XPA-2 vs Adcom GFA-555 mk II

[strindl]

I got the XPA-2 today and have spent a few hours comparing it head to head with the Adcom GFA-555 mk II that I have owned for years.

This is the setup in my bedroom:



The speakers are a three way tower using the case from vintage design Acoustic PS-103 speakers, but with all new drivers and custom crossover. It's using a high end Focal Audium 7" model 7Nv2 mid woofer, along with a Vifa D27TG silk dome tweeter. Those are mounted in a separate trapazoidal shaped enclosure mounted to the top of an acoustic suspension box for a down firing 10" CTS cast frame pro subwoofer with a 3 inch voice coil operating from 140 hz down. The rebuilt speakers are 4 ohm and have approximately a 90 db efficiency. They weigh 60 pounds each.


The Adcom sounded good with the speakers, but I noticed that they didn't have the bass depth and solidity that they had when being driven by a Threshold amp in my living room. The XPA-2 changed all of that.


Not only is the bass as deep and solid as it was with the Threshold amp, I am hearing more detail in the music...things like the strumming or plucking of guitar strings are more realistic than they were with the Adcom.

The amp has been on now for about three hours, and the top is not even warm..it's actually still cool to the touch. That's quite a change from the Adcom driving the same speakers. With the efficiency of those speakers, the XPA-2 is not even breaking a sweat..the most I have been able to light up on the power meter, is two led's per channel..and that was LOUD.

Here is a better shot of the speakers in the living room system:




I'll be doing more playing around with things over the next couple weeks. I want to take the XPA-2 into the living room and see how it sounds on the Magnepans there.

[strindl]

Oh..the preamp is a Harmon Kardon Signature 2.0 running in two channel mode and the music source is a Logitech Duet streaming lossless audio.

[Mike Ronesia]

Nice review and setup. Enjoy!

[strindl]

I've had the morning to play around with the amp some more. The XPA-2 is without question a better sounding amp than the Adcom GFA-555 mk II, which I've always thought was a pretty good sounding amp.

I looked up the specs on the Adcom...it has a 700VA toroidal transformer vs the 1200VA one in the XPA-2, and uses 16 output devices vs the 24 in the Emotiva. Filter capacitance is about the same for each.

The XPA-2 doesn't have to work hard at all driving my 4 ohm 90db efficiency speakers, so it's not JUST the added power capabilities that explain the difference in sound between the two amps. This is enough of a difference that anyone could hear it in a few minutes of listening...it's not subtle at all.

Now I'm curious how the XPA-2 would compare directly against the Threshold S500e in my living room. Those amps have exactly the same power output ratings. I'm going to have to drag it in there and hook it up to my Magnepans.

[klinemj]

I'm glad to see your review...what you describe is exactly what I expected you would hear based on the indirect comparisons I referred to earlier in this thread. As you note, the Adcom is a nice amp. But, the XPA-2 is noticably different and better, for my tastes.

[strindl]

Nov 20, 2009 16:15:11 GMT -5 klinemj said:

I'm glad to see your review...what you describe is exactly what I expected you would hear based on the indirect comparisons I referred to earlier in this thread. As you note, the Adcom is a nice amp. But, the XPA-2 is noticably different and better, for my tastes.

Without question. That Adcom is a great amp and was a real value when it was introduced. FAR less expensive than the amps it competed with. Kind of like what Emotiva is today. I actually paid less for my XPA-2 in 2009, then I did in 1991 for that Adcom 555 mk II. And the XPA-2 sounds considerably better and has more power.

[jonsmith]

If you look at the distortion curves for both amps, you will see that the Emotiva is basically operating in class B, as distinguished by the fact that its distortion gets lower as the wattage goes up, which differs from class A, which has distortion INCREASE gradually as it nears its highest output level (when it then increases logarithmically, same as with class B). (And also the distortion characteristics of an actual sound wave in air when driven to distortion, which distortion increases steadily as it gets louder, and then in a logarithmic fashion at the distortion point.)

Having listened to both amps, I would say that at low levels I prefer the Adcom, although at high levels (not how I typically listen) I think that the Emotiva has a lot more "cajones", particularly in the bass region. And, if you have four of the big mono amps running two speakers like they did at Emofest, you can play it _really_ loud.

But the Adcom I own (not a 555II, so perhaps not an apt comparison) is biased to 15 watts in class A, so I am probably listening in a mode that has a dominant 2nd distortion characteristic, and there's plenty of headroom and bass there for the jazz and classical music that I prefer to listen to.

And if you wanted to make a more direct comparison, you would want to parallel TWO GFA 555's, which would then give you a roughly equivalent number of bipolar output devices and roughly the same size power supply. (The Emotiva has twice the power supply capacitance, dual power supplies, and twice the output transistors, which makes it far more powerful than a single 200 W Adcom, numbers notwithstanding.) This could be done for a total of about $600, which is roughly the same price as the (dual mono) Emotiva. If you wanted to compare a 20 year old amp (with brand new capacitors in the power supply) to a brand new amp with the benefit of 20 years of computer-aided design. I'd respectfully submit that if you compared one GFA 555, you are comparing apples to oranges: far less power, and different design. But you may prefer the sound, which is a valid observation.

