Thöress 300B monoblock power amplifier

I was weak and easily led.

In 1978, after enduring four or five years of wretched music made by men with long hair and beards and tendencies toward eonic guitar solos, I suddenly discovered that the only music worth hearing was made by clean-shaven men of limited musical proficiency. I embraced the Clash, the Pistols, the New York Dolls, the Ramones, and the Buzzcocks. I cut my hair and gave away some of my old records. I even threw out my copy of Jethro Tull's A Passion Play—which, now that I think about it, wasn't that bad an idea.

Then I woke up and remembered: I'd left the baby in the bathwater.

Many bearded, long-winded musicians from my past, such as Neil Young and Phil Manzanera, were no less worthy of my attention. And although the Clash and the Dolls (footnote 1) made some of the finest pop records of all time, punk wasn't all good. Punk wasn't even all punk: Some performers were attracted to the genre simply because it was easy to play, and many of those people turned out to be unoriginal posers with little or nothing to say. Still others turned out to be upper-class entertainment-biz hacks and aging arena-rockers who merely cut their hair and shaved off their moustaches—or, in the case of female performers, adopted motorcycle jackets and accident-victim hairdos—as camouflage. Thus I had to relearn something I seemed to know when I first started buying records: to try everything, and avoid the temptation to judge unfairly or too quickly.

Then, in the context of audio, I had to learn that lesson all over again. In 1995, after witnessing 10-plus years of audio components of ever-increasing price, size, complexity, and filigree, and with a brief and only partly satisfying tour of duty in the Flat-Earth trenches (footnote 2), I suddenly discovered that the only electronics worth hearing were hand-wired, single-ended-triode tube amps with no negative feedback. Of all the dogmas I've owned, that one may have contained the most truth.

But it was still a dogma. I then woke up and remembered that many of the amps I'd owned and loved in the past—like the Naim NAP-250 and the Conrad-Johnson MV-50—were rendered no less great by the existence of single-ended-triode amps. And it eventually became apparent that some builders were attracted to SET amps simply because they're comparatively easy to design, and a few of those people turned out to be unoriginal posers with little or nothing to say.

I discovered that, as with virtually every club, being a member of the He-Man Push-Pull Haters Club was just another way to avoid thinking for myself.

Thankfully, I no longer believe that an amplifier must be single-ended—or feedback-free, or hand-wired, or tubed, or class-A, or anything else—in order to be good. Which is not to say that the single-ended, feedback-free, hand-wired Thöress 300B mono power amplifier ($12,995/pair) isn't really special. Because it is.

The Thöress 300B mono power amplifier, made in Aachen, Germany, uses a single 300B triode tube, and is said to deliver up to 8W across a 16-ohm load. Each Thöress also contains a pair of new-old stock Telefunken EL803S pentode tubes, which designer Reinhard Thöress says are carefully matched at his factory, and used together as a single voltage gain/driver stage. In Thöress's design, both input pentodes are configured as triodes, with the B+ supply (210V) applied to anodes and screen grids alike. The input signal goes to the signal grid of the first EL803S pentode, the plate of which is coupled to the signal grid of the second EL803S; the cathode of that second EL803S is capacitively coupled to the signal grid of the 300B output tube.

The 300B tube selected for the Thöress 300B—the Chinese-made Full Music 300B/c, which Thöress buys in matched pairs—is operated in fixed-bias mode. The bias voltage is set at the factory, with the aid of hand-selected, high-precision resistors that are matched between the two amps of a given stereo pair. Reinhard Thöress chose this approach, he says, because it's reliable—a potentiometer is likelier than a discrete resistor to fail or drift in value—and because it generates less heat than auto-biasing.

All three full-wave rectifiers used in the amp's simple, robust power supply comprise individual diodes. Apart from that, the power supply is unremarkable, save for one refinement: The 300B is heated with DC rather than AC, and its filament supply contains a user-accessible hum pot.

