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|"With a MOSFET, that mode of failure just does not exist by design. This change completely solves the reliability of the output stage." François touched on the reliability issue of the original 300Ms. "At first we had a few problems with the IGBTs due to imperfect matching. This was quickly resolved. The other issue was the 6H30, an excellent preamp tube but as a driver, not ideal due to its internal construction which can be prone to grid current problems. When that occurs, unfortunately, the tube burns out in a few moments. The original tubes were carefully matched and the problem never occurred in the prototypes, which worked flawlessly for nine months.
"Unfortunately, some of the tubes in the production models were not as carefully matched and some problems developed in the field. While the old Tenor Company was in business, every amp was brought back and updated. Although some of the work could have been done in the field, we brought the amps back at our expense to offer top-quality service to our customers. We challenge you to find a customer who was not pleased with the service from the original Tenor Company."
One might question the launching of an all-out, top-of-the-line assault of a brand-new product in the skin of a defunct predecessor. But that, ladies and gents, is their marketing issue. The brain trust at Tenor obviously believes that visual similarity and resultant recognition is an advantage - not a disadvantage. There is a slight visual difference, noticeable only to those intimately familiar with both the old and the new; the faceplate of the new 350M has a far more elegant and luxurious finish than its predecessor. Other than that, the 300Ms and the 350Ms are visually identical.
The 350Ms add a couple of new features to the back - an hour meter and a 12V triggered remote turn-on, which actually will function between 3V and 30V. There are two ground switches, one for the balanced input that disconnects pin one of the XLR from ground; plus a three-way audio ground switch for standard, lift or ground through a 51-ohm resistor. The original 300M had one rear mounted on/off switch. The 350M adds a bottom front-mounted on/off switch parallel to the rear one.
Two pairs of parallel speaker outputs are available for bi-wiring. The speaker terminals are the top-line WBTs, with the Signature Series used in production models. These binding posts have ultra-high contact pressure allowing a maximum continuous current of 100A! This commitment to quality components is the mere tip of the proverbial iceberg as you will soon realize. Overall, the Tenor people are off-the-charts quality obsessed.
A rear input selector toggles between single-ended RCA and balanced inputs. Balanced feeds an internal Jensen transformer which Michel believes to be sonically superior. "My opinion is that the transformer is the only way to go for a balanced line. To do it electronically is extremely difficult because most of the time you lose the advantage of the balanced line due to common mode rejection ratio degradation. With the transformer, there is no such problem."
|Michel considers good specifications the byproduct of superior design. Tenor promotes the sound, not the specs. As Michel states, good specifications "are a gift at the end if the design is working. If your design is good and sounds good, you will have good specifications." The frequency response extends to 200kHz and then tapers off very slowly without an abrupt end to avoid phase problems. The Tenors are down -3dB at 200KHz and extend below 1Hz in the other direction. This extended frequency response helps to insure good phase response in the audible spectrum, which generally requires a response of ten times above and ten times below the audible response frequencies. The 350Ms use no global feedback, just a very small amount of local feedback to stabilize the gain of the first driver stage, more for dynamic than distortion control.
|Michel believes that connectors are inherently problematic. They can become loose, noisy and degrade the signal. To address this, Tenor routs critical signals through two, three or sometimes four parallel wires. Therefore, if one connection becomes loose, the signal still has one or more clean paths. This redundancy allows multiple connection|
|points to fail without any effect on the sound. For the audio signal, Tenor uses Teflon-sheethed silver-plated coaxial RF type cable connectors, with Mogami cables used for other internal signals.
Current units ship with Cardas Golden Reference power cables to make sure that the customer begins with a very good power cable. It is not Tenor's endorsement of this product or brand but it is their belief that a standard stock power cable is not appropriate for their amps. If the customer already owns a high-end power cable, Tenor will credit the cost of the Cardas and ship the amps without power cords. Either way, the cable must be 12-gauge 20 amps.
The 350Ms are designed to produce the same sonic signature into any load. Every internal circuit is powered by its own fully regulated power supply. The 350M essentially doubles power output down to two ohms. Did I hear you say that 1,000 watts into two ohms just won't do it for you? Your inefficient one-ohm speakers really need more power to kick butt? No problem, you'll want the Tenor 350Mhc high-current version which trades off reduced maximum power for the ability to double output down into one ohm.
