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Circuit design and build quality.
Part of the beauty and mystery of an exotic car is not just the raw performance but also the workmanship and technology. For those into a layman’s tour of Tenor tech, read on. For the techno adverse, feel free to skip ahead. As I said already, everyone has expectations. Still, after hours of discussions with Jim and Michel, most of my initial preconceptions were turned upside down. First, I expected a product brimming with the most expensive trendy exotic audiophile boutique components. As it turns out, that’s not Tenor’s design model. Everything—and I do mean everything—is chosen for two interrelated reasons: sound and reliability. Both criteria must be met. Circuits that push the limits of parts might sound good but have a risk of instability or reduced operating life. Those would never make it past Michael’s drawing board. But don’t think that component and build quality isn't exquisite. It is. Everything is either high-quality industrial or military grade. "We do this for reliability and to ensure that the component will be available in the future. Custom components specifically made to order arrive in the market and disappear quickly. Many of our parts have been available for years and will be available 10 years from now."

Jim was clear about the design goals. "The preamplifier was built without regard to price. The only consideration was to build the best we could. The second objective was to design and build a piece that matches the Tenor 350 both visually, sonically and on reliability and quality of construction." Yet during my long talks with Tenor, questions about design choices inevitably returned to the two criteria of sound and reliability. Glitz for its own sake is not Tenor’s philosophy. I’ve talked to many designers who rattled off parts suppliers as though reading from a "who’s who" list, implying that brand name components are key to good sound. Tenor is more about circuit design and execution than trendy custom parts.

Physically the twin-chassis preamp concept has become almost ubiquitous in high-end audio to separate power supply and gain circuitry which here connect by massive industrial cables. In addition Tenor believes dual mono design a necessity for superior quality and performance. Each channel is completely independent to eliminate crosstalk. The signal in one channel cannot influence the other because even the grounds are separate. As to power supplies, Tenor uses five transformers, two for each channel and one for microprocessor circuitry. Of course independent transformers require duplicate circuitry. The payoff is that each channel receives full energy from its own supply with the only shared element being the power cord. Although chassis considerations are nice, let’s get right to the heart of the design with the gain process. For basic topology there were only three choices – tubes, transistors or both. If you know anything about Tenor, you know that they are committed to the hybrid concept. "The mix of the two has an advantage. You can get the best of the tube’s musicality with transistors providing output current." According to Michel this approach produces the overall neutrality Tenor is famous for and is the same as used in their 350 monaural amps.

When you start with a clean sheet of paper and no real budgetary constraints, you have the luxury of thinking outside the box. Anyone who has the pleasure to talk with Michel knows that he goes beyond conventional wisdom. The examples would fill a book so let’s look at a few starting with the tubes. Open the top of a typical tube preamp and you’ll likely see some variant on the 6922, 6H30, 6C19, 12AU7 or 6SN7. Tenor on the other hand uses four 6463 per side. Doesn’t ring a bell as a familiar audio tube? It isn’t. It’s a computer tube. Michel often finds traditional wisdom misguided. His discussion on tube selection was revealing and refreshing. Some designers fall in love with a specific tube or tube family and build around that. Michel tested and analyzed over 25 tubes for specific elements of performance "including the harmonic distribution of the signal and the precise makeup of distortion. We then built a test circuit to compare sound to measurements and determine which tube best fit our needs. The search ended with NOS computer tubes manufactured in 1958 whose specified life is 10,000 hours. They're rugged and don't develop grid current problems since they were designed as a computer switch."

The 6463 met Michel’s requirements for distortion and performance characteristics, reliability and long-term stability. Is this obscure gem the perfect tube then? Not quite. Its biggest problem is microphony which has the tube transform mechanical vibration into electrical noise. Tenor’s solution was to build a mini anechoic chamber with a wideband speaker which injects specific noise at nearfield distance at the tubes in a test jig. The internal vibrations amplified by the tube are analyzed with an oscilloscope, voltmeter and spectrum analyzer. Tubes exceeding a test threshold are rejected. Because microphonic problems can develop over time, all test tubes stay in this apparatus for a significant period of time. Although counter intuitive, Michel states that once you eliminate microphony, the 6463s are very low in noise. The tubes passed are next tested for shock resistance, DC characteristics, gas transconductance and current. The majority fail this second round of testing with a >60% rejection rate. The good news is that these tubes are relatively plentiful and Tenor secured a large supply.

In their circuit the tube runs significantly below its maximum specification and heat sinks further increase tube life and reliability. With this combination Jim projects approximately 12,000 hours. At ten hours per week you have a 23-year tube life. This is just the tip of the iceberg. You could write a small book on Michel’s tube circuit theories and selection process alone. Hard-core rollers need to look elsewhere though. I’ve seen Tenor’s R&D lab. Without their selection and overall calibration process, the Line1/Power1 most likely will operate out of spec and more importantly, the use of your own tubes will void the warranty. At this level of performance and cost, it probably isn’t wise to second-guess the designers. Various methods for volume control stir debates almost as much as tubes vs. transistors. Here too Tenor takes a somewhat different path. Rotary stepped attenuators, relay-switched resistor arrays, DAC chips and even contactless light attenuators are all viable yet Michael opted for the more traditional potentiometer. With competing manufacturers touting their exotic solutions, I probed for some understanding of this choice.