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Well, some of what you say is true ... but not always the case with tube gear. Generalizing will lead to religious battles ... let's not go there ... however I must say that...Roger Modjeski's designs at Music Reference aim to be:1) Good value for the dollar2) Environmentally friendly and efficient3) Reliable ... both in the short and particularly over the long-term4) Sound goodI think Roger has specifically focused on these areas because he's been around the block, and at one point in his career repaired many a tube amp of other brands and makes, and saw exactly the downsides that you say. He tends to be very no-nonsense, no baloney type of guy (engineer) and has a track record to prove it with some very desirable gear floating around as part of his legacy. He's well respected (the RM-200 is Stereophile Class A rated) in audio circles though his positions on things hi-fi related also has created detractors.One of the reasons I've bought and use MR gear is because I like this no B.S. philosophy, and in particular, his willingness to speak openly about his design implementations and design objectives. Which is why his perspectives on some of the truths and myths I listed above I think are insightful to anyone interested in hi-fi. I'd be curious as to your thoughts on the above list of 10 points?
Tom Evans actually sells a tube power amplifier, though this preamp is SS.There is more information on his Lithos regulator and preamp design here.http://6moons.com/audioreviews/tomevans2/vibe.htmlQuote:"Tom claims that commercially available voltage regulators suffer voltage noise equivalent to the output of a moving coil cartridge. If you want to uncover all of the information contained in very small audio signals -- like those output by moving coil phono cartridges -- you want an audio signal with the minimal possible corruption and maximum possible dynamic range. This requires vanishingly low noise. If noise and signal are similar in magnitude, a significant amount of data is irretrievably lost. Off-the-shelf regulators also exhibit very slow transient response and recovery times, compounding the signal degradation begun with the noise issue. Tom decided that the only alternative was to develop his own high-performance high-speed regulators, which he christened Lithos. His first efforts at designing them yielded results that were a staggering 1000 times quieter, 53 times faster and 100,000 times more accurate than the best commercially available regulators used for audio applications at the time."Here too, is another after-market regulator, though, no astounding claims made by Allen Wright.http://www.vacuumstate.com/index.tpl?rubrik=13&lang=2Both of these caught my attention, because (in relation to the 10 truths and myths), its only recently that I've seen any emphasis on power supply regulation as having much of an effect on sound in a preamp or amplifier. For the longest time, power supplies seemed to be all about beefier power supply caps.
Quote from: 6BQ5 on 23 Aug 2008, 04:28 amTom Evans actually sells a tube power amplifier, though this preamp is SS.There is more information on his Lithos regulator and preamp design here.http://6moons.com/audioreviews/tomevans2/vibe.htmlQuote:"Tom claims that commercially available voltage regulators suffer voltage noise equivalent to the output of a moving coil cartridge. If you want to uncover all of the information contained in very small audio signals -- like those output by moving coil phono cartridges -- you want an audio signal with the minimal possible corruption and maximum possible dynamic range. This requires vanishingly low noise. If noise and signal are similar in magnitude, a significant amount of data is irretrievably lost. Off-the-shelf regulators also exhibit very slow transient response and recovery times, compounding the signal degradation begun with the noise issue. Tom decided that the only alternative was to develop his own high-performance high-speed regulators, which he christened Lithos. His first efforts at designing them yielded results that were a staggering 1000 times quieter, 53 times faster and 100,000 times more accurate than the best commercially available regulators used for audio applications at the time."Here too, is another after-market regulator, though, no astounding claims made by Allen Wright.http://www.vacuumstate.com/index.tpl?rubrik=13&lang=2Both of these caught my attention, because (in relation to the 10 truths and myths), its only recently that I've seen any emphasis on power supply regulation as having much of an effect on sound in a preamp or amplifier. For the longest time, power supplies seemed to be all about beefier power supply caps.I took a good look at Tom Evan's website and found the following spec on his Linear A amplifier.---LINEAR A SPECIFICATIONS---Frequency Response: 12Hz to 90kHz-these are 0dB down points! Flat.Power: 25.2wpc Class AOutput Noise: 150dB down 700microvolts(!)(not millivolts)Output Impedance: 0.5 OhmInput Sensitivity: .7millivoltsPrice: $8900His noise output is not referenced so I'll give him the most generous spec. and compare it to his 25 watt output. The answer is 86 dB (20 log 14volts/.7 mv= 86 db), not 150 dB. This makes me doubt his power supply noise claims also. The Frequency Response spec makes little sense "0 dB down" how about 1 or 3 like the rest of us do. My output noise is typically 150-300 uV, yes microvolts, not millivolts. The Input Sensitivity is likely .7 volts not millivolts.Hey, I've had some errors in my specs too, none of us is perfect. I hope he will clean this up especially since he is claiming noise numbers that are not achieveable with active regulators unless you have a resistor feeding a big cap filter after them which both slows them down and degrades the output impedance.
