David Bernings interest in sound reproduction began in 1962 when his father and grandfather made a turntable for him. Although it had a plastic tonearm and platter, it featured a nice wooden enclosure with a bass reflex speaker and to these young years, sounded great.
In 1963, when he was 12, a Voice of Music 740 tape recorder arrived, his father’s machine. Now it was time to begin taping. Dave used this machine so much that belt replacements and eventually a head replacement was necessary. His father wasn’t impressed.
Solution- He made his first tape recorder in 1967, based on the 740. Using a washing machine accessory motor and some parts made of wood, left a lot to be desired. Before the end of high school, 3 more refined models followed and V4, which is still in his possession can be seen in the picture.
As the tape recorders improved, it was now time to address the amplification and DB built his first amplifier, based on an audio output stage from a TV, a single-ended 6V6. He added input stages copied from the RCA tube manual, substituting 6SN7 tubes. This was a very successful build, which he later sold to a friend so he could build something more advanced.
With this encouragement, a push pull amp was built using 29 tubes. The end result, a lot of oscillation. The self taught student realized it was overcomplicated. He did not know at this time how to properly engineer an amplifier, so future builds contained both winners and losers. By now his father had given him a capacitor tester and an oscilloscope and the design was reduced from 29 down to 15 tubes. About this time, David was transitioning from high school to college. He graduated from the University of Maryland with a BS in physics. But during his last year he took some electrical engineering electives where he was top of the class and was able to put together a good knowledge base using a lot of experimental findings combined with formal circuit design techniques.

Now in a position to really start designing amplifiers, David was using hybrid designs using tubes and transistors for his own use. He knew nothing about the audiophile community or publications discussing audio components. He was also unaware of a renewed interest in tube amplifiers when he was introduced to an individual who was knowledgeable about audio and had many audiophile friends. This person talked David into helping him build his own tube amplifier. With the new amplifier in hand, they visited many audiophiles to try the amp in various systems. This resulted in some of these people asking David to build one for them. Thus, the David Berning brand of amplifier began in 1975. In 1976, Berning was issued a patent for his tube screen-drive using bipolar transistors. While David made a significant number of what was designated the Berning EA2-150B, he licensed the patent to Audionics of Oregon and they produced the BA-150 for several years. These amplifiers incorporated digital autobias. There were a very limited number of EA-2150A units that were all transistor except for the tube output stage and these did not have autobias.

In 1974 David was hired as an engineer to work for the National Bureau of Standards/ NBS/ National Institute of Standards and Technology/ NIST. He carried one of his amplifiers in for the interview as show and tell, and this may have been the reason for getting the job. NBS was looking for someone who could design and build test instrumentation for special research projects that was not available commercially. He worked there for 32 years and built many unique test instruments and was either primary author or coauthor on many publications. Most of the work was directed to characterization of power transistors and discovering the best measurement methods to disseminate to the semiconductor industry as a whole to facilitate improvements to these devices. Ironically, David designed many of these instruments in part with vacuum tubes to obtain a combination of speed, voltage, and power that could not be obtained at the time with existing solid-state devices. As a side benefit to his work in this area, he developed a very good understanding of not only the limits under which transistors could reliably operate, but limits for vacuum tubes as well. Even more important was an understanding of what phenomena really affected these limits. The value of this understanding played a very large role in improving David’s circuit design for new instruments but naturally extended to audio amplifier and power supply design. So David had two parallel careers going forward: the research position at NBS/NIST and making audio amplifiers for sale.

For the audio-amplifier business there were several inflection points of notable interest. One of these inflection points came in the late 1970s. Up to this point, Berning had a (largely) tube preamp called the P1 that was a two-box solution that separated a conventional power supply with a line-frequency power transformer from the preamp itself for hum reasons. It was time to make a new preamp, and he wanted to have only one box. To do this, the line-frequency transformer had to go, and a switching power supply could get him there. With a good understanding of power transistor limits, he figured that a switcher was in reach. But no respected high-end audio company, especially not one that made tube amplifiers, had done this. Clearly line-frequency hum could be eliminated, but what about the 30 kHz “hum” that would now be created? By itself it was above audible range, but could it affect the music? So, David did some experiments of mixing the music signal with varying amounts of 30 kHz from a signal generator to see what might audibly affect the music. He could not find any affect at all, so he felt free to go ahead with the switcher-powered preamp. This became the TF-10, and it was quite successful with three runs over the course of seven years. There were certainly naysayers, but the TF-10 got good responses from respected audio reviewers. The TF-10 was also unusual in that the gain stages combined p-channel FETs with tubes, described by another patent.

