Adcom GFA-555

fredv

Hi,

I am considering this:
http://www.craigslist.org/sby/ele/87448465.html

Do you mind to give me some opinions on this amp? Also, please let me know the major difference betweem the 555 and 555II?

Thanks,

aaharvel

omg. if you bridge those two amps then we are talking A SHITLOAD OF POWER!

here's a review of these amplifiers from Stereophile back in the day. It seems like a good deal- Power to $ ratio I wouldn't think you could do much better than that, but I could be wrong.

http://www.stereophile.com/amplificationreviews/678/

Spawndn72

That is a good price, like he said, fair. I love my 545. I am sure the 555 is just as good, just twice the power.

heiney9

A couple quotes 1 from Nelson Pass renowned amp desinger who built the original 555 prototype for Adcom. It's been said that as time went on Adcom moved farther away from HIS original design and it became a hybrid of Adcom's in house design team and Nelson's original design.

The 2nd quote is from a tech who repaired/repairs audio equip for a living and has seen many many Adcoms over the years.

I designed the 555. I believe the Mk II added a couple
of what were considered improvements, and it seemed
to me that it included triple darlington outputs instead
of 2 follower. I might have the schematic around, but
all told it was a simple bipolar design with a current
sourced input diff pair followed by a common-emitter
gain device that was also current sourced. This drove
the followers.

It was typical of all the 5XXX amps that I did the first one
and then they improved it until it had too many parts,
and then I would design the next in the series

Americo Borza sent me an old 555 in the hopes that I
would update it, and it still sits here, a perfect candidate
since so many of them were built.

Personally, I would score a few of these and run them till
they die and then update them. If you wait long enough,
an update will be issued.


Last edited by Nelson Pass on 08-16-2002 at 11:10 PM

Written by Nelson himself. I believe Americo Borza was the President of Adcom at the time.





The 555Mk.II was similar to the Pass designed 555 except for the drivers. In the Mk.II Adcom used beefier drivers i.e. 2sb817/2sd1047. These were 130 watt/ 140v devices. Also they used a 33 ohm resistance across the driver emitters. The heatsinks were much more substantial than the original and construction overall much better.

The 585 and 565 were similar to each other but radically different topologies that the 535/545/555's.

They still used the Toshiba outputs but changed the drivers to 2sc3907/2sa1516 (???) pair which was a 180v device...

I always liked my 555's with lessor NFB than stock. Instead of the 22k NFB resistance, i used 30 or 33kohm and got a sweeter sounding amp..

From a person who repairs Adcom among others for a living.

FWIW, there is very little difference between the 2. The triple Darlington outputs were used to gain more stability at extremely low impedence loads (around 1-2 ohms). They are also said to lower either THD levels which were already low enough in the original 555. Most people who have had multiple experience w/Adcom think the original series sounds better....who knows. This is probably much more info than you expected or needed...but I'm bored so there you have it!!:p

H9

heiney9

Crap, got so wrapped up in my prev post I forgot to comment.

Not sure what you're going to use this with but you can't go wrong with a well maintained GFA series amp from Adcom. I used to sell them; I own one; and have friends who own them. Very musical and solid. Adcom has kind of fallen by the wayside these days. If it HAS been abused chances are it's still ok...they can take a lot...and can still be repaired for a reasonable amount.

You can buy 2 and bridge each one for mono use, but if you have SDA's this will cause a problem...no more common ground amp when bridged. Something to keep in mind.

What set-up are you going to run this in?

H9

fredv

Thanks for the info, sent email to the seller, let's see I have the first dibs or not

aaharvel

Originally posted by heiney9
Crap, got so wrapped up in my prev post I forgot to comment.

Not sure what you're going to use this with but you can't go wrong with a well maintained GFA series amp from Adcom. I used to sell them; I own one; and have friends who own them. Very musical and solid. Adcom has kind of fallen by the wayside these days. If it HAS been abused chances are it's still ok...they can take a lot...and can still be repaired for a reasonable amount.

You can buy 2 and bridge each one for mono use, but if you have SDA's this will cause a problem...no more common ground amp when bridged. Something to keep in mind.

What set-up are you going to run this in?

H9

excellent advice Heiney, as always.

