Their internal grounding system uses shielding to attack noise and hum. Grounding the tube coolers might seem excessive but is just one example of preventive measures used in their high-sensitivity circuits. In the audio part of the circuitry at the rear panel, Tenor use a four-layer board containing various ground planes and signal traces. All audio boards use silver plating atop copper, not gold. Several factors led to this decision. The use of tin is now restricted and gold plating uses a nickel alloy which is ferromagnetic. Those ferrous inclusions attract magnetic noise to the parts of the board which are nickel plated. Also, copper, nickel and gold thermocouple to create parasitic voltages. This does not occur with silver plating which lacks nickel. "We did a test using gold plating. It was a catastrophe. So we redid the boards with another technology. I chose silver plating because the bonding between copper and silver is very good and doesn’t develop any thermo coupling effects. It also diminishes the resistivity of the traces at least at the surface. When you deal with very low-impedance signals, you have to diminish the resistance of connections to avoid losses."

Tenor’s end-user ground switching is flexible like a Swiss Army knife. There are enough options to counter virtually any hum. Michel recognizes that any audio system might include an assortment of components, each with their own grounding issues. As he explained, Tenor employ a variety of grounding and switching options that can solve virtually any ground-loop problem. The setup menu allows the user to change the grounding between normal, grounded or lifted. Specifically
    • The ‘grounded’ position ties together the audio and chassis ground
    • The ‘lifted’ position completely separated audio and chassis ground
    • In the ‘normal’ position a resistor inserts in parallel with the capacitor between both grounds
In a few minutes of simple trial and error, one can choose between the two ground posts and cycle through the three grounding options. This is a continuation of the grounding options first introduced in the Tenor amplifiers. Although common in professional equipment, this flexibility is somewhat rarer in consumer hifi. Those enthusiasts who have spent hours, weeks or months fighting ground issues will find these features alone nearly worth the price of admission.

Under the hood. Ultimately the purity of the audio signal is limited by the purity of the power supply. It all starts with the power transformer. Tenor’s oversized custom transformers are substantial and here of the sort one might find in a power amp, not the typical phono stage. They are of inherently ultra-low noise. Tenor then add dual electrostatic shields and seal each in a steel can with special epoxy resin for further noise reduction. The DC supply is independently filtered and regulated for each channel. Medical-grade filtering on the mains removes remaining noise and according to Michel creates "essentially pure DC like a battery". Many manufacturers promote toroidal transformers with their limited stray magnetic fields. As with many design elements, Tenor take a different path. "We use a conventional transformer with very low magnetization which does not radiate a lot of interference. Since it is of conventional construction, bandwidth is very narrow. By design this excludes higher AC harmonics and HF noise. If we used a toroid, we would need external filtering to remove possible high-frequency content coming from the grid. The bandwidth of a toroid transformer is very large to catch every possible stray inference. Therefore we are very happy with our choice of conventional transformer."

The front section contains a separate standby power supply for the microprocessor board. A battery powers the processor and is used for stability, protection against spikes or interference that could affect the microprocessors. It is separated from the audio circuits by a layer of steel shielding to avoid magnetic field interference. The processor runs off the battery all the time, with the charger coming on only when necessary. If you lose power during listening, the microprocessor detects the power loss and shuts down the preamp in a specified manner to avoid any clicks or pops. Additionally, a second small battery acts as backup for real-time clock and memory services to additional settings. On top of the large mains transformers sits a magnetic loop as essentially a humbucker circuit tuned to eliminate residual electromagnetic noise. It is an adjustable coil capturing electromagnetic interference from the transformer, then re-injecting the now out-of-phase signal at a specific circuit point to cancel out residual noise. "These are the same power transformers we use in our line stage. When you put this large a power supply into the same chassis as your audio circuit, you're confronted with things you never had to contend with just by virtue of physical separation. This was really a reaction to the fact that the magnetic field of these transformers was interfering a little with the tube circuit and it is a practical problem given that everything is living closer together. But to put this into perspective, if you removed this humbucker loop, you would hear nothing. You can only measure the effect." That nicely illustrates their philosophy. For Tenor, good enough never is, be it audible or not.