As to the sound, perhaps you all prefer the sound of class B, and its dominant odd-level distortion characteristic over the sound of class A? Many people do.

There are a ton of great sounding amps built along those lines, including Emotiva and Musical Fidelity Titan and its progeny. That's also why Musical Fidelity and Pass Labs, among others, make amps that are "pure" class A (with even order distortion, like real sound waves), or A/B with the emphasis on class "B" (which have odd-level distortion characteristics and decreased distortion with higher sound levels) which is what Emotiva makes.

Just my $.02.

[terentellie]

I know this is an old thread but I couldn't help myself.
This 555MKII is a dual mono block design with dual 1Kva transformers 100mfd of storage on the mains plus some bypass caps and upgraded 20amp Motorola transistors.  I would love to compare the XPA-2 to this and a stock 555MKII to hear the differences. 

[Deleted]

Strindl "The lack of bass depth" is the first time I have heard that description applied to an Adcom amp. Read even just a few reviews. I have owned several Adcoms over the years, and agree with most who say that Adcom's low end prowess is one of the things that these amps are noted for. So something is wrong here. What year was your Adcom made? I am guessing that it is in excess of 20 years old. If that is the case, and nothing has been done to it, it may be suffering from "tired" circuitry. After about 10 years I had the caps, transistors replaced, along with the bias reset on one of my GFA-7500s. The results were quite impressive. I had no idea that the sound had changed that much over the years. In an old Stereophile mag years ago, the writer (KR maybe?) said the GFA 555 MKII sounded like a Krell amp. It may not be entirely fair to compare amps to far distant in age, if that is the case here, if no upgrades were done to the older amp. Of course, and ultimately, system synergy is paramount.

[pedrocols]

Wow this thread is almost 4 years young...

[terentellie]

Nothing like resuscitation. I am reworking the input board on my 555MKII and ran across this thread while doing so research. I am also looking at this guys rack in the second picture and in the left cubby. I do see an Adcom but to me it looks more like a 545 than a 555... Maybe it's just not in the photo. Also when comparing the output transistors the Adcom uses TO-3 Darlington metal cased transistors and if I understand correctly Darlington transistors are really two transistors in one case. The Emo's use silicone. I would like to know what the model of the output transistors are in the Emo. After looking up the transistors used in my modified amp I found they are 16 amp not 20 as I previously mentioned (my bad). Mine uses Motorola MJ21194 and MJ21193 matched pairs in the output section. One thing I think would be crazy! I noticed on the heatsink of the Adcom there is an open spot for another set of transistors... I have a feeling though to utilize another pair on each channel would mean re-engineering the whole amp.  It would probably be easier to buy a 585 and mod that

[DavidR]

I'm loving my XPA-2 Gen2. I had all of the experiences you had: great bass (even at low volume), superb imaging, clarity and was hearing detail like never before. My speakers are a difficult load with (2) 10" woofers in parallel @ 4 ohms; not the worst to drive but difficult none the less. I'm very happy with my choice.

[terentellie]

I do not discount Emotiva at all. I think they are a great company. If I were looking for a 2 channel beast to replace my modded out Adcom I would most likely buy an Emotiva. I really like the looks, I like their attitude and most who buy them are very happy most especially when they consider what they paid.

[splash51]

Hi all, thought I would chime in here. I recently purchased an XPA-2 gen2 to replace an Adcom 555 II. The Adcom still sounds great but I was ready to try something from this decade.  I have done a side by side with these 2 amps.  My 1st impression is the  Adcom seems to have a little more presence at a low volume.   Highs and lows are more defined and the sound stage is a bit better.  The XPA really show its stuff with higher volume.  It just gets better and better as the volume rises.  I am not saying the XPA is bad at low volume, not at all.  It sounds very clean and clear, its just a little more subdued than the Adcom at a low volume. Overall I like the XPA better I am not disappointed at all!!    

[Cogito]

Jan 12, 2015 12:41:54 GMT -5 splash51 said:

Hi all, thought I would chime in here. I recently purchased an XPA-2 gen2 to replace an Adcom 555 II. The Adcom still sounds great but I was ready to try something from this decade.  I have done a side by side with these 2 amps.  My 1st impression is the  Adcom seems to have a little more presence at a low volume.   Highs and lows are more defined and the sound stage is a bit better.  The XPA really show its stuff with higher volume.  It just gets better and better as the volume rises.  I am not saying the XPA is bad at low volume, not at all.  It sounds very clean and clear, its just a little more subdued than the Adcom at a low volume. Overall I like the XPA better I am not disappointed at all!!    

I recently upgraded my 23 y/o GFA-555 II with a XPA-2 Gen.2. I can't honestly say I can hear ANY difference between the two amps. Yes, the XPA-2 does have a tad more "breathing room" with it's extra power reserves and it's a bit quieter. But when it come to sound quality, they are basically the same to MY ears. This is really a testament to Adcom's 25 y/o design which can actually compete with modern designs. The most astonishing thing, is that I paid LESS for my Emotiva than I did for my Adcom 23 years ago!

