The Berning Zero-Hysteresis ZH270
A New Kind of Amplifier
The design of the Berning ZH270 power amplifier represents a radical departure from that of other amplifiers. The ZH270 is 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.
Audio-amplifier design engineers have long sought to eliminate the audio-output transformer because of the restrictions that it has always imposed on amplifier performance. Both leakage inductance and interwinding capacitance limit the high-frequency response of the transformer, while core saturation and magnetizing current limit its low-frequency response. Transformer-core hysteresis causes certain kinds of distortion of the non-symmetric and transient waveforms so characteristic of musical reproduction.
Output-transformerless (OTL) tube amplifiers are not new. Julius Futterman pioneered, and many others have copied, an amplifier topology that uses many power tubes connected in parallel to obtain enough output current to drive a speaker. To be able to truly match the tube impedance to the speaker using this method would take an impractical number of large power tubes, and a compromise is reached by forcing a smaller number of tubes to deliver the required current with high-feedback techniques. If one listens to OTL amplifiers on the right speakers, they can be quite good.
While the high-feedback used in the OTL amplifiers lowers the small-signal output impedance, it does not provide true impedance matching for large signals. As a result, the traditional OTL amplifier does not work well with most speakers. Major reliability issues surround OTL amplifiers, and power consumption is very high.
The ZH270 takes advantage of new technology in order to eliminate the audio-output transformer. The application of this technology is the subject of a pending patent. The ZH270 uses radio-frequency to change the voltage-current transfer characteristics of the output tubes from their normal impedance-plane to one suitable for driving a speaker. The radio frequency impedance conversion is implemented using special high-frequency power-conversion techniques. The high-voltage, low-current tube impedance-plane is changed to the high-current, low-voltage speaker impedance-plane through special transformers at a constant carrier frequency. Because the audio signal is riding on a carrier, it is not subject to the parasitic elements of the transformer that would otherwise distort the audio signal. There is no low-frequency limit for this impedance conversion, and the ZH270 is dc-coupled, with the exception of a small, high-quality, dc-blocking capacitor at the input of the amplifier.
The impedance matching in the ZH270 is precisely determined by the effective turns-ratio of the RF conversion transformers. Both the push and pull portions of the amplifier are handled in a symmetric fashion, as they are in conventional transformer-coupled amplifiers, and negative feedback is not required to balance the positive and negative drive to the speaker. The impedance matching ratio is 16 k ohms plate-to-plate into 4 ohms, or 32 k ohms plate-to-plate into 8 ohms in the ZH270. These unusually large transformation ratios are possible because there are no limits imposed by parasitics present in audio-output transformers. The large ratios offer advantages of higher current delivery to the speaker, higher tube efficiency, reduced speaker loading of the output tubes, and low reflected plate resistance. Like other Berning amplifiers, the ZH270 uses a special triode-connected screen-drive arrangement that offers greatly enhanced reliability and also improved linearity at low idle currents.
The ZH270 includes two sets of inputs, along with an A-B selector switch. A high-quality volume control is also provided, and the ZH270 has sufficient gain to boost many high-level signal sources to full power without pre-amplification. One possible use of the A-B selector feature might be to feed the signals from a CD player into the A inputs, and to feed the signals from a pre-amp into the B inputs. This configuration would simplify the signal path for the CD player and should provide the best sound reproduction of CDs. Stereo systems having only one or two high-level signal sources may not even need to include a pre-amp.
A three-position feedback switch is included on the ZH270. The range of feedback settings is from 12dB to 3dB with an eight ohm speaker load connected. This variable feedback feature gives the user flexibility in matching the speaker damping and tonal balance to the listening room and personal taste.
In addition to the user features included with the ZH270, there are a number of internal house-keeping features too. All tube biasing is done automatically with peak-holding sampling and servo-control circuits. Biasing is updated on a cycle-by-cycle basis, even when the amplifier is being driven hard.
A pair of LED indicators on the front panel indicate proper biasing by turning from red to mostly green during amplifier warm-up. Should the ZH270 not be able to set the bias on all the tubes, one of several error-sensing circuits will place the power supply in a fold-back current-limiting mode in order to protect amplifier components from possible damage.
Another pair of servo-loops perform automatic de-offset-null functions so that no dc voltage is fed to the speakers. These servos are also tied into error sensing so that the power supply is cut back if the dc-offset cannot be nulled.
The error sensing and protection circuits are not in the signal path. A momentary short circuit at the speaker terminals will not harm the ZH270 and will not activate any protection. The ZH270 can be operated without a load.
The ZH270 is built on a circuit board, selected portions of the audio signal path are constructed using point-to-point wiring.
ZH-270 Dual 70-Watt-Per-Channel Zero-Hysteresis Amplifier
- Single-ended only inputs, all balanced amplification.
- Low negative feedback, no phase compensation.
- Constant output impedance over audio spectrum.
- Brown-out protection.
- Four-stage power line filter and surge suppressor.
- Non-magnetic chassis.
- Power consumption: idle-100W, max-300W.
- Power required: 100-130VAC or 200-260VAC.
- Input impedance: 50k.
- Signal to noise: broadband (typical) 60dB, all feedback settings. 20kHz bandwidth (typical): 94dB, NORM feedback; 91dB, MED feedback; 85dB, LOW feedback. All figures unweighted.
- Distortion (typical, both 4 and 8 ohm loads at 70W): 1%, NORM feedback; 1.5%, MED feedback; 2%, LOW feedback.
- Typical output power at onset of clipping: 8 ohms, 84W; 4 ohms, 110W.
