Reprinted with permission from http://www.stereotimes.com, "The Complete Audiophile Magazine"
The Berning Zero Hysteresis ZH 270 OTL Stereo Amplifier
Rated Power: 70 Watts per/channel with normal feedback position (3 User Switchable modes) 1% Normal, 1.5% Medium, 2% Low Input impedance: 25k Frequency response at 1 W, 8 ohms; +0, -1dB: 1.5 Hz to 60 kHz, NORM feedback; 1.5 Hz to 35 kHz, MED feedback; l.5 Hz to 21 kHz, LOW feedback. Full power bandwidth (-3 dB), 8 ohms: 2 Hz to 80 kHz, NORM feedback; 2 Hz to 50 kHz, MED feedback; 2 Hz to 35 kHz, LOW feedback. Typical output impedance (measured at 1 amp, 60 Hz): 1.8 ohms, NORM feedback; 3.8 ohms, MED feedback; 8.7 ohms, LOW feedback. Size: 12.5 x 15 x 4.5 inches Weight: 10lbs Finish: Black Anodized aluminum with red window Warranty: Two-year, non-transferable.
Address: 12430 McCrossin Lane
We've all heard the battle cry of the two camps of audiophiledom: solid state vs. tubes. Solid state is becoming more tubelike as tubes are achieving solid state power and control. Both camps have made great strides in getting closer to the sonic truth (I'm not going to attempt to define that--to each his own.)
Then, along comes a product with an almost mythical background--the output transformerless tube amplifier. Output transformerless (OTL) tube amplifiers are not new. Julius Futterman pioneered, and many others have copied his amplifier design that uses a multitude of power tubes connected in parallel to generate enough output current to drive loudspeakers without output transformers.
Why this desire to eliminate the transformer? Audio designers have long recognized the limiting factors and distortions that audio output transformers impart to sonic performance. Problems arise in both high and low frequency reproduction due to transformer core hysteresis and saturation, just to name a few.
Previous OTL designs proved unreliable and difficult to live with. The power tubes, running at very high operating temperatures, created many tube replacement problems. In addition, matching speaker impedance's became critical, and greatly limited speaker choice.
Enter David Berning and his unique ZH 270 OTL stereo amplifier. He claims his design has eliminated all of the pitfalls of previous OTL designs while maintaining all the advantages. I couldn't wait.
Tubes had always fascinated me, but I was a little reluctant to give up the ease of use of solid state--you know, tube biasing adjustments, limited tube life and all that heat. With all that, tubes seemed to possess a harmonic rightness in presenting the music that was at times intoxicating. Thus I looked forward with anticipation to exploring this amplifier.
The day arrived when the amp was delivered. I unpacked the box and was stunned when I picked up this featherweight amplifier--it weighs only 10 lbs. My preamplifier is heavier. Could this really be a power amplifier? After a thorough audition lasting several months, this certainly is, and in spades.
I don't want to get ahead of myself, so let me continue with the review. Berning's design has a bandwidth, in normal feedback position, of 2Hz to 80kHz sans power transformers, improved bass response, speed, and clarity (not to mention weight). Power output is 70 watts/channel into 8 ohms and 110 watts/channel into 4 ohms (both in normal feedback position).The ZH270 is also more than a basic power amp. It offers almost the flexibility of an integrated. It has two sets of inputs with an A/B switch on the face of the unit. One can run a pre-amp into one set of inputs and a digital source directly into the second set of inputs. The amp also has a high quality volume pot that adds additional flexibility vis a vis equipment sensitivity issues.
The ZH270 uses a pair of 6JN6's, connected as triodes, in a push-pull mode to develop its power output (such small tubes!). The input/driver stage uses a pair of 12AT7's and one 12AV7 dual triode for each channel. According to Berning, power consumption is a mere 100 watts at idle and a maximum of 300 watts at full power. In addition, the amp incorporates its own brown-out protection, along with a built-in, four-stage power line filter and surge suppressor. The amp has continuous automatic tube biasing and tube life--Berning claims 20 years--clearly longer than anyone would have to be concerned with.
