I don't have the interest or patience to give a detailed explanation of 'motorboating' as applied to power amplifier design. However, motorboating is a low frequency stability problem that happens when there is too much gain and too much phase shift due to coupling caps, power supply caps, and transformers. It is an old concept, that has not been very applicable for many decades. That is why many here are ignorant of it.
Some here might just think it through and learn something new.
Some here might just think it through and learn something new.
john curl said:
Maybe you should too, and reconsider your claim that "Tubes almost always use 20dB or less" due to "Motorboating".
Advice like that doesn't help anyone learn something usefull.
Thank you for your interesting input. Motorboating is still the usual reason that 20dB or less negative feedback is used in conventional tube power amps.
Your article also mentions 20 dB, but was more vague as to whether the low frequency oscillation or the high frequency oscillation was the most difficult to address. At least, that is what it appears to say, at first glance.
Your article also mentions 20 dB, but was more vague as to whether the low frequency oscillation or the high frequency oscillation was the most difficult to address. At least, that is what it appears to say, at first glance.
john curl said:
Are you serious? The article isn’t even remotely vague with regards whether the low frequency oscillation or the high frequency oscillation is the most difficult to address and is almost entirely devoted to the latter.
I think I'll eat some Easter eggs and relax in front of the TV now.
I have a tube oscilloscope that is flat from DC to 80 MHz. Does it have feedback restrictions?
Tony said:
A common sentiment, but untrue. A tube like the 12AX7 (mu=100) will give you roughly the same gain per stage as solid state, with the exception of something like a current source used as an active load in a solid state circuit. At that point you can simply use a high voltage transistor to load the tube. Nothing to it.
Or you could just cram the thing full of gain stages in series the way solid state people do.
G.Kleinschmidt said:
When somebody gives “advice” in such cryptic terms, it’s probably because they don’t really know what they are talking about or are being intentionally disingenuous.
In valve power amplifiers it is the open loop bandwidth and phase limitations imposed by the output-coupling transformer that ultimately puts a practical limitation on the amount of global negative that can be applied before the amplifier becomes an oscillator (typically 20-30dB).
It is silly to infer that solid state amplifiers should therefore be designed with valve amplifier levels of global negative feedback due to the performance limitations of an obsolete technology.
Motor boating is a low frequency oscillation caused by the lousy power supply rejection of typical valve amplifier stages (often caused by supply rail fluctuations of the power output stage being fed back to the supply rail for the low power/level stages).
It is a potential problem regardless if the amplifier uses no (zero, nil, nada) global negative feedback or 30dB of global negative feedback.
Heavy RC decoupling of the low level (input stage / phase splitter / driver stage) plate supply rails avoids motor boating. Motorboating is a common fault that develops in valve power amplifiers in which the supply rail decoupling capacitors have dried up and gone high impedance with age (I’ve got one on my workbench ATM).
Applying high levels of global negative feedback to a valve amplifier can incur low frequency or subsonic oscillation if there are too many interacting low-frequency roll-offs in addition to that of the output transformer in the open loop response (each inter-stage RC coupling provides one high-pass pole). But this is simply solved by using large coupling capacitors and using direct coupling as much as possible, so that the high-pass pole of the output-coupling transformer is dominant.
For more info read this:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=118901
We used to have a member here (who shall remain nameless) who would go to great lengths to be a pain in the butt criticizing anything and anyone, then claim that it was everyone elses's fault for being so ignorant. You are cast in the same mold. Into every life a little rain must fall, and you are the rainstorm du jour. Eventually the sun will return and life will go back to being normal again.
--As most people other than yourself have realized by now, John "teaches" using the Socratic method, meaning that he asks questions in an attempt to get you to arrive at the answer on your own. I confess that I don't necessarily feel it's always the best way to go, but John is John and if I'm given the opportunity to pick his brains, I will, regardless of what form the answers come in.
--Clearly you have no clue about tubes. Here are three quick options to show just how lost you are:
1) You can use local feedback within the circuit and avoid the power transformer.
2) You can build an OTL tube amp and do without the transformer entirely.
3) Or you can simply choose to look at tube preamps, which don't use output transformers at all.
Any one of, or combination of these three points exposes your comments on tube circuits as nonsense.
As a practical example, my main tube amps have just shy of 150kHz bandwidth--and that's using EI output transformers. If I were to design them today, I'd use some of the output toroids now available and shoot for 250kHz bandwidth. Most people would regard either figure as respectable in a solid state amp--some would even go so far as to say that it's too much bandwidth.
PSRR in tube circuits is approached in the same way as in solid state circuits. To assert that tubes have some inherent limitation regarding power supplies that solid state does not is first rate foolishness. Here, let me make it easy for you--there's this thing called a current source. I know you're not familiar with them, but they're useful in certain circumstances. What you do is connect the grid to ground and choose a cathode resistor to define a given current. You take that current and use it to run a differential and suddenly your PSRR problems become a thing of the past. I'd like to point out that the same trick works in exactly the same way with a JFET; bipolars and MOSFETs have to be handled slightly differently, but they can make nifty current sources too...even though they aren't tubes.
As for motorboating, per se, you can make solid state circuits motorboat, too. I long ago went to isolating stages in my circuits, both tube and solid state. Viewed in a general manner, motorboating is just one specific example out of many of unwanted coupling between stages. Reduce the coupling between stages--be it a simple RC filter or something as elaborate as a regulated supply--and you improve the performance of the circuit overall.
I'd suggest that you spend a little less time pounding your chest proclaiming to all and sundry that you're an 'expert.' The content of your posts tends to indicate otherwise. The tone of your posts goes even further towards indicating that you're simply a pompous fellow with an inferiority complex.
