How then does one design a circuit that tailors the distortion characteristics to mimic the natural distortion curve of human hearing? That’s what Tenor call their secret sauce. They claim it to be extraordinarily difficult because their entire process is performed in the analog domain with no digital signal manipulation whatever. Furthermore, it is complicated by the brain’s nonlinear reaction to dynamic levels whereby low-level sounds and distortions are interpreted quite differently than loud ones. To top it off, the change between low and high levels is nonlinear as well. As Michel put it, "the amplifier must do something fundamentally different at 80dB than it does at 20dB." Jim used the analogy of a computer-controlled car engine. The hundreds if not thousands of adjustment parameters are significantly different for each RPM and torque level. Yet when programmed correctly, the net effect is a smooth and elegant power curve with adjustments that are invisible and transparent to the driver at any speed. The HSI circuit and design are Tenor’s analog of the computer-controlled car engine. Michel acknowledges that distortion is inherent in any amplifier design and passionately believes that feedback and other corrective measures ultimately corrupt the signal. Tenor prefer not to fight the laws of Physics in a futile attempt to eliminate distortion. Rather they opt to lower distortion with good circuit design, then tailor the remnants in a way that our brains view them as nothing more than naturally occurring ear distortion which gets ignored entirely. Now listening becomes a transparent automatic process. When the electronics are designed correctly, Michael asserts that the involved distortion filtering which our brains perform on the fly can’t be detected and doesn’t register as fatigue or effort.

A final element of the secret HSI sauce requires managing this harmonic envelope far beyond normal hearing limits. The Phono 1 has bandwidth to 1MHz with usable frequency response of 2Hz to over 100kHz. This runs counter to audiophile chatter on the inherent bandwidth of a vinyl record. A few years ago I experimented and viewed a cartridge output using Logic Studio software with a direct-to-disk recording. There was significant musical information above 30kHz. Those who remember the old discrete four-channel Quadradisc of the 70s know that these records included a separate 30kHz carrier recorded on each groove wall. Vinyl's ability to transmit signals above a typical 15kHz point is very real. Tenor's position is that harmonics of real instruments can extend far beyond our hearing range and without this information, timbre is negatively affected. Maintaining and presenting the proper harmonic structure past audibility and managing the distortion in the brain/ear canal mechanism is the essence of their proprietary design philosophy, which they claim differentiates them from their competitors.

Reliability. Gone are the days when consumers accepted lack of reliability as tradeoff for sonic excellence. Today’s state of the art demands both. When you are dropping $50K on a phono stage, you have the right to expect perfection. Although impossible to verify, Tenor claim that their testing and QC procedures exceed that of any competitor. They go to almost ridiculous lengths to ensure reliability. For example, they no longer worry about tube failure. Their key is stressing each tube during initial testing to eliminate all subsequent tube failures from infant mortality. Each tube undergoes a proprietary testing regime including in-house burn-in up to 700 hours depending on tube type. Tubes are then subjected to an acoustic chamber with white, pink and brown noise whilst being attached to a measuring device. Output, distortion and microphony are measured and categorized, with as many as 60% of the tubes failing. This combined with Michel’s conservative circuits suggest a useful tube lifetime of more than ten years of daily use.

Features. One might assume that $50k should net every feature and flexibility imaginable. Others would expect sonic performance to overrule all else. Tenor split the bill. The Phono 1 delivers flexibility that should satisfy 95% of the fussiest users. However, if there is a conflict between flexibility and sound quality, in Tenor’s world sound wins out each time. They concluded that costly variable equalization curves were no smart move when the overwhelmingly majority of records use RIAA. Why add circuitry, switching and additional costs? In general, Tenor tend to exotic engineering and expense that offer sonic improvements, not feature glitz. Whilst you might find another phono preamp with more inputs and equalization curves, the Phono 1’s features are certainly not Spartan. I came from arguably the most stripped-down, hot-rodded, high-end phono preamp in the world, the Walker Reference, which heretofore had beaten back all sonic challengers for 15 years, be it Audio Research, VTL, Aesthetix, darTZeel, Pass, Manley and others I no longer remember. The Walker with its minimalist design has zero flexibility, one input, one output, one loading option (1kΩ), manual DC offset, no mono and no gain adjustment. Yet with the right cartridge--it loves Clearaudio--it delivers utter magic. The new Tenor combines the perfectionist model of a Walker with enough flexibility to satisfy all but the most compulsive. Features include:
  • Microprocessor control with front panel VFD display
  • Internal USB port for software upgrades
  • A three-position grounding selection to equalize ground potentials and eliminate ground loops
  • Gain adjustment selector (55db, 60db, 65db,70db for MC )
  • Menu-selectable loading at 100Ω, 200Ω, 300Ω, 400Ω, 500Ω, high and custom
  • Two different passive EQ curves (RIAA standard 1954, RIAA + IEC) +/- 0.1dB
  • Absolute phase switching
  • Mono/stereo selector
  • Switchable balanced and two single-ended inputs
  • Switchable balanced and two single-ended outputs
Some will note that missing from this feature set is a moving magnet input. Michel explained that the intended audience of the Phono 1 will likely use a moving coil exclusively. Adding the expense, circuitry and switching for MM could potentially degrade the sound and was not deemed a worthwhile tradeoff. However, if a customer demanded a moving magnet input, Michel indicated a possible custom solution and suggested potential users contact Tenor directly for options.