SCompRacer

Originally posted by heiney9


You can buy 2 and bridge each one for mono use, but if you have SDA's this will cause a problem...no more common ground amp when bridged. Something to keep in mind.

In cases like that, I just reach for my Polk Non-Common Ground Amplifier Interface. Or you'd just have to make one.

The guy I bought my SRS 2's from had a pair of GFA-555's in bridged mono. He sold me one of the Adcoms but I found that I preferred Carver's magnetic field juju instead.

EDIT: BTW, those are very fair prices. I paid $350 for mine in primo condition and sold it for $400.

fredv

I have a SDA SRS 2.3 and plan to bi-amp with the adcom's because I don't have the AI cable. SCompSpacer, (understood that everyone has his/her own preference) why do you like Carver more?

SCompRacer

Hi fredv. Your right about his/her own preference. The Adcom GFA-555 sounded grainy to me. Now I tried a mosfet Adcom and liked it better. Could be I'm not a bipolar amp type of person. I'm not ecstatic about the TFM-35X either, going to try tubes.

I'd jump on the pair of Adcoms for that price and try them. In good condition, you wouldn't lose and would most likely gain if you resold them.

aaharvel

=Now I tried a mosfet Adcom and liked it better. Could be I'm not a bipolar amp type of person.[/B]

amen. i love my 5300 WHICH is a MOSFET design. perfect for my stereo rti4's for critical 2ch. listening. My bi-polar 7300 works great with the center, sides, and rear speakers for home theater however.

marker

Even though I now like NAD better, probably Adcom's single biggest competitor, my first set of true seperates were Adcoms, and the GTP-400 tuner/pre and GFA-535L power amp still work to this day about 15 years later. They will still best any new stereo 2 channel receiver like those from Onkyo, Denon, HK, etc. too. The Adcom GCD-575 CD player long ago bit the dust however.

I have nothing but respect for Adcom, and fully endorse them, especially at their price points, both new and used.

george daniel

Can anyone point me in the right direction/person for adcom modifications,have 2 555's and have read about mods, but have not heard any @ this point and I am somewhat reluctent to spend alot of money. Thanks in advance.

fredv

I was late to ask the seller, not even close to 2nd dibs. However, thanks to everyone providing me such valuable info. I am pursuing a Carver TFM-55x now. This should be on par of the GFA-555.

heiney9

george daniel wrote:

Can anyone point me in the right direction/person for adcom modifications,have 2 555's and have read about mods, but have not heard any @ this point and I am somewhat reluctent to spend alot of money. Thanks in advance.

My best advice is to visit this forum

www.diyaudio.com

Search some of the old threads in both the solid state amp area and the Pass labs area. After that maybe post a thread about mods and I'm sure you'll get some dialog going. Only mods I've ever seen are Stan Warren mods, however I've never heard one of his pieces. There were a few of his modded pieces FS on Audiogon recently maybe you can get some info from there. I know there are lots of mods that can be made and since the original series is such an uncluttered design anyone with decent soldering skills should be able to do it no problem. If you search out there I'm sure you'll find some info. Problem is the info is older so it's a bit harder to find.

I know many like to mess with the bias current to get a better sound, this can be tricky w/o the right equipment. There are some shops that say they do mods to all kinds of equipment. Maybe start w/ your local repair shop and pick their brain a bit.

Hope this helps.

H9

SCompRacer

Here are a couple of the threads at DIYAudio that heiney referred to. I had them bookmarked cause I was thinking of doing some mods to my 555 before I sold it. Though the one thread is about the 555II, there are references to the 555 mixed in.

http://www.diyaudio.com/forums/showthread.php?threadid=13695&highlight=

http://www.diyaudio.com/forums/showthread.php?threadid=5280&highlight=adcom

Musical Design is one place that performs mods to Adcom amps but it's costly.

I am pursuing a Carver TFM-55x now. This should be on par of the GFA-555.

The 55X is a powerful amp. Specs are 380 WPC @ 8 ohms and 500 WPC @ 4 ohms. IIRC, it can pull around 1500 watts at max power so make sure you have an ample power circuit for it.

I borrowed a second Carver so I have pair of TFM-35X's in bridged mono on my SDA SRS 2's now due to the 55X's going for way more than I want to spend on the bay. In all honesty, one was plenty for normal listening.