The ZH230 is a stereo vacuum-tube amplifier that uses the output transformerless ZOTL technology (US Patent 5,612,646). The ZH230 uses moderately high bias currents in the output tubes compared with some Berning products. This is possible because of the lower power of the amplifier. The ZH230 is designed for loudspeaker loads ranging from 4 Ohms to 16 Ohms, and can be operated without a load. Single-ended RCA type inputs and binding post outputs are provided.

The ZH230 is designed to accept various tube-type combinations, and will auto-bias itself for these combinations. The ZH230 is shipped with a medium-high-gain tube combination. These include a 12AX7 for the first stage and a 12AU7 for the second stage. The 33JV6 output tubes cannot be changed to different types. This combination provides a medium amount of feedback and one-ohm output impedance for moderately high speaker damping.

The Proven ZOTL Technology, Now in the ZH230

The design of the Berning ZH270 power amplifier represented a radical departure from that of other amplifiers. The ZH270 was the first amplifier using all-tube amplification to properly match 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. The technology is detailed in Berning US patent 5,612,646, and a simplified overview is given below. The ZH270 has gained much respect for its transparency, versatility, and reliability, and had enjoyed a production run of over 10 years. The ZH technology has also proven itself in other recent Berning products as well.

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.

Tube Amplifiers have been previously made that drive dynamic loudspeakers without output transformers. These amplifiers are called output-transformerless (OTL) amplifiers. A well-known pioneer of this type of amplifier was Julius Futterman, and there have been many amplifiers made by various companies over the years that are based on the Futterman principles. The Futterman design incorporates two separate banks of parallel-connected tubes. The two banks are joined in series by connecting the effective cathode of one tube bank to the effective plate of the other tube bank, and driving the loudspeaker in a push-pull fashion from this junction point directly without a transformer. Popular alternatives to the Futterman use low-impedance triodes, but the basic principals are the same. Given a sufficient number of parallel-connected tubes in each bank, enough drive current can be obtained to drive the speaker.

There are several problems associated with the OTL amplifier that have limited its application. The OTL amplifier is a “brute-force” methodology, whereby the banks of tubes are forced to deliver the required drive-current by high-feedback techniques. The natural impedance of the cathode “push” is much lower than the plate “pull”, and neither one is anywhere near the actual speaker impedance. In the Futterman amplifier, a large amount of negative feedback is required to force the push and the pull to work together properly, and to provide sufficient damping for the speaker. Some of the alternative OTLs that use the low-impedance triodes can operate without requiring feedback because the gain of these triodes is so low that there is little difference between the plate impedance and the cathode impedance, so the mismatch between the push and the pull is minimal. However, even with the lowest impedance triodes available, there is still a basic impedance mismatch between the tubes and the speaker in the OTL circuit. The success of mating OTL amplifiers to dynamic speakers is very dependent on the characteristics of individual speakers. Major reliability issues surround OTL amplifiers, not only because a large number of power tubes are required, but also it is necessary that the tubes need to be driven hard to obtain the required output current. Power consumption is high for OTL amplifiers, often exceeding one kilowatt for a stereo pair. Noisy fans may be needed in these amplifiers. Additional room air-conditioning may be needed, further increasing the listening-room noise level.