Berning ZH-230Reviewers' ChoiceMany manufacturers claim that their products are “innovative,” but few merit that description more than the amplifiers designed by David Berning. His recent power amps have used a circuit he describes as a “unique Impedance Converter that replaces the traditional audio power output transformer and greatly extends and improves amplifier performance. We call our technology ‘ZOTL’ for Zero-Hysteresis Output Transformerless. Operating at a fixed high frequency without traditional audio output transformers, the ZOTL Impedance Converter eliminates the frequency-dependent performance limitations inherent in all transformer-coupled tube amps.

“Berning amplifiers using the ZOTL technology exceed the performance of traditional OTL tube amplifiers by properly matching the tube impedance to that of the speaker. Berning amplifiers depart from traditional OTL amplifiers in that they do not require a large number of hot power tubes to supply adequate current for driving speakers.”

If you’re interested in delving deeper into the technology, see Berning’s website. My efforts here focus on describing how the ZH-230 ($8360 USD) amplifier sounds rather than how it works. Well, mostly; there are some things about how it works that you need to know because of how they affect its sound.

The input circuitry, too, is innovative. It basically uses a dual-triode tube for input voltage gain, followed by a dual-triode driver tube. “And just what,” you may wonder, “is new about that?” The innovation is that you can use 12AX7, 12AU7, or 12AT7 tubes (or their equivalents) to change the amplifier’s gain and feedback. The only restriction is that you can’t use a 12AX7 as a driver tube. The ZH-230’s amazingly thorough manual includes a table showing the gain and damping factor for each combination of tubes.

Another innovation is that the output tubes are 33JV6s, a type I’d never heard of -- it’s not your typical output tube. I found some 33JV6 tubes at for $5.95 each -- a welcome respite from the high prices we’re used to paying for premium KT88s and KT120s. There are several brands of 12AX7, 12AU7, and 12AT7 tubes currently in production, and numerous new old stock (NOS) tubes are still available, though the versions more in demand usually command higher prices.

My favorite innovation in the ZH-230 is that its tubes are operated very conservatively: David Berning predicts a tube life of 10,000 hours. If you run the ZH-230 four hours a day, five days a week, you won’t need to replace your tubes for 9.6 years -- really good news for someone who recently had a single-ended-triode amplifier whose tubes failed annually and cost $450/pair.

Berning ZH-230

Like most Berning amps and preamps, the ZH-230 has a switching power supply. If that conjures up images of cheap wall-wart supplies, worry not; Berning’s switching power supplies are cut from different cloth. Berning designed electronic equipment for the National Institute of Standards, and his power supplies are the state of the art. Because the ZH-230 is a switching design, its power supply accepts voltages from 100 to 240VAC at frequencies from 50 to 440Hz -- talk about a universal power supply! The use of switching power supplies means that there are no heavy transformers -- the ZH-230 weighs only 15 pounds. When I accepted the amplifier from UPS, the box was so light that I at first thought it was empty.

The ZH-230’s anodized silver case measures 16.5”W x 6.75”H x 12”D, including connectors and feet. There is no visible clue that the ZH-230 is a tube amplifier -- the case completely encloses the tubes. To try different combinations of low-level signal tubes, you’ll have to remove the 14 Phillips-head screws that secure the top plate, on which is engraved “David Berning.” Some might think the ZH-230 looks ordinary; I thought it was tasteful and elegant, in an understated way.

To eliminate vibrations transmitted by whatever surface the ZH-230 sits on, its footers are three Stillpoints Universal Resonance Dampers. I’m a big fan of Stillpoints, and appreciated not having to struggle with positioning my own Stillpoints under this amp.

Berning ZH-230

The ZH-230 is claimed to output 30Wpc, though I’m told that it will typically produce 36Wpc at clipping. Because it has no output transformers, there is only a single pair of output terminals per channel, these usable with speakers with nominal impedances of 4 to 16 ohms. The ZH-230’s input impedance is 50k ohms, which should be easily driven by any preamplifier or other source. The input sensitivity at 8 ohms ranges from 0.4 to 0.75V RMS, depending on the tubes used and the load impedance. Only unbalanced inputs are available, on RCA jacks.

