Siegfried by Berning

A Single-Ended, Output-Transformerless (OTL) Triode Amplifier

The Zero-Feedback Single-Ended Amplifier--A Primer

The zero-feedback single-ended triode amplifier has gained a following from music lovers who feel that high-quality sound reproduction at natural levels is more important than reproduction at loud levels. To these listeners, midrange quality is more important than room-shaking bass.

The single-ended amplifier uses a single output tube, or several in parallel that act as one, to amplify both the positive and negative polarity portions of the music signal. In contrast, most amplifiers are configured so that the positive polarity of the signal is amplified by one output device and the negative polarity is amplified by another device. This second type of amplifier is termed push-pull, and the music signal is handed from one device to the other, and back again with each cycle of the frequency being amplified.

Triodes are used in single-ended circuits because they are the most linear voltage amplifying devices available, and they have low output impedance for good speaker damping without requiring negative feedback. Although triodes are more linear than other devices, they do exhibit compression at the high-voltage, low-current portion of their transfer characteristics. This compression results in low-order harmonic distortion on the order of 5% for large voltage swings. This distortion could be largely eliminated by using negative feedback, but most listeners find that feedback introduces more problems than it solves.

Directly-Heated Power Triodes and Single-Ended Circuits--The Old

In the earliest days of electronics all amplifiers were built with directly-heated triodes. These early circuits were all single-ended, and by definition, class A. Negative feedback was also unknown to the early designers. As the field of electronics advanced, new tubes with multiple grids made it easier and cheaper to make amplifiers with more power that ran more efficiently. This trend continued with the introduction of push-pull circuits, and negative feedback techniques brought measured distortion to very low levels. Transistorized amplifiers continued the move to higher power, higher efficiency, lower measured distortion, and lower cost.

By the early 1970s, the manufacturing of hi-fi tube amplifiers had for all practical purposes ceased. The tube filament did not quite go out, and from that point of low interest in tube amplifiers there has been a steady reemergence. At first, the designs of most of these amplifiers picked up where tube-amplifier technology had stopped in the 1960s. They were based on multi-grid tubes and used feedback to the maximum extent possible.

While many amplifiers built today are built based on the 1960s techniques, a growing number of critical listeners and designers have been discovering that the further back they go in the history of amplifier development, the more they like the sound reproduction of the early circuits. The directly-heated triode amplifier with no negative feedback reappeared on the market a few years ago, this time sporting exotic components and a high price tag.

Why, one might ask, should these low-power amplifiers built on such simple principles cost so much more than higher-powered modern designs, especially since they were in common use 70 years ago? The answer is they do not have to be expensive if one is willing to accept the high hum levels and limited frequency response of those early amplifiers. In fact, single-ended amplifiers can be very inexpensive indeed. Single-ended amplifiers were used for audio-output in virtually all television sets, clock radios, etc. until transistors replaced the tubes. Single-ended class A audio-output stages even remained common in these consumer items for several years after transistors had replaced tubes. It was not until the widespread usage of integrated circuits that class B replaced class A at the lowest cost end of the spectrum.

High hum levels and limited frequency response are not acceptable for amplifiers designed for high-fidelity sound reproduction. It is very expensive to eliminate these deficiencies in the single-ended amplifier, especially if negative feedback is not used. In order to correct the frequency-response problems, the output transformer must be made very massive, and with great complexity. An air gap is required to prevent core saturation from the required unbalanced dc current in the single-ended amplifier, and the air gap makes it much more difficult to achieve a wide frequency response than is the case with a non-gapped push-pull transformer. This air-gapped transformer is really responsible for the love-hate relationship often experienced with single-ended amplifiers. The air-gapped transformer operates in a more linear fashion than does a push-pull transformer because the ungapped push-pull transformer is so easily saturated. The gapped transformer gives the single-ended amplifier a wonderfully pure midrange. Unfortunately the air-gapped transformer can not match the performance of the ungapped push-pull transformer at the frequency extremes, and as a result, the single-ended amplifier sounds soft in the highs and weak in the bass.

