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Don't call it a Circlotron
"Regarding the Sumo story, I analyzed the patent of Mr. Bongiorno in detail in my own filed patent (see EP 04030390). The examiners of the European Patent Office rated the Bongiorno patent as well as the other documents only in the class "technology background" (category "A"). That means there isn't a single published document which alone (category "X") or in connection with other patents, documents or products (category "Y") could be used as a guideline for building an amplifier like the TEM 3200. In short, my patent really is a new thing." Frank Blöhbaum works as an engineer in a 5,000 employee strong company in Freiburg/Germany. He is the head of the New Technologies and Basic Research division. He performs systems analysis with applied mathematics and simulations and prototyping of promising solutions. He also is head of the company's Analog Design group and their senior analog simulations consultant. His expertise is in precison time-of-flight laser radar sensors. Their sensor/target/sensor reflections occur in 6.66 picosecond windows. "To measure that under all conditions even in dusty industrial environments at low cost is a real art because the fastest affordable electronics are about 1000 times too slow." Blöhbaum also works on ultrasonic sensors for gas monitoring, 2D and 3D cameras, complex mixed signal processing and such where operating bandwidths range from DC well into the GHz range.
What separates his circuit from the earlier Circlotron variants based on the 1955 Electro Voice precedents (Experience Electronics, BAT's VK-60, VK-75 and VK-150) is that those require tubed output stages since semi conductor biasing isn't feasible. AC coupling of the voltage amplifier is mandatory as is a center-tapped output transformer or low-resistance voltage divider across the load. Long-term thermal stability of the operating parameters is lacking. Atma-Sphere's main patent describes how to create two virtual floating power supplies from one real power supply by means of decoupling resistors and capacitors. "That really works only with preamps and is utterly useless for power amps. The basic structure of such amps is the same as that of the Electro Voice and other standard push-pull parallel amps. The main distinction is that Atma-Sphere eschews the output transformer. Again, valves in the output stage are mandatory, AC-coupling of the voltage amp is a must as is a low-resistance voltage divider across the load. There's thermal and operating point drift, output voltage offset drift, high levels of odd harmonics due to mismatches with common speakers (Graaf GM-20, Croft Dakshini, Einstein The Final Cut) or huge paralleling of output tubes (Graaf GM-200) where output tubes "are squeezed to deliver abnormally high current levels for questionable reliability".

Hybrids like Einstein's Absolute Tune use a classic tube voltage amp AC-coupled to a balanced/bridged floating MOSFET output stage with a bipolar driver stage where biasing occurs with a source degeneration resistor, meaning both halves of the floating output stages are independently biased for two dissimilar quiescent currents. This causes output offset currents and limits the topology to "fairly low power" (50-watts into 8 ohm for the
Sidebar I: James Bongiorono's Sumo 9 and 9+ were a major step forward away from the limits of pure tube technology but could only be realized with bipolar transistors (BJTs) in the output stage... continue
Einstein). Ultra-power output MOSFETs can't be reliably biased and local as well as global feedback loops are "nearly impossible due to bias problems and AC coupling. This causes sonic coloration from the output MOSFETs which are contributing the majority of THD. That is in stark contrast to Einstein's marketing claim that only the input tube stage defines the sonic characteristics".

Lastly, the 300, 600 and 1000 Series of former East-German Vermona amps were all bipolar transistor amps with two Triple-Darlington output stages independently biased to cause bias drift and large non-linear distortions. And while Gamut does use the beefy SOT-227 packaged power MOSFETs also used in the Thorens amps, "Gamut uses an old-fashioned quasi-complementary 1970's circuit so they have a very good output device coupled to a poor driver stage". The Blöhbaum circuit meanwhile can be realized with all BJTs, all MOSFETS, a mix thereof or as a full tube amp (which is exactly what the matching Thorens TEP 3800 preamp does). And while Tenor Audio claims exclusivity for coupling a powerful tube gain stage to a transistor power buffer with zero voltage gain, "the TEM 3200 has all the voltage gain in the tube stages too and was on the market 2 years prior to the Tenor amp".

