John Curl's Blowtorch preamplifier part II

Status
Not open for further replies.
Jump to Latest
#38,801
bcarso said:
I pulled out my copy of Sze, Physics of Semiconductor Devices, early this AM to see a bit more about Schottky and others, and in particular to make sure there weren't two of them (the shot noise one and the diode one). What was fascinating was the theory of the rectifying metal-semiconductor junction, and the references and some resemblance to thermionic emission. Sze is great by the way, although a bit out-of-date understandably --- but one of those "if you have only one book..." I suppose by now there may be a later edition that the second that I have.
Click to expand...
Wow...there was a second edition?? Mine's from '69

jn
 
#38,802
Waly said:
That would be the excess noise, otherwise 1/f noise is generated in any condensed matter system, that's what I was taught in the solid state physics course. Or perhaps it's the high 1/f corner frequency which is very high in tubes?
Click to expand...
Yes, it is excess in the sense that no one has satisfactorily modeled it as a physical mechanism, although there are perennial assaults on the fortress. Maybe some day. One physicist friend believed that 1/f phenomena were indeed limited in low-frequency extension, but the time constant would turn out to be the age of the universe 😀

And yes, in tubes the corner does seem to remain stubbornly high frequency, and still difficult to predict. I think there is a recent piece in Linear Audio (?) in which the author, after running down various things known, winds up saying you have to test and optimize the operating point for any specific tube sample to get the lowest noise. There are some remarks in Cherry and Hooper about the dependence being to some extent one of noise proportional to anode current, which may represent an uncomfortable tradeoff in a given circuit.

Another option that is unlikely anyone will fund, but was explored many years ago for other purposes: an optoelectronic cold cathode. I don't recall if the results of the experimental devices had reduced 1/f noise or not.
 
#38,804
Sorry, here is the next page:
 

Attachments

  • skov3.jpg
    skov3.jpg
    629.7 KB · Views: 265
#38,805
jneutron said:
Wow...there was a second edition?? Mine's from '69

jn
Click to expand...

My copy was about as well-hidden in a storage box as to make me think I had lost it somehow, and I had the first edition accessible, but yes, it finally was unearthed. The second edition dates from 1981: ISBN 0471056618

EDIT 1: Ah yes thanks Chris. I guess I had seen that third ed. in a search, as it was clearly a transitional one with the co-author. Sze, born in 1936, could well be graced with a collaborator now.

EDIT 2: Just ordered for about 77 bucks and shipping. We shall see. What is interesting sometimes is what gets left out from previous editions of things. It was disappointing that latest editions of Gray and Meyer (et al.) don't barely even acknowledge JFETs, for example. So one does not want to throw old editions away, many times.
 
Last edited:
#38,806
Straying from the well defined, one thing that vacuum valves have, and modern devices usually have not, is wasteful profligacy. Lots of heat, lots of electrons, "headroom" of another sort. JC recommends running his output devices at 30 or 50mA, when less than 5mA would drive anticipated loads - so who knows? We're complex pieces of mud.

Thanks,
Chris
 
#38,810
bcarso said:
Yes, it is excess in the sense that no one has satisfactorily modeled it as a physical mechanism, although there are perennial assaults on the fortress.
Click to expand...

This may lead to some confusion with regard how excess noise is used with regard to resistors. In that case, it is related to current, and some of the mechanisms involved are well understood (e.g. granular structure of carbon composites, impefections in metal films). The fact that resistor excess noise also has a 1/f character is just the way nature works. Better find a better term to describe the tube case.
 
#38,811
vacuphile said:
This may lead to some confusion with regard how excess noise is used with regard to resistors. In that case, it is related to current, and some of the mechanisms involved are well understood (e.g. granular structure of carbon composites, impefections in metal films). The fact that resistor excess noise also has a 1/f character is just the way nature works. Better find a better term to describe the tube case.
Click to expand...

Don't agree. the mechanisms are NOT well-understood. Merely waving hands about granularity and film imperfections doesn't explain spectra going down to lower and lower frequencies indefinitely, and saying that it is "just the way nature works" is no more of an explanation of the stochastics than if I said 1/f in tubes was due to porosities and chemical changes in the cathode material.

Another wonderful reference in the aforementioned compilation of Gupta's: Radeka's 1/|f| Noise in Physical Measurements. It also includes some bit of the discussion following Radeka's reading of the paper, which is very funny.
 
#38,812
SY said:
I paid under $200 for the pair I used, which compares very favorably to the cost of a ready-to-go 20dB gain stage (including power) with equivalent input noise density of <0.1nV/rtHz. That would take a whole ****-ton of low noise FETs...
Click to expand...

SY said:
I used Sowters, which are a pretty decent brand.
Click to expand...

That oughta be your lucky day, 'cause I just looked up my neighbours in Ipswitch and they would charge me 200 squid for a pair of 9570's (including VAT). That would be over $300 - about the price I would expect for a decent MC preamp.

Of course, that kind of money might not be an issue for a boutique fashionable tube MC preamp, selling in the 1000's range.
 
#38,816
scott wurcer said:
The 1/f nature of natural phenomena has been studied from many angles and this property is rather well accepted. I have measured exact compliance down to 11 micro-Hz (1 cycle per weekend) for bipolar current noise.
Click to expand...
Yes, it is truly ubiquitous and still baffling. My point is that a real mechanism in any particular case is elusive. And this is true whether or not we know how it may be affected by current flow, etc. So we can make it larger with more current (perhaps). That's a start, but not an explanation.
 
#38,820
bcarso said:
Yes, it is excess in the sense that no one has satisfactorily modeled it as a physical mechanism, although there are perennial assaults on the fortress. Maybe some day. One physicist friend believed that 1/f phenomena were indeed limited in low-frequency extension, but the time constant would turn out to be the age of the universe.
Click to expand...

1/f Noise?the flickering candle | EDN....interesting comments section.

Dan.
 
Status
Not open for further replies.
Top Bottom