The ZH-230’s only control is the small on/off toggle switch (with red indicator LED) centered low on its front panel. Unlike most tube amplifiers with output transformers, the ZH-230 won’t be harmed if it’s operated with no speakers attached.

Even with my high-sensitivity horn speakers, the ZH-230 was almost totally silent -- when I put an ear just a few inches from the speaker’s driver, I heard only a faint bit of noise.

Setup and use

I slid the Berning ZH-230 onto the bottom shelf of my equipment rack, where it had sufficient clearance for ventilation, and connected it to my Audio Research LS27 line stage and Affirm Audio Lumination speakers with Clarity Cables’ Organic unbalanced interconnects and speaker cables. I’d been told that my review sample was already broken in, but as I do with every component, I played it for several hundred hours to be sure it was well broken in. Like most tube gear, the ZH-230 needed some time after turn-on to sound its best -- about 40 minutes.

The ZH-230 ran hot, but not unusually so for a tube amplifier -- my Atma-Sphere S-30 Mk.III, another OTL design, runs much hotter. The Berning’s input-sensitivity spec seemed rather high, so I switched my line stage’s gain to Medium so that I could use a higher volume-control setting. Even so, my typical volume settings were from 20 to 30 out of a possible 104 -- still pretty low.

Berning sent along a pair of Electroharmonix 12AU7s so that I could experiment with different combinations of tubes. I used the ZH-230’s stock Sovtek 12AX7WA input tubes and Penta Laboratories 12AU7 driver tubes for the bulk of my listening, then did some tube swapping. I was tempted to dig into my stock of NOS tubes to see if I could improve on the Berning’s sound, but that sort of thing has no place in a review, since it turns an amp into a one-of-a-kind unit that users may not be able to easily duplicate. But as the manufacturer had sent me those 12AU7s . . .


When the Berning arrived, I was reviewing a pair of speakers that sounded decent. The ZH-230 transformed them into speakers that sounded seriously good. The bass was now supercharged, with considerably more extension and weight. I ended up swearing a lot as I put this amp through its paces, saying things like “I can’t believe this amp sounds this @#%&* good!”

Far from being a liability, the ZH-230’s switching power supply was one of its glories. That’s because, unlike linear power supplies, it delivered power right now. That speed made it the most effortlessly dynamic-sounding amplifier I’ve ever heard, and it seemed much more powerful than its rated output. Heck, it sounded more dynamic than my Audio Research VS115, which puts out 115Wpc to the Berning’s 30Wpc. And it wasn’t just the ZH-230’s ability to play loudly -- the microdynamic changes within musical passages that constitute a musician’s phrasing were somehow more clearly defined than with most other amps I’ve heard. There was a sense of unlimited dynamic reserve -- as if every other amp I’ve heard has been dynamically compressed.

But I don’t think those amps were actually compressed -- I think they were slow. I can’t emphasize enough how the ZH-230’s dynamic agility enhanced the music played through it. And I don’t mean just a few orchestral blockbusters; it applied equally to every album I listened to. Dynamics are a huge part of music, and doing them better makes the music sound better.

Nor was the ZH-230 a one-trick pony. Its frequency extension was excellent, and not just for a tube amplifier. When I played “The Panther,” from Jennifer Warnes’s The Well (16-bit/44.1kHz WAV, Sin-Drome), the opening percussion instruments were as detailed as I’ve ever heard them, and their high-frequency extension was just right -- not peaky, not rolled off. The bass drum in Folia: Rodrigo Martínez, my favorite track from Jordi Savall’s La Folia 1490-1701 (16/44.1 FLAC, Alia Vox), had lots of weight and extension, and its descent to its lowest notes rivaled what I heard through First Watt’s J2 amplifier, which had been the bass champ with my speakers. But the real news revealed by Folia: Rodrigo Martínez was not the ZH-230’s bass performance, but the amount of detail it produced. Although depicted with extreme detail, the instruments of Jordi Savall and his band didn’t sound the least bit threadbare or etched -- I could easily follow each throughout the piece. Usually, some instruments, such as the castanets or wood blocks, fade into a background haze -- but even when they were played more quietly, the ZH-230 still let me distinguish them in the mix. And as the musicians’ musical lines ebbed and flowed, the Berning’s microdynamic capabilities made these continuous variations in their instruments’ volumes very easy to follow. When an amplifier reproduces the most fleeting volume changes, you hear more of the performers’ intent. That’s what high-end audio is all about.