Because the Thorens' monolithic output transistors never switch off to have a constant well-defined drain-source current, there are no crossover or switching distortions and "the amplifier behaves sonically like a class A amplifier but with much lower power consumption. The beefy output MOSFET has an incredible transconductance of about 40A/V. One volt charge of gate-source voltage causes a change of 40 amps in the output circuit." With 132000uF of storage capacitance, foolproof protection circuitry is naturally critical. Then, with a >100MHz open loop bandwidth, "the global feedback loops work nearly perfectly. I argue that people who claim that global feedback amps don't work have never used it properly." Finally, the TEM 3200 circuit creates self-cancelling magnetic fields and the slew rate is symmetrical while "almost all common complementary amps suffer a slower falling than leading edge because the pnp or p-channel devices are slower than their npn or n-channel counterparts."

To create a balanced signal from unbalanced inputs, the TEM 3200 uses a custom ultra-quality input transformer. The gestation of this amplifier took one solid year of pure calculations and simulations and another 7 years of implementation. Lastly, Frank doesn't like to apply the Circlotron term to his amp because that Electro Voice concept applies only to all tube circuits. It relies on a center-tapped output transformer with a fixed grid bias and/or cathode resistor and an AC-coupled voltage amp. Blöhbaum is free to use every known electronic part as the active decive and he employs no output transformer, center tap, low-resistance voltage dividers for virtual center taps etc. "The only thing my circuit has in common with a Circlotron is the cross-connected output stages fed by two floating power supplies. It's really a completely new system but since I didn't find a cool buzz word for it, I simply called it 'Blöhbaum System'. With that name, I didn't expect any problems registering the brand."

Heinz Rohrer meanwhile joined Thorens by mid 1999 to reorganize the brand's export activities and invest in production support. "I knew that the Thorens organization in Lahr was in trouble. The turnover was 85 % domestic with a lot of unpaid OEM invoices. As a security for my investments, I used shares of the Swiss Thorens brand-holding company. 6 months later, the production facilities at Lahr declared insolvency and liquidation followed within a few months. Then a battle ensued with a part owner but I did not wish to invest myself on solely a minority ownership. A cash infusion was sorely needed which I raised but at the time only against ownership shares. By early 2003, I was finally the sole owner of the Thorens brand name and completely restructured the company. The production facility at Lahr was hopelessly antiquated, tools were broken and all the useful engineers had left or retired, with the remaining 2 or 3 people useless.

"I restarted negotiations with OEM suppliers who had worked with Thorens in the past and founded a new company for producing the new Thorens line. Thorens Engineering was no longer in place as I was forced to make plant changes by the end of 2004. Fortunately, I had the support of the retired old factory manager who still loved Thorens. Some two years ago, a new Thorens subchassis player was introduced, an outside runner named T 350. I also wanted to have a traditional inside runner so we introduced the TD160 HD half a year ago.

"I met Frank about three years ago. He wanted to associate with a traditional German brand for his amps. Alas, we rather did underestimate the work load involved and how a production line assembly had to be organized. I did not pressure anybody because I wanted to have a mature product. The amplifiers were introduced some 2 years ago and with some delay, we recently added the TEP 3800 preamp. The Thorens Holding domicile is in Basel and Thorens Export is in Giebenach so Thorens has returned to Switzerland. But we keep saying made in Germany. For a long time now, Thorens hasn't produced anything in Switzerland proper.

Sidebar II: I grew up in Leipzig, East-Germany. When I was 14, a friend of mine gave me a non-working but fascinating old tube radio from the early fifties as a present. I thought I could repair it but that was one big false estimation... continue for Frank Blöhbaum's hifi roots.

"My concept at the beginning was to get Thorens turntables back on their feet as quickly as possible and to adapt to market response first before creating a new product concept. I answered the trend for high-mass decks, then followed with the new modern acrylic models and built out the classic triple sub-chassis players with modern materials and drive units. Put simply, I want Thorens to once again be a global presence. I want growth but as a niche player, I have limits. Hence my growth is based on market presence. I want to become an important player again in the turntable market where Thorens was always based and where I believe we have a future market. I'm also working to become a full supplier of analog gear. Thorens will remain analogue but after the electronics launch, who knows - might there also be a speaker line to carry the Thorens brand name?

Sidebar III: For our technically inclined or curious readers, Frank Blöhbaum has penned a 9-stage manifesto that presents the core attributes of his circuit and explanations for how it works... continue
"Development of the home electronics has to be monitored carefully. Thorens is too small to lead but has its niche. I want for Thorens to become strong and solid and transcend the 90s when people no longer knew whether Thorens still existed or not. One important mile stone has been turned already. As a personal newcomer to this trade, I am still here after 7 years - and I don't want to say Heinz is still here, I want to say Thorens is still here."