![Rogers Background Rogers High Fidelity- founded in 2009 Roger Gibboni- President ,[object Object],equipment design and manufacturing for military and space applications ,[object Object],with RCA and General Electric ,[object Object],15 years Rogers High Fidelity’s mission is to design and manufacture the highest quality amplifiers using military and spacecraft design heritage at an affordable price.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Rogers High Fidelity EHF100 Amplifier
- 1. Engineering Considerations for HiFi Amplifier Selection Roger Gibboni President, Rogers High Fidelity
- 7. Vacuum Tube Amplifiers - Why? Thermal noise voltage– v²= 4K * Temperature * Impedance * Bandwidth Noise voltage is an electric field quality and is less affected by a tube grid than a semiconductor junction
- 10. Power Output
- 11. Amplifier Class
- 12. Single Ended vs Push-Pull output
- 13. Triode vs Pentode modes
- 16. Amplifier Output Power Amplifier Output Power = (output voltage)²/ output impedance Output power is the sum of all of the energy in a given bandwidth. Amplifier output power is critical—not because we want to drive our speakers with a single signal of very high volume BUT because we have to deliver audible signal levels across a broad frequency range! 20 Hz 20 KHz
- 17. Amplifier Classes Class A Class B Amplifier conduction cycle or “on” range Class AB Class C Class A- highest Linearity, lowest efficiency—best quality audio reproduction Class AB- lower linearity than Class A but more efficient, used where power output cannot be achieved with a Class A design Class B- Lower Linearity but higher efficiency- introduces cross-over distortion Class C- Used for RF amplifier service—highest efficiency The best HiFi amplifiers are Class A through the full operating range. These are typically the most expensive and run the hottest!
- 18. Single Ended vs Push-Pull Output The key value of the push-pull configuration is the “artifact” cancellation.
- 19. Triode vs Pentode Output Stage The best solution is the “Ultra-Linear” pentode configuration achieving the linearity of the triode with the output power of the pentode!
- 21. Non-Linear DistortionThe key design consideration for minimizing amplifier distortion is to design all components to operate well within their linear operating range- This drives cost but yields the best listening results.
- 23. Noise is the increase, above the stage gain, of the naturally occurring thermal noise. Thermal noise increase is controlled by careful component selection particularly in the highest gain stages. Noise and hum are controlled using good engineering and layout practices.
- 24. Bias: Fixed Bias or Self-Bias Class A amplifiers can utilize self-bias. Class AB must be fixed bias. Self biasing allows the grid to correct to Class A as the tube ages and the plate characteristics change.
- 27. Signal Resistors- Metal film provides the best aging characteristics
- 28. Power Resistors- Wirewound provide the longest life
- 29. Coupling Capacitors- Polypropylene or silver mica
- 30. Power Supply Capacitors- High quality electrolytic
- 31. Tube Sockets- Ceramic age better in the high heat environment of a power tube
- 34. Hardware- The EHF 100 uses all military grade stainless steel hardware. All loose hardware incorporates the use of locking washers and Locktite Thread Locker providing exceptional vibrational stability and long amplifier life.Good mechanical design is critical to amplifier performance and long life
- 35. Mechanical Considerations This is a partially assembled view demonstrating the sound mechanical design of the EHF-100
- 37. Key Performance Comparison Rogers EHF 100- a very serious amplifier at a not so serious price!
- 38. Retro Appeal! MOG offers music subscription for $5 a month- USA Today 2/17/2010 MOG CEO David Hyman, photographed in his office in Berkeley, Calif., is a stereophile. He has a Japanese tube amp, made by Wavac, powering MOG into speakers.
- 39. Rogers Contact Info Thanks! Rogers High Fidelity 52 Kain Rd. Warwick, NY 10990 845-987-7744 Roger@rogershighfidelity.com See us at: www.rogershighfidelity.com www.audiogon.com
Hinweis der Redaktion
- We’ll talk today about a series of technical and engineering subjects related to vacuum tube amplifier design. The purpose of this workshop is not to teach you how to design amplifiers but rather to form some reasoning behind a lot of industry terms that affect our buying decision. Our industry is unique in that we have a strong, right brained, sense for listening quality but we’re also left brained, techies. It’s that left brained, techie side that I reach out to today so that you leave with an understanding of the technical qualities and decisions that go into an amplifier design and how that affects what we buy and the real value of certain technical aspects.I’ll also describe for you our engineering logic behind the EHF-100 amplifier and why we believe that we are making the best amplifier on the market today!
