John Curl's Blowtorch preamplifier

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#3,524
john curl said:
Did you now?😀
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i found his writings in the AUDIO magazine in the 80's, i even built a lot of his "doubled barreled amps"....

i do hope he posts here....


@ grey,

i hope you don't get offended but there is another poster here who posts longer than you sometimes, PRR, his posts are full of substance, reading his is a joy and is very educational....not at all confrontational...
 
#3,532
GRollins said:
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.
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Oh spare me the boring diatribe. I'm not going to great lengths to criticise anyone. I'm pointing out the fact that the claim made with respect to global negative feedback and motorboaing was wrong.
It is a claim that requires a bit of a detailed technical discussion to expain why it is false.


GRollins said:
--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..[/B]
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John can impart knowledge any way he sees fit. When he makes claims that are false there is a possibility that someone may call him up on it. Why on earth his fan boys find this so offensive is truly beyond me.


GRollins said:
--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..[/B]
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LOL! I'm pretty sure I know a hell of a lot more about tube amplifier design than you do.
Your points 1-3 three are completely irrelevant.
My comments were explicitly concerned about valve power amplifiers that have output coupling transformers within the global feedback loop, as it is these amplifiers which have rather tight limitations on the amount of global negative feedback that ca be applied, and it is these amplifiers which were under discussion (20dB quoted). Please try reading for comprehension.
The claim was made that the global feedback limitations of such amplifiers was due to "motorboating". This claim is false. I explained why and I provided an indepth technical article which support my point of contention.


GRollins said:
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...[/B]
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So what?


GRollins said:
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..[/B]
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What on earth are you talking about. I'm not familiar with a current source? Are you serious?
The vast majority of valve amplifiers ever made (regardless if they use LTP inputs with a current source in the tail or other elaborate methods employing current sources or sinks) have at least one stage that is capacitively coupled to the next stage from a plate resistor referenced to a positive supply rail. This gives little power supply rejection and is the potential source for motor boating in common valve amplifier designs.


Originally posted by GRollins 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...[/B]
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Of course motorboating can be a potential issue in some soild state designs. I didn't say otherwise and you "point' here is still completely irrelevant.


Originally posted by GRollins 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.
[/B]
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That really is pathetic and incredibly ironic. Please don't project your frailties onto me.
Closing this thread, I’ll just point out that in you condescending post full of personal insults, you really haven’t added anything of merit to the point of contention under discussion, or expanded upon any of the technical issues I detailed or those provided in the technical article I provided.
 
#3,533
john curl said:
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'. 😀
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Back in the 50's the audio engineering fraternity did indeed have the knowledge and tools at their disposal to build valve amplifiers with relatively high levels of global negative feedback.
I can scan for you a number of articles from the era detailing designs with levels of feedback well above 20dB and bandwidth’s over 100kHz - the principal restriction to the amount of GNFB is almost always the HF response of the transformer – not motorboating, which is easily addressed with attention to PSR and supply rail decoupling.
The low frequency response of the transformer can be an issue in some circumstances (I have a rather high global NFB Mullard design in which the open loop LF response is deliberately tailored to increase the low frequency phase margin, but this type of low frequency stability concern is a distinctly different one to motorboating, which is oscillation incurred by feedback via a modulated powersupply.
 
#3,537
john curl said:
Show me some power amp examples, please.
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SY said:
Krohn-Hite comes to mind. Futterman also, but no OPT.

At line level, there's the Philbrick op-amps.
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Some of the Mullard power amplifier designs from the late 50's fit the criterion and they all use output coupling transformers.
Preamplifiers (especially RIAA) used gobs of GNFB.

Details of Mullard’s 1959 20 W power amplifier, from chapter 5 of Circuits For Audio Amplifiers:

Closed loop bandwidth 100kHz
GNFB (loop gain) – 30dB at 100Hz, rolled off to 24dB at 20kHz
Graph attached below

Note that this open loop response corresponds to a unity loop gain of less than 200kHz, which (contrary to what some may say) is absolutely blown out of the water by moden solid state amplifiers, with unity loop gain frequencies typically between 1 and 2 MHz. As far as bandwidth is concerned, even with a modern torodial coupling transformer, a valve amplifier still won't match even a mundane solidstate design using low fT transistors in the output stage.
 

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#3,538
I give up, you win. I don't know what you have won, except to confuse a number of other people, but let us move on. What do I care? I have NEVER designed a tube power amp in my lifetime, even though I have 7 on hand. I was just trying to help others understand one of the differences between solid-state and tube amps and why the OP AMP concept became popular.
 
#3,540
john curl said:
I give up, you win. I don't know what you have won, except to confuse a number of other people, but let us move on. What do I care? I have NEVER designed a tube power amp in my lifetime, even though I have 7 on hand. I was just trying to help others understand one of the differences between solid-state and tube amps and why the OP AMP concept became popular.
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I wasn’t trying to "win" John, just attempting to back up my argument with factual data. You know, if you were more interested in discussing technical points of contention (and you can hardly expect people to always agree with what you say) in more of an impersonal, matter-of-fact manner instead of constantly evading, complaining and trying to “win”, threads like these might be a less prone to going nowhere.......
 
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