Adcom 3 # amps v.s 4# amps

[rselby]

Adcom bi-polar amps V.S Adcom MOSFET amps!

ran across an adcom GFA-555 amp that I may get, saw on their web site that the older amps(GFA-555) uses bi-polars( whatever this is) v.s the newer amps (GFA-5500) which I believe is the the same except uses MOSFET's...NOW is one better than the other, I have never owned an adcom amp before...so wanna ask you guys opinions...thanks

 

[hjames]

I got a pair of early Adcom GFA-555s ... had them for a couple of years running my JBL 4341 studio monitors when I had them biamped.
- love them to death with those JBLs. We never play real loud and it they were a very good match.
Currently, one is running the front mains (Vandy 2CEs) in my home theater, the other is set in bridged mono (est 400+w-mono) running an 18 inch JBL subwoofer ...
They do look a little different. I have seen 3 different styles for the original GFA-555s,(not counting the GFA-555 vII).
I included a picture from when they were in the biamp rack - they are the lower 2 amps

One of mine has big flat handles on a smooth front panel and built-in rack mount ears.
Its the lowest one in the rack. The one just above has the usual Adcom look.
I think that one is a bit newer, it has the fins on the upper front panel and add-on rack mount ears.

I've got a B&K ST-202 Plus amp on the way to try out on the Vandersteen 2CEs ...
its 200w/ch - the same power as the GFA-555, but a bit warmer.
Its supposed to be a better match for the Vandys.

I think which Adcom you'll like depends on what speakers you pair it with.

 

[rselby]

the one Im looking at looks like (your normal looking one) was just wondering if there is any sound, power differnce between the bi-polar and the MOSFET amps..

 

[hifitommy]

i own a 555II which sounds wonderful and my friend had a 5500 and a 5400. he preferred the 5400 which is a VERY powerful sounding 125wpc.

i will bet that the vandys will respond nicely to a 5400 but then they do well with tubes too.

 

[frommerstop]

I tried a borrowed original 555 on Vandersteen C2i , and found them an OK match, but not as good as my Audire - Good bass, but a bit bright, bordering on a tiring quality. I later tried a 555 MkII, and it sounded much better in my system... good enough that I bought it, and sold off the Audire. I've always found Adcom in general, a little forward, but the 555 MkII sounds a bit sweeter than most of their other "larger" amps - I also like the little 535, I think, I get their models confused. As for Mosfet vs bi-polar, I'm noncommittal, seems there are good & not so good amps in both camps so I don't pay too much attention to the drivers.

 

[musichal]

Found this from Tom's Hardware, posted either by someone with the handle "Anonymous" or anonymously posted, or both - unclear to me which:

"Greetings,

I've always felt that if audio manufacturers truly aspired to achieve
100% accuracy in electronic components, everything would slowly begin
to sound the same. In my quest for a new amplifier, it seems the use
of varying technologies flys in the face of accuracy, and creates
certain sonic "signatures".

I've read that the matching of an amplifier to a set of speakers is
critical, and that some speaker designs lend themselves to BiPolar
amplifier designs, while others match better with MOSFET amplifier
designs. It is said that BiPolar amplifiers sound "faster" and
"punchier," and MOSFET designs are "softer".

Based on some general research, MOSFET's devices are technically
superior to BiPolar devices in just about every parameter:

Mosfet devices are inherently self regulating. Since they exhibit a
positive thermal co-efficient, they don’t suffer from the dreaded
BiPolar thermal runaway.

Mosfet devices are very fast and switch several Amperes in nano
seconds, which is 30 to 100 times faster than equivalent BiPolar
devices.

Unlike BiPolar devices, Mosfets will handle short overvoltage and
overcurrent operating conditions without terminal failure.

In practice the reliability of Mosfets in surviving real world abuse,
usually overheating and driving output shorts, is vastly higher than
that of comparably rated Bi Polar output stages, even with their usual
extensive protection.

