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A perfect amplifier?
As exciting as valves with brass balls sound, they can be red herrings. A recent e-mail promised them from Dintec Plumbing Co. in Ningbo city, Zhejiang province. Back on the ball. Regardless of persuasion, the perfect amplifier faces conflicting requirements. It must span the nano to macro range from a fraction of the first watt to the high-draw 3-digit bursts of climactic low-impedance bass transients. It must be brutishly load-invariant and elegantly sophisticated at once. The appeal of single-ended amps is purity. They don't rely on multiple output devices that need matching nor circuits that induce switching. Inherently, their power is limited to what a single device can generate and, with tubes, output impedance is high and self noise likely. Load invariance relies on massive power supplies and headroom. High output power is generated by massively paralleled output devices where even minute offsets from 100% matching will counteract low-level clarity in the nano range.
The ideal amplifier should operate with zero noise since the micro activities within the first watt are audible only above the noise floor. Its high amplification factor should enlarge the tiniest of input signals as substantial output events to make them audibly relevant. Naturally, high gain not only magnifies micro details but amplifies noise by the same factor. The power supply should be stiff to not sag regardless of load and the circuit should be as agile and responsive as a fierce stinger. Its operating points should be stable and the amplifier protected against fault conditions without sonic compromise. Distortion should be as low as possible, bandwidth expansive.

This bundle of demands is inherently at odds with itself. Still, the Thorens amp concept makes a most convincing case for bundling just such perfection and crossing off all the requirements. It creates high voltage and current gain from a single-ended push-pull circuit which enforces stable, self-monitoring symmetry down to the micro level. (Nelson Pass' aptly named SuperSymmetry attempts the same with massively paralleled output devices). The output MOSFETs' extreme specifications insure that despite the high output power, the devices are run conservatively within their linear range. The S/N ratio is high (Stereoplay measured 93dB), the claimed transfer function ultra linear. The very high transconductance of the gain devices turns a 1milliVolt input signal into a 40mA current at the output to promise excellent first-watt reflexes. Speed and phase linearity are covered by exceptionally wide bandwidth. Surprisingly, this is achieved with tubes. The last unconventional amplifier to cross my path which combined tubes, high power and true load stability was B.K. Butler's Monad A100. While it used a single 300B in a novel current-mirrored semiconductor output stage, it sounded more transistor than valve. What would the six valves in the Thorens contribute sonically?

For the true measure of the TEM 3200s, one would confront them with 15-inch woofers and known beastly loads. That's what the German Stereoplay team threw at these monos, declaring them victorious over the big McIntosh MC501 AC monos (500 watts from 8 to 2 ohms). I however don't own such speakers. This limits my ability for a complete assessment. What I'd focus on would be the amps' first-watt-plus prowess. As stated, what traditionally guarantees superior nano excellence (circuit simplicity) can confound muscle amps by design. Should the 3200s acquit themselves with high honors on speakers amply served by 2-watt
SETs or 30-watt T-amps, one could triangulate such results with already published findings on big Isophon, Thiel, Sonics and similar speakers (Stereoplay's test bench measured the Thorens amps as good for 614 watts into 2 ohms with a damping factor of 800 into 100Hz). In combination, this would determine whether these unique monos indeed have the makings to be amps for all seasons. More than always, you'll have to square up and triangulate my review with other findings for the complete picture. (As an aside, Frank's circuit appears in all tube guise in the matching Thorens TEP 3800 preamp. Its output impedance without an output transformer is <1 ohm, its bandwidth in excess of 600kHz. The Blöhbaum circuit does defy expectations particularly when realized with valves.)
While Frank Blöhbaum claims class A-reminiscent behavior for his class A/B amp because he avoids switching the output devices, two Nelson Pass papers available on the Pass Labs website sketch out a few rule-of-thumb specs on what a non-technical shopper should be looking for with pure class A amps: The "purity" of Class A designs has been at issue in the last few years, with "pure" Class A being loosely defined as an idling heat dissipation of more than twice the maximum amplifier output. For a 200 watt amplifier, this would be 400 watts out of the wall on a constant basis. The best power transformers are toroids with donut-shaped magnetic cores. They pack the most power for weight and size and they make less noise. Toroidal transformers have to be rated at a minimum of several times the intended wattage because the power is delivered in short pulses to the capacitors. For a 200-watt class A amplifier, this means about a 3000-watts power transformer for continuous operation. A toroidal transformer delivers about 30 watts per pound, so a 3000-watt toroid will weigh about 100bs, maybe more. The rest of such an amplifier will probably weigh about as much so if you are looking at a 200wpc stereo class A amplifier, you will want to see if it weighs at least 200lbs.
At 61.7lbs and 200 watts, the Thorens TEM 3200 clearly lacks the physical heft and heat-sink mass to generate and dissipate full power in pure class A. Nor does it claim to. What it claims are similar sonics but from a more efficient, cooler running amplification approach which is quite in keeping with today's energy conservation attempts. And this completes the introduction, historical and technical presentation. Save for a last note on how Frank Blöhbaum landed at Thorens: "As so many things in life, there was a good portion of luck. I spent about 5 years from the first concept to a completely working prototype. Then I filed the patent and sent out a short specification brief to various companies. Uwe Bartel, the German distributor for Thorens, rendered the introductions
to Heinz Rohrer. Heinz liked the idea very much to combine the relaunch of Thorens with a truly new kind of amplifier and gave me the freedom of designing the perfect amplifier for him.


"Yes, the perfect amplifier was the goal, not a cost target. That of course is the dream of every desinger. I already had a well-paying job at a good company. Designing a dream amplifier then had to be pleasure and art, not mundane work. So this setup was perfect and I very much respect Heinz for making a high-risk decision. It could have ended with disaster, waiting a long time for a mediocre amp or nothing at all. But he trusted me and I appreciate that tremendously. As it turned out, it took me two more years to perfect the TEM 3200 in all details. Spending at least 7 years on developing an amplifier sounds crazy, even stupid. Perhaps only audio nuts and music addicts could waste so much time and effort..."