|John Broskie's Guide to Tube Circuit Analysis & Design|
10 February 2009
Stimulus: It's Not What It Used To Be
I belong to that small group of cantankerous guys with voices hoarse from saying "I warned you" so many times. Actually, although I might be cantankerous, I haven’t done a lot of gloating (my wife and close friends may offer a different opinion, I admit), as I do not enjoy what I see happening, so I keep to my low profile. On the other hand, if you were lucky/unlucky enough to receive one of my e-mails, laden with graphs, predicting the current housing and economic downturn, count this as my “I told you so.”
But I must also come clean: while I predicted the dot.com and housing bubble bursts, I was wrong about the date, being one year too early in both cases. Now, being right too early is the same as being wrong, except that you can walk away still feeling a bit smug. Though, to be frank, even being smug is difficult these days.
The economy sours. Jobs disappear. Businesses close. Families lose their houses. Politicians party. Rohm Emmanuel, president Obama’s chief of staff, tells us "we cannot let this financial crisis go to waste." No, sadly, there is little chance that any opportunity to expand government will be lost. It was Robert Higgs, the gifted economist who wrote Crisis and Leviathan: Critical Episodes in the Growth of American Government, who clearly explicated how governments make use of all crises as excuses for them to grow in size and power, robbing us of not just our money, but our liberties, much like a boa constrictor taking advantage of its prey’s exhalations, squeezing tighter at every opportunity, until its meal is dead.
History is filled with examples and the only real arguments are on whether the ratchet must work in only one direction, or if governments can be shrunk in the absence of crises. A more interesting question, in my view, is whether in this age of media we inhabit, can we ever be crises-free. Just think back a few years ago, when few foresaw the coming meltdown, were we not pummeled with crises? From Ms Spears missing hair and panties to congressman Foley’s bawdy phone messages, when were we free from hysteria? Hasn't the sky always been falling? Remember, a good crisis need not be real, as the imagined ones are just as effective.
All of this leads us to president Obama’s stimulus package. I will spare you all the easy jokes about hope, audacity, and change; instead, I must admit that I am more shocked than disappointed by the enormity of it. It was just so naked, so bald, so obvious. By the way, that was “enormity” in the old dictionary sense of “1) The quality of passing all moral bounds; excessive wickedness or outrageousness. 2) A monstrous offense or evil; an outrage,” not the current meaning of being big, really big, really really big.
I used to work for the government and I will never forget the unspoken commandment, “If you have a chance to screw over some poor civilian, you have an obligation to do so.” So I am not surprised at finding some pork in the stimulus package, but by there being nothing but pork in it, including the lips, ears, and tail. (Where do we buy an “It’s the pork, stupid” bumper sticker?) The more I read about the fine details of this stimulus package, the more I began to worry perhaps “stimulus,” like “enormity,” no longer means what I had known it to signify. So I went looking in the dictionary and between “stimulants” and “sting” (ironically enough) I found “stimulus,” but the old meaning held constant. No doubt future editions will include the new meaning of “huge cash payouts to favored political allies and close friends.”
Okay, it is far too easy to be just nagging and cynical; it’s much harder to actually make sober suggestions. Here are my six off-the-top-of-my-head solutions to our current woes:
So if I am often right, but too soon, might all of this be too drastic, too much real change for anyone to audaciously hope for? Perhaps. But I do know that it would actually stimulate real economic growth and greatly improve the quality of life in the USA. Instead, however, I expect us to follow the Obama economic plan of being wrong and too late. Recessions can be painful, but they are also often necessary, as nothing else allows business to reboot once it is stuck in an unresponsive program, such as the railway investment dead end in the 19th century or the dot.com bubble in the 90s. If we just let the economy reset itself, things would improve in a year or two. Instead, we want the same craziness that led to the current fiasco to continue forever. No nation has ever triumphed by suing itself silly and having most of its people work for the government.
Wait a Minute...
Getting Electronic Parts
Of course, what makes things worse is that I am so picky about parts. I know that the HER108 rectifier is the one to get, while the UF4007 is the one to leave behind (we won't even mention the 1N4007). Another example is the 105°C capacitor I stock, the Nichicon UHE1C103MHDIf 10kµF/16V capacitor that comes in all the regulator kits. It is a high reliability type that boasts a 10,000Hr lifetime, not the usual 1,000hrs (and 85°C rating). If I were selling radar jamming kits, such careful devotion to to getting the best parts would make little sense—and my life would be much easier, as I can source a few million generic 1N4007s in a few minutes. I must mention that the hands down winner in speedy delivery has been Jameco.com. The only faster shipping I known of is Uline, as thier shipments not only arrive the next day, they arrive as early as 7am, but then they do not sell electronic parts.
By the way, if you do not use OctoPart.com, you should. This site will search through many more electronic part suppliers than you can remember. For example, I performed a search for 5687s and Octopart revealed that an industrial instrument supply house had 8 NOS RCA 5687s for only $6 each collecting dust for the last 30 years. Think outside the the Mouser/Digi-Key box.
Back to Power Boosters
As we can see from the chart, a 1024W amplifier only puts out 32 times more voltage than a 1W amplifier. Worse still, our ears do not perceive it as being 32 times more powerful. I would still welcome only a voltage or current rating on power amplifiers, as it would help flatten the exponential climb. In other words, no one can hear the difference between a 60W and a 64W amplifier, which a voltage rating of 21.9Vrms and 22.63Vrms, respectively, would better help to reveal.
The next step is to factor a loudspeaker’s SPL into the mix. If you double the output voltage, the SPL gains 6dB. For example, an 88dB efficient loudspeaker will put an SPL of 88dB at one meter with a 2.8Vrms input signal. Doubling the signal voltage to 5.6Vrms increases the SPL to 94dB and the wattage from the power amplifier goes from 1W to 4W. The following graph includes SPL.