Berning ZH-230

I love Allegri’s Miserere mei Deus, and especially in the very fine recording by the Tallis Scholars (24/96 FLAC, Gimmell). I also have a wonderful performance by A Sei Voci (16/44.1 FLAC, Astrée E 8524), ornamented in baroque style. While it doesn’t have the soundstage depth of the Tallis Scholars’ recording, it still presents a realistic image of a small chorus performing in a room of moderate size. The Berning ZH-230 made it possible for me to locate and follow the voices of individual choristers as they performed this challenging work. If I understood Latin, I could have made out the words they were singing.

Orchestral music fairly glowed with tonal richness. Some single-ended-triode (SET) amplifiers are at least as rich, perhaps due to an abundance of second-order harmonic distortion, but the ZH-230 realistically presented tonality in time and space -- instruments sounded right. Rachmaninoff’s final composition was his Symphonic Dances, and I’m a huge fan of Eiji Oue and the Minnesota Orchestra’s recording, especially the version on high-resolution disc (24/176.4 WAV, Reference HR-96  HRx). Through the ZH-230, it sounded extremely spacious and tonally spot-on. The alto saxophone in the first dance suite sounded just right -- through many amps, it sounds like a synthesizer trying to imitate a saxophone. The ZH-230’s instantaneous dynamics benefited this music’s lightning-fast dynamic swings. I seem to be saying that a lot.

With the Electroharmonix 12AU7s replacing the 12AX7WA tubes in the input position, some of the circuit parameters changed: The ZH-230’s output impedance is 1 ohm, its total harmonic distortion is 0.3%, its feedback into 8 ohms is 20.5dB, and its noise is 0.15mV. With the 12AU7 tubes in place, the output impedance rises to 2.7 ohms, THD to 0.7%, and noise to 1.0mV. Those changes could result in audible changes in some systems, but I didn’t find noise to be a problem.

Changing the tubes didn’t audibly change the ZH-230’s frequency response, but there were some other, modest differences. The bass response seemed even more powerful, the location of performers in space seemed better defined, and, to repeat a cliché I used to use when reviewing SET amps, tonal qualities seemed lit from within. I can’t say I was surprised -- I’ve usually preferred the sound of 12AU7s to 12AX7s. If I were to keep the review sample, I’d leave the 12AU7 tubes in place. I’d also try 12AT7s as driver and input tubes. I’ve got some NOS RCA 12AT7s I’m dying to try, along with some NOS Telefunken ECC82s (12AU7 equivalents). An enthusiastic tube roller could go crazy trying different combinations.


The Atma-Sphere S-30 Mk.III ($3300 when produced) is another 30Wpc, OTL amplifier. Unlike the Berning ZH-230, it’s a fully balanced design with five output tubes per channel, which produce a lot of heat. On its front panel are two on/off switches: one to turn on the heaters in the tubes, the other to turn on the B+ high voltage so that the tubes will operate. Like the ZH-230, the Atma-Sphere uses unusual tubes (6AS7G dual triodes or their Chinese equivalents), and its input sensitivity is 2.8V, which is much lower than the ZH-230’s. But if you use a balanced preamp with the Atma-Sphere, its output may be higher, so it’s hard to predict whether or not the volume setting will need to be higher.

The Atma-Sphere’s styling is quite retro, which I kinda like. All of its tubes are on the top of its case, unprotected by any sort of cage. The input jacks are on the front panel, which is a pain -- you have to route interconnects from the back of your preamp, over the Atma-Sphere’s very hot tubes, and down to its input jacks. It helps to have very flexible interconnects, and they may need to be longer than the usual 1m. With my preamp sitting on the shelf above the Atma-Sphere, I could just get by with 1m interconnects.

Like the ZH-230, the Atma-Sphere produces excellent bass. With Folia: Rodrigo Martínez, there was plenty of weight and the bass went quite deep. There was lots of detail, but while dynamics were quite good for a normal amplifier, the Berning’s dynamic speed seemed to make the music flow just a bit better. Still, I was surprised by how close the Atma-Sphere came to the Berning in this regard.

Likewise with Jennifer Warnes’s “The Panther”: the Atma-Sphere exhibited extended high frequencies, but they seemed very slightly more threadbare than the Berning’s -- and never before had I thought the Atma-Sphere sounded at all threadbare.

