Versatility, power, great bass and clean clear mids and highs, big bang for the buck

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Wow, is all i have to say!
i am runnining it thru my sansui 717, no negs at all! bought it for 200, what a steal!
would recomend it, to anyone

I purchased this roughly a year ago after reading favorable reviews. Needed adequate power to run magnepan MMGs. This was big improvement over the Denon AVR891 I was using. Claims 135 watts per channel but I'd bet it's around 70 real watts. I started reading about numerous ways to upgrade the adcom and found someone in Indianapolis who had 10 years plus years of experience in modifying these amps. So I took the plunge.
This is what he did:
1. Made it dual mono with two 800va toroidals.

2.Made the amp DC couple input to output by bypassing the input coupling cap with a jumper. This jumper can be removed to restore stock operation since the capacitor was not removed.

3. Added to the existing 47uF power supply bypass caps, 330uF 100v caps to make it a total of 377uF per rail for the 2 rails of each channel. i.e. 4 rails.

4. Reduced NFB by adding 6.8k resistance to the original 22k in series.
Replaced the ageing 2sb817/2sd1047 , 140v 120 watt drivers with the latest ON/Motorola devices NJW0302G and NJW0281G 250v 150watt devices that have superior hfe under higher current delivery.

5. Biased the amp to 23mV across each ER instead of the standard 10mV. This softens the sound, makes it cleaner to listen to and drives the amp to 3 watts Pure class A.

The result was outstanding. Gone was the graininess that so many people talk about and the dynamic range is now incredible. The guy that modified my amp says it should be around 500 watts a channel on a 4 ohm load. I am able to hit 95db before clipping with my very inefficient magnepan MMGs. I have yet to listen to any amp under $1000 that can touch it. Just for kicks I hooked amp up to a very efficient pair of infinity sm125 which are best for "party" speakers. Loud but not accurate speakers. Made my house shake hitting near 110db before clipping. Anyone considering Adcom GFA555 ii should seriously consider upgrading it before getting another amp. I have $650 invested in mine after upgrades and after upgrade I give it 5 plus stars!!

Rest of associated equipment:
1. Sonos connect
2. Denon DVD5000
3. Rotel RSP1068
4. Adcom GFA555 ii highly modified
5. Velodyne VT 10 inch powered sub
6. Monster power HDP1800 conditioner
7. All audio quest interconnects and speaker cables

I've been using this adcom for 5-6 years, I bought it used for USD $ 350
If you had been heard several "high end" power, you will know that your adcom have many weakneses, it sound harsh, have punchy bass but not low enough, not really sweet vocal etc.
So I decided to make several modification and upgrade the component.
I added SCR / Solen (from 22 uF to 68 uF) bypassing on supply capacitor, replace critical standard capacitor with blackgate F caps, replace feedback resistor with audionote tantalum resistor, bypassing input cap with dynamicaps caps, replacing standard KOA wirewound resistor with the better ones, adding capacitor smoothing for input electricity.

The result is very satisfying, the adcom could beat BRYSTON 4ST (yeah you;re not misread....hahaha)
sounds very smooth from top to bottom, still dynamic, still punchy with deeper bass, and detailed

I'm using magneplanar 1.6 and fostex 206 modified with scanspeak 6600 tweeter
and the loudspeaker cable is the beautiful sounding cardas golden hexlink 5

Want to make this amp really sing, replace the driver stage caps with Nichicon Muse Fine Gold caps. I had my service tech do it for a mere $175. Let the amp then burn in for at least 50 hours sit back and be in awe!! Individual instrument placement and definition is unprecedented and I have owed many amps over the years. I have yet to experience any listening fatigue since the upgrade. Low level listening, high level listening, it all sounds great. Bass is airy, no sense of boom, mid-range is so detailed now I can hear every word spoken, guitar riffs are just eye opening. If you are not a believer that caps make a difference modify this gem and you will become a believer. My tech wasn't a believer until I had him modify this 555II, he is now!! He currently has one of my Rotel amps that I am having him replace the caps in, looking forward to hearing how that turns out. Before the mod the Rotel bested the Adcom, after the mod the Adcom smoked the Rotel. Curious what the caps will do for the Rotel. The Rotel uses Black Gates too!! After hearing the Nichicons in the Adcom the black gates are being pulled. So I looked in my Arcam CD73 to see what kind of caps were in it and low and behold I find Nichicon Muse Fine Golds!! Wonder what my McIntosh C220 preamp has in it?