Note how much power is required to get to 118dB. Also note how starting with a higher-efficiency loudspeaker, say 100dB, would require only a 64W power amplifier to reach the same 118dB SPL. On the other hand, an 82dB loudspeaker would need a 4096W power amplifier! (By the way, what is the limit to a loudspeaker’s efficiency? If I remember correctly, about 128dB SPL would equal unity power transfer.)
Now the interesting question is what happens if two 88dB loudspeakers were placed in series, effectively presenting a 16-ohm load to the amplifier, what would be the effective efficiency of the pair? If a solid-state or tube OTL power amplifier were used, each driver would see half of the 2.8Vrms signal, so each driver would lose 6dB in SPL, but at the same time we would have doubled the driver surface area, which would give rise to a +6dB boost; thus, we would end up where we started, 88dB for 2.8Vrms.
If both drivers are placed in parallel, each driver will still see the same 2.8Vrms, but the doubled surface area will deliver its +6dB boost, so the SPL will increase to 94dB. We didn’t just get something for nothing, as the current into the loudspeaker pair has doubled (4 ohms versus 8 ohms). Of course, if our amplifier can deliver the doubled current, we will gladly take the increase in SPL. Now what happens if we place four drivers in series/parallel, for a combined impedance of 8 ohms?
Each driver will see half the signal voltage, so each driver’s SPL goes down 6dB, but we increased the surface area by fourfold, so we gain +12dB of SPL, assuming that the drivers are close to each other and the frequencies reproduced are large relative to space between drivers. So, -6dB added to +12dB yields a final SPL increase of +6dB, so our normally 88dB SPL speakers will behave as 94dB cluster. In addition, the power amplifier delivers the same voltage and current into this cluster of drivers that it would into a single driver.
The next 8-ohm combination of drivers is nine 8-ohm drivers, with three rows of parallel atop each other, as 8/3 x 3 = 8.
Each driver will now get its third of the power amplifier output voltage, so each driver will lose 9.5dB of SPL, but with nine times the radiating surface area, we gain +19dB in SPL, so the final SPL for 2.8Vrms will be +97.5dB, with our 88dB drivers.
Okay, this is all interesting, but where are the tube power amplifiers with 2A3s or 300Bs? Where can I buy a “Where’s the tubes?” bumper sticker? We can now return to the idea of using a flea-powered tube amplifier with a big solid-state beast of an amplifier. If you are as old as I am, you will remember the huge fuss the Bose 901 loudspeakers created decades ago. Back then, when you told someone that you were an audiophile, they would more than likely instantly respond with, “Oh, you own Bose 901s!” I was never a fan, but then I was in the minority. Dr. Bose used exploding wires, if I remember correctly, to discern the ratio between direct and indirect sound from a point source, the imagined theoretically perfect loudspeaker polar dispersion pattern. His conclusion was that only 11 percent of the signal reaching our ears was direct, the rest coming from reflections. Thus, he placed one driver facing the listener and eight facing backwards.
Now, imagine a modern day copy of the famous 901, with the same driver placement, but with the front-firing driver getting its signal from our 2.5W flea-powered champ and the remaining eight drivers configured as a 4-ohm cluster, driven by a 100W solid-state brute of an amplifier. In other words, our 2.5W has effectively become a 200W amplifier (4-ohm load for solid-state amplifier double power output). But won’t all that stinky solid-state power just overwhelm the bejeweled glory from the tube amplifier’s output? Perhaps, but I doubt it, as the ear will give the first arriving sounds precedence over the latecomers. This just might work and today’s full-range drivers are vastly superior to those made 30 years ago.
Since we are getting whimsical here, I will mention that I would like to build a line-source dipole loudspeaker that held nine full-range drivers (and one 98dB horn tweeter), which would allow switching between 8 ohms or 72 ohms, with all the drivers in series. Who the heck would want a 72-ohm loudspeaker? I would, as I could then build a simple OTL amplifier that would welcome the high load impedance. And at the flick of a switch, the loudspeaker would return to 8 ohms, for easier selling or to bring over friends house to try with their solid-state monsters. By the way, 88dB would still equal 88dB with the 72-ohm configuration; but with the 8-ohm configuration, +9.5dB boost in SPL would result. On the other hand, our OTL amplifier would certainly swing much more voltage into the 72-ohm load than any solid-state power amplifier, so effectively the tube amplifier play plenty loud.
The Last Idea for Now
The two power transistors are turned off at idle, but when the OpAmp’s output voltage signal gets high enough they begin to conduct, greatly increasing the possible power delivery into the low-impedance headphones. At low signal levels, however, the transistors contribute nothing to the output signal. Think of them as turbo chargers. Now imagine a 1-ohm loudspeaker, such as the old Apogees. Such a loudspeaker will kill even the burliest of solid-state power amplifiers. Well, that actually isn’t true. All amplifiers can drive a 1-ohm load—they just cannot drive it very far. For example, if the power amplifier holds a 2A current limit, only 2Vpk will be possible into the 1-ohm speaker, which equals only 2W. But if we put in place some big power transistors, our 2A current swing could equal 25V of peak voltage swing into the 1-ohm load, which would equal 312W of power. Ideally, the transistors should reside in the 1-ohm loudspeaker’s cabinet, but such an arrangement would make hooking up the small power amplifier’s feedback all but impossible. And if the transistors fell outside the feedback loop, the distortion would be high, as the switching transition is a harsh one. Perhaps staggered arrays of MOSFETs would prove more effective at lowering distortion.
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