Z-Infinity Audio was founded in 2001 by Zsolt Mathe with the goal of offering perfectionist tube gear that is designed, tested, and hand-assembled in the U.S. Zsolt, who is of Hungarian descent, has designed audio and other electronic circuitry throughout his life. The connection with his European heritage is reflected in his focus on execution and quality. Distribution is currently limited to direct-to-the-consumer on-line sales, though there are plans to expand nationally via showrooms at select locations. The Z120 amplifier is available either as a 60Wpc stereo amp or as a 120W bridged monoblock. Optional configurations include minimal or no global feedback and either 4- or 8-ohm impedance taps.
The circuit is quite traditional in appearance, as befitting a design vision that can be summed up as the highest quality sound with the simplest circuits. According to Zsolt, “many other amplifiers have complex filtering, feedback and correction circuits; we don’t. Even when using negative feedback, it is done so sparingly. With that said, ultimately there is nothing groundbreaking about our actual designs, following the simplicity principle of not complicating the amplifiers unnecessarily.” The input voltage gain stage is a grounded cathode 12AX7, directcoupled to a 12AU7 long-tailed-pair phase-splitter. A pair of KT88 beam power tubes are operated push-pull, connected in ultra-linear (UL) fashion to the output transformer, which are sourced from Hammond Manufacturing. KT120s may be substituted for slightly more headroom but at the cost of slightly increased distortion.
UL remains a popular output configuration even 60 years after it was popularized by Hafler and Keroes. Numerous amplifier classics from Dynaco, Heathkit, EICO, and Marantz serve as a testimonial to its commercial and sonic success. Connecting the KT88 screen grids to taps on the output transformer obviates the need for screen bias supplies and may be thought of as the application of negative feedback to the screens. None other than Herbert Keroes noted in 1958 that in this paradigm, “the feedback is of the power type rather than the more usual voltage or current feedback…It can be demonstrated mathematically that when power feedback is applied to the screen grid of a tube, the linearity of the plate characteristic curves can be improved over and above the amount normally to be expected by a consideration of voltage feedback only.” Another important benefit of UL is a source output-impedance that is much lower than that of a pentode connection, being closer to triode in value. That makes for a much higher damping factor relative to pentode, even with only moderate global feedback levels.
The 4-ohm Z120 uses 19.3dB of global negative feedback, a reasonable level that is pretty much in line with what was used in many successful vintage tube amplifiers, and well below extreme levels in excess of 30dB. While triode amplifiers may be able to get by without any global feedback, feedback is essential for pentode and beam-power-tube output stages to maintain a decent damping factor, reduce distortion and noise levels, and increase bandwidth. Anecdotal evidence suggests that between 10 to 20dB of feedback may be ideal in most circumstances. Feedback levels much above 20dB, aside from stability issues, appear to constrain the soundstage as well as compress microdynamics. I asked Zsolt about the possibility of incorporating a feedback switch to provide a couple of options. His response was as follows: “The question has come up a few times internally. The reason why there aren’t switches to change feedback options, triode/pentode/UL operation, and the like is because it takes more than switching a few wires around to tune the amplifier for a particular mode. For example, stability, frequency response, and impedances change drastically when changing negative feedback, requiring several components also to be changed. We wanted to avoid making a compromise amplifier, and instead, we are making a tuned amp for the particular configuration. This is unfortunate for all the ‘tweakers’ out there, but we feel that quality over tweaking ability is the winner.”
The output stage is fixed bias, adjustable via individual bias pots for each KT88. However, there are neither test points nor a built-in bias meter. In order to monitor or adjust the bias voltage you will need to purchase a suitable bias meter. I use and recommend the Compu-Bias meter. It monitors both the cathode to plate voltage and cathode current for up to two power tubes and calculates and displays the total tube dissipation dynamically in real time. To use it, power down the amplifier, remove a pair of power tubes and insert the probes into the tube sockets. Plug the tubes into the probe sockets and power up the amplifier. Wait a couple of minutes and then adjust the corresponding bias pots as necessary to bring the bias to about 60mA. You then repeat the process for the second pair of power tubes. The location of the pots is not particularly convenient and requires the reach of a long screwdriver. Out of the box, I measured a bias current (plate plus screen) in the range of 60 to 63mA for both monoblocks and a cathode-to-plate voltage of 393V RMS for all of the KT88 tubes. The average idle plate dissipation works out to a very reasonable 24 watts. The power supply features a toroidal power transformer, fast-recovery rectifier diodes, and plenty of filter capacitance. The preamp tube-heater supplies are DC and are regulated using zener diodes. There’s also a relay-activated soft-start circuit that ensures tube filaments heat up before high voltage is applied—always a good idea that should significantly increase tube life.
Customers must specify either 4- or 8-ohm impedance taps. A few tube amplifier manufacturers have opted to optimize the output stage for a single nominal load impedance, sometimes 6 ohms, which splits the difference between 4 and 8 ohms. In reality, speaker impedance often varies by over an order of magnitude between its minimum and maximum values. The applicable international standard for specifying nominal impedance (IEC 268-5) states that the nominal impedance shall not exceed the minimum value by more than 25%. According to this standard, a speaker with a 4-ohm minimum should be rated no higher than a 5-ohm nominal load. There are a ton of 8-ohm-rated speakers out there with an impedance minimum of 4 ohms and lower, meaning that very few manufacturers abide by this standard. From a user perspective, it is always advantageous to have access to multiple impedance taps. That gives the opportunity to experiment with a specific load and a better chance of optimizing the amp/speaker interface.
