The following article from FI Magazine, March 1998, is reprinted by the kind permission of Dick Olsher and FI Magazine, and is copyrighted © 1998 by FI Magazine.
OTL & Berning: Output Transformerless (OTL) Tube Amplification and the Berning ZH270
Ubiquitous. Output transformer.
When it comes to vacuum tube power amps, these words are synonymous. The chassis of amps that glow in the dark is typically dominated by a big, hefty output transformer (or a pair of such devices in the case of a stereo amp). And that's as it should be, because sound quality is crucially dependent on the transformer's sonic performance. Of necessity, transformer weight and cost escalate proportionately to the amp's power rating. If you're into tubes like Toobman, you probably recognize that weight is a transformer's hallmark. The bigger the core, the less likely it is to saturate. Core geometry, core materials, special winding techniques, winding inductance and capacitance, and even the conductor material are factors that influence performance and sound quality. The truth of the matter is that output transformer design is an art, and lately it seems mostly to be a lost art.
Sadly, the enormous pool of knowledge developed in the '40s and '50s has by and large failed to survive the generational gap. There are presently only a handful of individuals who can fashion classic iron. The point I'm getting to is that even expensive transformers are sonic bottlenecks -- limiting factors in tube amp performance. The full-power bandwidth of a typical tube amp is no better than 20 Hz to 30 kHz. Contrast that with solid-state performance and you're looking at 2 Hz to 80 kHz. Bass performance is almost always hampered by core saturation, hysteresis, and loss of damping. At the high frequency extreme, transformers are prone to resonances which can intermodulate into the audible bandwidth. The obvious question by now is if output transformers are so heavy, costly, and restrict bandwidth, why use them in the first place. Why not cast away the iron yoke, and have the power tubes communicate directly with the speaker load?
As long-play records and home audio in general began to mushroom in the '50s, industry executives began to think along the same lines. Eliminate the transformer and you reduce cost and weight and increase your competitive edge in a fast-growing consumer market. Unfortunately, eliminating output transformers is easier said than done. They are necessary for the same reason a bicycle is designed with a gear box or an automobile with a transmission. To transfer power efficiently from the high-voltage and low-current plate circuit of a power tube to a 4 to 8-ohm speaker requires a transformer with the right turns ratio so as to "gear-down" from high to low impedance.
Dave Berning and his Most Unique OTL
To my mind, the most exciting chapter in the OTL story has been written by Dave Berning with the recent introduction of the ZH270 stereo power amplifier. It's a case of Godzilla (my favorite sci-fi icon of the'50s) meets the Pentium II chip: a new paradigm of OTL technology wherein the vacuum tube meets highfrequency power conversion.
The ZH270 idles at a mere 100 watts. A single pair of 6JN6 "baby" TV tubes operating in a conventional push-pull mode develops over 100 watts into a 4ohm load without the plates showing any color. This is a consequence of Berning's patented (US Patent No. 5,612,646 issued 3/18/97) power conversion technology. Another amazing figure: the amp weighs a mere 10 pounds! That's lighter than any of the preamps I've got in-house. Yet the ZH270 can reproduce bass frequencies at full power into an 8-ohm load down to 2 Hz. That's almost DC! No other tube amp can touch that level of performance.
At the heart of the ZH270 is the radio-frequency switching power conversion circuitry which impedance-maps the high voltage and low current domains of the output tube onto the high-current and low-voltage domains of the speaker. Though the ZH270 is technically an OTL, there is a small RF conversion transformer in the signal path which precisely controls the impedance matching. This transformer is wound with an effective overall turns ratio of 64:1 -- far greater than what's possible with a conventional audio transformer-because it is optimized for operation at an RF frequency of 250 kHz. The audio signal flows across the converter on the RF carrier.
The input/driver stage uses a pair of 12AT7 and one 12AV7 dual triodes per channel. The output tubes are connected as triodes, but as with all Berning amps, are driven with screen-grid drive -- another Berning innovation (US Patent No. 3,995,226). The control grid of the 6JN6 is connected to the cathode and drive voltage is applied to the screen grid. Benefits include increased reliability and improved linearity at low idle currents. The idle plate dissipation for the 6JN6 is a remarkably low seven watts, and their heaters are operated below rated voltage. Expect to re-tube these amps once in a blue moon.
Another reason for the ZH270's low weight is the use of a resonant type switching power supply which eliminates the need for a large power transformer. Because the amp can pass DC, two speaker protection mechanisms are incorporated. The first is a DC voltage offset sensor that shuts down the power supply if a continuous DC voltage is present at either speaker terminal. The second is a current imbalance sensor that in the event of a catastrophic failure can shut down the power supply in about one-fourth of a second.
A volume pot is provided along with a pair of line-level inputs and selector switch. So in reality the ZH270 is a bit more than just your basic power amp. You could run a CD player (as I did) directly into it and still have room to accommodate a tuner or a tape deck.