The Atma-Sphere captured quite well the overall orchestral weight of Rachmaninoff’s Symphonic Dances. But the alto saxophone sounded just a tiny bit hooded, not as open and realistic as the ZH-230.

Overall, the Atma-Sphere and Berning both sounded splendid, and illustrated that omitting output transformers from tube amplifiers can be an excellent way to overcome the shortcomings of traditional tube amps. The differences I’ve cited are pretty small -- the Berning sounded just a little faster and more harmonically sophisticated. But my Atma-Sphere S-30 Mk.III is not the current version; there have since been two updates, and the latest is said to sound better than mine. It’s also a lot cheaper than the Berning: $3990 for the S-30 Mk.3.2 vs. $8360 for the ZH-230. And the Atma-Sphere is fully balanced, which may or may not be important to you; to me, a balanced amplifier usually sounds better, assuming that all other components in the system are balanced as well. On the other hand, the Atma-Sphere produces a lot more heat, a little bit more noise, and it’s a bit of a challenge to set up. But if $8360 is a bit high for your budget, the Atma-Sphere S-30 Mk.3.2 is a superb amplifier.

Bottom line

I was totally smitten by the David Berning Company’s ZH-230. It solves some of the significant drawbacks of tube amplifiers -- short tube life, heavy weight, limited bandwidth -- while retaining the tonal luminosity, detail, and smoothness for which tubes are prized. Its ability to provide almost instantaneous power made it sound more powerful than its rated output. If you really need more power, Berning also offers the 200W Quadrature Z monoblock amplifier, though at a much higher price.

As a reviewer, I get to audition a fair number of amplifiers, and have tried to survey 30Wpc amplifiers for the SoundStage! Network. I’ve also reviewed amplifiers ranging from low-powered SETs to fairly high-powered push-pull models, and have auditioned (but not reviewed) other types of amps, including class-D models. I haven’t listened to every power amp out there, but the Berning ZH-230 is the best I’ve ever heard. And while all the technical hoo-ha is fascinating, what counts is that the ZH-230 made music sound right. For me, that meant it was more fun to listen to music with the Berning -- my listening sessions with it ran unusually late. David Berning’s ZH-230 has made it very hard for me to listen to other amplifiers.

. . . Vade Forrester

Associated Equipment

  • Speakers -- Affirm Audio Lumination speakers
  • Amplifiers -- Audio Research VS115, Atma-Sphere S-30 Mk.III, Art Audio PX-25, Pass First Watt J2
  • Preamplifiers -- Audio Research PH5 phono, Audio Research LS27 line stage
  • Analog sources -- Linn LP12 turntable on custom isolation base, Graham Engineering 2.2 tonearm, van den Hul Platinum Frog cartridge, Sony XDR-F1HD tuner (modified)
  • Digital sources -- Meridian 500 CD transport; Sony SCD-XA5400ES SACD/CD player; Hewlett-Packard dv7-3188cl laptop computer running 64-bit Windows 7 Home Premium and Foobar2000 music-server v.1.1.17; Auraliti PK100 music server; all servers and digital players connected to an Audio Research DAC8 DAC
  • Interconnects -- Audience Au24 e (balanced), Clarity Cables Organic, Crystal Cable Piccolo (unbalanced), Purist Audio Design Venustas (unbalanced), TG Audio High Purity Revised, Wireworld Cable Gold Eclipse 7 (balanced)
  • Speaker cables -- Audience Au24 e, Blue Marble Audio, Clarity Cables Organic, Crystal Cable CrystalSpeak Micro, Purist Audio Design Venustas
  • Power cords -- Audience powerChord e, Blue Marble Audio Blue Lightning, Clarity Cables Vortex, Purist Audio Design Venustas
  • Digital -- AudioQuest Coffee and Diamond USB; Wireworld: Platinum Starlight USB, Gold Starlight 6 S/PDIF, Gold Starlight 5 AES/EBU

David Berning Company ZH-230 Stereo Amplifier
Price: $8360 USD.
Warranty: Two years parts and labor (90 days for tubes).

The David Berning Company
12430 McCrossin Lane
Potomac, MD 20854
Phone: (866) 823-8720, (301) 926-3371