- A little about me and our company (READ)Our mission is clear and simple: To design and build the best amplifier on the market leveraging our military and spacecraft design and manufacturing experience. We believe we’ve achieved this goal with the EHF-100.
- Lets take a look at why some audio enthusiasts prefer tube amps to semiconductor amps. Our industry tends to use subjective terms to describe certain listening qualities—lets take a look at the wiki definition. Now-Lets take a closer look at what drives our definition and our design approach for amplifiers.
- Noise is purely an electric field quality. Read the ChartSemiconductor devices require a current to flow from the base to the emitter in order for the device to conduct current from the collector to the emitter. To generate a current, the junction provides an impedance to convert the electric field to a junction current. The impedance affects the “quality” of the noise. In a vacuum tube, the plate to cathode current is controlled by the electric FIELD between the grid and the cathode. Ideally, there is no grid current flowing. Therefore, there is a truer representation of the noise quality from a vacuum tube amplifier because the grid draws no current and therefore presents no input impedance to modify the noise.
- Lets spend a few minutes to discuss some key amplifier design considerations and how they affect our listening quality and why they are important to us as vacuum tube amplifier consumers and listeners.
- These are the major design categories for HiFi, vacuum tube amplifier consideration. They’re all trades of design excellence versus cost. Our mission at Rogers is to develop the best amplifier possible at an affordable cost---however cost is always the secondary consideration, design excellence is always 1st. We’ll talk a little about each of these in the succeeding slides. Some of these subjects are hallowed battlegrounds for our industry. I expect we may have folks on both sides of the tradeoffs here today. Without intentionally polarizing the discussion, I’ll discuss the technical reasoning process that engineers use to evaluate each of these parameters, but more importantly, I’ll explain why we, at Rogers, made specific selections to design what we believe is the best hi fidelity, vacuum tube amplifier made today.
- A standard audio amplifier has 3 stages: a gain stage, a phase inverter , if it uses a push-pull output and the output stage. Each stage can be one or more tubes but our diagram shows the basic stages of operation. Each stage has a key role and a unique selection criteria. The first stage provides all of the signal gain for the amplifier. For that reason, the tube should be a high gain, low noise tube. Typically a high mu (or gain) pentode is used. Rogers uses a series of internally shielded hi gain pentodes designed specifically for ultra low noise audio reproduction. Because of the low signal levels and extremely high gain of this first stage, Rogers adds additional tube shields to further reduce the noise pick-up from the tube environment. This gives the EHF-100 a noise floor as close to thermal, or as low, as possible.The phase inverter provides unity gain amplification but creates 2, out of phase outputs for the push-pull power stage.The output stage provides all of the power gain. This stage converts the power supply high voltages and currents into output signal. Rogers uses KT88 output tubes in the EHF 100 rated at 85 watts of plate dissipation each. They are supplied by a robust 500 watt power supply and deliver a nominal 100 watts per channel rms with a 150 watt peak capability.
- Lets first define amplifier output power---read equations. This is a key concept that we need to understand. We often consider the output power capacity of an amplifier as the ability to drive speakers louder. This can be the case but in HiFi applications, power output capacity allows the amplifier to deliver signals, at their appropriate recorded level, across a wide bandwidth without saturating---or running out of steam. Total power output is the SUM of all of the energy in a given bandwidth. In a listening environment, this translates as follows: imagine a recorded piece that has powerful lows but has key high components as well---Dave Brubeck’s Take 5 is a great example. This piece has strong sax content but also has continuous cymbals going. If an amplifier reaches it’s output power saturation level, the sax would overwhelm the speakers and the listener would not hear the cymbals—at least at the level that Brubeck intended. So Power Output is important to faithfully reproduce music content with broad spectral density or in other words, lots of frequency content. The Rogers EHF-100 develops 100 Watts per channel (RMS) with a peak capacity of 150 Watts. With appropriate speakers, this will yield the best reproduction available. So, When you evaluate an amp, listen to music that you know the expected spectral density—and then listen for those frequencies and their relative levels.
- Amplifier classes are an engineering process to categorize the tradeoff between linearity of reproduction and amplifier efficiency. The tradeoff the designer is faced with is the higher the linearity that he desires, the lower the stage’s efficiency and the higher the cost to produce. With a lower efficiency, the designer has to increase the power supply capacity and select higher power tubes and transformers to achieve a desired power output.Lets take a look at this linearity-efficiency tradeoff and the amplifier classes. (read chart). The amplifier class is selected by biasing the control grid of the output tube into the range to select the appropriate conduction cycle.—more about the bias laterThe Rogers EHF-100 is a fully Class A amplifier. In order to get 100 Watts per channel, we need to use KT88s at the their highest design range and large output transformers. We also have designed a robust 500 watt power supply to support this design. With our design, we get 100 Watts per channel with a peak of 150 Watts all with less than 0.1% distortion!
