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Originally Posted by Bolder /img/forum/go_quote.gif
Hmmm. Looks rather close to this classic Optimized Morgan-Jones circuit
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Nothing except the basic topology is the same.
Its much closer to the 6n1p amplifier one line down on the project pages but with the addition of an active voltage regulator.
The supra PS is funky, but everyone wanted more voltage more voltage more voltage and never really looked at what it did they just wanted more voltage because more voltage is moar betterer. The key to building an amp like this better is not more voltage but more current. The weakest variants of this kind of amp can drive any known 300 ohm headphone loud enough to overcome the noise at an NYC meet
voltage output is not an issue with a WCF unless you are especially daft. The issue is the current and how the amp deals with low-impedances. A 6sn7 at the sane limit of its current (say 14mA/135V rather than 10mA/160V which is what the SP supra ran... the MPX3 was about 8mA/160V {per section in the output stage}. in HV mode, the bottom tube which sets the current "sees" the same voltage and sets the same current but the top tube has about 100 extra volts to deal with and the corresponding currents 6sn7 is fuxored for plate dissipation so you get the fun of 6bl7/6bx7), but very conservative voltages would be pimp in a WCF, but we all formed a line demanding more voltage. In the end Mikhail as a designer holds infinitely more blame for caving to these demands than non-techies are for making them. He cranked the voltages up so high so that an amplifier which could have been made (was made) well with 6sn7 had a mode of operation that would only run properly with big power type tubes.
Nobody ever sought a high mu/low Ra/high current tube for this type of amp which would have rocked it for output impedance. 5687 with mu of 20 has good Ra, and good current capacity, but 20 isnt exactly high mu... try something with mu in the 40 ish range. maybe a triode strapped small signal pentode
At that point, we must ask, is this design REALLY worse than the alternatives in an OTL all-tube audio stage using small tubes of the types commonly used in audio preamps? SRPP? the sad joke of a class-AB played on those who are not careful enough to use it with an appropriate load - this joke is played on this forum daily so how could anyone point a finger at mikhail for using a circuit that will never run outside of class-A (IE a WCF CAN NOT run class-AB) in any acceptable fashion. Pure cathode followers? an option with big tubes, but then we get into all of that....
Eventually the more voltage line grew kind of thin because you can only go so high before things start to blow up even if you were already exceeding part ratings by a little bit, and looking at it backwards it really was a very weak idea from day 1. Somewhere in there the desire for a very high current amp came up for people who need more than the 16mA (peek to peek ) output current an MPX3 can dish out at a reasonable price. Behold! The Extreme. nobody said hey can we get more voltage? This time they said more current! Perhaps too far the other way considering the great lengths mikhail went to to **** himself with this amp. Some of it could be written off as ignoring the necessity of a safety factor on some parts, others are just dum. But we got an amp that could dish out well into the 100mA output current range. Because we need 100mA of output current (yea, sarcasm) to drive headphones.
Every design topology has its compromises. Mikhail's problem wasn't one of using inherently poor topologies, clearly I and many others who have heard his amps without knowing about some of the monsters that live in these little cases would argue that his chosen topologies are nicer than some other designs that only have military grade circuit boards (whatever that means) going for them, his problem was his supreme ability to take what was a nice design and screw it up.
Someone above asked if an MPX3 would be a nice amp to clone:
Yes. If you are comfy building a HV amp like that, do it! spend a few hours with a calculator and sort out some of the screwups in the supra PS or just use the MPX3 PS... The casual observer will note that it has 2 RC filter cells before the active filter where the supra only has 1. more filtration IS moar better.
here is what the SP MPX3 toaster had if someone would be kind enough to draw a schematic:
600vct transformer
full wave rectifier, CT grounded
similar 4*10uf cap arrangement to the supra
300 ohm 12W
big blue cap. 450uf/450v?
300 ohm 12W
big blue cap. 450uf/450V?
similar voltage regulator to the supra, with the exception that B+ on the "amp side" of the reg is 350V.
I did not note the sizes of the "big blue" caps on my schematics so your on your own... If someone with a toasterstyle MPX3 could verify {or not} my values I think the community would benefit from it. as a bonus it dosnt even require opening the amp, just reading off the values.
ooh, one last note, I think B+ on my supra was 350V, but maybe my memory is foggy. I like 300V better anyways, and have seen references to 320 elsewhere on these forums. Most likely like many things singlepower this simply varied from unit to unit so don't be surprised if you find that while trying to diagnose a screwed up amp.
