Yay for craigslist!, Adcom GFA-555 as 600w mono blocks!

Just picked up a second Adcom GFA-555 which has currently replaced my mofset based GFA-5500 (non-bridgeable) to supply, as the owner's manual would suggest, 600w each to my AR9LS! Now if I can only find the time to give them a good listen

i think you will like the way the 9's open up bi-amped, and with the added power too.

whats the spec on the GFA-555 when bridged to mono, can they take a 4 ohm load?

From the manual:
Bridged mode power of 600 watts, 20Hz-20KHz, into an 8 ohm load with less than .25% total harmonic distortion.

http://www.adcom.com/data/manuals/GFA555manual.pdf

200 Watts per channel, 20Hz-20KHz, into an 8 ohm load with both channels with less than .09% total harmonic distortion.

I take it the 9's are 4 ohm? Do they have a split crossover? It might be healthier for the amps to bi-amp them as oposed to bridging them.

Do you have muffin fans on them? I picked up a thermally operated 120mm muffin fan for my 585 on that auciton site, you can adjust the temperature that you want the fan to come on at, runs very quietly too, put on a different circuit (outlet) than the amp you can have them run after shut down to more quickly bring the am p back down to room temp, they od draw in dust and you would need to clean periodically.

I did notice that they are now selling a 555se, which would actually version 3.

Jim

It says no where in the manual what impedance limitations it has bridged to mono, if any, and the 555 in stereo mode has been a stalwart performer with more difficult impedance loads so I'm not too concerned. I would love to biamp them but for whatever reason Teledyne decided not to include that feature in the AR9LS! (I'm secretly jealous of your 9's, Scott, just for that reason... amongst others ) Still might try biamping the DQ-20is although I seem to remember reading somewhere that most DQs sounded better running single amps?

Also from the manual, 325wpc 20Hz - 20KHz into a 4 Ohm load with both channels driven with less than 0.25% thd, 600wpc 20Hz - 20KHz into 8 Ohms bridged to mono with less than 0.25% thd! Remarkable for a 15 year old amp that cost less than $1000 when new.

No cooling fan, at least not yet. The 555 just never seems to get very warm, especially compared to the 5500 which I also have. That thing could make a nice toaster, even at an idle.

I do think I hear a slightly improved prominence in some of the frequency range, maybe a couple of dbs louder at the same pre-amp levels, but honestly I only had about a half hour to listen to them before I had to go to work.

This post has been edited by Oktyabr on August 20, 2009 05:33 am

Here is an interesting thread on the topic:

http://www.polkaudio.com/forums/showthread.php?t=76289

This post has been edited by Oktyabr on August 20, 2009 05:24 am

Cool, let us know how it works out for you.

Jim

sweet, stable to 2ohm as well...

i think to get the best out of the 9lsi's you'll have to get them alone on the back wall. you'll be able to set them wider apart and there wont be other speakers in proximity detracting from the sound.

Yup, I think so too. I sold my Cerwin Vega D-9s yesterday which freed up a LOT of room in my listening area. I brought my DQ20is back into the house from storage but haven't placed them yet. Just the AR9LS all alone, something I'm not sure I've done with them since I bought them!

There is definitely an increase in db's (running the same baseline source volume) and while I haven't tried really pushing them "loud" yet, an hour and half of my David Gilmour collection at a comfortable listening level sounds sweet and the 555s are barely notably warm to the touch.

Did some Yes and all of U2's "The Joshua Tree" at higher volume and the heat sinks are now warm to the touch but still not what I would call hot and still no where near the temps produced by that 5500! I also noticed that U2's "Moment of Surrender", a song I love but have also criticized for crappy mixing and mastering (the whole album) sounds a bit more restrained and listenable now. I'm not sure what produced that, or why, but I definitely hear a difference on this particular track and a few others?

A friend of mine just got his 555's back from Echowars, he to is running his pair brigded, here is how they checked out after repairs.

Quote from Echowars:

"The amp that was busted is a 555 of the original design, and I measured clipping at 218WPC into 8 ohms. At the rated 200WPC, THD+N measured 0.0144% @ 1KHz. The second amp that was working is a newer design of the 555, and I measured clipping at 232WPC. At the rated 200WPC, I measured 0.0118% THD+N @ 1KHz. I tested both in bridged mode, but not to full power, as my dummy loads cannot take the bridged power of these amps...I simply verified that the bridged mode was functional."

Jim