EAR/Yoshino M100A monoblock power amplifier Page 2

A distinguishing feature of the M100A is its transformer coupling at the output. (The only other solid-state amps I'm aware of that use this topology are some of the McIntoshes.) The documentation explains that this "offers advantages to the designer who can produce suitably high-quality transformers. Tim de Paravicini, probably the leading authority on audioband transformers, is such a designer." It's good to believe in oneself, no?

The other salient feature of the M100A is that it's single-ended, "a configuration which is seldom used in transistor amplifiers." TdP goes on to explain that without the benefit of transformer coupling, "single-ended outputs are both exceptionally inefficient and difficult to make work properly. And even with an output transformer, there is a penalty to pay in terms of efficiency, hence the very substantial heatsinks on the M100A."

TdP continues to develop the M100A's raison d'être by explaining that "single-ended operation inevitably means that the amplifier is working in pure class-A. The output devices are never completely cut off, and at idle are right in the middle of their linear operating envelope. This in turn ensures that at low signal levels the amplifier is as linear as can be, in contrast to more conventional push-pull class-AB topologies, which always risk producing relatively high distortion at critical low levels." An unspecified number of power MOSFETs each has its own bias and drive components, ensuring "accurate current sharing between the devices...irrespective of variations between the MOSFETs." Feedback is taken from the primary side of the output transformer, "eliminating low-frequency stability problems often associated with secondary-derived feedback. Only about 15dB of [negative] feedback is required, as the amplifier is intrinsically highly linear."

Hence the huge, thick, hollow aluminum heatsinks, square-sectioned and open-topped, that populate the M100A's side panels, creating a chimney effect to throw off the prodigious heat produced by these suckers. You could float a hot-air balloon from here to Laguna Beach nonstop on the heat thrown off during a night's sustained listening session. Manual: "Cooling should be adequate even for use in non–air-conditioned rooms in the warmest climates." For the amplifier maybe—but for the user? I never schvitzed so much from the heat from any amp I ever used in my life, and that includes the mighty VTL Wotans. Touching the heatsinks or ½"-thick faceplate during operation is possible for only the briefest moment before le sizzlement sets in.

The power supply uses a choke filter, once again in vogue. Such a filter "gives maximum rejection of mains variations and minimum peak current demand on the mains supply," reports the manual. Following is a large bank of filter capacitors, another choke, and yet another bank of capacitors, "with the result that power-supply ripple is held to exceptionally low levels of approximately 0.3mV, and hum at the amplifier's output is almost nonexistent." True—the M100As were really quiet.

TdP says that traditional "brute force" capacitor-input filtering would require a stack of capacitors "about the size of two M100As in their flight cases set side by side." Lemme outta here! Total energy storage in the power supply is said to be near 200 joules.

The manual cautions the user not to bother with power conditioners, which are rendered unnecessary by the monoblocks' superior in-house–designed mains transformer and power supply. But as you'll read below, the manual's claim that TdP "designed this amplifier to give a lifetime of trouble-free performance" proved not quite fulfilled.

Setup and Unexpected Troubles
The user is instructed to turn the volume down before powering up each M100A. You're advised to then check the lower meter and make sure it reads somewhere near mid-scale. My impression was that I didn't have to turn the volume down when powering up the amps—after all, importer Dan Meinwald hadn't when he'd set them up. But each time I turned the left channel on with the volume up, a brief electronic zing emanated from the JMlab Utopia's tweeter—not too loud, no real problem, no flaming tweeter in the lap, but still...disturbing. Interestingly, the other M100A didn't exhibit this anomaly. Also, for the first week or so, the power meter on the left amp didn't budge. I rapped it a few times, after which it moved for a while, then stopped. I gave up. A few days later, it started working normally. Go figure.

I also had a problem with the dCS Purcell/Elgar Plus combo, which, unlike the Accuphase DC-101, is very sensitive to voltage transients and DC. Whenever I turned the M100As up loud, the Elgar Plus went nuts, lost its lock, muted, relocked, then started playing again. Jeez, now what? I thought. Might it be the DC current in the shields of my Synergistic Research Active Shielding Designer's Reference interconnects? But when I turned off the Master Control Center for Discrete Shielding only, the locking hiccups continued unabated. (I had tried Tara The One and Cardas interconnects before the Synergistics, and the same problem had been evident.)

I then pulled the connectors from all the Synergistic interconnects and unplugged their Master Control Center, but the dCS still hiccupped away. The sound was also unacceptably lean, even on vinyl (through an XLO Reference Phono cable), so out came all the Designer's References, however guiltless. In went the previous combination of TARA The One ISM Onboard and Cardas Golden Reference interconnect and speaker cable. (I used single-ended connections because the M100A is a single-ended amp, though there wasn't much difference in sound with balanced connections.) The sound was better, less lean and more soothing, but the dCS still hiccupped like a drunk at anything over 5W.

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