Adcom GFA-555 II vs 5500 or MosFET vs BiPolar experience and opinion

[Rein E. Narma]

Interested in comparing Adcom GFA-555 II vs 5500 or MosFET vs
BiPolar experience and opinion. I am curious whether my experience
relates to all/many mosfet designs (Hafler 9505 etc).

System is: Pioneer Elite PD-59 >AA DDE 3.0 > DTI Pro 32 > AA DLC >
Adcom 555 II or 5500 > VMPS Super Tower III.

cables dyi: interconnects (mic cable) and 4ft CAT 05 UTP and/or 8ga
line cord. Connection config is either biamped pairs (555 or 5500)
of amps or a single 555 or 5500 amp driving both speakers.

I am hearing much larger differences (pro and con) between my 555 II
and new 5500 than I expected regardless of mono or biamping. The
amps have nearly identical power ratings and very similar specs.

The fundamental technology differences are: bipolar vs MOSFET
(Hexfet) output devices and much higher (50 wpc) class A bias on the
5500.

The 5500 is smoother and exhibits less grain especially on female
vocals. It also seems to provide less dynamic contrast, "slam" and
speaker control particularly at higher volume levels. Loud passages
and transients seem to have less power and impact when compared to
the 555 II in each config. (I am going to do some measurements of
instantaneous current and voltage levels on peaks.) The sound seems
almost compressed, louder but with less and less dynamic range as if
it were "soft clipping". All this is happening at 200w peak levels
but without exciting the "distortion alert" leds.

Having said the above I still prefer the 5500 smoothness over the 555
brute force for most of my listening. I would be curious about
pairing 555 IIs with 5500s in a biamp config.

Any of this sound familiar?

Thanks,

Rein

[Randall Bradley]

The differences that you are hearing is likely to be more
related to the specifics of the two amplifiers that you
are comparing than to some general principle underlying
a bipolar vs. Mosfet.

Even so, in my experience there are some qualitative
differences between amps made with the two different
output devices. Mosfets do often tend to sound less
"grainy" although there are some bipolar amps that
are surprisingly free of grain as well.

There is the issue of how these two amps are biased and
how the feedback loops are closed at DC. That issue
will make a significant difference in the overall
performance of the two amps.

Running the amps at or near the rated power will accentuate
the the bias/feedback/clipping/power supply issues.

_-_-

[G. Bandru]

Rein

I can speak a little from experience and a little from education
(EE). There are fundamental doifferences between a FET and a bipolar.
FET's are voltage gain devices where bipolars are current gain
devices. As this is the case FETS have much more in common with Tubes
than thaey have with bipolars (which may explain why some people who
like tubes also like FETS but thats another story). The FETS and
bipolars distort differently, FETS generally softer at the peak while
bipolars clip harsh. In addition PNP-NPN junctions of bipolars utput
stages have nonlinearities at the zero crossing (hnce the biasing to
Class A to remove some of this). With FETS in a push pull this is
less of an issue. In a nutshell they will sound different from
bipolars in most if not all cases.

As for experience I own a 555 II and love it. I have auditioned the
5500 and, while I liked it, didnt feel it was "better" then the 555
II, so I didn't buy it. I have read comments from others that are
similar to yours..in fact many have said they prefer one or ther
other but never both. If you like the 5500 better..use it. They are
both fine amps for the money and either sound drive your speakers
well. As for biamping, I'm generally found it doesn't make a huge
amount of difference. I would stick with one amp to drive oth
speakers, OR, if you really want power take the 555II and brige them
and use one for each channel. Hope this helps. Happy listening.

Greg

Rein E. Narma wrote in message <6hr10f$b...@news01.aud.alcatel.com>...

[Rein E. Narma]

Thanks Greg and to Randall who also responded.

I have done some more critical listening since my post, based on which I
will now have to turn in my "Amps-Sound-Alike" membership card. The
more I listen the more differences I hear. My 70's vintage EE didn't
cover power MOSFETs and my only experience has been in small signal RF
work. Also, I don't recall my old Hafler DH-500 sounding that hugely
different from my Ampzilla and Dynaco 400. The limiting factor then may
have been the lower efficiency and dynamic range of the Dahlquist
DQ-10's vs the VMPS. My next steps will be to do some single blind
listening using two different outputs from my AA DLC after carefully
level matching the outputs with a test CD steady state signal and
O'scope.

Back to the listening. I expected the slight differences in grain and
detail with 555 II being more etched and the 5500 softening vocal
sibilance and cymbal brush strokes. I even expected some softening of
the bass. When listening again I heard what sounds like a looser "grip"
and reduction in dynamics and contrast even at moderate levels. On
some recordings this adds ease and smoothness. On others, it adds a
layer of thick gauze over rim shots and guitar licks. This was not
subtle! Less air is being moved.

I already know that "distortion alert" LEDs don't react (or more
correctly have little or no peak hold circuitry ) to very short term
clipping on 555 and probably don't on 5500. If my listening tests
continue to show the differences I will see if I can measure major
differences in current capability, slew rate and short term dynamic
power. I also need to see if I am having some line voltage problems
with the pair of 5500s as they draw a few hundred watts more on average
perhaps resulting in some power supply sag and early onset of clipping.

The Adcom folks have been very helpful and generally agree that the two
amps sound very different. Nothing in the specs hint at the obvious
dynamic contrast and "power" differences I am experiencing.

More to follow when my measurements are complete.

7 Line voltage behavior
7 Internal power supply behavior
7 Peak short term Power / Current
7 Square wave response

I know the high-end generally dismisses measurement as a key indicator,
but this is too large and obvious difference in performance for it not
to have some clearly mesurable cause. Specifications, on the other
hand, seem not to always be a reliable predictor of sound.

Thanks again.

Rein