- For the sake of this comparison, I assume we all understand the structural differences between a single ended configuration and a push-pull design. Do we need to spend a few mins on this for clarity? OK so lets look at the performance differences. Harmonic distortion and clipping are key differences but really only encountered in an overdrive state. Generally, good practice for HiFi amps is operation well below the overdrive level so these non-linear characteristics minimally come into play. The primary difference however is the second harmonic content. In the single ended configuration, this is the primary distortion component. This is cancelled in the push pull design where the third order harmonic is the first artifact. The 3rd order is generally down from the second order quite a bit. As a side note, musically, the ear tolerates the second harmonic better than the third.The primary reason Rogers utilizes our unique push-pull design is “artifact” cancellation. By artifact, we mean anything introduced to the output stage input other than the desired signal. These artifacts could be distortion from previous stages but more importantly, it could be hum or noise. In a single ended configuration, these pass right thru and are amplified just like the desired signal. Because of the phase cancellation properties of the push-pull configuration, these artifacts get cancelled---completely!The Rogers EHF-100’s push-pull output is designed for maximum output power from the KT88 with a robust output transformer that operates in a linear range throughout the dynamic range of the amplifier.
- Again, focusing on the output stage configuration, we trade off between a triode or a pentode. Some amps are designed specifically for a triode or pentode configuration while others are switchable by connecting the 2nd and 3rd tube grids to the plate in a pentode. The trade-off is primarily between power output and linearity or distortion---cost again! There is however an alternate configuration that’s not shown here. It’s called an “Ultra-Linear” pentode configuration. In this design, the screen grids are nearly at the plate potential by sharing a portion of the output transformer primary winding with the plate circuit. By using taps on the output transformer primary, the designer achieves the linearity of the triode with the power output capacity of the pentode---and some negative feedback enhancement as well! Rogers feels this is the optimal solution in a push-pull design achieving distortion free power output and maximum power.
- Lets start out by agreeing on a definition for distortion. Distortion is the compromising of the original signal by phase related artifacts. These could be harmonics or overtones or could also be non-linear modifications due to clipping, saturation or cutoff. Distortion is distinctly different from noise or hum which are also sources of signal degradation but are introduced from external sources not by the non-linear reproduction of the primary signal. There are two flavors of distortion—harmonic distortion and all other non-linear forms of distortion. The two flavors are somewhat inter-related but we’ll separate them for this discussion because they are minimized by different design practices. Harmonic distortion is naturally present in any amplifier design but harmonic content can be minimized through good design practices---such as push-pull vs single ended. The other sources on distortion come from designing amplifier components to operate in non-linear performance regions. These could be tubes, transformers or amplifier class selection. By selecting components with ample “headroom” in the linear range, the design minimizes the introduction of non-linear artifacts—but also drives the cost.
- Noise and Hum are again, artifacts unrelated to the primary signal that introduce distortion and degrade listening quality. Noise is the increase in the naturally occurring thermal voltage, that we discussed earlier, above the gain of the stage. This is often referred to as the “noise figure” of the stage. This value is expressed in dB. So if an amplifier stage has a noise figure of 5 dB and a stage gain of 30dB, the noise floor will increase 35 dB. So the designer always tries to achieve the lowest stage noise figure. Rogers utilizes a 1st stage, EF86, that has an extremely low noise figure and was designed with internal shielding for ultra-low noise audio applications.Hum is the amplifier pick-up of stray 60Hz power line radiation. Again, good design practice and layout reduces hum significantly but drives cost and assembly complexity. Rogers EHF 100 uses design practices to neutralize all 60 Hz components within the chassis and also reduces stray magnetic coupling. Rogers utilizes RF coaxial cable for all high gain signal line connections. The hum and noise in the EHF-100 are so low, they are immeasurable and certainly are inaudible!