At first glance, the design of the 300B's chassis seems intended to maximize rigidity while avoiding excess mass. Thöress modestly dismissed that notion: "There is no big philosophy behind my chassis. I just try to keep things simple and functional." The bottom, top, and sides of the 300B are made of aluminum, shaped with curves and louvers both attractive and functional, and beautifully lacquered and polished inside and out. To the bottom plate are fastened the three largest and heaviest components—the power-supply choke, mains transformer, and output transformer—all of which are designed and made in-house by Thöress. Also fastened to the bottom are five very large filter capacitors, made in Italy by a company I'd never before heard of: Comar Condensatori.

Just under the 300B's top plate is a sizable chunk of circuit-board laminate, about 3/8" thick and measuring some 19" long by 5" wide. All three tube sockets are moored here, as are two long rows of terminal strips, between which are arranged the input section's resistors and capacitors cordwood-style.

I would not hesitate to describe the build quality of the Thöress 300B as masterful, in every respect: construction and finish, quality of wire dressing and soldering, layout of circuits and parts. It appears to be built to last a lifetime.

Installation and setup
With its one RCA jack for input, one pair of binding posts for output, and complete lack of bias controls, the Thöress 300B might be regarded as distinctly easy to set up. It was. Each amp of my review pair was packed in its own well-designed carton, with tubes tucked away in their own subcartons. With the amps unboxed and placed on the floor—at 13.4" high by 5.9" wide by 23.3" deep, each 300B is far too large for the average equipment rack or domestic tabletop—the tubes were easy to install in their top-mounted sockets: No tools of any sort were required.

After that, only one chore remained: Once each Thöress is connected to the system and powered up for the first time, the user must trim the filament supply of its 300B tube, listening for hum and adjusting the rear-mounted knob until all is silent and well. This was easy and unambiguous, and took less than 30 seconds for each amp. From that moment forward, the Thöresses were completely free of hum, and no noise intruded on my listening, save for the scarcely perceptible, gamelan-bell–like sounds one often hears immediately after powering up an amplifier that uses one of today's premium 300B tubes.

The proprietary output transformer of the Thöress 300B has multiple secondary taps, and can be configured for use with loudspeakers of 4, 8, or 16 ohms. Output impedances are set with jumper wires soldered into place—but because the soldering of transformer leads is best left to experts, I left my review samples at the 8-ohm settings bestowed on them at the factory. (The respective nominal impedances of my Altec Valencia and DeVore Fidelity Orangutan O/96 speakers are 16 and 10 ohms.)

Footnote 1: I am unrepentant in holding to the unpopular opinion that the Dolls' second album, Too Much Too Soon, is better than their debut.

Footnote 2: My experiences were satisfying inasmuch as they taught me the worth of certain products and points of view associated with that movement. But that satisfaction always hit the wall whenever the doctrinaire attitude of so many of the movement's proponents and adherents raised its ridiculous and sad little head.

Reinhard Thöress
US distributor: Audioarts, Inc.
210 Fifth Avenue, Suite 301
New York, NY 10010
(212) 260-2939

manisandher's picture

I've just received my Thöress 300B monos, set up by Reinhard to work with Elrog 300B output tubes. Feeding my Tune Audio Anima horn speakers (109dB/W@1m), I have to say I have the most satisfying sound I've ever managed to achieve in this hobby. I concur with Art's description of the sound, though the Elrog tubes are substantially better than the regular Full Music tubes.

Reinhard took a frequency response measurement of the amps before shipping them out to me. The measurement was taken on the PRIMARY side of the OPT, with a 5 Ohm wire wound resistor as dummy load across the secondary. (He believes this is the best way to take the measurement as the OPT secondaries do not carry a ground potential.)

The amps measure -3dB at 7.5Hz and 150kHz. Also, there is virtually no OPT resonance above 150kHz. Pretty good for a SE amp with zero feedback! And as I said, they sound utterly fantastic.


directdriver's picture

What is the point of measuring the amp at the transformer's primary when your speaker is connected to the secondary? You're bypassing the output transformer to get better results. That's cheating!