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Power amplifier: ADCOM 555 vs 555 II


aquila2010

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Hello,

as far as I know the 555 was designed by Nelson Pass, but I don't know who was the 555 II designer.

Have you ever compared them driving the biggest AR speakers (3a, 10Pi, 11, 9, 91 etc.) at the same time?

What are the sonic differences between 555 and 555 II? They say they were very different sonically.

What about their reliability? They are old and their failure could seriously damage our beloved speakers.

Thank you

 

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Hi Luigi

John Atkinson wrote:

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.

You can check out the Stereophile article here: http://www.stereophile.com/content/adcom-gfa-555-power-amplifier-john-atkinson-1990#fxzrvUtuag7O5g3P.97

According to this page http://www.kenrockwell.com/audio/adcom/gfa-555-ii.htmthe 555-II was "designed from the ground up" by Walt Morrey.

Any old amp should be checked out by a qualified tech. I drive my AR-3a's with an Adcom 555 II but I installed AR's recommended fuses on the speakers just to be safe.

-Kent

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I almost finished recapping my 555 II today. I put one mod in. I'll let you know how it turns out. I always thought the bass was thin on this one compared to my old SAE amp. I've blown a lot of speaker fuses with this one but it drives four sets of speakers without complaining :rolleyes:

Roger

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Adcom pulled an AR-esque revival here with the 555 SE: http://www.amazon.com/Adcom-GFA-555SE-2-Channel-200-Watt-Amplifier/dp/B001OT90DY

The reviewers comments are interesting.

And here is the manufacturer's listing: http://www.adcom-usa.com/gfa-555se

I haven't had the opportunity to look inside the original 555 but I did read Nelson Pass's comments over on DIYaudio. The MkII version was apparently redesigned by in-house engineers using some upgraded components.

Here is my recently recapped and slightly modded 1994 version of the 555 MKII. The amp had seen some major service prior to me and was very dirty with some rust however it did clean up quite well. More mods to come in the bias circuit and input stages when I get around to it. I picked up an old Fisher integrated amp at GW yesterday which looks like it has a nice heat sink I can use here.

The replacement ps caps have slightly more capacitance and were one inch taller necessitating raising the torroidal transformer to avoid rewiring. Lot's of space in the cabinet so no problem there.

Haven't played it yet though to see how it sounds ... maybe later today :).

post-173498-0-37754600-1440683535_thumb.post-173498-0-32846300-1440683544_thumb.

Roger

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The original 555 used fairly advanced transistors for the time, it ran on high rail

voltages, and it had no output current limiting other than fuses - so there is no

false triggering of output protection. Many of the originals were built with marginal

driver transistors that fail into a short circuit or a very low impedance. They upgraded

the drivers during production and issued an update for older units to correct this issue.

It was advertised that it would drive speakers that dipped to 1 ohm with the upgraded

drivers and upsized rail fuses.

The 555-II employed a triple darlington output stage rather that the conventional double

in the original allowing it to drive low impedance loads with higher current and lower

distortion.

I own a pair of original 555s and have helped people repair them on DiyAudio. I'm PB2 there:

http://www.diyaudio.com/forums/solid-state/157682-another-high-dc-adcom-gfa-555-a.html

If you buy a 555 made sure that it has, or do the driver update.

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I have used an Adcom GFA-545 (100 w/c into 8 ohms; 150 w/c into 4 ohms) and a GTP-450 pre-amp/tuner to drive my AR90's since the 1990's. Sometimes I switch over to some Polk SDA-2B's. The AR90's have been recapped, and are a bit more efficient and a bit more forward in the midrange than the Polk's, which have the interconnect cable and the upgraded tweeter mod.

I also have a GFA-555 (it was designed by Nelson Pass) but don't use it because, at the listening levels I use nowadays, any power advantage of the GFA-555 is never utilized. I do not know if it has the mod. These amps sound the same to me.

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I own a pair of original 555s and have helped people repair them on DiyAudio. I'm PB2 there:

http://www.diyaudio.com/forums/solid-state/157682-another-high-dc-adcom-gfa-555-a.html

Holy crap! Over 600 posts with lots of expert advice (mostly beyond my technical savvy) and the poor schmo had counterfeit parts. Bummer :(

But you guys really saved the day.

Any advice re the 555/II? I had mine checked out by a pro and it works great, so you might say "if it ain't broke....." but I'm always interested in tweaks ;)

-Kent

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Holy crap! Over 600 posts with lots of expert advice (mostly beyond my technical savvy) and the poor schmo had counterfeit parts. Bummer :(

But you guys really saved the day.

Any advice re the 555/II? I had mine checked out by a pro and it works great, so you might say "if it ain't broke....." but I'm always interested in tweaks ;)

-Kent

Well, I haven't hooked mine back up yet but I put the 22MFD caps across the power supply caps. I'll let you know how it sounds soon ... :)

Roger

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Holy crap! Over 600 posts with lots of expert advice (mostly beyond my technical savvy) and the poor schmo had counterfeit parts. Bummer :(

But you guys really saved the day.

Any advice re the 555/II? I had mine checked out by a pro and it works great, so you might say "if it ain't broke....." but I'm always interested in tweaks ;)

-Kent

The only problem with the 555 II that I know of is that some were built with "bad caps"

(google it if you don't know about it) that leak all over the circuit board and make it

very difficult to repair. Big amps are not something to learn on, when things go wrong

they go wrong in a very big way usually with lots of smoke.

That repair should have been much easier, but he decided to "shotgun" it and replace

parts that were not bad, installing bad counterfeit parts. Very bad move on his part.

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Here is a reference to the cap problem on an Adcom restorer's site although his reference is a bit ambiguous:

In fact, Adcom got hit with a batch of leaky capacitors in the early 90s that caused many GFA-565 amplifiers to send DC current to the speakers! The electrolyte leaks out onto the circuit board and forms a conductive film, making electrical connections where there should not be! This is the same issue that caused great embarrassment and loss for companies such as Dell and Apple. See “The great capacitor plague” for more. If you have a GFA-565 that has not had these capacitors replaced, I urge you to do so!

http://hoppesbrain.com/adcom-service-and-refurbishing/

Roger

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Thank you everybody for all the useful information on these amps you are giving.

In other old threads I have read about using professional amps (especially Crown and QSC) for our beloved AR. They would seem to be as well sounding as home amps and especially more reliable becouse of their numerous electronic protections. They have reasonable prices too. Have you ever listened to them in comparision with home amps? What do you think about them?

thank you

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