David Berning ZH-230 Stereo Power Amplifier (TAS 210)

The State of the Art in Medium-Power Amplification

In my book, a new product introduction from the creative mind of David Berning is an event, a happening if you will. His designs are far from being yet another twist on a 1950s circuit, which to be perfectly honest is what most of tube audio is all about nowadays. Although the ZOTL technology has been around for over a decade and could be considered mature, it still looks and sounds cutting-edge. It represents David Berning’s severing of the Gordian Knot that is a conventional audio output transformer. The output stage is push-pull. The beam power tubes (a pair of 33JV6 per channel) are connected to two full-bridge power converters, which reflect their conductance through a pair of RF transformers (each power converter has its own dedicated transformer) operating at a fixed carrier-frequency to match the impedance between the tubes and loudspeaker. It’s fair to say that the ZH-230 represents an update of the venerable ZH-270 (long out of production), albeit at a lower power output. Most notably, the RF carrier-frequency has been increased from 250 to 500kHz and the source impedance has been reduced for increased damping factor.

According to Berning, “the ZOTL is in reality an emulation circuit for an ideal output transformer. It matches impedance just like a transformer, and it is a two-way communication between tubes and speaker as is a conventional transformer. The beauty of it is that the ZOTL does not suffer from the frequency-dependence issues of a transformer that is expected to work over orders of magnitude of frequencies. The ZOTL transformers just operate at a single frequency, and any non-ideal issues with these do not affect the audio response as they are constant and would be like having a constant resistor somewhere in the signal path.” He is quick to emphasize that the two-way communication is a critical design feature. This is the thing that separates the ZOTL from a hybrid amplifier. The ZOTL “should not be considered a tube amp followed by some other kind of output buffer—it is not. By having the two-way communication, the speaker is fully controlled by the output characteristics of the tubes, and that includes all-important damping. This is not the case with the hybrid amplifier that is simply tubes with an output buffer.”

This is one of the few power amps I can easily carry under my arm. That can be explained not only by the lack of a conventional output transformer but also by the absence of a power transformer. There’s no heavy iron here. The power supply is a switching type and incorporates a five-stage power-line filter and surge suppressor. The amplification circuitry is fully balanced. The input signal is AC coupled to a differential phase-splitter, followed by a driver stage. Out of the box, the amp is outfitted with a 12AX7 input and a 12AT7 driver. Since the ZH-230 is auto-biasing, these tubes may be changed without having to worry about manual bias-calibration. A 12AT7 or 12AU7 may be substituted for the input tube, while only a 12AU7 may be substituted for the 12AT7 driver. The user manual outlines the performance of the amp with various tube types. With the stock tube complement, the feedback is highest, resulting in the lowest output impedance (about 0.6 ohms for an 8-ohm load). The use of lower-gain tube options (e.g., 12AU7 for input and driver) yields the lowest feedback and highest output impedance (2.7 ohms for an 8-ohm load). My preference is to keep the source impedance under 1 ohm to increase the damping factor and minimize load interactions. After an initial stint with the stock tubes, I rolled in (with Berning’s blessing) a quartet of NOS Philips 7062/E180CC. These are computer-grade tubes, similar to a 12AT7, but with slightly lower gain and somewhat higher filament current (200 vs. 150mA). These tubes yielded richer, more nuanced harmonic texture, and I can safely recommend the 7062 as a worthwhile upgrade. Note that the 33JV6 may not be replaced with a different type. Prior to replacing tubes, be sure to explicitly follow all of the safety precautions outlined in the owner’s manual, as high DC (900V) and high AC voltages (1600V) are present inside the chassis. Since the idle-plate dissipation of the power tubes is only 7.5W and their heaters are operated below the rated voltage, their expected lifetime, according to Berning, is 10,000 hours.

Choice of matching load is always an important consideration for low and moderately powered tube amps. The amp is designed to work with nominal loads in the range of 4 to 16 ohms and, ideally, would appreciate a speaker sensitivity of at least 90dB. My intention all along was to mate the ZH-230 with my Basszilla Platinum Edition Mk.2 DIY speaker, a 96dB-sensitive load that has performed well in the past with such amps. But along the way I had the opportunity to drive the Salk SongTower and the DALI Helicon 400 Mk.2, both being 4-ohm nominal loads and rated at a sensitivity of 88dB.