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Reviewer: Srajan Ebaen
Financial Interests: click here
Source: APL HiFi NWO 3.0-GO; Ancient Audio Lektor Prime; Raysonic Audio CD-168; Yamamoto YDA-01
Preamp: ModWright DM-36.5; Hegel P-10 [on review]; Red Wine Audio Isabella [on review]

Amp: First Watt F5; Yamamoto A-09S; Red Wine Audio Signature 30.2 [on review]
Speakers: Acoustic System Tango R
Cables: Acoustic System Liveline interconnects; Crystal Cable Ultra loom; Crystal Cable Reference power cords

Stands: 2 x Ikea Molger with Ikea butcher block platforms and Acoustic System footers
Powerline conditioning: 2 x Walker Audio Velocitor S
Sundry accessories: Furutech RD-2 CD demagnetizer; Nanotech Nespa Pro; full-house installation of Acoustic System resonators, noise filters and phase inverters
Room size: Sound platform 3 x 4.5m with 2-story slanted ceiling; four steps below continues into 8m long open kitchen, dining room and office which widen to 5.2m with 2.8m ceiling; sound platform space is open to 2nd story landing and 3rd-floor studio; concrete floor, concrete and brick walls, converted barn with no parallel walls nor perfect right angles; short-wall setup with speaker backs facing the 8-meter expanse
Review Component Retail: €15,000

Frank Blöhbaum's name could make him a stranger still in hi-end circles. That's because he works behind the scenes as a contract engineer. In publishing terms, he's a ghost writer. But it's his patented floating bridge bias control circuit that runs both the Thorens TEM 3200 monos already reviewed and today's matching TEP 3800 preamplifier. And similar to Goldmund's Marcel Croese with his Adaptive Auto Bias board for PrimaLuna's Chinese Spark/Cayin platforms, it's Blöhbaum's precision bias solution that governs the TAC 60 atop Sheng Ya's AT-661 platform. The TAC 60 is a new valve integrated from the Tube Amp Company. And that is a subsidiary brand of Sintron which also owns the 14-year old Vincent brand. Both Sintron brands are built by China's Zhongshan Shenya Technological Co. Ltd*. And, Sintron is the German importer for Thorens. That's how this little connective detour neatly doubles back to our Thorens review subject. Phew.

* Before you hope for Blöhbaum's no-holds-barred Thorens circuitry in a value-priced TAC product, that particular circuitry has been exclusively licensed to Heinz Rohrer of Thorens. Sorry, no dice. This advanced stuff costs the long green. Blöhbaum's already registered "BiasWizard" module for Uwe Bartel and TAC is depicted below in the first TAC amp to incorporate it. You can read the German Stereoplay review of it here: "The Sheng-Ya AT-661 was produced in about 20 units only as a first test run. Now there is only the TAC V60."

Back to the TEP 3800. It's a DC-coupled (no signal-path capacitors), dual-mono, all-tube 'Circlotron' circuit; or as Blöhbaum prefers, a counter-parallel floating bridge affair. It adapts his TEM 3200 power amp implementation to an all-valve circuit. The monos end in single 800-watt monolithic transistors per phase to make the desired power. The preamp replaces those transistors with glowing bits - 6E5P tetrodes in triode mode*. There are, per channel, also 2 frame-grid C3g pentodes in triode mode for voltage gain. And two E95F/6AK5W 'drivers' - or more correctly, potential shifters since with 8 watts max of anode power, the Russian output tubes require no boosting. If you've kept count, the total tube count here is a square dozen - 3 x 4 since as a differential circuit, each channel needs "two of everything".

* Blöhbaum's chase for the perfect preamp power tube had consecutive prototypes run the 6S19P in copper chimneys; the EL82 in triode mode, and the 6P41S Magnoval in triode mode, the latter shown to the right and below in 'prototype 3', the version Frank brought to Chardonne together with the current production review unit.

Clicking on the above image opens a graph of the curves of the C3g as archived by this website

Like the C3m in my Yamamoto A-09S, the C3g stems from the German postal/telephony service and dates back to approximately 1954. Yamamoto's tube [upper left left] has been extracted from its 'cage' (which then also removes the guide pin), Blöhbaum's remains in the original cover. The 6E5P and E95F are to the upper right.

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The output valves' input capacitance is lower than the power amp's Mosfets by a factor of 1000 of course. Far more importantly, their output current is 30mA. And their output impedance 0.67 ohms. That latter is a shockingly low figure for a direct-coupled tube circuit. It means phenomenal drive over even endless interconnects. Or 600-ohm load happiness as active pro monitors present. Yousa! And for the really curious, yes, the 6E5P's close electrical -- but not pin-compatible -- cousin is the E810F which has a higher transconductance but less anode power dissipation. This preamp circuit would be capable of swinging an ungodly max +/- 60V but is naturally limited by the input signal. A standard 2V input signal translates into roughly a voltage swing of 10 (gain is 16dB). That means overkill headroom for true dynamics. And an unconditional incapacity for clipping the output stage. Ever. It's really a power amp wolf in preamp sheep skin. Blöhbaum runs his original TEP 3800 prototype [lower rights] to drive AKG K-1000 headphones with. Those are beastly inefficient and usually coupled speaker-level to amplifiers putting out 10 watts.

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Expensive AD 8620 op amps -- "real divas with a propensity for 500MHz oscillation so they need to be domesticated", punned Blöhbaum; "but they'd make an excellent preamp all by themselves and don't rely on optimized source impedances like input transformers" -- those op amps' only job is to convert all incoming RCA signals to balanced immediately after entry before they encounter the motorized 4-channel Alps pot. All signal amplification occurs in the differential domain, hence the RCA outputs are preceded by the same kind of op amps to go from dual-diff back to single-ended again.

For the whole business of floating power supplies to work (after all, they ride on the signal and must track it with a lock jaw), accurate symmetry of the power supplies back to the 2nd coils of the overbuilt 60-watt power transformers is key. It means tricky grounding solutions and mandates absolute path length equality of all traces and wiring. Then it leads to self-cancelling magnetic fields and complete current separation such that no current flows from the power stage to the power supply on the ground plane. It's this latter part that, admits Blöhbaum, baffles even bona fide engineers. But it is one of the secrets of his circuit. The other is his patented bias solution. For an in-depth report on that plus further background on Blöhbaum, refer to the relevant side bars of my TEM 3200 review.