Established in 1974, The David Berning Company manufactures vacuum tube audio amplifiers based on an unique Impedance Converter
that replaces the traditional audio power output transformer and greatly extends and improves amplifier performance. We call our technology "ZOTL" for Zero-Hysteresis Output Transformerless.
Operating at a fixed high-frequency without traditional audio output transformers, the ZOTL Impedance Converter eliminates the frequency-dependent performance limitations inherent in all transformer coupled tube amps.
Berning amplifiers using the ZOTL technology exceed the performance of traditional OTL tube amplifiers by properly matching the tube impedance to that of the speaker. Berning amplifiers depart from traditional OTL amplifiers in that they do not require a large number of hot power tubes to supply adequate current for driving speakers.
Berning amplifiers convey the musical transparency of tubes without the limitations imposed at the frequency extremes by audio output transformers. This limitation at the frequency extremes is a major contributing factor to what some negatively call "tube sound". ZOTL extends the desirable musical transparency of tubes to the frequency extremes and provides the quickness of the high and low frequencies that are cherished by those who prefer solid-state amplifiers.
ZOTL Technology vs Audio Output Transformers
This paper demonstrates the electrical performance of an amplifier using the ZOTL technology versus the identical amplifier using a high-quality audio output transformer and presents these differences using oscillographs.
Electronic Devices and the Amplification Process
This paper shows how a curve tracer can be used to explore the attributes of the ZOTL technology and contrast it to the shortcomings of the audio output transformer. Also shown is how this tool can be used to optimally design an amplifier.
5,612,646 Output Transformerless Amplifier Impedance Matching Apparatus
This patent teaches the principles behind the Berning ZOTL technology that can properly match the impedance of vacuum tubes to the impedance of speakers without using audio output transformers. The impedance conversion is done with dc-dc switching converters operating at fixed frequencies well above audio frequencies. The actual voltage and current impedance transformations are done via special high-frequency transformers. This is an RF carrier system that eliminates the frequency-dependent distortions present in audio transformers, and can enable the amplifier to have a wider frequency response and be dc coupled.
4,163,198 Audio Amplifier
This patent details a hybrid tube-FET amplification stage that forms the basis of the Berning TF-10 preamplifier. An important advantage of this amplification stage is the elimination of the parasitic feedback capacitance and thus providing a stage with improved transient response.
3,995,226 Audio Amplifier
This patent describes the screen drive philosophy that has become an important part of many of the Berning power amplifier designs to the present day. The screen drive allows tubes to be operated at much higher efficiencies, much higher power output, and at greatly enhanced reliability. Whereas the patent shows transistors driving tubes, the same principles apply to presently implemented versions of tubes driving tubes. This is also known as the "enhancement" amplifier.