- The DC bias voltage applied to the control grid of the output tube centers the operating point on the characteristic curve. Lets take a look at an EL34 tube curve.(define grid) The signal voltage is an AC voltage applied to the grid and swings the grid voltage between cutoff and saturation. Ideally, the operating point is selected to give linear operation over the entire grid voltage cycle. This is how the amplifier class is set. As the grid voltage swings with the input signal, the plate current swings, per the tube characteristics, and creates gain.The key criteria for self bias vs fixed bias lies in the aging characteristics of the tube. In a Class A design, if the bias is fixed, as the tube ages the operating point moves closer to cutoff or saturation making the amplifier more Class AB. If the tube is self biased, the change in plate current, augments the bias voltage and keeps the operating point centered on the curve.The Rogers EHF 100 is designed to be full Class A over the entire dynamic range of the amplifier and is self biased to maintain this performance over the entire life of the tube. The Rogers self bias design also alleviates the need for tube gain matching making individual replacement possible.
- Now---Component selection
- Component selection is the key to an amplifier providing long service life. Again, this design trade-off is between design excellence and affordable cost. This information provides some insight to the design goals and quality of the manufacturer when selecting an amplifier.The first item is tubes. Rogers buys the best tubes that are available. We tested and sorted many manufacturers and found price, like other things in life, is not necessarily an indicator of quality. The first choice, when available, is new-old stock (NOS) or JAN military tubes. The next would be eastern European or Chinese manufacture. We’ve found these tubes to have acceptable quality and aging characteristics.Before we discuss individual components, a few words about the environment. The component environment---heat, humidity, and dust dictate the aging and overall life characteristics of the components. A tube amp is generally in an indoor environment so its dry. Rogers chassis design is sealed to provide a dust free environment. The real challenge is heat. A Class A linear amplifier runs hot. There’s just no way around that. The tubes run hot, the transformers run hot and the power resistors run hot. The designer must take the high heat environment into account in every phase of the component selection. Rogers uses all military grade components to assure the longest possible amplifier life and provide years of trouble free listening.Now lets talk about resistors. Signal resistors are best in metal film. Metal film resistors are ultra stable in heat, age well and are close tolerance. Carbon composition or metal oxide decay with heat and drift in value over time.Power resistors are best in low inductance wire-wound configurations. Rogers uses high dissipation ceramic resistors in the power tube cathode circuits and chassis mounted, heat sinked power supply resistors.Coupling capacitors- the best coupling capacitor is no capacitor at all! Capacitors have reactance that is frequency dependent. Frequency dependence yields higher circuit Q and consequently bandwidth limitations. Rogers EHF-100 uses a novel “direct coupled” approach eliminating coupling capacitors in the gain stages and resulting in a bandwidth in excess of 100 KHz. Where coupling or bypass capacitors are used, polypropylene or silver mica are best. They’re age and temperature stable.Power supply capacitors are high capacitance values and are limited to electrolytic selections. Good, military grade capacitors are available and will last the life of the amplifier.Tube sockets are best in ceramic rather than phenolic. Ceramic are heat and age durable and available as military grade.Our last category is wire. Don’t under estimate the value of good, Teflon coated wire. In a high heat environment, teflon wire will last a lifetime. In a past life, We designed spacecraft wire harnesses using teflon wire and they went to the moon! I’ve restored 1960’s vintage military radios, assembled using teflon wire, and when these radios are opened, they look like the day they came off the assembly line. The wire isn’t dry and cracking; it looks like new! Teflon wire is about 10 time more expensive than organic jacketed wire but well worth the price.
- Now we’ll talk about the mechanical considerations for a well made amplifier
- Good amplifier design isn’t limited to circuit design excellence---mechanical design is central. Like all other design considerations, the use of good mechanical design, excellent quality materials and a design goal of long amplifier life is the core mission at Rogers. At Rogers we prefer chassis mounted tube sockets and point-to-point wiring and limit printed wire boards to holding small components only. Some manufacturers use tube sockets that are mounted directly on the PWB. You can identify these because the tube base sits below the chassis level. This assembly process is cheaper to manufacture but can lead to problems. Our experience is that PWB’s can fail from excess vibration. Having the tube mounted directly to the PWB levers a lot of mass on 8 solder joints that are prone to cracking. Hand wired, point-to-point assembly eliminates this risk.Good chassis and hardware selection are also central to design excellence and long amplifier life.
- Here’s a semi-completed view of the underside of an EHF-100Point out:Ceramic Tube socketsTransformer grommets1% power resistorShielded meter housingComponent staking
- Now that’s take a look at the competition….
- This comparison spells out the value difference for the Rogers EHF 100! It’s the only full Class A amplifier available for an affordable price. We believe that we’ve designed and built the best amplifier for quality listening and long life.
- This was an article in Wednesday’s USA Today. Tube Amps have retro appeal!