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