This review page is supported in part by the sponsors whose ad banners are displayed below

By the time our pure 2kHz sine wave passes through the circuitous passages of the ear this wave has become a highly distorted sound with added harmonics up to 14th order in some cases. Remarkably, over the millennia your brain has developed a mechanism that instantaneously filters out its own ear-generated harmonics. The result is the brain’s recognition of the absolute purity of the 2kHz tone, not the harmonic distortion induced by your ear canal. The human ear/brain has evolved a built-in mechanism that detects and recognizes the original signal and rejects all other natural noise and distortion. Even with complex natural sounds the brain is able to treat a large amount of information in the same manner it treats a single pure tone. It’s very complex BSP (biological signal processing).

Our naturally occurring hearing distortion can be 5% or higher. Which begs the question that if every sound we hear is distorted and then filtered by our ear/brain interface, why are small amounts of amplifier distortion any issue? The answer lies in the word ‘natural’. The human hearing process filters distortions which are naturally formed in the ear and the environment. That is why the normal sounds of life never fatigue. One can listen to the sounds of the ocean, mountains or forest indefinitely. Has anyone every complained about the sound of crashing waves being irritating? But start adding artificial sounds including and especially electronic amplification and things change. Subtle electronically induced distortions are not what the brain has been conditioned to automatically filter. Therefore it doesn’t. This results in hardness, lack of purity, fatigue and unnatural sound. Attempting to reduce electronic distortion with feedback and other means produces a complex signal of music and "distortion which the brain doesn’t recognize as natural." That is profound.

The pattern of ear-induced harmonic distortion is complex and non-linear in both frequency and amplitude. However the structure of this distortion is well defined and experimentally proven. Now we come back to Tenor’s design philosophy. Instead of artificially trying to reduce amplifier distortion with feedback, what if you could develop a circuit that had intrinsically low distortion and then tailored the residual distortion curves to mimic those naturally occurring in the ear? In theory the brain should discard the amplifier’s self distortion and be left with only the purity of the music. If the amplifier's own THD distribution matched the natural curve of our ear distortion, the brain would instantaneously and transparently filter it out. You'd be left hearing the accurate timbre of spectacularly clean music. This is Tenor’s unifying design philosophy which they summarize as HSI.

Michel contends that the execution of their HSI theory is extraordinarily difficult. The trick is to design circuitry that mimics both the amplitude and harmonic structure of the natural hearing process in the analog domain without DSP trickery. According to Michel the issue is not just the frequency but also dynamic domain. "The natural ear distortion at low volumes is quite different than at 90dB and the change between the two is not linear." The circuit thus must be able to deal not only with a steady-state tone but complex dynamic musical signal. As a layperson we tend to think of dynamics in linear terms. A music crescendo or volume increase is the same sound only proportionally louder. But it is not. "The amplifier must do something fundamentally different at 80dB than it does at 20dB." The harmonic characteristics of a louder signal are completely different from those of a lower level. Another dimension in addition to frequency and level is that the fundamental curve or shape of the distortion characteristics fluctuates/shifts dynamically.

Jim Fairhead offered this analogy. "Think of it as a computer-controlled engine where the computer manages multiple parameters of the engine completely differently at all RPM. Therefore if the circuit follows these changes, you will not be able to detect distortion at any volume level regardless of the dynamic content of the music. As the musical dynamics change, the amplifier still produces this benign harmless distortion curve which the brain instantaneously and transparently filters all in real time without temporal distortions or time delays. You are presenting the amp’s self distortion to the brain in a manner that is easy to process and integrate as part of the natural hearing process which discards that distortion. This is not forced or needs to be learned. It is learnt already and as such natural, effortless and without any fatigue."

Every amplifying device generates distortion to one degree or another. Most high-end manufacturers attempt to lower that distortion in a manner which, for lack of a better word, is artificial by using some degree of feedback, feedforward mechanism or correction type that ultimately corrupts the signal. Michel's view is that an amplifier must have distortion. It is a byproduct of the laws of Physics. Tenor’s goal is not to artificially eliminate distortion but to manage it such that our brain perceives it as normal ear distortion to ignore it. You end up listening to the purity of the original signal without realizing how because it's an automatic subconscious process. Done correctly Michel claims that such an amplifier’s self distortion becomes inaudible or outside our brain's ability to perceive.

Jim claims their design to be unique in that it is virtually immune to listening fatigue. "Ours is the direct opposite of an amplifier or preamplifier that produces unnatural distortion through feedback and other mechanisms the brain recognizes as distortion and does not inherently filter or cancel. The brain knows something is wrong, can’t deal with it and you get tired and fatigued. You end up missing the purity of the performance because the brain is working to cancel distortion it cannot. This generates listening fatigue."

To achieve their goals, Tenor manages the harmonic envelope far past the normal human hearing limits. Analog recordings present the signature of each instrument into the ultrasonic range. "There are a lot of instrumental harmonics extending to 40kHz or 60kHz. Although you cannot hear them directly, this ultrasonic signal does interact with the audible signal and affects its timbre. Unless these harmonics are properly managed, it is as if the soul, personality and ambience of the original tone has been removed." It is Tenor’s belief that they are the only company which intentionally designs its products to maintain and tailor the correct harmonic structure for the ear/brain interface [Audiopax would disagree – Ed.]. While other products might use some of these elements, Tenor has built their entire company around this concept and considers their design philosophy of tailoring distortion characteristics to the human hearing mechanism to be quite unique and proprietary. Tenor claims that while DSP designers have applied many of these theories, analog designers have not yet integrated established psychoacoustic research into modern circuit design. Clearly Tenor believes themselves to be not just manufacturers of hardware but builders of an interface between music and human hearing.