I installed the new amp into my system in place of my Balanced Audio Technology VK 200. My digital front end, consisting of the Camelot Uther DAC with its own remote volume and balance controls, was used alongside the Sony DVD/DVP S7000. ( transport only) Bypassing my Audible Illusions Modulus 3 preamp, compliments of the Uther, allowed me to connect directly into the Berning amp. Between the DAC and the amp, my system contains a True Dimensional Sound unit (audiophile version). This device is totally passive and has a bypass switch. For evaluation purposes, I left the switch in the off position. All interconnects and power cords are by Harmonic Technology, feeding the Total Media Systems (TMS) Adiabat 8.5's, which have been modified by designer Mike Levy in a major way (more at a later date).
For quite a while I've been drawn to the sound of tubes but reluctant to give up the dependability and ease of use of good solid state. After living with my very fine solid state performer by Balanced Audio Technology VK200, the switch to the Berning ZH270 proved very illuminating. (The BAT VK200, a very fine amp, approached a "tube-like" sound in a solid state design with many desirable characteristics that put it up there as a favorite of many.)
My system, however replaced with the Berning ZH270, immediately became more alive, with a good dose of vibrancy, not to mention a metaphoric veil was lifted between the music and the listener. This dear reader added an increase in palpability to instruments and voices with each and every disc.
Under the amp's normal feedback position, the sound has clarity, speed, and realism. The soundstage is much more open, possessing more air that few amps can match. The mid-range, where the majority of music happens, is special. Clarity and articulation go hand in hand with imaging and depth. Instrumental timbres and voices are detailed, three-dimensional, and accurate, with dynamics to match. Control of lower and mid-bass is articulated, with all the virtues of the grip of a solid state performer. (Nothing mushy, boomy, or vague here.) High frequencies are clear and bell-like, and free of any grain, without that electronic etching that can irritate--I'm talkin' making music.
Recently, I received a gift of Chesky's The Ultimate Demonstration Disc (UD95). This disc is a good test for any audio system. The Berning proved to be extremely accommodating to all the musical requirements that this disc presented. Rebecca Pidgeon's version of Spanish Harlem put you into that special place--that place we love so much when things sound so right! Solo base and drum set cuts were startling in their realism. I could go on and on , but I think you get the point.
In addition, the amp has a total of three feedback settings: normal, medium and low. Medium and low positions tend to give the amp more of a traditional tube sound. By adding variable levels of feedback (which essentially is a form of distortion), one can match the speaker damping and tonal balance to the listening room and personal taste. Once again, this gives the buyer more flexibility in equipment matching and sonic preference.
I'm not a technical person and will not attempt to explain how David Berning has solved the OTL conundrum--after all, it is patented--but suffice it to say, he has. In short, Berning's all balanced, OTL amp is true to the musical source, without any euphonic enhancement. It offers the reliability of the best solid state amps and the harmonics of tubes--definitely cause for celebration.
On an aesthetic note, the amp has a utilitarian, no-nonsense, understated look that's all business. The faceplate has a large glass window for tube viewing, but is oddly deep red in color, obscuring the tube view. Unless there is a functional reason for the red color, keep it clear--to match the sound.
With all of the performance characteristics and flexibility this amp offers, it is incumbent upon anyone ready to audition a new amplifier that the Berning ZH 270 is mandatory.
Clement Perry adds a footnote
I've personally listened closely to Martin's setup before and during the review of the Berning amplifier. I, for one, have always been some fan of David Berning and his eccentric designs. (Once again he claims a 20-year life span on these tubes!) This amp is very different from anything I've seen in that it's about the size and weight of most separate power supply's or line conditioners. It's sound is nothing lightweight however. From the very start of the first disc I brought up for a listen, (Fairfield Four's Standing In the Safety Zone) from the first track, I could tell that this amp was special. The sound it seems to convey is one of a quiet, noticeably clean, yet un-sterile, and very well balanced and musical device. The OTL design employed here, I'm sure, is a good reason there's so much ease and relaxed presentation to the overall character. The Berning amp was certainly a welcome addition to Marty's system proving superior in overall sound performance to the BAT VK200, which is an incredible amp in its own respect.
Personally, I am hoping to hear this amplifier in my system. Marty... can you spare an amp?
Martin's Associated Equipment: BAT VK200 Camelot Uther DAC, Sony DVP S7000 as transport, Harmonic Technology cables, power cords, TMS Adiabat 8.5 modified speakers, Bybee interconnect filters, TDS audiophile version, Audible Illusion Modulus 3 preamplifier, Black Diamond Racing Cones