The single-ended amplifier is very sensitive to power-supply hum, and the low-impedance of the triode makes the situation worse. Any hum in the power supply is coupled directly to the speaker in the triode single-ended amplifier. By comparison, multi-grid tubes used for TV audio reject much of this hum, push-pull circuits largely cancel the power-supply hum, and the use of negative feedback surpresses hum. If triodes are to be used without feedback in a high-performance single-ended design, the only option left is to build an exceptionally clean power supply. This is costly regardless of the technology used.

Directly-heated triodes have an additional problem regarding their heaters. In this type of tube, the heater is not isolated from the cathode as it is in more modern tubes. Electrically, these tubes work best when the heater is operated from an ac source derived from a center-tapped transformer. This provides for an average equipotential cathode for the audio signal. Such operation introduces hum, and most modern amplifier designs use dc on these heaters to avoid the hum, even though the transfer characteristics of the tubes are altered by such operation, and there is uneven wear on the tube.

Directly-Heated Power Triodes and Single-Ended Circuits--The New

In 1996 Berning introduced a radically new technology (US patent no. 5,612,646) for tube amplifiers that eliminated the problematic audio-output transformer. This amplifier was designated the ZH270, and it is a push-pull design. The ZH270 was the first amplifier using all-tube amplification that properly matches the high-voltage, low-current operating parameters of vacuum tubes to the low-voltage, high-current drive requirements of dynamic loudspeakers without using audio-output transformers.

Berning has now applied this new technology to the single-ended amplifier, and Siegfried is the result of this effort. Prior to the Berning impedance conversion technology, it was simply too impractical to even consider building a tube OTL single-ended amplifier.

Siegfried uses a regulated resonant switching power supply to achieve extremely low hum levels. This supply operates at 250 kHz. An important side benefit is that the heaters of the output tubes are heated with this RF through properly configured center-tapped transformers. No hum is generated, and the tubes have the proper equipotential cathodes with their true transfer function.

Siegfried 300B

Features and Specifications

On board level control--can be used without a preamp.
Tube complement per channel: 6SN7 differential input; 6SN7 balanced amplifier; 6J5 follower-driver; Svetlana 811-10 output triode, operated at less than one half its rated plate power for long life. (Optional 300B output, see below).
Point-to-point hand-wiring for audio circuits.
Single ended inputs only.
No negative feedback of audio. Entire amplifier dc-feedback stabilized.
Unique brown-out protection circuitry.
Built-in four-stage power-line filter and surge suppressor.
Non-magnetic chassis prevents skin-effect induced distortion.
Power consumption: 170 W
Power required: 100-130 V ac or 200-260 V ac, 50-440 Hz.
Signal to noise: (typical) 92 dB, 20 kHz bandwidth. RF carrier: -56dB (0.5 MHz). Unweighted.
Line-frequency hum components: 60 Hz: -94dB; 120 Hz: -100 dB; 180 Hz: -104 dB.
Distortion products (typical, into 6 ohm load): 4% 2nd harmonic, 1.5% 3rd harmonic, 0.23% 4th harmonic, 0.26% 5th harmonic, 0.16% 6th harmonic, at 10 watts output (THD= 4.3%). These products are reduced at lower output power.
Typical output power (per channel) at onset of clipping: 6 ohms-12 W ; 8 ohms-10 W ; 4 ohms-8 W
Full power bandwidth (-3 dB), 6 ohms: 0.2 Hz to 45 kHz.
Typical crosstalk: At 1 kHz: -60dB; 10 Hz: -42dB; 100 Hz: -57dB; 10 kHz: -43 dB.
Typical output impedance (measured at 1 amp, 60 Hz): 1.5 ohms.
Sensitivity: 0.6V RMS for 10 W out.
Input impedance: 50K.
Size: 33 cm wide, 43 cm deep, 23 cm high, ( 13 X 17 X 9 inches), not including connectors.
Net weight: 8.2 kg (18 lb.).
Finish: black anodized aluminum with gold-plated brass tube cage.
Available equipped with Western Electric 300B tubes (requires circuit changes) on special order. Power output at onset of clipping: 8 ohms-7W; 16 ohms-5W; 4 ohms-4W.
Limited Two-Year Warranty. (WE 300B tubes are warranted for one year by Westrex Corp., all others covered by the Berning two-year warranty).