At the same time that Berning was making the TF-10, he was making the very popular EA-230. This had a conventional power supply, but did use the screen drive but now done with a tube driver stage instead of the patented transistor drive. One thing worth mentioning here is that the screen drive provides a much more linear transfer characteristic that is triode-like but still has the rather high output impedance that occurs with pentode operation and normal grid drive. So feedback is still needed to get a reasonable low amplifier output impedance. But the tube idle current can be one tenth of what might normally be used. In the EA-230 this current was only three mA, very close to ideal Class B. This did two things: extend tube life and better control output-transformer saturation.

Next up was a new and larger power amplifier, later called the EA-2100 and followed up by the EA2101. UPS had a 75-lbs weight limit, and a 100-watt tube amp with its large output transformers and large power transformer would be too heavy. This time it was weight, not hum, that made David look to another switcher. 500 watts was needed and the first generation of MOSFETs capable of operation from 120V/240V line voltage had just become available. These early devices were very fragile, expensive, and had reliability issues related to fabrication processes. It took developing a complex slow-start procedure to get them to work. During development of the power supply things blew up many times destroying $ 80 of transistors plus some other components each time. Success finally prevailed, and the EA-2101 replaced the EA-2100 as better MOSFETs were available and the 6LF6 output tubes used in the EA-2100 were becoming scarce.

Both of these amplifiers use an unusual type of switching power supply called a parallel-resonance topology. Instead of the popular pulse-width modulation this uses frequency modulation for control. There are several nice features that make the added expense worth it. Largely sinusoidal waveforms go through the power transformer instead of fast-rise pulse waveforms thus reducing noise. The other big deal for audio is that there is no limit to the amount of energy storage that can be used, and high amounts of energy storage improves bass impact. Berning amplifiers would all incorporate this topology moving forward.
Concurrent with the EA-2101 David made the TF-12 preamp. An all-tube design with an unusual 80-dB stepped attenuator volume control with digital control.

Around 1995 David was getting tired of the audio business and figured he would quit after all remaining TF-12s and EA2101 amplifiers were sold. But curiosity got the best of him and he started experimenting with trying to use the same parallel-resonance principles used in his power supply to make a power amplifier. The attractive characteristic that he was trying to make use of was the fact that in this topology the full operating current is recirculated continuously, but power not used by the load is returned to the power supply capacitors rather than being dissipated as waste heat. Perhaps this could have the virtues of Class A without the heat. David actually got it to work but there were distortion issues that he didn’t think he could solve. In the process of experimenting, he tried using a tube as a load for an unregulated square-wave inverter and noted that the tube load would transfer to looking like a load on the power being supplied to the inverter. This had nothing to do with a non-dissipative emulated Class A, but interesting. He wondered if this discovery might work sort of like an output transformer. After a few more experiments, David decided to see if he could build an output-transformer-less amplifier (OTL). He would target a stereo 50-watt unit with resonant switching power supply that would be very light and small.
As an avid cyclist, David had many times carried amplifiers by bicycle and if this designed worked that would be a great improvement.
The NIST office Christmas party was coming up and he always provided the music. He figured that the amplifier could not be very good as it was absurd to think that he could make a pair of switching inverters compete with a good audio-output transformer. But the people at NIST were quite technical and might be fascinated with such a light-weight and small amplifier using tubes. They would be drunk anyway and if it didn’t sound that good it would not matter.

After completing the amplifier and doing some bench tests, David hooked it up to his speakers and was astonished with what he heard. He had a Shostakovich string quartet LP that he could get only two of the four instruments to sound convincing through his transformer-coupled amps. He could change some parameters and pick up another instrument but he would lose another one in the process. It was very frustrating. But with this crazy new amplifier all four instruments sounded right. At this point David decided not to quit audio, but file a patent instead. The patent issued in 1997 and the ZOTL was born. The late Harvey Rosenberg came up with the ZOTL name, combining OTL with Zero hysteresis as it eliminated the hysteresis present in audio output transformers. The new technology greatly extended the potential frequency response of tube amps and could now pass dc thus allowing good autobias to be included. The first production amplifier was the highly successful ZH270 which is a compact integrated dual 70-watt amplifier that came out in 1996. It has a reliability track record that far exceeds that of any prior Berning transformer-coupled amplifier in spite of its complexity. All Berning amplifiers made since this time have been of the ZOTL type.
DB is of the opinion that ZOTL designs are primarily for amplifiers and not necessary in his preamplifier designs.
The external, switching power supply used in the DB2 tape head preamplifier is very sophisticated and effective but does not use this technology.