Running Adcom GFA-555

cnh · March 2009

Hi,

I'm picking up an Adcom GFA-555 from Audiogon on the cheap...god there are a ton of Adcom amps for sale these days? I was reading reviews of this unit and a number of owners say that the large caps bleed when you shut it down...meaning that they continue to discharge and play for a while when powering down..no big deal there.

But some people decide to leave them on 24/7 (which aside from wasting a lot of energy) they seem to prefer; claimning that it keeps those caps, etc. fully charged and is less stressful?

Does this make any sense to anyone here? Because I don't see any problem with powering up and down?

thanks,

cnh

ben62670 · March 2009

I leave mine on 24/7 they draw little power at idle.
http://www.adcom.com/manuals.aspx
A little food for the head:)

heiney9 · March 2009

cnh wrote: »

But some people decide to leave them on 24/7 (which aside from wasting a lot of energy) they seem to prefer; claimning that it keeps those caps, etc. fully charged and is less stressful?

Does this make any sense to anyone here? Because I don't see any problem with powering up and down?

thanks,

cnh

That can be said for ANY piece of electronics. If it is possible and you have no worries.....leave it on 24/7. I don't because of an outside chance something might 'eff up. Ask Joe Parski about that. The stress from turning off/on is negligible, but it could extend life a bit. Certainly nothing to loose sleep over.

The fact is electrolytic caps have a life expectancy of 12-15 years on avg. Will leaving them on all the time significantly extend life......probably not. I'd worry more about the power switch wearing out than the caps.

cnh · March 2009

Thanks to Ben and Heiney9

I just downloaded and read through the manual. Will probably leave the amp on once its set up. Am picking it up in person Monday or Tuesday..will demo at that time. Seller looks like a nice guy, he's upgrading.

cnh

ben62670 · March 2009

Are you anywhere near me?

cnh · March 2009

ben62670 wrote: »

Are you anywhere near me?

No I'm in Maine (see state motto: the way life should be for my location) and I'm driving to visit my sister in Pa. The seller lives in Pa. so it's a wonderful coincidence that allows me to make the pick up. A good friend of mine, my best friend moved to Atlanta a couple of years ago so I do get down there once in a while...

cnh

ben62670 · March 2009

Stop in some time when you are down here. Bring your new amp. A recap of a 555 takes no time at all:)

Systems · March 2009

ben62670 wrote: »

A recap of a 555 takes no time at all:)

How about a full on hi current regulator for the output stage?:eek:I can supply a schematic.

vijayl · March 2009

cnh, If I can ask how much did u pay 555? I'm looking for one as well..

Lowell_M · March 2009

Its funny... I leave my GFA-7000 on all the time in the HT because everything else is remote controlled (shut up H9), but I ALWAYS turn off my GFA-555 in my living room off when I'm done because I see the red light on the front and think "man I bet that uses a lot of electricity keeping those big caps charged." I guessing the 7000 probably uses more power at idle.

Lowell_M · March 2009

vijayl wrote: »

cnh, If I can ask how much did u pay 555? I'm looking for one as well..

Gen I 555's go for $300ish. 555MKII's usually a little more... maybe $350.

Systems · March 2009

Its 24/7 for me,Im triamping using Class A/B and D .If I were using Class A it/they would be shut off when not in use.

cnh · March 2009

ben62670 wrote: »

Stop in some time when you are down here. Bring your new amp. A recap of a 555 takes no time at all:)

Thanks Ben,

I'll be sure to do that. I'm sure it could use some fine tuning.

cnh

cnh · March 2009

vijayl wrote: »

cnh, If I can ask how much did u pay 555? I'm looking for one as well..

I got it for about 300 roughly. Seller came down a bit because I said I would pick up in person. Nice guy...he's even going to Demo it. He used it to power some very hungry JBL 4425s (I think).

cnh

cnh · March 2009

GV#27 wrote: »

How about a full on hi current regulator for the output stage?:eek:I can supply a schematic.

Sounds good I'd have to pass that to Ben above though.

Thanks you've all been great. I spent a lot of time before making my move. This site has been an invaluable source of info.

cnh

Systems · March 2009

cnh wrote: »

Sounds good I'd have to pass that to Ben above though.

Yeah I thought while he was replacing the caps for you,he could take a few extra minutes and add this mod aswell.:D

Enjoy your 555 cnh.

leroyjr1 · March 2009

vijayl wrote: »

cnh, If I can ask how much did u pay 555? I'm looking for one as well..