Owners Manual PDF

Siegfried Reviews

Single-Ended Amplifier. 5 watts. ZOTL Technology.

"A most amazing amplifier. Output transformerless using one Western Electric 300B output tube per channel. The amazing thing is it sure doesn't sound like what one would imagine a low powered amplifier should. It is, quite simply, the most dynamic (that's right), realistic presentation I've heard from any amp. It's all there from the frequency extremes to an eerily realistic midrange. Just magic." --Weldon K. Jackson, 19 Jan 1999, source

Siegfried review by Harvey "Dr. Gizmo" Rosenberg

"When I auditioned all the ZOTL circuits (push/pull, 811 and 300B) with master tapes there was no doubt that the Berning circuit was the best at creating the illusion of the Steinway concert grand that we just recorded, and all transformer coupled amplifiers sounded slow, sloggy and distorted. ... the ZOTL much more dynamic in the way single-ended circuits have that amazing midrange dynamism without sacrificing what ZOTL does that no other transformer coupled amplifier can do...complete balance from top to bottom. The upper octaves are there as are the bottom octaves. While listening to a wide range of music I noticed (now get this Pearsonism) there was an inner glow to the vibrato of strings, a shimmering grace fell over the harmonics of Cassandra Wilson's voice, it was as if an Angel of Tone was sprinkling fairy dust over each note, but what was most stunning was the silence between the notes, which become the background for a palpable sense of depth that I had previously only experienced when staring at Dolly Parton's bosoms. My epiphany was having goose bumps. This combination of harmonic purity, clarity, dynamism, and wave front wholeosity is completely unique. And let me go on record once again and state that David Berning pisses me off that this amplifier weighs under ten pounds."

"So let me state once again: this amplifier is not for everyone. It is too good for most systems. It is the kind of device that forces us to reconsider everything. It has given me a much greater appreciation how the best Wadia digital gear can reveal the subatomic nature of digits...and that is both good and bad. You absolutely must have the finest input devices for this amplifier. You must have high efficiency speakers of high refinement, and they are rare. This is absolutely the most stunning view of the edge of the art of tube amplifier. There are many good reasons that David is making other audio designers emotionally unstable."

"Only the courageous will use this amplifier, because not everyone is ready for triode heaven on earth."

Siegfried - 811 Svetlana reviewed by Harvey "Dr. Gizmo" Rosenberg

"When Mark Conese and I compared the sound of David Berning's new ZOTL single ended circuit 811 amplifier to the 300B version we thought that there something wrong with the 300B amplifier... Marco the pianist completely agreed with us. The 300B amplifier sounded warm and fuzzy compared to the 811. The 811 was much closer to the sound of the Steinway, and the 300B was not!"

Siegfried - Directly Heated Triode review by Harvey "Dr. Gizmo" Rosenberg

"David's patented circuit, for the first time, solves the impedance problems endemic to all conventional OTL circuits; lack of stability at low impedances: all OTL amplifiers are very weak in the bass region. The ZOTL circuit is the first OTL circuit that doesn't go bonkers into low impedance loads, and this is the only tube amplifier on the face of earth, since the Futterman OTL-1, that doesn't have an output transformer and has regulated power supplies, and ZOTL is far superior in every regard! And this is a major big deal..THIS IS A DIRECTLY HEATED TRIODE OTL... it lets us hear the tone of directly heated triodes in an OTL circuit. This is unquestionably one of the major breakthrough in the thermionic arts. Those of you who are using OTLs that have video compactron/pentodes output stages are in for a big thrill when you hear the totally superior tonal qualities of directly heated triodes. But rather than rave about this circuit here, just check out Dick Olsher review in March 98 FI Magazine (also available on this site) where he puts David's amazing technical wizardry in perspective."