So what does it sound like? In several respects, the ZH270 pushes the state-of-the-art envelope forward. It is utterly free of tube glare, brightness, treble resonance, zip, edge, grunge, or whatever your favorite adjective might be for upper octave irritants that can quickly take the fun out of any high-end listening experience. For that reason alone I found it to be an extremely easy amp to live with. Feel free to use it with bright or etched sounding speakers without fear of compounding their sins. The first and lasting impression was of speed and control-speed beyond ordinary tube amps and control in the class of the finest solid-state designs. It is felicitous of treble transients like almost no other amp in my experience. Transients were articulated with delicacy and a silky smoothness, unfolding with a totally believable organic wholeness. Plucked strings, in particular, were reproduced with convincing attack and decay. Midrange textures took on an electrostatic-like clarity. It was easy for me to resolve harmonies or to make out densely layered orchestration. Low-level detail and the decay of notes into the noise floor of the recording were clear. I often complain about the inability of transistor amps to paint sufficiently vivid harmonic colors. Soprano voice or violin overtones are usually bleached out, lacking in sheen and natural sweetness. True to its tube origin, the ZH270 dished out a lifelike harmonic palette. Tonal balance proved to be load dependent, varying from laid-back to almost total neutrality, though most of the time the balance tended toward a laid-back perspective. The source impedance of the ZH270 is a bit highish at 1.8 ohms, even in its Normal or highest feedback position. The source impedance climbs much further in the Medium feedback position (3.8 ohms), and reaches an astronomical 8.7 ohms in the Low feedback position. Expect a significant interaction between the ZH270's output impedance and the load's impedance magnitude. The change in frequency response is small where the speaker's impedance magnitude is much larger than the amp's source impedance, and large where the impedance drops to a value comparable to that of the amp's source impedance. Most dynamic speakers exhibit their lowest impedance in the lower midrange. Not suprisingly, the ZH270 usually sounded a bit thin in the lower midrange. This is not a lush, romantic sounding tube amp in the tradition of vintage tube gear. But it tends more in that direction when it is operated in the Medium and Low feedback positions when harmonic distortion rises a bit and the power bandwidth shrinks considerably.
Feelings and emotions are buried in the music's microdynamics. The Berning had no difficulty at all in retrieving the music's full emotional gamut. It excelled in highlighting inflections in human voice and the filigree-like nuances in bowed strings. It could deliver quite a punch into a compatible load and recovered quickly from overload conditions. It didn't seem to mind most typical dynamic loads, but lacked bass punch into low-impedance loads, doing considerably better with 8-ohm rather than 4-ohm loads.
Pitch control and definition of bass lines were exemplary. The bass range was airy and precise, in general portraying a convincing orchestral foundation. The ZH270's combination of speed and control extended to the bass range where double bass and cello were given the sort of solidity I've rarely experienced from tube amps. This is one savvy amp when it comes to preserving the music's rhythmic drive. It just never seemed slow or out of breath. Image outlines were tightly focused in space with the sort of presence only tube amps can generate. The soundstage was brightly lit with absolutely no veiling of any sort. The ZH270 seemed to bring out the best in speakers known to image well. It was able to paint a panoramic soundstage that at times threatened to envelop me within its confines.
David Battles Goliath
Are we ready for a confrontation of biblical proportions? I was. I just could not resist. I was too curious to find out if David would be slain in the epic confrontation of the Berning amp (David) and the Sound Lab A-1 (Goliath) ESL. Visually, it appeared to be quite a mismatch, David -- a mere pimple on the reference room floor, sandwiched between two towers of power. Could David hold his own with a speaker known to be a notoriously tough and inefficient load? After all, the A-1 had vanquished numerous tube amps, including fine specimens from Sonic Frontiers and Jadis. It only took me a few seconds to realize that the ZH270 was destroying everything that had come before it -- at least in a couple of performance areas. Soundstage transparency was just unbelievable. I had never before heard the A-1 so vividly energized through the midrange. The dividing line between the speaker and the room almost disappeared completely. The inner recesses of the soundstage were lit up by bright searchlights. Clarity, or the ability to resolve the harmonic structure of each instrument, was startling in its intensity. It became obvious that here was a new breed of A-1 with an immediacy I never experienced before. The Berning did have some problems, however, at the frequency extremes. Unlike my experience with dynamic loads, it slightly brightened harmonic textures, and thereby reduced apparent smoothness. It also had problems controlling the deep bass, although the midbass was exceptionally precise.
Although it impressed me overall as more refined than brawny, the ZH270 did manage with a compatible load to unlock the gates to sonic nirvana like only a handful of amps out there. Technically, it makes most other amps look like relics from the distant past. This is a MAJOR design with a blend of solid-state and tube strengths that should appeal to a wide audience. Finally, an OTL that can drive dynamic as well as electrostatic speakers. An OTL without the fuss and maintenance headaches of conventional OTLs. When I look at its performance to price ratio, I see a screaming buy recommendation. With so much bang for the buck, you've got to give it a listen.
FI SPEC SHEET
OTL power amplifier
The David Berning Company
12430 McCrossin Lane
Potomac, MD 20854
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