If possible local pickup is the best. You save shipping and you can check it out first . With that being said check craigslist in your area for good deals. I current have two in perfect condition that I got for less than $300.

cnh · March 2009

That's a great price, but I'm happy at 300. These things are built like tanks, parts are easily available, and it's hard to find much comparable for that price. A little tender care and some slight mods as the guys mention above and you're good for years to come or until your hit by upgraditis once more, and then again, and yet again...and....

You know what I mean. And it's not an Emo..? You know I'm kidding, right?

cnh

cnh · April 2009

Ok guys,

I finally got my Adcom GFA-555 up and running. At the moment it's running M70s with a Denon AVR 2807 serving as pre-pro.

On to a few issues that have been discussed here of late. Am running reasonable quality 12 gauge speaker wires with gold pins. And twisted pair I/Cs mid-quality. For those who think that AVR receivers don't sound any different when running a power amp all I can say is...WHAT? The difference in clarity, depth, punch, imaging, musicality...where do I stop...is VERY noticeable with the Adcom in place. It's like having a new pair of speakers. Who would have thought M70s could sound like this? And those aren't even my best speakers.

Does the Denon affect the general sound..yes...the sound is still Denon-like only...more like a Denon on steroids, but also mellower than the Denon's own internal amplification. A bit 'warmer' than the Denon. I couldn't be happier...even listening to FM stations is better, though the Adcom 'reveals' more of the transmission interferences from the station if it's not extremely clear and locked in. And CDs, even on my cheaper Yamaha universal player sound much better...will look to upgrade there later. But good for now.

Well I'm off for now. But I have to say that I do think Denons from 2800 and above make pretty good processors. Are they the best, NO. But definitely 'acceptable'.

cnh

Lowell_M · April 2009

CNH, Congrats and welcome to the next step down the steep slope to audio insanity!!!

comfortablycurt · April 2009

Congrats on the 555 cnh.

I figure this is as good of a place as any to ask this question...

Could someone explain to me what the difference is between the 555 and the 555 MkII. Is there a preference between either version?

cnh · April 2009

Hi Curt,

I can't answer this as well as Heiney9 can. Somewhere there is a post of his in 2005, that addresses the history of this Adcom unit. The 555 is an original design for Adcom by Nelson Pass. As time passes Adcom tweaks this design until it becomes a bit too far removed from the original. At one point the head of Adcom gives Pass an old version of the Adcom to improve on and heiney9 posts a response by Pass himself on letting the 555 sit there...because there may not have been much of a need to improve on his design? See if you can find that post. I don't remember where it is. But it is unbelievably informative.

I think we can say that the original 555 is, perhaps, the best of the lot. There were some improvements made in the MKII but, at present I don't remember exactly what those were (possibly mosfets, etc.?).

cnh

heiney9 · April 2009

comfortablycurt wrote: »

Congrats on the 555 cnh.

I figure this is as good of a place as any to ask this question...

Could someone explain to me what the difference is between the 555 and the 555 MkII. Is there a preference between either version?

Curt do a search.............I've explained atleast 3 times on the forum. If I run across the thread I'll post.

Systems · April 2009

cnh wrote: »

I think we can say that the original 555 is, perhaps, the best of the lot. There were some improvements made in the MKII but, at present I don't remember exactly what those were (possibly mosfets, etc.?).

cnh

There may be other changes but the MKII version added a DC servo to allow for direct coupling thus eliminating the need for a coupling cap at the input.Aswell it uses a triple darlington confirgured output stage that gives it better low impedance drive capability.However I don't think the original 555 was any slouch in this deparment and the triple darlington arrangement adds complexity which being more complex may affect SQ in a negative way.The outputs are stll BJT's not FET's.

comfortablycurt · April 2009

heiney9 wrote: »

Curt do a search.............I've explained atleast 3 times on the forum. If I run across the thread I'll post.

I searched...and I found. Thanks Brock. All my questions have been answered now.:)

http://www.polkaudio.com/forums/showthread.php?t=53981&highlight=adcom+555

cnh · April 2009

Here's even more Curt, from the horses mouth!

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

comfortablycurt · April 2009

There is no such thing as too much information. Thanks cnh.:)

From everything I've been able to find, it seems like the 1st gen 5XX amps are the way to go. Good to know.

Running Adcom GFA-555

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