1st stage. The design requirements of a high-gain phono preamp pushed Tenor in a different direction from their linestage. The Phono 1 uses an active transistor gain stage followed by a tube gain stage, then a solid-state buffer without voltage gain, finally triple-shielded transformers for the input and output. Due to the inherent nature of the input transformer, it significantly reduces common mode noise and acts as an RF filter. If you ever picked up radio signal with your cartridge, you know what that means. "Since it is a 1:4 step-up, you get four times the voltage at the output. This helps reduce the need for active gain. We get a ‘free’ 12dB from the transformer. At the same time, it acts as filter. You get common mode rejection because the transformer is constructed to reject 125dB of common mode noise. We can have a very bad environment and the transformer still takes care of it. That's part of the reason that we are pro-balanced coming in. Part of the benefit is to have a low step-up ratio for much wider bandwidth and better phase response. This is why we chose not to use for example a 1:12 ratio transformer whose high-frequency specs and phase response would be much lower than our 1:4. It is also an extremely low-distortion transformer, albeit quite expensive."

The solid-state first gain stage generates about 36dB using extremely low-noise bipolar transistors acting as two symmetrical amplifiers in parallel to further diminish noise. The user-adjustable gain and loading occur in this first stage. The Phono 1 is actually capable of 80dB of gain but intentionally limited to 70dB. A potential problem with all high gain is that it amplifies any inherent noise along with the signal. "We had to make sure that the first amplification stage was extremely low in noise. The residual noise of the preamp is determined by the first amplification stage. That is why the first active stage is solid state and in our case uses mostly discrete components, which is important. Some designers use multiple transistors in parallel to reduce the noise. I prefer to use one extremely low-noise transistor and make a symmetrical circuit to further cancel noise."

2nd stage/tubes. The second stage uses a pair of Tektronix 8416, a bottle with two triodes packed side by side in each glass envelope. It is robust and stable with low microphonics. The 8416 was developed by Amperex for Tektronix oscilloscopes. It is very low noise, tested individually for Tektronix with a very low rejection rate. The 3rd and final gain stage is a pair of Russian 6n6p. Measured with an integrated elapsed time counter, Michel estimates tube life to be 15’000 hours or two hours/day for 20 years. The main board is isolated and floats on a suspension with a resonance tuned to 20Hz, avoiding the possibility of microphonics traveling from chassis to circuit. Tap on the chassis and you’ll hear nothing at the output. Passive EQ is applied after the first gain stage. Tenor’s proprietary RIAA equalization is broken down into two parts to reduce noise and increase precision. The high-frequency part of the EQ is between first and second stage and makes up the 75µs time constant. After the second stage come the 318µs and 3180µs points which together form the RIAA curve. The other EQ curve is the IEC and its time constant sits after the third stage. This is a European curve that was made to reproduce very inexpensive records rolling off the subsonic output caused by disk warp, turntable rumble and off-center records. This curve can be used as a subsonic filter if needed.

Output buffer. The output buffer is solid state and pure Class A designed for transparency. The distortion is so low that Michel claims it is difficult to measure with any equipment; on the order of 2 parts per million. It runs at unity gain with no feedback. An output transformer follows this stage delivering an output impedance of 10Ω.  The advantage of the output buffer aside from its noise and sonic attributes is minimization of cable effects and the ability to drive long runs. This is very similar to the buffer used in the Tenor line preamp.

The sound. No one will spend $50’000 on a phono preamplifier for just elegant appearance and innovative engineering. If it doesn't deliver sonically, you have nothing but an expensive box filled with exotic components. In this rarefied strata, the preamp must provide sonic magic. Does the Phono 1 deliver? Having lived with the Walker Reference Phono Preamp for so long, I initially tried to imagine what the Tenor could possibly bring to the party which the Spartan yet amazingly accomplished Walker could not. Amazingly, the Tenor took the best of the Walker and expanded upon it with air, a touch of richness and prodigious bass. It connected me with the music in a very deep, personal and emotional way. Let’s look at the specifics.