- Frequency response at 1W, 8 ohms; +0, -1dB: 1.5Hz to 60kHz, NORM feedback; 1.5Hz to 35kHz, MED feedback; 1.5Hz to 21kHz, LOW feedback.
- Full power bandwidth (-3dB), 8 ohms: 2Hz to 80kHz, NORM feedback; 2Hz to 50kHz, MED feedback; 2Hz to 35kHz, LOW feedback.
- Typical output imedpance (measured at 1 amp, 60Hz): 1.8 ohms, NORM feedback; 3.8 ohms, MED feedback; 8.7 ohms, LOW feedback.
- Sensitivity, 70W, 8 ohms: 0.7Vrms, NORM feedback; 0.35Vrms, MED feedback; 0.2Vrms, LOW feedback.
- Size: 32cm wide, 38.6cm deep, 11.5cm high (12.5 x 15 x 4.5 inches), not including handles, connectors and feet.
- Net weight: 4.5 kg (10lbs.)
- Finish: black anodized aluminum with clear window.
- Limited Two-Year Warranty.
- List Price $5,670.
Owners Manual PDF
ZH270 commentary and comparison by StereoMike on ZeroGain audio forum, August, 2007.
Starting at the bottom, the bass is wonderfully rounded and perfectly damped, yet solid as a sledgehammer unlike any valve amplifier I had heard before or since. ... Indeed the midrange and treble are utterly natural and delightfully transparent. People talk of veils. With this amp, all the veils are off before the first note's finished. ... I have not heard an alternative amplifier with this speed or openness regardless of price. And this speed is coherent - not varying at all across the audible response. This coherency all adds to the illusion of real musicians playing real instruments - something the Berning is extremely good at purveying. ... A remarkable performance for a 70wpc [push-pull output stage] device that can drive real world loudspeakers to real listening levels.
ZH270 reviewed by Rick Becker, Enjoy The Music, December 2006
"...more importantly, waves of goose bumps rolled over me as I listened... outstanding focus, combined with the outstanding transparency... the estimated tube lifespan being in the neighborhood of 15 to 20 years. Compare that with your own life expectancy. ... Acoustically, it leaps over almost the entire field, landing in the upper echelon of the finest sounding amplifiers."
ZH270 reviewed by Gary Beard, Positive Feedback, Issue 5, 2002
"First and foremost, this amp is musical. It draws you into the music and makes it difficult to tear apart the sound in the typical audiophile way. It does not seem to have a confining musical signature to which one can constantly point. Unlike the VAC, which was seductively rich and beautiful no matter the source, the ZH acts like a mirror of the original, yet does so with a purity of tone and grainless-ness that won't leave your ears scarred from a near-death experience if the source is less than perfect."
The Berning showcases the beautiful tone and timbre of instruments and voices. It is exceptionally clear, and has excellent inner resolution. When you least expect, it will thump you in the chest with deep tuneful bass, serenade you with a beautiful midband, and give you chills with exquisitely delicate treble. The attack and decay of notes are excellent, and the amp's speed and ability to present dynamic contrasts can be startling. All the airy images and soundstaging that I've come to expect of a first rate amplifier are present in spades. All these audiophile qualities add up to an amplifier that is wonderfully musical and engaging.
"'Visceral, delicate, beautiful, rhythmic, dynamic, in a word, incredible' were the descriptors I used in an email sent to another Berning owner shortly after my first taste of the sound of the ZH270. Today I would add 'lightning fast, extended, and musical.' In the high-priced world of audio, I believe the Berning ZH270 is a great value. It is a unique product, built by a designer dedicated to finding new and better ways to reproduce sound, and yet, unlike so many other 'new and different' components, it is one that fits within the real world."
ZH270 reviewed by Charles Hansen, Glass Audio, vol. 12 iss. 1-2, 2000
"Right out of the box, the ZH270 was a sonic pleasure. ... The bass was tight, far more so than I have ever heard on any tube amplifier. Acoustic bass was woody, resonant, and full of life. The extra harmonic richness in electric bass came through in spades, and kick drum had a lively pop...the bass never became tubby or boomy. The soundstage was spacious, wide, and deep. You could hear each individual instrument in even the most complex orchestral crescendo. Piano was portrayed as though you were standing right above the soundboard. You could hear each hammer and pedal in realistic detail. The sound never became confused, nor did the soundstage retreat even the slightest bit. This amplifier was absolutely unperturbed at even the highest volume. Complex pop music with layer after layer of synthesizer was more detailed than I have ever eheard. Yet the ZH270 also tames the bright, edgy CD mixes that seem to be in vogue these days. The best of both worlds!"
ZH270 reviewed by Martin Appel, stereotimes.com, 1999
"My system, however replaced with the Berning ZH270, immediately became more alive, with a good dose of vibrancy, not to mention a metaphoric veil was lifted between the music and the listener. This dear reader added an increase in palpability to instruments and voices with each and every disc. ... The soundstage is much more open, possessing more air that few amps can match. The mid-range, where the majority of music happens, is special. Clarity and articulation go hand in hand with imaging and depth. Instrumental timbres and voices are detailed, three-dimensional, and accurate, with dynamics to match. (Nothing mushy, boomy, or vague here.) High frequencies are clear and bell-like, and free of any grain, without that electronic etching that can irritate--I'm talkin' making music."
ZH270 reviewed by Dick Olsher, Fi Magazine, 1998
"In several respects, the ZH270 pushes the state-of-the-art envelope forward. ... 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. ... True to its tube origin, the ZH270 dished out a lifelike harmonic palette. ... 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."
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