I will start with a disclaimer: NB (note well) that I am not a digital amplifier designer, that I have only a passing interest in them and I have no opinion about how they sound. If any class D amp designer cares to state how he has solved any of the problems below I welcome his information. I also welcome any class D amp designer to state his noise specs, frequency response graph, output impedance from 20-20 kHz, short-circuit protection and level of emitted radiation at the switching frequency. Does anyone make on that will drive a 10 uF capacitor (big electrostat) at 20 Khz?I was working on an unusual class D amp about 19 years ago with Mac Turner, a very intelligent EE. Power MOSFETs were just coming online. The Motorola first edition data book that I have is dated 1984 and is about 1/2 inch thick. By the time Mac and I were at it the IR book was about 2 inches thick and MOSFETS were cheap and easy to get. The earliest commercial Hi Fi class D amplifier I know of was the Infinity SWAMP which I saw at CES in 1975 driving their SERVOSTATS. Kudos to them for trying and putting a lot of money at it but they were just too soon, the fast switching MOSFETs we have now were not available. They used the fastest switching transistors available but they were not nearly as fast as what we have now.I know there are many terms for these amps and many designs floating around now; B&O, Tripath and others. I would not call these "digital amps" though I see that term used now and them. They are actually closer to analog amps until you get to the output stage and that's a BIG, FAST Switch. Keep in mind that the switching needs to be very fast just to get the amp to go to 20 KHZ. Fast switching also simplifies the output filter, which is the biggest, and not often mentioned, problem in these amps. Also note that these amps are not necessarily "faster" than other amps and often do not have even as wide bandwith as many tube amps notwithstanding their often derided output transformer.Now here's a real bugaboo. Many of these amplifiers have feedback before one gets to the output filter thus the filter is out of the loop. What happens then is the amp becomes sensitive to load impedance and will either peak up if the load is higher impedance than the design load or roll off if it is lower. I do recall seeing a test of such an amp in Stereophile in the past few years and it did just that.
I hope he will clean this up especially since he is claiming noise numbers that are not achievable with active regulators unless you have a resistor feeding a big cap filter after them which both slows them down and degrades the output impedance.
The story goes that audio-legend Paul Klipsch wore a button under his lapel that said Bullsxxt. He would flash it to someone who would engage with him on some audio-related topic (when appropriate of course).I'm not at all an engineer, but like to read and learn to educate myself the gear I use and how they work (Roger's product manuals are great that way).Over the years, I've come across various statements frequently made on discussion boards regarding amplifier design. As with all things, there is a bit of truth and some B.S. with each in the list below. These are NOT my ideas. Curious as to what others think ...1. Transformers: The bigger “iron”, the better, hence massive power supplies and output transfers sound best. 2. Rectification: Tube rectification is better than diode, preferably with a choke in PS3. Tubes: NOS is generally better than today's Chinese, Russian, Czech and former Yugo production. It's more than just testing well.4. Parts I - Caps : Good ol’ Sprague caps sound better than new expensive exotics 5. Parts II - Materials: Silver is better if you can afford it. Silver transformers, wiring, etc…6. Design: Fewer stages the better7. Negative Feedback: Preferably none. Zero NFB is best8. Design II: Class A … nuff said, generally sounds better than AB, B and other variants9. Construction: Hand wired is generally better than PCB10. Straight wire with gain : Tone controls are generally bad, another complexity to degrade the signal from "purity"11. Transformer II: Transformer-coupled amps perform better than those using a cap between stages12. Made in the USA: The Chinese don't care about quality of products. Made in the USA is best if you want your amp to last.I would say with all of these, its a matter of implementation, thus "generalized" statements are mostly untrue. However, what is the "real story" with these? They can't be entirely right or wrong either.
