John Curl's Blowtorch preamplifier

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#3,341
Hog-out chassis (2)

Here's the top view of the machined casting.
 

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#3,342
GRollins said:
Scott,
Ah, but you're still assuming that the Greeks knew what cause and effect were in the case of lightning. They didn't. And neither did the Romans, the Saxons, the Druids, the Mongols, the Vikings, nor the Cossacks. That didn't stop them from acknowledging that lightning existed. Of course, a lot of utter hogwash was uttered over the centuries as people attempted to explain what lightning was and what caused it. The Greeks thought cause and effect were explained by invoking Zeus's wrath. Did that make it right? Of course not. But did lightning cease to exist simply because they had their explanation wrong? Of course not.
To this day, we still don't know everything there is to know about lightning. People launch rockets trailing wires into clouds in an attempt to get lightning to strike in front of test instruments. We're still learning things about lightning.
Odd effects exist. To say that absolutely everything is known about any given subject is the height of arrogance. Physics was quite famously declared to be 'done,' and people believed it to be so. Of course, that was right before the bottom fell out and Newtonian physics got shouldered aside by these little itty-bitty particles that no one had thought possible previously. And we're still learning things. Sometimes the discoveries were predicted ahead of time and the experimental results merely served as confirmation. Sometimes the predictions didn't work out. Sometimes serendipity intervened and the experimenter said, "That's funny..." in the best Asimovian sense.
Are there undiscovered distortion mechanisms in audio? I'd have to say yes. Are there counterintuitive things going on in audio? Definitely. After all, caps were proven to be different from one another. That certainly set some people back on their haunches. Absolute phase was proven to be audible. And along the way a lot of strands of spaghetti got thrown against the wall that didn't stick. Does that in any way invalidate the strands that did stick? No.
Oh, and Snagglepuss? He caught up with Practicalus just as he got to the bottom of the hill. They went into town, bought a bottle of wine, and spent the evening discussing the origin of species. They were laughed at, mocked, and criticized by their contemporaries...no surprise, I guess.

Grey
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Grey,

Your analogy is completely off. Of course people acknowledged lightling, even if they couldn't explain it: they actually SAW it and in some unfortunate occasions FELT it. One would be utterly stupid to deny its existence.

What you want us to do is to accept your audio anecdotes without any evidence that it actually exist, and often with strong indications that it goes against what we do know exist and can explain.

Completely different situation; so, nice story about history, but irrelevant for it's purpose.

Jan Didden
 
#3,344
janneman said:


Of course people acknowledged lightling, even if they couldn't explain it: they actually SAW it and in some unfortunate occasions FELT it. One would be utterly stupid to deny its existence.
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Indeed. You are close to my point, but not quite there yet.
Audio effects are there to be heard if only the listener would make the effort to "climb the hill" and hear them. But no, sadly, they allow others to tell them that there's nothing to be heard and they never make the effort to go to the hill. I could have written that part of the story, but it would have taken more time than I had available.
Yes, we are in agreement. It is pretty stupid to deny the existence of audible phenomena. Once you admit their existence, you can begin searching for proper explanations. Denying their existence only prolongs the process.

Grey

P.S.: I've gotten a surprising number of e-mails wanting to know where Practicalus and Skepticalus came from--what play, and so forth. They're not from any formal play. I made them up on the spot. The names were derived (I thought it was clear, but perhaps not) from the words practical and skeptical. The hill is, perhaps, a bit more murky, but it's a metaphor for making effort. Note that, at least for the purposes of this one-act play, Skepticalus is not seen making any effort--he arrives at his 'answer' based on what he's been told by others. Practicalus is seen to be making the effort. There are other things buried in the story at various depths and admittedly lightning isn't necessarily the best analogy, but I liked the idea that it at least invoked the use of electricity. I'm an author. This is what authors do. They use words to create a picture and tell a story. Sometimes the story has meanings beyond entertainment.
 
#3,345
Chuck, or the Charles Hansen of 'Audio Express' Thanks for the reminder of Ed Oxner's book. I'm pretty sure that I have zeroxed most of it, but if you find something extra special, I might just get my own copy for reference. Usually, when I have a real fet question, I just as Ed by phone and he gives me the best explanation possible. He is still alive and well in Texas and is still associated with Interfet and Siliconix.
This month, I got another book on fets, 'Noise in Electronic Devices and Systems' By M. J. Buckingham. Now that is one special book! Cost me $175 plus postage for a used paperback, but worth it. An associate of mine, a design consultant for Linear Systems also bought the book and we are reveling in it. Of course, what turns us on would bore many here, as it is usually concerned with neat and sneaky ways of getting more out of fets, than usually considered possible.
Thanks also for the casting example. This is one of our interests, also.
 
#3,346
john curl said:
I wonder if all quality IC's are the same also? Some cost more than others, but is it really worth paying a premium, when another manufacturer can sell something a few dollars cheaper? What would I look for? Scott, any answers?
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Well I have to admit that is the way it works for the vast majority of our business. No one pays a premium for performance that they can not quantify in a hard tangible repeatable sense. The people that buy amp A over amp B on intangibles is so far beyond six sigma that it just does not matter.
 