Setup. Setting up the Phono 1 was quick and intuitive. All functions are performed from the circular eye on the front panel. Press the setup button, then scroll through the various options including loading, gain, input and output selection. Rarely used parameters are handled in the setup routine by software. More frequently used functions such as mute, mono/stereo, phase and power all have dedicated front panel buttons. The three inputs can be customized with their own unique gain, load impedance and grounding. These settings are stored in permanent memory to allow multiple turntables or arms. It's all quite logical. You can be up and running in a few minutes even without instruction manual. The front panel can be configured to display operating parameters, the Tenor logo or nothing, with adjustable brightness for each screen. One interesting option are the balanced inputs. Whilst a balanced output is fairly standard, balanced inputs on a phono stage are somewhat rarer. Michel’s position is that an MC cartridge is inherently balanced, hence a balanced input provides the highest sound quality. Tenor have a simple schematic for the conversion. Since most customers will likely use a single-ended input instead, that’s what I used for the review as well. The switching relay matrixes are microprocessor controlled, with their lines beneath the chassis to not inject distortion into the audio circuit. The logic system controls multiple relays in a very specific sequence to avoid potential noise on the output. When changing gain for example, the microprocessor quickly mutes the outputs whilst deselecting one gain value and entering another. Hence there is no need to turn down your preamp to avoid clicks and pops. As with their line preamp, a network board handles initial programming whilst a USB port allows end-user field upgrades.

Mated with the Clearaudio Goldfinger, 500Ω offered the best balance and was the logical choice based on the cartridge’s own impedance of 50Ω. I used the 60dB gain setting. The Goldfinger has a relatively high output of 0.7mV, allowing the use of any of the available gain options. Whilst the Clearaudio did not need 60dB per se, it seemed to deliver a touch more sparkle and excitement with it. Or, due to the lack of level matching, it could have been reacting to a slightly louder output. As to which output, balanced was the clear winner, with slightly better transparency against a richer blacker background. When not used, the Phono 1 typically remains in standby whilst maintaining low-voltage power to the processor battery and startup relay. No power is applied to the amplifier circuits or tubes. Pressing ‘on’ activates a start-up sequence to ramp up the supply voltages and stabilize the circuits. This initialization takes 100 seconds and its countdown displays on the front screen.

Noise control. The control of noise is critical in any phono preamp due to the very high gain involved. Although the Phono 1 specs out at 70dB of max gain, due to the EQ curve it’s actually 84dB at 60Hz and 90dB at 20Hz. Whilst noise control and low-level signal integrity are like motherhood and apple pie, in this environment it’s exceptionally challenging to achieve. Discussions with Michel dove deeply into Tenor’s intense obsession with distortion, noise and hum control. One gets the unmistakable impression that this is an engineering commitment, not mere specmanship. For example, Tenor use 27 different ground planes across four layers for maximum shielding, to make the connections in the most symmetrical way and to avoid electromotive voltage differences. Thermoelectricity is the direct generation of electricity from heat wherever two dissimilar metals join. If one junction operates at a different temperature, it creates a current. The greater the temperature differential, the greater this current. This effect is well known and actively exploited in measurement circuits or to supply small amounts of remote power. In audio however, its unwanted effects require mitigation in precision circuits. You must insure that one side of the junction won’t develop more parasitic voltage than the other. Every time two dissimilar metals meet to create a heat change, one risks ghost voltages that could distort the audio signal. In essence, Tenor design to compensate for and balance the voltage potential of their solder joints on each side of the circuit. While stray thermoelectric voltages have been mentioned by a handful of audio designers, Tenor appear to be one of the few to actively design toward mitigating their effects.

Whilst the voltages generated by dissimilar metal contacts are small (one hundred to one thousand times lower than the output of a MC cartridge), noise and distortions are always cumulative. Further, they mask and smear the lowest-level signals which define the spatial cues that help differentiate the great from the merely good preamps. Michel discussed the effect of these parasitic voltages. "We did tests and found that if you do not achieve perfect cancellation of the contact’s parasitic voltages but run very high gain, the signal will distort and all the detail become unclear almost like a blurry bad photograph. If you can eliminate that phenomenon, you get a clear photograph of the musical event. We then tested the limits of the preamp’s resolution and input a signal of 300 nano volts to clearly see the signal come out of the noise with absolutely no harmonic distortion. This is incredibly difficult to do as all of the switching has to be symmetrical to avoid the development of thermal EMF voltages which will destroy the low-level signal." Whilst thermoelectric EMF might read somewhat esoteric, garden-variety hum is another great destroyer of fine detail. Like other forms of noise, hum is insidious. Once introduced, it becomes a physical part of the signal to reduce and obscure fine detail. Hum is generally caused by unwanted currents and voltages in the equipment’s ground plane/s. The causes are numerous: multiple paths to ground, incorrect component design, components being plugged into multiple circuits with different ground potentials. All of it can induce ground-loop hum. Most of us, especially those in the analog realm, have fought this battle. There have been times in the past when I personally gave up, accepting defeat with the inevitable slight hum as byproduct. Tenor are especially cognizant of such grounding problems. They attack them both via circuit design and end-user flexibility.