To start with, the shipping crates offer initial clues as to Tenor's quality within. While your plebeian neighbors might get wooden crates with their Wilson or Lamm equipment, Tenor is at a different level with their custom rolling aluminum cases, which are elegant and no doubt very expensive. Removal is easy with multiple locks releasing a two-piece top revealing the form-fitted Tenors wrapped in ultra-soft linen cloth.
Once unpacked and placed on my Walker Prologue stands, operation was quite simple - but then how difficult can an amplifier be? Turn the master rear-mounted power switch to the 'on' position and the power rocker switch located on the front belly is energized. Press the button and the amp begins a three minute warm-up, allowing the tubes to gradually heat up while preventing in-rush current which could trip house breakers. The amp remains in mute during this period and the front Tenor logo glows red. After three minutes, the logo switches to blue and you are ready to go. Full heat stabilization of the output stage takes about 20 minutes, after which the amp performs at its maximum transparency and dynamic headroom.
Everything starts with the power supply. No matter how good the gain stages, no matter how well the tubes are matched, the old adage stands - garbage in, garbage out. Fed from a 20-amp IEC connector and cable, the power supply is so powerful that the amplifier's output is not limited internally but by the wall outlet itself. If you could supply more power, the amp would use it. The main supply for the power stage can output 100 amps continuously.
Regulated power supplies the tubes at +/- 285V balanced, in a configuration giving ultra-low noise of 40 microvolts under full RMS power. Heater voltage is separately regulated at both 6.6 and 12.6V; yes, it's a small detail but another step to lower noise. Michel claims the unique power supply configuration is a primary key to the extraordinary low noise performance:
"Each [amplification] stage is independently regulated for high voltage and low filament voltage to eliminate any AC noise. Our supply transformer is very unique. We considered multiple transformers but our final answer was to use two toroids in the same can. One is the big toroid that powers the power stage; the other powers all the other voltages, including high-voltage, filament and logic supply. A big design problem is interference between two transformers so we developed a magnetic shielding system. Additionally, where the wires exit the transformer, radiation interference can occur. Since we have two sets of wires exiting the transformers, the wires are kept at 90° angles to cancel interference. Because of this, if you have big variations in current between the two transformers, you cannot interfere with the transformer that powers the input stage."
If there is interference between the power transformers, the power will not be pure and in turn modulate the input signal with various types of distortion not easy to measure. Poorly designed power supplies can create intermodulation throughout the system, which according to Michel is a "form of hidden distortion".
"Most of the time when you try to regulate high voltage, let's say at 300V, it's difficult to get low residual noise. However, with the design of this circuit, we have less than 100 microvolts of residual noise for a power supply of 285V. This is extremely low for an amplifier of this power. Furthermore, we have only 300 microvolts of output noise at the speaker line, again extraordinarily low especially for an amplifier with no global feedback since global feedback, as you know, usually lowers noise. At 10 watts or normal listening level, the noise is down 94dB."
The power supply is dead silent, both physically and electrically, and transmits no noise to the chassis. Any possible residual noises are filtered through the suspension system of the gain stages. In amplifier design, the importance of power supplies cannot be overstated. An undersized power supply combined with large power demands specifically from the bass can have the effect of distorting the soundstage and reducing the control and articulation of the lower octaves. Properly designed regulation will produce a constant voltage regardless of the external and internal power factors and influence.
Amplifying Gain Stages
I guess the first question is, with all the possible circuit topologies, why choose OTL? Overall, Michel believes that tube amplifying stages make it inherently easier to compensate for distortion characteristics. "Tubes are much easier to control or compensate than transistors. It is not the same kind of distortion. That's why I wanted to use them as the basis for the amplifier and only use solid-state circuitry for the big power at the end. Specifically, why OTL? A properly designed OTL circuit is very transparent, with less coloration than other tube amplifiers. We've simply taken the known characteristics of OTL and built upon them by further regulating our power supply and building a completely symmetrical circuit."
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