Adcom GFA 555 II

Agree with roberjerman, any bridged amp should not be used with a speaker having an impedance of less than 8 ohms.

daywalker702,

As I understand it, the Adcom 555 II amps were designed by Nelson Pass and are stable down to 1 ohm. However, even Nelson Pass has stated on DIY Audio that the Adcom 555 was not a great bridged amp.

https://www.diyaudio.com/forums/solid-state/53551-gfa-555-upgrade-substitutions.html?perpage=35&...

I also understand that Magnepans in general present a relatively benign and stable impedance load to amps of about 4 ohms without big peaks or dips.
Based on my experience driving my older 4 ohm Magnepan 2.7QR speakers with a stereo Adcom 555 MKII (200w @8 ohms and 400w @4 ohms) amp, I’d suggest a better solution would be a powerful class D amp.
I’ve used 3 class D amps on my Magnepans with all of them outperforming the Adcom 555 MKII. I initially used a ClassD Audio SDS-440-CS stereo amp (440 w/ch at 4 ohms), then an Emerald Physics EP100.2SE stereo amp ( 150 w/ch at 4 ohms) and currently a pair of D-Sonic M3-600-M monos (1,200 w at 4 ohms).
All 3 of these amps provided better bass response, a lower noise floor, a more detailed but still smoother mid-range/treble response and much improved dynamics than the Adcom.MKII in my system.  All 3 were also capable of very high volume levels in my spkrs without clipping or distortion (at levels far beyond my normal listening level during testing).
I’m fairly certain you’d notice similar performance improvements with a good class D amp driving your Magnepan MG-12QR speakers.
I have no associations with any class D companies; I’m just a big fan.
Tim

Huge Thanks  noble100  wow i didn't even think about checking out class D amplifiers, that opens up so many more options and the prices aren't too bad on the ones you listed above. I will be checking the local shops to see if they carry any class D amps at all just to get an idea of the characteristics and performance of them, if not i'll check with a few guys that i have dealt with here in Vegas to see if they have a class D in their collection that i can check out. Also can you please advise on why you moved on from the ClassD Audio SDS-440-CS and the pros and cons that you experienced with it.

"Also can you please advise on why you moved on from the ClassD Audio SDS-440-CS and the pros and cons that you experienced with it."

Hi daywalker702,
It was mainly out of curiosity and a desire to gain more experience with class D amps that I moved on to different amps.
But, I think I was also spurred on to try another D amp by the fact I wasn’t thrilled with the physical looks of the ClassD Audio SDS-440-CS. It’s got a basic and utilitarian appearance that reminds me of Soviet Russian styling even though its sound quality is very good.
This is a somewhat embarrassing admission since I do consider myself a serious audio enthusiast concerned mainly with sound quality. However, I’m not blind and I also appreciate good styling, especially in a component in my system that is visible on a constant basis.
So, I used the SDS-440-CS to drive the rear surrounds in my combo music and ht system, replacing an older Adcom GFA-335 class AB amp located behind a closed door in my system rack.
Next, I bought the Emerald Physics EP100.2SE stereo class D amp rated at 150 w/ch at 4 ohms and within a much more attractive chassis.
This amp had all the good qualities of the ClassD Audio amp (good bass response, very low distortion,a dead-quiet background noise level, a highly detailed but still very smooth mid-range/treble response and a very neutral overall sound quality) but it lacked the powerful dynamics on my Magnepans that I found so natural/realistic and enjoyable using the CassD Audio.
So, I decided to use the Emerald Physics in bridged mono configuration (350 w at 4 ohms) to drive my Magnepan CC3 center channel spkr, replacing an older Adcom GFA-545 class AB amp also behind a closed door in my system rack.
Based on very good pro reviews, I then bought a pair of D-Sonic M3-600-M mono blocks rated at 1,200 w at 4 ohms within plain but attractive chassis.
These amps had all the good qualities of my first two class D amps but with significantly better bass response and dynamics than either. The D-Sonic monos are the best amps I’ve ever heard thus far on my Magnepan 2.7QRs by a wide margin.
So, I mounted them on matching maple isolating amp stands with brass cone footers and placed them on the open top walnut shelf of my system rack. They sound and look great and I’ve seriously enjoyed these for about 3 yrs now.
A Goldilocks ending to a class D journey. But a journey that’s likely to continue given the seemingly constant improvements in class D technology and performance.
Sorry this was so long but I wanted to give a thorough response to your question.
Tim

I guess I been screwing up since 1990 running 2 gfa 555s driving two 8ohm speakers and two 4ohm speakers? There is some kinda special cord I was sold back then to mix 4s with 8s?