Mosfet devices require very small amounts of current to operate
correctly, allowing a much simpler drive circuit to be used,
maximising reliability and performance.

Mosfets exhibit a "soft clip" characteristic when driven into clipping
levels. This is predominantly made up of even order harmonics rather
like that of Valve Amplifiers, not the harsh odd order harmonic
clipping produced by BiPolar Amplifiers.

The misconception that Mosfet Amplifiers have by definition low
damping factors was primarily due to early low output Hitachi device
based amplifiers with marginal Power Supplies. This is not a factor
with today’s Third generation high output Mosfet devices that are used
throughout the ARX range.

If MOSFET designs are technically superior, why do companies like
Levinson and Krell use BiPolar designs?

Also, is there truth to the contention that BiPolar designs are
"faster" sounding?

Thanks!"

Personally, I've heard well-designed and well-executed (in manufacturing) amplifiers that sound great of many various types, of course including various tube designs. More than one way to skin a cat. Love my Adcom GFA-5500 and 5503 amps - maybe someday I'll have to replace them, but not as an attempt to upgrade. They are close enough to perfect for me.

 

[fiddlefye]

i own a 555II which sounds wonderful and my friend had a 5500 and a 5400. he preferred the 5400 which is a VERY powerful sounding 125wpc.

i will bet that the vandys will respond nicely to a 5400 but then they do well with tubes too.

I don't know a whole lot about Adcom having never owned anything by them, but I run my Vandy's on CJ tube power these days and even at 45 w/ch they math really beautifully. Mind you I'm not trying to fill a really huge space.

 

[hjames]

I don't know a whole lot about Adcom having never owned anything by them, but I run my Vandy's on CJ tube power these days and even at 45 w/ch they math really beautifully. Mind you I'm not trying to fill a really huge space.

Yep - had them on Jolida 40w/ch tube amps when I first got them - but when I brought them downstairs i found having more power did open up the sound quite a bit. Again, I don't play loud at all, but the bass is a lot fuller with more capacity.

And, while there is nothing wrong with the Adcoms, I'd heard the B&K ST-202 Plus amps were a bit warmer (a little more tube-like) compared to the Adcoms, and that sounded good to me!

 

[JoeESP9]

ran across an adcom GFA-555 amp that I may get, saw on their web site that the older amps(GFA-555) uses bi-polars( whatever this is) v.s the newer amps (GFA-5500) which I believe is the the same except uses MOSFET's...NOW is one better than the other, I have never owned an adcom amp before...so wanna ask you guys opinions...thanks

Buying an amplifier because it has Bi-Polar or MOSFET transistors is IMO silly. buying an amplifier because you like the sound makes sense to me.

For example, Krell and Levinson use Bi-Polar transistors while B&K, Unison and Jolida use or have used MOSFETs. Adcom has used both.

Adcom amps are IMO good sounding reasonably priced amps. The older 5x5 series should have the capacitors checked as they are nearing the end of their life cycle and when they go they put DC on the speaker lines. Adcom amps are all wide bandwidth designs with no protection on the output stages. They are relatively insensitive to resistive loads and can drive very low impedances. They can go into ultrasonic oscillation when confronted with a highly capacitive and or reactive load.

With all that said:
I have two GFA-545's in daily use driving the rear speakers (Acoustat Model 1's, Acoustat SPW-1, Paradigm X-30 electronic crossover) in my system.

 

[n8thesk8]

I just picked up the new Adcom 565 SE amp and i have to say it is a great sounding amp. I have the 545ii and a pair of GFA-1's and the 565 has a nicer sound. It's a beast 250 wpc and 51 lbs.

 

[hifitommy]

http://audiokarma.org/forums/showthread.php?t=460717

the best sound i ever heard on VIIcEs was with an ARC classic 60 sold by optimal enchantment. then another friend, a doctor said he was driving his with a 30wpc musical fidelity tube amp so tubes is probably the best way to go with them.

the fact that nelson pass had a hand in the design of some of the mentioned adcoms may explain the great sound they produce.