#3,347
GRollins said:



Indeed. You are close to my point, but not quite there yet.
Audio effects are there to be heard if only the listener would make the effort to "climb the hill" and hear them. But no, sadly, they allow others to tell them that there's nothing to be heard and they never make the effort to go to the hill. I could have written that part of the story, but it would have taken more time than I had available.
Yes, we are in agreement. It is pretty stupid to deny the existence of audible phenomena. Once you admit their existence, you can begin searching for proper explanations. Denying their existence only prolongs the process.

Grey

[smip]
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Grey,

Of course there are audible effects that exist. And there are effects that are not audible. What I protest is the expectation that we accept that *any* proposed effect is audible if we would only look for it.

No rational person would spend his time chasing everything that is thrown at him or her, only because it cannot be ruled out yet that it could exist. Any rational person would think whether a certain proposed effect can be reasonably expected to exist given experience and current knowledge. What I protest is the uncritical running around after *anything* that is being thrown up without some rational judgement. Based on experience and knowledge, I do not believe, for example, that Bybee stuff makes an audible difference so I won't chase after them.

Of course, once in a while you make a mistake and run after a ghost or you miss something that really is existing. I take that risk above uncritically chasing after *everything*. To the dismay of snake oil salesmen, but that's not really my problem.


Jan Didden
 
#3,349
actually the LT1028/1128 are the "industrial" part equivalent to the "audio" spec'd LT1115 and I have selected between the LT parts and the AD797 in a strain gage amp application - I found input current noise and 1/f corner frequency differences were significant with the source impedance and signal frequencies I was designing for.

The AD797would win in a higher gain at high frequency application due to its flexible external (de)compensation, but I use composite amplifiers with a cfa op amp adding loop gain in such applications so the input op amp choice relates more to the input noise specs with the expected source impedance
 
#3,351
john curl said:
Jan, why do you persist in questioning anything that we do or have knowledge of?
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John,

I hope I don't! What I do is ask 'why' if someone comes with the latest trick to significantly improve his/her system with an unlikely change. If someone tells me the color of the wire is audible, and when asked why, answers: "because I clearly heard it", I shrug.

I learned some interesting things from you in that discussion on input long tail pairs some time ago. That was a factual discussion with engineering arguments from you that made a lot of sense.

But then when you say that output coils are audible but cannot provide any arguments beyond "I clearly heard it, so you better accept it as The Truth (tm)" I'm disappointed. You're not just any old DIY-er John, you're John Curl! People look up to you for explanations and arguments, not "because I say so, and if you don't accept that, hey, tough luck". And if you don't know, I don't think anybody has a problem with "I don't really know, I'm not sure, possibly I'm wrong". Saying you don't know if you don't once in a while only makes one more convincing when you say that you're certain. But people very quickly see when one is tapdancing.

You're John Curl. Noblesse Oblige.

Jan Didden
 
#3,353
john curl said:
Which was better JCX? Noisewise.
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my choice wasn't relevant to audio apps-

our strain gage transducers use 350 and 700 Ohm bridges and we are looking at “DC” (most users collect <100 s data sets) -1KHz so both of these "low noise" op amps are marginal in that i_n is dominant over v_n and the mix of bridge impedances requires a compromise solution

my reading of the LT1028 data sheet specs edged out the AD797 - but I'd guess process variations for both companies could make it go either way

since we're not directly competing on the “DC” noise spec the cost to characterize parts on a lot by lot basis and wouldn’t be justified

I'm interested in the new OPA211 in the same application

if pressed for better DC performance the next step would be to go to AC bridge excitation - demodulation becoming "trivial" in software as DSP hardware is already used for filtering/decimation - but mechanical effects like creep and thermal drift limit transducer performance so pushing further for low “DC” noise isn’t too important in our applications
 
#3,354
Thanks, JCX, I did notice that the current noise was about 1/2 as much in the 1028 type product, yet the voltage noise was about the same. Offset is not too important to me, and the only other factor that really concerns me is the Iq of the output stage. I believe that both are seriously compromised, so I am putting a current source to -V on the output. Now, to optimize that.
 
#3,356
Unusually, both LT1028 and AD797 data sheets show the output stage bias current in odd corners of the data sheets - they both indicate 500uA is used in the output Qs

with 500uA bias 1mA Class A push-pull output is available

the LT1028 over-comp pin is used with a Cherry-style C around the output stage

the AD797 also has a unusual output stage error canceling scheme
 
#3,358
Well, that's really inside knowledge... I would use, as an educated guess, 1/10th or so of the short circuit current as external class-A bias, which is hopefully close to the beta and fT max but if it misses it's probably on the more benign left side of these curves.

- Klaus
 
#3,359
But Scott DESIGNED the AD797 IC. He can tell me, and it would help me optimize his part further. This can't be a secret, I should think. Better to ask the designer, than to speculate.
Just the NPN should be OK, unless the PNP is even a more linear and better device.
 
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