Full review in AUDIO magazine (Germany), Issue May 2008

Accuphase Monophonic Power Amplifier M-6000

On The Safe Sideby Malte Ruhnke

Nothing was left to chance: the mono blocks M-6000 are going to set a new
benchmark in the portfolio of Accuphase. And they've got the making tobewitch even the most discriminating listeners.
Giants, a reason for celebration, the best power amps of all times - with superlatives
like these AUDIO was welcoming Accuphase's monophonic amplifiers M-8000 in theyear 2002. With all due respect, where and how can this renowned manufacturer top
these amps by launching the M-6000 now in 2008? Actually, I always thought that
progresses in amplifier technology could have been only marginal in the past sixyears while the top dogs up to now are surpassed by the newcomers neither by their
sheer size nor by power specs.
Well, the virtues of these Japanese power plants simply cannot be debatedeven by down-to-earth contemporaries: their stability under any condition guarantees
that even with critical, demanding speaker systems the sonic results are not spoiled
by interactions between amp and speakers. The overwhelming output power willbring also low-efficiency transducers out of their shell. Month by month the newest hifi appliances are introducing themselves in the AUDIO listening room and hence forcomparison purposes a power amplifier, which is stable under any conditions, isworth its weight in gold.
DA CAPO
Talking about value - this seems to be the biggest difference between a pair of
M-8000 and M-6000, whereby the latter, with nearly the same promising power,
comes with a price tag which is about 35% less expensive than the one of its bigger
brothers. No question, this is still a lot of money for two monophonic power amps, yet,like with any other audio component from Accuphase, when put into perspective as tothe stable value over the time and outstanding workmanship, one might think a bit
different here.
Very few changes can be noticed when the M-6000 is viewed from the outside.
Except for some prominent features on the front panel, which in the M-8000 are
decently hidden, namely the switchable input sensitivity, absolute phase and aselector switch for balanced or unbalanced inputs.
PRELUDE WITH TECHNOLOGY
Inside, however, we are to encounter some big changes indeed. Instead of bipolartransistors there are field-effect semi-conductors, a.k.a. MOSFETs, taking care of thefinal amplification. While conventional solid-state devices of the bipolar variety aretrying to amplify low base currents, the work of MOSFETs can be compared with atunnel for electrons of which width and thus flow rate is regulated by an electric field.

Rather, the function principles are reminding us of those going on in a thermionicvalve, and this is also why many high-enders believe the sound of MOSFETs coming
close to the sound of valves.
Instead of employing a pure Class-A circuitry, in which the power transistors
are permanently wasting half of the maximum output power in form of heat,
Accuphase has gone for a push-pull circuitry in A/B mode of operation in the two big
mono blocks. An array of 16 MOSFETs amplifies the signals below the cross-overline while the other batch of 16 devices is doing the same above the line.
Such an elaborate topology, the more since it features altogether 32transistors, is nevertheless subject to cross-over distortions. In order to keep them as
low as possible, the engineers at Accuphase have split the MOSFETs into twoparallel groups with 16 transistors each and connected them with two driver stages
and feed-back loops. This layout was dubbed "Double MCS" (Multiple Circuit
Summing-up) by Accuphase and it is said to be extremely precise and smooth whencomparing the input with the output signal, whilst taking care of the best possible
signal-to-noise ratio. This all takes place in a balanced signal path throughout inorder to process the positive and negative half-waves independent from earth.
In our lab the two mono amps delivered nearly perfect measured values - yetthey can still do more. At a load of one Ohm or even lower the voltage is supposed tostay completely stable. There is also a facility to have two mono blocks bridged inorder to mobilize altogether a hefty 2,400 Watt per channel.
GETTING RID OF THE TROUBLEMAKERS
A system at this high level would certainly not tolerate any bad conditions as theAccuphase mono amps were clearly demonstrating after we connected them to ourKEF Reference 207/2 speakers. On the surface, this combination was able to deliver
powerful and quick-off-the-mark sounds, yet no one of the listening panel was really
happy with the inharmonious basses and the altogether rather harsh sonic picture.
Owing to our joyful anticipation of listening to music the AUDIO crew has simply
forgotten to remove some super-speakers which were still lingering about in ourlistening room. Actually, those speakers were behaving like uncontrolled bass traps
whilst absorbing decisive energy for the sonic picture.
This speaks for the quality of the chain, yet on the other hand makes alsoquite clear that in spite of all enthusiasm for this fine amplifier technology, the bigsonic differences are actually to be found in the speakers and room acoustics
themselves.
GRAND FINAL
Of course, the bad conditions were immediately optimised whereupon two highly
experienced amplifier specialists, namely Bernhard Rietschel and Lothar Brandt,
went to evaluate the subtle differences between the Accuphase amps and the current
reference mono blocks "Max" from Audionet (see review in AUDIO 03/2008). Inparticular the sonic beauty of the Audionet amps initially deemed unbeatable to thefour expert ears. A fine resolution, tenderness and a very natural sound, that's how
they characterized the rendition of Dvorák's 8th Symphony (AUDIO pure music #1,Bose) through the Audionets and KEF. With respect to the M-6000, they attested abroader and in the higher frequencies very realistic reproduction, although not quite
as extravagant.

A matter of taste? Yes, perhaps, but I, being the specialist in speaker testing,would always prefer the mono amps from Accuphase because of their slightly fuller
and warmer sound. Eventually, even my colleagues would admit that the M-6000 are
getting a tiny bit closer to the tonal truth.
This was definitely approved when we played "Snow (Hey Oh)" from Red Hot
Chilli Peppers ("Stadium Arcadium", Warner) and additional control and stability wasasked for. The two M-6000 left absolutely no doubt that they would depart from theirrich, dynamically controlled but still indefinitely transparent style of playing by noteven a micrometer. Indeed, with such music material they even sounded morehonest and more precise than the pair of Octave Jubilee valve amplifiers (see page110) at almost double the price, yet to which they came stunningly close in terms ofoverall sonic rendition.
Even more so when the vintage Isophon Vertigo speakers (tested in AUDIO
02/1994) - considered a true amplifier-killer, owing to their minimum impedance of
1 Ohm - took over and were electrically pulling the speaker binding posts of theM-6000. When we played Stravinsky's "Sacre du Printemps" (Sir Simon Rattle, EMI)
the Accuphase monos were able to disclose that this "old hero" is tonally no longer
up to date. Nevertheless, the band-pass drivers of the Virgo, which are known as
being pretty difficult to drive and control, seemed to be tamed under the authoritarianleadership as they were emitting the many thundering tympani eruptions in therecording in a very clear and dry manner.
In combination with any such demanding speaker system, the gigantic power
reserves of the M-6000 really make sense. When switched to "peak hold", the bigVU-meters on the front panel are capable of documenting the "loudest" deflection ofthe indicator. Well, during the AUDIO test this was usually beyond 50%.