[strindl]

Oct 31, 2014 11:28:45 GMT -5 terentellie said:

Nothing like resuscitation. I am reworking the input board on my 555MKII and ran across this thread while doing so research. I am also looking at this guys rack in the second picture and in the left cubby. I do see an Adcom but to me it looks more like a 545 than a 555... Maybe it's just not in the photo. Also when comparing the output transistors the Adcom uses TO-3 Darlington metal cased transistors and if I understand correctly Darlington transistors are really two transistors in one case. The Emo's use silicone. I would like to know what the model of the output transistors are in the Emo. After looking up the transistors used in my modified amp I found they are 16 amp not 20 as I previously mentioned (my bad). Mine uses Motorola MJ21194 and MJ21193 matched pairs in the output section. One thing I think would be crazy! I noticed on the heatsink of the Adcom there is an open spot for another set of transistors... I have a feeling though to utilize another pair on each channel would mean re-engineering the whole amp.  It would probably be easier to buy a 585 and mod that

Good eyes...that "amp in the cubby" in that system is an Adcom, but that is an Adcom 2535...a 3 or 4 channel configurable amp rated at 60 watts a channel in 4 channel mode. At the time that picture was taken, that amp was in three channel mode with two of the 60 watt channels driving my Polk rear surround speakers, and the second pair of 60 watt channels bridged into one 200 watt amp at 8 ohms for my, at that time Infinity IL36C center channel speaker. shortly after that picture was taken, I put a Magnepan center channel speaker in place of the Infinity, which sounds much better matching with the Magnepan 1.7 mains.

That Magnepan center channel, the MGC3, is a power hungry 4 ohm speaker..the Infinity was 8 ohms. That bridged Adcom 2535 was not at all happy driving a 4 ohm speaker. It ran too hot and would shut down when driven hard on that Magnepan. So..what I did was take the center channel speaker off of the 2535 and just use two of it's channels to drive the Polk surrounds..which still works great. I then brought in my Adcom GFA 555 Mk II, which had spent most of the previous decade stored away in it's box out, flipped the bridging toggle on the back and turned it into a single channel 500 watt amp to drive the Magnepan center channel speaker. The maggie sounds way better with that kind of power driving it and the Adcom 555 works perfectly in that application, even driving a 4 ohm load while bridged.

Today, I have the 555 mk II in the cubby where the 2535 was in the previous pic, and the 2535 in the shelf directly above where it had been. The vintage Threshold s500e still drives the Magnepan 1.7's. Good power amps last a very long time...that Threshold was purchased new in 1985...as an S500, then sent back to the Threshold factory in 1992 to be upgraded to an S500e. It still works perfectly after all these years. The inside still looks like new:





My XPA 2 is driving a pair of Magnepan 1.6's in my two channel bedroom system, and doing a very fine job of it.

[strindl]

Oct 30, 2014 21:48:50 GMT -5 @islandman2020 said:

Strindl "The lack of bass depth" is the first time I have heard that description applied to an Adcom amp. Read even just a few reviews. I have owned several Adcoms over the years, and agree with most who say that Adcom's low end prowess is one of the things that these amps are noted for. So something is wrong here. What year was your Adcom made? I am guessing that it is in excess of 20 years old. If that is the case, and nothing has been done to it, it may be suffering from "tired" circuitry. After about 10 years I had the caps, transistors replaced, along with the bias reset on one of my GFA-7500s. The results were quite impressive. I had no idea that the sound had changed that much over the years. In an old Stereophile mag years ago, the writer (KR maybe?) said the GFA 555 MKII sounded like a Krell amp. It may not be entirely fair to compare amps to far distant in age, if that is the case here, if no upgrades were done to the older amp. Of course, and ultimately, system synergy is paramount.

I've owned quite a few Adcom components over the years and was always very happy with them. I even owned a pair of their rare GFA 565 monoblocks for a while. I bought that 555Mk2 in 1991 if my memory serves me correctly. I used it in a variety of applications for a couple years then went whole hog into Threshold power amps. So..the 555 was put in it's box and not used, except occasionally to compare it to other amps, for more than a decade. Mine should have lots of life left in it yet. I know that Nelson Pass, of Threshold and Pass Labs fame, was involved in some of the Adcom amp design, and he knows his stuff.

For the last few years my 555 has been happily doing duty in bridged mode driving a Magnepan center channel speaker in my living room.

[strindl]

Oct 30, 2014 12:58:38 GMT -5 terentellie said:

I know this is an old thread but I couldn't help myself.
This 555MKII is a dual mono block design with dual 1Kva transformers 100mfd of storage on the mains plus some bypass caps and upgraded 20amp Motorola transistors.  I would love to compare the XPA-2 to this and a stock 555MKII to hear the differences. 

wow...that's quite the transformation of your stock Adcom. Your picture is a bit small to see the detail...is that built on two separate chassis side by side? What did you do with the circuit boards to add the extra output transistors? What sort of power output does it have?

[strindl]

Here is a current picture of that living room system...with both Adcoms in use

[strindl]

And the current incarnation of the bedroom system..with the XPA-2 driving Magnepan 1.6's.