Now run along and play. But play nicely.
Grey
Thanks Grey, and well said.
I REALLY do want many here to think for themselves. I used to to hate that when I was in school as well, but it is a good way to REALLY understand something, rather than just have it for the next test. Also, I am limited in energy to have to continually answer many of the same questions that I have already answered over the years, on the internet.
In fact, I have answered so many questions that Dimitri a contributer here, put together a edited compilation of my answers over the years that amounts to almost a book. Most of the answers are in there, and many of you know how to access it.
I REALLY do want many here to think for themselves. I used to to hate that when I was in school as well, but it is a good way to REALLY understand something, rather than just have it for the next test. Also, I am limited in energy to have to continually answer many of the same questions that I have already answered over the years, on the internet.
In fact, I have answered so many questions that Dimitri a contributer here, put together a edited compilation of my answers over the years that amounts to almost a book. Most of the answers are in there, and many of you know how to access it.
John, slightly OT, but since you brought it up...
I do not believe that the feedback limitation in most tube amps is because of motorboating- it is pretty trivial to put in stiff, low Z supplies, and the low frequency rolloff from the output transformer is first order. More commonly, it's because of the HF limitations of the output transformer. Williamson's original article is pretty explicit about this (although he made an error which actually did make his amp susceptible to motorboating).
The HF rolloff of an output transformer is relatively low in frequency and second order. So it's a challenge to use more than 20dB of feedback without enormous complication. And even then, it's pretty much impossible to maintain that feedback level through the top octave or two.
I do not believe that the feedback limitation in most tube amps is because of motorboating- it is pretty trivial to put in stiff, low Z supplies, and the low frequency rolloff from the output transformer is first order. More commonly, it's because of the HF limitations of the output transformer. Williamson's original article is pretty explicit about this (although he made an error which actually did make his amp susceptible to motorboating).
The HF rolloff of an output transformer is relatively low in frequency and second order. So it's a challenge to use more than 20dB of feedback without enormous complication. And even then, it's pretty much impossible to maintain that feedback level through the top octave or two.
Mr Curl, please rest assured that many here do appreciate the food for thought you care to continue to provide and the style you choose to provide it (same is true for Mr.Pass and some others). It really makes one think, and that is right because it sometimes seems to be "too far out" or too laconic, at times. Consensus of what is considered to be the "mainstream state of the art" doesn't help us too much...john curl said:
- Klaus
Please SY, you only confuse the situation. Look in 'Electronic and Radio Engineering' By F. Terman (1955) and note that EQUAL weight is given to motorboating as to high frequency stability considerations. Just because YOU know how to avoid it, does not mean that others in that time period had the knowledge and tools at their disposal. Please remember we are talking about a triode amp designed by Sid Smith in the 1950's, NOT one of your latest designs. In fact, I don't see why we could not extend the high frequency performance as well with a paralleled high freq only transformer, in parallel with a good audio transformer. Then YOU TOO could add more negative feedback, with all its 'advantages'. 😀
john curl said:
i remember back in the late 60's while connecting feedback to the OPT secondary for the first time, the amp motorboats when the phasing was wrong....no big deal there...you either got it right or else just reverse the leads...
😀
a 12AX7 is limited only to x100 max in gain and in practice with real circuits hardly attained, even with active loads, transistors otoh can do a lot more with just a resistor load.....
courage said:
Another chapter in Grey's boring episodes.....
courage,
If it comes down to a choice of keeping you happy and keeping John happy...it's a no-brainer. I've learned more from him than from you.
Or, as I've said elsewhere and elsewhen, if I write long posts, people complain because they aren't short. If I write short posts, people complain because they aren't long. Them's the breaks, I guess.
john curl said:
John has designed some pretty nifty stuff over the years. John is here at DIY. John is willing, within limits, to talk about his designs and the philosophy that leads to those designs. Given those factoids, what's the best possible use of John as a 'resource'...a free one, no less?
1) Mock him, argue, make fun of his every utterance, and generally make a butthole of yourself in an attempt to build yourself up by putting him down...
--or--
2) Listen to what he has to say.
You're under no obligation to use everything he suggests. If you don't like it, don't use it. If it sounds interesting, give it a try.
Me? I'm determined to tilt at the balanced phono stage windmill. John (and Nelson, for that matter) have suggested that it's more trouble than it's worth. Will I let John's position keep me from giving it a try? Hell, no! I may end up getting my nose bloodied when I hit the windmill, but it's something I want to attempt, nonetheless. Why? Because it's there. And I'll learn something either way.
An unsolicited bit of advice: Everybody has his own personality. Nelson will spontaneously tell you things with no prompting. Charles will tell you some, but requires priming the pump. John is less forthcoming, so I have been trying a sort of "interview" technique. By this I mean that I ask questions during lulls in the conversation. They may not even be questions that are foremost in my mind. I try to take an average of the way the last dozen posts are headed (disregarding the spiteful, useless ones, natch) and kinda prod things a little further in that direction.
You never know what might be useful down the road.
Tony said:
And I suppose that radio (just to take one example) had to wait for the development of the transistor before they could build those big ol' 100kW transmitters...?
Build a few real world circuits before you continue with your current line of reasoning. No, simulation doesn't count.
Grey
Well said again, Grey, and it’s not boring at all.
I think Mr. Curl is teaching us to use as little feedback as possible, either tube or SS.
I think Mr. Curl is teaching us to use as little feedback as possible, either tube or SS.
i learned about this from the articles written by prof. marshall w. leach of georgia tech.....
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