web counter
History | Master of Perfection! © Tenor Inc. 2000 -2024


Tenor OTL Amplifier Technology


The Tenor Audio journey began with the innovative Output Transformer-Less amplifier known as simply the Tenor OTL in early 2000. These amps took the world by storm as the premier tube amplifier of its time and gave Tenor Chief Engineer, Michel Vanden Broeck, a well deserved place in audio history. 

The OTL is a unique design concept that is different than traditional vacuum tube amplifiers.  The Output tubes coupled directly to the speaker with no output transformer thereby avoiding the various pitfalls of traditional tube convention. While not an entirely new concept in audio, many believed Michel’s OTL to be the pinnacle of technology at the time - as do many still to this day.  It is not unusual to see vintage Tenor OTL’s trading for almost their initial selling pricing some 14 years after inception.  OTLs are objects of desire - both soncially and visually!

Many argue today that Tenor OTL's sonic purity is still without equal.  75 watts of power is practically the upper limit as the physics of size, energy consumption and heat dissipation make anything beyond that impractical. While perfect for fairly efficient speakers or average efficiency speakers in smaller rooms, the 75 watt OTL Tenors simply could not satisfy the current demand of regular speakers.

Many audiophiles with a modest speaker load concluded their elusive journey to audio nirvana with the Tenor OTL. For the rest, the journey continued for the search of the illusive amplifier that would have the liquidity of a tube but the higher power capability of a solid-state design.  

Pursuing a more powerful OTL was simply not sensible.  Could the sonic purity and liquidity of the Tenor OTL circuit somehow be mated to a solid-state output stage with a synergetic result?

Initial experiments convinced us that the voltage rich Tenor OTL input stage could easily power a band of high current output devices at a constance impedance thereby achieving more power through current (Watts = Voltage x Current) . Furthermore, and perhaps more important, we discovered that tubes and solid-state devices each have areas where their technology enhances the other.

The OTL input stage, or what we refer to as the tube audio board, is the foundation that is critical in defining the neutrality of the entire amplifier and can only be handled with vacuum tube circuitry.   This is where we apply our famous Harmonic Structural Integrity or HSI that results in Zero Perceived Distortion.

While not suitable for powering speakers, this voltage rich, high impedance Class A amplification stage is perfect for driving a constant impedance bank of output devices.  Preservation of the audio signal’s delicacy, transparency and resolution is accomplished with additional power provided by the current of the output section in a Unity Gain configuration making a rather formidable combination.  The sound of a tube with the power of solid state!

Tenor Hybrid Power Amplifier Technology


The first commercial application of the hybrid technology for Tenor was the hp300 mono block introduced at CES in 2003.  (The hp150 stereo power amplifier was introduced shortly thereafter). This first generation hybrid technology, while an immediate international hit, taught Tenor many lessons.  The most important lesson was that pedigree and power demands responsibility!  In our case, protection from real life use, or possibly even abuse of the owner.

Tenor 300 Poster Final copy 2

Similar to early production super cars, the hp’s advanced precision and technology made driving close to the red line effortless and fun. No audible clues are being rendered at all that driving a little faster and a little harder is about to cause misfortune. When the music stops it is too late to worry as the output devices have surpassed their design specification and the laws of physics have rendered the final judgment.  The lesson was harsh but not final.  Fortunately, the hp series of amplifiers can be resurrected using modern day technology.  Replacing output devices with mosfets insures the second coming will be long and prosperous as mosfets have a natural tendency to shut down as they get driven to the edge and have tube-like distortion characteristics thereby improving the performance of the hp series in every way and extending the life of truly a classic amplifier. 

Tenor Hybrid Protection Technology

Intelligent automatic protection circuitry became a key design element for the next generation Tenor. Without interfering with the audio signal, 6 harmful conditions need to be monitored in real time:

1. Low AC main voltage
2. DC offset at speaker terminals 3. AC at speaker terminals
4. Static discharge
5. Ground pollution
6. RF (Radio Frequency) levels

If any of the above conditions exceed prescribed levels, the amplifier will automatically protect itself by “muting” and only going back on line when the condition is remedied. The benefits are fairly obvious as your investment is protected and your enjoyment is maximized.

In 2006 the pinnacle of modern amplifier design was introduced as the Tenor 350M mono block and 175S stereo.  These amplifiers were redesigned from the ground up using mosfets and proprietary Tenor protection circuitry.  

Unbelievable transparency and resolution coupled with sufficient power to safely drive any speaker to perfection are just a few of the accolades spoken by those fortunate enough to spend time listening to the new Tenor. Non-invasive protection circuitry so advanced that the purity of the input signal is unaffected yet so intelligent that when a problem is about to occur the protection circuit control safely shuts down the amplifier coming back on line only when the offending condition is remedied.

10 years of evolution and advances in amplifier design taught us many things. The most important revelation was our expertise was applicable to innovative audio circuit design not just limited to power amplifiers. We think of ourselves as the Ultimate Reference Amplification Company!

Tenor Hybrid Preamplification

Following a 5 year intensive R&D project, Tenor announced a new two chassis reference line stage (Line 1/Power1) and single chassis reference phono stage (Phono 1).

Similar to the 350M and 175S power amplifiers, the Line 1/Power 1 represents the cutting edge of modern technology and sets a new standard for Reference Preamplification. The new Tenor Line 1/Power 1 makes a formidable sonic and esthetic contribution to any system but takes a Tenor power amplifier to nirvana!

Where vinyl is your passion, you will be completely overwhelmed with the new Tenor Phono 1. You will hear nuances left dormant by all previous amplification technologies that are now only revealed for the first time in the full splendor of the original recording.

Michael Fremer, Senior Contributing Editor, Stereophile while listening to Tenor electronics including our new Line 1/Power1 and Phono 1 sums it up perfectly:

“Best Sound of the Show!” (CES 2009, Las Vegas, Nevada)  Another New Standard for Tenor Ultimate Reference Amplification!

At Tenor we set a very high standard for both function and form. An integral element of all Tenor designs is the natural wood faceplate and rails that dominate the physical beauty of every Tenor.  Expensive and luxurious in the somewhat austere world of high-end electronics, we insist on cherry wood as we find the organic beauty of the wood connects people visually with the incredible chemistry of the Tenor sound. A simple flip of a switch unleashes an all encompassing and natural experience that many describe as simply “organic”!

I encourage you to audition our amplifiers and preamplifiers as I think the sound will change your life as I know it has for me.  I also leave you with the definition of the word “organic” which I hope will prepare you for the experience that awaits the fortunate few that can experience the world’s Ultimate Reference Amplification by Tenor!

or·gan·ic adj

  1. relating to, derived from, or characteristic of living things
  2. occurring or developing gradually and naturally, without being forced or contrived
  3. forming a basic and inherent part of something and largely responsible for its identity or makeup
  4. consisting of elements that exist together in a seemingly natural relationship that makes for organized efficiency

Encarta® World English Dictionary

Best regards,
Jim Fairhead, P.Eng 

President, Tenor Inc. 

©2014 Tenor Inc.

“Select Language” below