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Now, you can see the difficulty of using just one output device to generate gain and deliver a low output impedance. Adding one extra gain stage effectively amplifies the Gm of the output device by its gain. Now that extra gain stage might be found in your line stage for free or it might have to included in the amplifier itself.
November 29, 2002
The new position I have is at Beige Bag Software, the people who make the excellent B2 Spice A/D program. And by the way, I thought their program was excellent long before I began working there. In fact, two years ago, I wanted to do a review of several inexpensive SPICE programs and I quickly discovered that inexpensive and SPICE seldom went together, that $30,000 to 75,000 per seat installations were common, and that finding a capable SPICE program for under $1,000 was not easy. At first I was wowed by a competitor’s $1,000 dollar program — the manual alone had to be worth a few hundred dollars, big and glossy, snuggly fitting in its shrouding box. Well, a month later, the only thing I still liked about the program was its manual. Why? I couldn’t quickly draw complex schematics. I draw up to ten schematics a day and it was nerve racking to use that program. With the B2 Spice A/D program I was able to draw a complex hybrid amplifier in 15 minutes and I had a useful test running in the next 3 minutes. So what you can expect see in upcoming articles (beyond the usual schematics) is B2 Spice A/D circuit files, which means you can download the circuit and run your simulations on it (and no, I will not be offering the circuits in P-SPICE or Protel or EWB or any other version of SPICE.) To see what this might be like, check out the two case studies I have posted on the Beige Bag website http://www.beigebag.com/resources.htm The first study covers RIAA preamps and the second covers headphone amplifiers. If you don’t see a tube for the first few pages, do not despair: they are there.
Email? Several readers have asked where the Tube CAD Journal’s mail from readers section has gone. Beyond the work required to assemble (and edit) the letters and replies (and draw the illustrations), something else has bogged down this portion of the journal: many of the emails received have been secret: |
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“Help! Our amplifier sounds kind of dull and lifeless compared to other 2A3 amplifiers and sometimes it even breaks into oscillation; it also hums, so much so that I am almost embarrassed to charge $6,000 for the pair. Maybe the transformers are to blame? Who makes the best output transformers? Which one should we use? I hear that 3k is a good impedance. We are using US Army pulls RCA 2A3s; should we be using 211s instead? Or would a new input circuit be better? What is a White cathode follower? Could it be used? Unfortunately, 70% of our parts budget goes to the single gold-foil, extra-virgin olive oil coupling capacitor (NO OTHER CAPACITOR IS WORTH LISTENING TO), so any modification that costs more than $40 is going to hard to justify. We were thinking that power supply regulation is needed. Is a shunt or series regulator better? What is the difference between the two? Could a regulator be added for less than $40 in parts? How would we hook it up? Can you draw us a color-coded diagram? Could the 211 or 845 be used so as to make our Japanese customers happy. Please provide a circuit and WIRING diagram, Phil, who does all our wiring can't read schematics that well. Speaking of Phil, he is convinced that you do not know what you are talking about, as you wrote that it was impossible for a single 2A3 to put 27 watts and he says that since our amplifier sounds much louder than most single-ended 2A3 amplifiers, which everyone agrees put out 25 watts, ours must be putting out at least 30 watts. Do you fully realize the legal consequences of slandering a company’s product in that way? Our attorney tells me that he could easily win a six-figure judgment against such a slander. IMPORTANT! Do not publish or show anyone else this email or the attached schematic. TOP-TOP-SECRET. Reply expected no later than tomorrow (use only DFX files for schematic revisions and the chassis layout). Sincerely
Joe Smith Owner Acme Tube Design The makers of the world’s best-designed, best-sounding tube amplifiers See and hear our amplifiers this weekend at the CES, Hilton, back of room 1132. |
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An exaggeration, of course, except the line specifying Top Secrecy. And it isn't always a business that is shy about sharing schematics. Many individuals have submitted circuits under a cloak of top-secrecy and seldom (with a few glorious exceptions) have the circuits been anything but derivative (and I am not talking about the ratio of the change in a function to the corresponding change in its independent variable). So I am not sure why secrecy was required. Still, most individuals do not retain an attorney. Once, in a moment of extreme stupidity, I signed a non-disclosure form, as the business owner who was so nervous over his highly original tube circuitry that he could not speak to me otherwise. For about six months afterwards, I was nervous about including a tube differential input stage in this journal, so as not to bring the wrath of voracious lawyers upon me. Yes, that was his big trade secret, two triodes sharing a common cathode resistor. Yes, that circuit has appeared in this journal prior to his showing me it; it also appeared thousands of preexisting products, BW TVs, oscilloscopes, voltage meters, test equipment, and dozens of amplifiers, but would any of that necessarily hold off a hungry lawyer or persuade the jurors of the OJ trial? I can imagine a dramatic scene in which my attorney would bring in a large wheel barrel overflowing with old electronic textbooks and user manuals. “Your honor, each and everyone of these old books holds a drawing of a tube-based differential amplifier and we have seven more wheel barrels in the backroom, should this evidence prove inconclusive.” Unfortunately, I can also easily imagine the prosecuting attorney’s reply: “Well, ladies and gentlemen of the jury, I think we all know what the accused thinks of you all. He thinks you and all those fine, honest, hard-working folk who served on the OJ jury are idiots. He’s said as much in his journal. But we are not idiots are we? We know he didn’t sign a non-disclosure form with the writers of all those books in the wheel barrel. Of course, believe me when I say that I’m sure of this: all those writers are sure happy they didn’t sign one with him.” Beyond legal hassles, there is the general problem of constraining this journal’s scope, which is at odds with its mission to inform and reveal the art and science of tube circuit design, not to form an exclusive club or, even less, a confessional booth. For example, a perfectly good natured reader writes showing and describing his new circuit, but requests secrecy. Now, even if his circuit is not as newborn as he imagines nor as not obvious as thinks, I do not want him to feel betrayed, when I publish the circuit or a similar circuit in this journal; yep, it’s the differential circuit all over again. And what is imagined to be wildly bold and startlingly creative can be as trite and tiresome as using a 100k grid resistor instead of a 1M or 47k resistor. So rather than risk court time or hurt feelings or a constrained journal, please do not send me any schematics that you are not willing to share with your fellow readers. |
