplifier is dark-sounding; instead, it has what I feel to be significantly less grainy high frequencies. It's fair to point out, however, that many listeners unfamiliar with the subtleties of live piano sound will prefer the original '555 as being more clear, more vivid.

The new amplifier again threw a better sense of depth than the old. Anna's Steinway was set further behind the plane of the loudspeakers, despite its now having a more robust, more forward midrange tonality.

Both amplifiers sounded pretty well equivalent in the bass with these kinds of music. I therefore reached for something with a bit more low-frequency oomph: Jeff Beck's Guitar Shop album (Epic EK 44313), which has some killer drum sounds. Here, the GFA-555 Mk.II had as good an LF extension apparent, but its upper and midbass registers were, depending on your tastes, better defined/less weighty than its Mk.I incarnation. Both, however, were more than adequate in presenting Terry Bozzio's thunderclap drum sound with more than sufficient weight and dynamic impact.

To sum up, I feel the GFA-555 Mk.II to be significantly more neutral as regards midrange and treble tonal quality than the Mk.I, while preserving its virtues: excellent imaging specificity, a deep, well-defined bass, and a superb sense of dynamics. Currently ranked in Class C of Stereophile's "Recommended Components" listing, the Mk.II '555 could well deserve an even higher rating. I have passed it on, therefore, to Tom Norton for a more detailed evaluation.—John Atkinson

engtaz

heiney9
As usual you come thru with flying color. Thanks for the valuable info. This is why I and other look to you for Adcom advice.

engtaz

ben62670

venomclan wrote: »

Where is Ben? He is our Adcom expert.
V

H9 has been my teacher either directly, or indirectly leading me to different sources for research, and help.:)

Thanks H9;)

venomclan

ben62670 wrote: »

H9 has been my teacher either directly, or indirectly leading me to different sources for research, and help.:)

Thanks H9;)

Oh no, you have been dethroned!

dudeinaroom

I can not remember the site, but it was a DIY site that Nelson Pass(designer of the 555 and others) is a member of and had the differences laid out. He said that Adcom mucked it up (in short) and should have left it alone (if I remember correctly)

Lowell_M

H9 is the ADCOM Master!!! He has become extremely helpful to me in helping me learn the ways of the audiophile. ...and exactly the reason why I have a GFA-555 in my system now. I can't say enough good, and can't find any bad about that amp. I really can't believe what it can do with those RTi70's. There are a couple seemingly good buys on the 'gon right now.

I'm hoping to get H9's critique of my new DAC setup along with the GFA-555 with my HK as a pre soon!!! (We'll probably have to hook up the Nak CA5 also)

Anyway, listen to H9. He is wise. In my own personal opinion, I haven't heard the mkII, but you can't go wrong with the prices that you can get a good MKI at. (~$300)

... All hail his Heininess.......

heiney9

dudeinaroom wrote: »

I can not remember the site, but it was a DIY site that Nelson Pass(designer of the 555 and others) is a member of and had the differences laid out. He said that Adcom mucked it up (in short) and should have left it alone (if I remember correctly)

There is some truth to that. They especially (according to NP) really took the 555 II beyond his initial simpler more straight forward design. He basically said they made it more complicated than neccessary.

All the original Adcoms were much closer to the original design he did for them. You really have to do some digging (mostly reading) at the Pass Labs forum at DIYAUDIO.COM to get really good insight. NP tends to be a man of few words in most cases. Kind of like that one teacher you ahd in school who would never give you the answer but point you in the right direction.

heiney9

Come on guys you are embarrassing me. I'm certainly not the most knowledgeable (I do read a lot) and Adcom certainly isn't the be all end all of amplifiers. They certainly are worth their money and in many cases you'd have to spend a lot more to get better and even then in some cases it's only slightly better. Until I can afford/decide to spend $2000 or more on a replacement I'll keep my Adcom. 21 years and counting

As always YMMV

H9

Dexter

Thank you H9

John in MA

If you don't mind me jumping here, I had a couple 555 questions. I'm looking at one for sale locally. It's a GFA-555 rackmount version, with the smooth face and rack handles. Two things I'm wondering about:

1. Each pair of speaker terminals are vertically aligned (one above the other) but all the 555's I've seen pictures of have them horizontally. Any idea if the one I'm looking at is very early or very late? Any particular reason I should avoid or grab it?

2. Do the rackmount models differ internally at all?

Thanks

Lowell_M

John in MA wrote: »

If you don't mind me jumping here, I had a couple 555 questions. I'm looking at one for sale locally. It's a GFA-555 rackmount version, with the smooth face and rack handles. Two things I'm wondering about:

1. Each pair of speaker terminals are vertically aligned (one above the other) but all the 555's I've seen pictures of have them horizontally. Any idea if the one I'm looking at is very early or very late? Any particular reason I should avoid or grab it?

2. Do the rackmount models differ internally at all?

Thanks

You looking at the rackmount on Audiogon? I've wondered the same thing because that one has been on there for a pretty decent price.

John in MA

Not the one on Audiogon, but almost identical. The posts are in the same place but they have more ribs.

heiney9

The ones with the smooth faces are the very earliest prodcution runs of the 555. I am almost certain they used the older style cabinet (like the GFA 1 and GFA 2 amps) but it had the 555 electronics. The original GFA1 and GFA 2 are much different than the 555. There may have been a few small tweaks to the 555's with the ribbed face, but I doubt it. Merely a cosmetic issue, however the smooth faced units are the earliest units produced.

Rack mounts are/were avail for both styles not sure if they are interchangeable. That's really all I know for sure

H9

John in MA

Thanks, the only rackmount 555s I've seen have smooth faces, regardless of back panel design. The face itself has the rack holes and handles.

I don't think a GFA-1 cabinet has much in common with any GFA-555. My GFA-1 is a little cube the size of a large shoebox, the 555 is a full-size amplifier.

heiney9

I think the rack mounts on the smooth faced units were built in and on the later ribbed faces units they were removable. I’ve never seen a GFA 1 or 2 in person didn’t realize they were that short. I was more referring to the cabinet style rather than hard dimensions. Good to know.

dee1949

http://washingtondc.craigslist.org/nva/ele/474686195.html

also includes an Adcom Line conditioner.

enjoy the music

ben62670

dee1949 wrote: »

http://washingtondc.craigslist.org/nva/ele/474686195.html

also includes an Adcom Line conditioner.

enjoy the music

Run, don't walk for that one:eek:

torchum

I bought a GFA1 a few years ago with intentions of using it for a sub. I bought a whole 5.1 Swan set up and have not done a darn thing to the Adcom except break off one side of the still attached handle. Other than the handle it is in super shape.I hate for it to not to be utilized and would like to sell it to someone that might appreciate it for what it is. Problem is that I do not know what a fair and respectable price would be. I know that it is not made of Gold but I just don't want to give it away either. Any info would be appreciated.

lightman1

Uhh-ooooh!!

torchum

I was hoping to read something a little more helpful and less gutteral. Not sure what kind of response that was meant to be. I thought H9 might be the person to ask but maybe Tarzan could translate the last post.