This unit is in mint condition and purchased only a few months ago, simply stunning in terms of speed, transparency and timbre. You won't find a better tube amp than this at this price point. We need more than 30 Watts to drive our demo TAD speakers, this is the only reason why it is for sale.
Specs and description taken from manufacturer's website:
All balanced amplification.
No phase compensation for feedback used.
Constant output impedance over entire audio spectrum.
Built-in five-stage power-line filter and surge suppressor.
Non-magnetic chassis prevents skin-effect induced distortion.
Power consumption: idle-110 W; max-300 W. Power factor corrected.
Line voltage required: (universal) 100-240 VAC, 50-440 Hz.
Signal to noise: broadband (typical) 60 dB, all feedback settings. 20 kHz bandwidth (typical): 85 dB to 105 dB, depending on tube complement. All figures unweighted.
Distortion (typical, at 8 ohm loads at 30 W): 0.3 % to 0.7 % These double at 4 ohms.
Typical output power at onset of clipping: 8 ohms 36 W; 4 ohms 36W.
Frequency response at 5 W, 8 ohms: +0, -0.6 dB @ 30 kHz.
Frequency response at 5 W, 4 ohms: +0, -2,7 dB @ 30 kHz, +0, -1.5 dB @ 20 kHz.
Low frequency limit: 0 dB down @ 20 Hz, but repetitive waveforms into 4 ohms below 20 Hz activates speaker protection.
Typical output impedance (measured at 1 amp, 60 Hz): 0.6 Ohms to 2.7 Ohms, depending on tube complement.
Sensitivity at 30 W output: 8 ohms: 0.4 V RMS to 0.75 V RMS, depending on tube complement and load impedance.
Input impedance: 50 kOhm
Size: 42 cm wide, 30 cm deep, 17 cm high, (16.5 X 12 X 6.75 inches), including connectors and feet.
Net weight: 7 kg (15 lb); Boxed, 9 kg (20 lb).
Finish: silver anodize
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.
Often it is desirable to change the speaker damping to obtain a more natural sound. The impedance of many speakers varies with frequency, and often this shows up most dramatically with a rise in impedance near the bass resonance frequency. A tube change can be used to reduce or increase the feedback and thus raise or lower the output impedance of the amplifier, and this affects the system frequency balance by coupling more or less power into the speaker at those frequencies where the impedance is higher. Speaker designers generally try to design for a flat frequency response using an anechoic chamber. Home listening environments generally fall far short of the ideal acoustic properties of the anechoic chamber, and the ability to change feedback gives you, the amplifier user, a means to modify the normal speaker response to compensate for room acoustics and personal preference.
Technically, speaker damping relates to the control that the amplifier has over the position of the speaker diaphragm. High damping applies tighter voltage control over the speaker and causes the amplifier to absorb and stop speaker cone motion due to the speaker cone overshooting its desired position in response to transients. However, many speakers are somewhat over damped by design and rigid voltage control is not optimum. Lower feedback in the amplifier reduces the tightness of the voltage control and applies a mixture of current and voltage control. If the speaker is over damped, the cone will not reach its proper position quickly in response to a transient. With the right amount of damping, the speaker cone will neither overshoot nor undershoot its desired position. This is often termed critical damping. High damping is likely to give a tighter, dryer, and more analytical sound, and low damping is likely to give a more open and resonant sound. The sonic attributes of damping show up most dramatically in the bass.
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.
The Berning ZH amplifiers take advantage of new technology in order to eliminate the audio-output transformer. These amplifiers use radio frequency to change the voltage-current transfer characteristics of the output tubes from their normal impedance-plane to one suitable for driving a dynamic loudspeaker. The radio frequency re-mapping is implemented using special high-frequency power conversion techniques. The high-voltage, low-current tube impedance-plane is re-mapped to the high-current speaker impedance-plane through a special transformer at a constant RF carrier frequency of 250 kHz (500 kHz for the ZH230). Because the audio signal is riding on a carrier, it is not subject to parasitic elements of the transformer that would otherwise distort the audio signal. There is no low-frequency limit for this impedance conversion, and these amplifiers are dc-coupled, with the exception of a small, high-quality, dc-blocking capacitor at the input of the amplifier to prevent the amplification and application to the speaker of any dc that might be present at the input.
The impedance matching in the ZH amplifier 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 a conventional transformer-coupled amplifier, and negative feedback is not required to force the positive-output current capability to be similar to the negative-output current capability.
Audio-output transformers have a limited impedance conversion range imposed by the parasitic elements. Audio-output transformers also have limits imposed by voltage breakdown between primary and secondary, and required insulation also affects the parasitic elements adversely. It is difficult to make a high-fidelity transformer with much more than a 25:1 overall primary-to-secondary turns ratio for the above reasons. These restrictions are eliminated in the ZH amplifiers, and larger effective overall turns ratios are applied to use the tubes more efficiently than they can be with the smaller ratios. One implication of this change is that the reflected plate resistance of the output tubes is lowered, making the tubes appear more like triodes to the speaker.
Another implication of using a higher-effective turns ratio is that the output tubes operate at lower current and higher voltage than they otherwise would. As tubes become weak with age, the current capability is reduced, but the voltage capability is not affected. Longer useful tube life is expected because less current is required for a given output power. The opposite situation applies to the traditional OTL amplifier.
Audio-output transformer-coupled amplifiers should not be operated without a load connected because the inductance of the transformer's primary can cause the plate voltage to reach a sufficiently high voltage to break down insulating materials, resulting in amplifier failure and even fire. Some amplifiers are even unstable without a load. Unlike output-transformer-coupled amplifiers, the ZH amplifiers can be operated safely without a load, as the voltage cannot exceed its normal designed-in operating voltage.
Other Features and Specifications
Like other recent Berning products, the ZH230 incorporates a fully regulated, resonant-type, switching power supply. The use of regulated switching power-supplies in Berning amplifiers over the years has resulted in these amplifiers obtaining a reputation for having better control in the bass than tube amplifiers with conventional power supplies. The use of these supplies has also resulted in unusually lightweight and compact amplifiers for their power capability because there was no line-frequency power-transformer in these units.
The ZH230 follows other Berning designs in that the tubes are operated at relatively low temperatures for long life. The idle plate-dissipation for the 33JV6 output tubes is 7.5 W each, and their heaters are operated below their normal rated voltage. Routine tube replacement on the ZH230 should not be needed with normal usage, and 10,000-hour lifetime should be expected. It is recommended that the ZH230 be powered down when it will not be used for several hours or more, otherwise it can be left powered.
The audio circuit board in the ZH230 is a special low-loss ceramic material and provides for the amplifier having a more transparent sound with additional presence. The ZH230 is also equipped with Stillpoints TM OEM Universal Resonance Dampers.