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.

Features

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.

Specifications

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.