The Z120 made an excellent impression right out of the box, driving the Basszilla Platinum Edition DIY speakers. Midrange textures were pure and sweet sounding. The overall tonal presentation was suave and refined. It became crystal clear that this Class AB amplifier was being biased heavily into Class A, which was responsible for a benign distortion spectrum—even at moderate volume levels. For the first time in ages I wasn’t tempted to roll-in tube substitutes and was happy to let the stock Russian Tung-Sol reissue preamp tubes simply do their thing. I’ve come to like and respect these tubes for their smooth textures and full-bodied tone. Ditto for the JJ Electronic KT88, which reinforces the virtues of the Tung-Sol preamp tubes. Resolution of complex orchestral passages was nothing short of sensational, and there was plenty of low-level detail. Note that detail flowed naturally from the fabric of the music without giving the impression of being in my face.
There was no tube glare or brightness, which aided the reproduction of hot recordings. In my book, bright-sounding amps are a major annoyance. They may sound exciting in the short term, but a persistent presence region emphasis invariably leads to sensory overload. In fact, the Z120 was found to err in the opposite direction yielding a slightly laid-back presence region. As a consequence, the tonal balance shifted to the core of the midrange, reinforcing the sensation of a smooth and suave midband. Since I happened to have the Bob Carver Cherry 180 amplifier in the house, it gave me the opportunity to let these amps duel it out on three speaker loads; fair competition as both amps are similarly priced. On the Basszilla, the Carver came across as livelier sounding with better boogie factor and depth perspective. But the Z120 was cleaner, evincing a purer musical tapestry from top to bottom. And being a bit mellower, it shifted the listening perspective toward the rear of the hall. The Carver had a slight edge in dynamics from soft to loud, though both amps fell short in terms of microdynamic conviction relative to a good SET amplifier, at least in the context of a high-sensitivity loudspeaker.
The Z120 didn’t much care for the MartinLogan Summit X ESL. This combo sounded far too bright and dynamically compressed. However, the Z120 fared much better partnering the Analysis Audio Omega planar speaker—a true 4-ohm load— and the load I had I mind for the Z120 all along. Here it left no doubt about transient clarity, textural purity, and dynamic prowess. Bass lines were amazingly solid, which implied a high damping factor. Bass drum was reproduced with stupendous punch, and in general, power reserve did not appear to be an issue. Transparency was another highlight. The soundstage was brightly lit making it easy to visualize its inner recesses, and image outlines were palpably localized within a convincing spatial expanse. However, the Carver Cherry 180 provided the more spatially immersive experience, a you-are-there sensation that surpassed that generated by all other amps I’ve auditioned to date with the Omega. Here too I wasn’t entirely satisfied with the Z120’s reproduction of the presence region. It came across as tonally darker than the real thing, reducing the sheen of massed strings, and slightly hardening piano tone. And finally, in head-to-head competition with the far more expensive Lamm Audio M1.2 Reference monoblocks, which feature a solid-state output stage, the Z120 was found to lack the M1.2’s bass precision and punch. But it answered back with tube virtues such as enhanced tonal color saturation, and superior depth perspective and image palpability.
The Z120 is a superb tube amplifier that is sonically competitive with any tube amp I’ve auditioned in the sub-$10k price range. It is eminently listenable and brings plenty of tube charm to bear on conventional speaker loads. I’ve enjoyed my time with it and shall miss it. It is certainly deserving of a serious audition and should work wonders for a brightly voiced box speaker.
SPECS & PRICING
Power output: 120W RMS mono
THD: 0.01% at 1KHz, 1W and 0.3% at 60W
Input sensitivity: 0.7V RMS
Frequency response: 3Hz to 50kHz (-1dB)
Output impedance taps: 4 or 8 ohm
Supply voltage: 120V AC
Power consumption: 250VA maximum
Weight: 37 lbs.
Dimensions: 16″ x 7″ x 10″
Price: $3299 ea.
MartinLogan Summit X, Analysis Audio Omega, and BassZilla Platinum Edition mk2 loudspeakers; Pass Labs XP 30 preamplifier, Experience Music Passive Aggressive autoformer volume control ; Sony XA5400 SACD player with ModWright Truth modification: Kuzma Reference turntable; Kuzma Stogi Reference 313 VTA tonearm; Symphonic Line RG -8 Gold MC phono cartridge; SoundTradition Live! MC-10 step-up; FMS Nexus-2, Wire World, and Kimber KCAG interconnects; Kimber KCAG speaker cable; Bybee Speaker Bullets; Sound Application power line conditioners
By Dick Olsher
Although educated as a nuclear engineer at the University of Florida, I spent most of my career, 30 years to be exact, employed as a radiation physicist at the Los Alamos National Laboratory, from which I retired in 2008.More articles from this editor
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