What a gem! This thing has more power than a demented dictator. I purchased this unit today on Craigs List for $250 in mint condition! I jumped on it based on the reviews that I read here on Audio Review and Stereophile. It just churns out the music with fantastic airy highs and thunderous bass. Placement and separation of the instruments are phenomenal! I am awe struck over the soundstage that it reproduces. I lost my job a couple of years ago and had to give up my Audio Research, Krell, and Rowland equipment! I reduced myself to Kenwood and Sony. Audio suicide! I have been trying to economically put together a system that would at least satisfy my ears. When I came across the Adcom my mission was completed. If I were to take the blindfold test I would not be able to tell it from my Krell or Rowland. Oddly enough it sounds like the two have combined! It has the same type of Krell bass slam and the finesse of the Rowland 5. Now I can sit down and actually enjoy the music rather than criticize it and complain about what I need! I would love to find another one and run them as mono amps. What a treat for the auditory canal that would be. I highly recommend this unit to whoever is considering listening to it. It will not disappoint, just delight! I have had over 100 different combinations in the past trying to achieve a certain sound that I have in my head. Mission accomplished. The price is a spit in the ocean compared to the past amps that I have owned. My hat goes off to you Adcom.

Versatility, power, great bass and clean clear mids and highs, big bang for the buck

As to what?

I buy and sell classic audio gear on eBay among other places. This means that I am constantly changing speakers, PreAmps etc. I need a good powerful amp of signifacant quality, one versatile enough to play any speaker on the planet. I owned several of Adcom's earlier amps including the GFA-1 and 1A. I gought the first series Definitive Technology BP-20s when they first hit the market so I bought a 555 Adcom to power them and was never disapointed. Always plenty of power to drive the BP-20s to wickedly loud levels cleanly and effortlessly. As time passed and after continuous upgrades into a full blown HT setup I found I longed for the good stereo sound again, simple and uncluttered in my office/computer room. I remembered my experience with the 555 so when my friend was selling mint 555IIs I bought one. I remembered my old JBL L-112s and found a pair of them. Next I needed a decent PreAmp when along came a Sony TA-E77ESD, now we are talking pushing a classic pair of JBLs with enough power to light a small town everything came back. The JBLs were a bit dated and I wanted a more hi-res speaker so I put a pair of NHT Super Ones on this 555II and WOW! those who think the NHTs are a little thin havnt heard them behind a 555II. After dozens of different speakers I now use Sonys SSTX7S a overseas MTM model with glass cones and I am here to tell you that the sound is extraordinary, comming is a hand built pair of 7in 2ways that my friend is building for me and I expect this 555II to sing with these as well. I read every review written here about this great amp. I hear things like grainyness etc. when I hear nothing but clean, clear music out of everything I put on it. Of course there are differences between different speakers but I never thought for a moment that any thing other than good was comming from my 555II. I see people comparing this 500 dollar amp to multi thoasand dollar amps, somehow the Adcom can't cut it when comparing it to a amp costing ten times the money, call it mid-fi, low-fi or what you will. I call it great. A great amp at a great price period.

Similar Products Used:

Fort'e model 3, Adcom 555/565, Rotel RB-981/1080/985thxmk2/Anthem/ Accurus 200x3

controll, absolute power, everything

thump when turn of, but whatever.

awesome amp. crazy with power and hifi...

Similar Products Used:

yamaha

Fairly solid sound with good full bass and some continuity and good tonal quality.

Somewhat grainy, lacks bass control, congested with large orchestral works, OK treble.

In 1995 I was auditioning stereo power amplifiers for my Carver Platinum Mark IV speakers. I owned a pair of Carver Silver 7t's which were not a good match for my speaker as they both had a peak around 3kHZ. I borrowed these used 555II amps from a local dealer to audition. Right away I could see that they had a richer tone and better bass than the Carver amps. However there was also a noticable grain that was not evident with the Carver amps. At least the low treble peak was reduced. I then obtained a late production Parasound 2200 stereo amp and elected to go with this as it was as smooth as the Carver but had the richness and freedom from low treble boost as the Adcom. The Parasound amp also appeared to be more refined. Later after I biamped my speakers I was able to audition a Rotel 990 amp that was faster than the previously mentioned amps although a little roughness compared to the Parasound or Carver products. The bass was better deliniated also. All these amps tended to congest with large scale orchestral works. I liked this amp but it is far from perfect. My speakers, especially since I biamped them, can reveal differences in amps. A Krell FPB 200 that I tried on the top end of my speaker made significant improvements in many areas (although it is forward in presentation).

Similar Products Used:

Parasound 2200 (late model), Rotel 990, Carver Silver 7t

Adcom's best sounding power amp ever. (can be compared to much more expensive amps). Sound quality, Build quality, Great bottom end.

Absolutely none.

The Adcom GFA555II never ceases to amaze me. It has to be one of the best values out there in amplification. I have tried much more expensive amps only to fall back on this one. The real key to deriving the best sound from this amp is to match it with the right speakers and preamp. The Adcom preamps do a fine job with this power amp but after experimenting with different preamps, I found the Audio Research LS1 pre is the best match for high end sound. The LS1 with it's one tube creates a much more lush midrange and far more grainless highs. (I also tried the LS3 and it wasn't the better of the two with this amp). I never thought the 555II could be so capable of this calibur of sound! I have four large (floor standing) Klipsch speakers and it is well known that Adcom and Klipsch make a great synergy match. I feel now that I have a very musical system without mortgaging the house. I highly recommend this amp to anyone including those that intend on spending much more to check out this one first.

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Bryston, McCormack, PS Audio, Conrad Johnson