Adcom GFA-5500 vs. GFA-555 II - AVS Forum
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post #1 of 8 Old 12-16-2009, 09:58 PM - Thread Starter
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I've done some searches across this forum and others comparing these 2 amps, and have read opinions, many contradictory. Can someone with first-hand experience with both help me understand the sonic differences between these two amps?

The power outputs of each seem similar enough that they would be almost negligible. I presume each amp's sonic signature can be chocked up to its different circuitry - I know the 5500 has a MOSFET topography - but not quite sure how that affects it vs the 555.

Thanks in advance! I'm looking to pick up one of these to drive my magnepan 1.6's in a home theater. Ultimately, would you have a preference for one vs the other?
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post #2 of 8 Old 12-16-2009, 10:29 PM
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I found this thing where some dude talks about both amps -

http://www.audioreview.com/cat/ampli...5_1583crx.aspx

FYI - the usual word is topology. Topography is usually related to the study of the earth's surface, or maps showing contours Also, the use of MOSFETs is not what I would call an amp topology as such. Amp topology is usually given as a class of amp like class A, AB, D, etc.

"But this one goes up to 11"
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post #3 of 8 Old 12-16-2009, 11:55 PM - Thread Starter
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Quote:
Originally Posted by MichaelJHuman View Post

FYI - the usual word is topology. Topography is usually related to the study of the earth's surface, or maps showing contours Also, the use of MOSFETs is not what I would call an amp topology as such. Amp topology is usually given as a class of amp like class A, AB, D, etc.

Oh man, guess my noobie is showing...

Thanks for the link. I was perusing audioreview earlier and found a few comparisons under the 5500 reviews, most people knocking the 5500 compared to the 555, but also some conflicting opinions. Was just hoping to get some fresh opinions.

Oh, and if it makes a difference, I intend to mate this amp in a home theater with a 5-channel Adcom GFA-7700, so also thoughts on which might pair better.
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post #4 of 8 Old 12-17-2009, 02:48 AM
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You might contact Adcom. I find it interesting how close the specs of the two amps are. They have identical 8 ohm power, and different 4 ohm power. They seem to have different pricing based on my cursory search.

"But this one goes up to 11"
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post #5 of 8 Old 12-24-2009, 04:42 PM
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I've had both and ran Maggies with them too... I've had a LOT of Adcom gear!

You sound like you have read plenty on both of these amps Both are high current and capable of handling a wide impedance range with ease.

The 555mkII uses bipolar output transistors and the 5500 uses MOSFETs.

The 555mkII, like the original 555 before it, is bridgeable to up to 850w @ 4 Ohms! I've actually ran two of these as monoblocks on a pair of 400w/4ohm AR9LS and was blown away by the control offered even at high volume. This is a key feature missing on the 5500.

The 5500 on the other hand is said to have faster transient response due to it's improved class A front end and MOSFET outputs. Some will say this makes the 5500 sound a bit brighter than the 555 or conversely that the 555 sounds "grainier" on the mids and highs than the 5500 (I would say the latter is true IMHO). Both are excellent performers into the deep end. The MOSFETs draw a LOT of power and it doesn't matter if the amp is pushing music or just sitting idle, it still sucks down the same amount of juice. It (the 5500) also gets notably hotter than the 555, also due to the MOSFETs, and ventilation is thus a bigger concern.

I personally prefer the sound of the 5500 over the 555mkII enough that I'm working on a trade of one of my 555mkIIs for another 5500 as I type this. I might not be able to monoblock the 5500s but bi-amping is certainly a sweet thing.
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post #6 of 8 Old 12-24-2009, 08:23 PM
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I have noted that you can read 10 articles on bipolar vs MOSFET and get almost that many opinions.

There are advantaged and disadvantages to both. Neither should "sound" like anything. It's the job of the amp designer to make them both behave in a linear manner, amplifying the input signal to a higher amplitude, otherwise idential waveform.

Audible distortion, if any, is likely not the fault of the transistor itself, but result of inherent non linearities in amp design, for example crossover distortion which occurs in all class AB amps.

I suggest the Amplifier Design Handbook (I think the name is,) by Douglas Self for a treatment of MOSFET vs Bipolar transistors in terms of their use in amplifiers if you are curious about the technical side of things. Otherwise, I see no reason to prefer MOSFET or Bipolar designs assuming competent engineering. You will note that Bipolar amps are the more commonly used transistor in receivers and home amps. I think I see MOSFET used more in car amps, but that's not based on any hard evidence.

"But this one goes up to 11"
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post #7 of 8 Old 12-24-2009, 10:43 PM - Thread Starter
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Thanks for the info.

Michael, I took your advice and emailed Adcom about this. Their reply is below:

The GFA-555se, end of story! This amplifier would be the perfect compliment
to the GFA-7700 for a pair of front speakers. It has the power and finesse
to do the job without reservation, sonically integrating with your existing
amplifier.
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post #8 of 8 Old 05-15-2014, 10:35 AM
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Quote:
Originally Posted by RushM3 View Post

Thanks for the info.


Michael, I took your advice and emailed Adcom about this. Their reply is below:


The GFA-555se, end of story!

amplifier.

LOL No way! You mean to tell me that they recommended the model that is currently on the market? I'm surprised. I would have expected them to recommend that you buy one used from someone else. Never thought they would steer you towards their newest and most expensive model. (slathered in ultra thick sarcasm)biggrin.gif
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