John Curl's Blowtorch preamplifier part II - Page 2317 - diyAudio
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Old 18th May 2012, 12:05 AM   #23161
1audio is offline 1audio  United States
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Make this simple-
Pick an appropriate alloy, wind it in a noninductive form, expose it to RF and heat it to the curie point. The additional heat from the signal passing orthagonally won't change the temperature because of the Curie threshold.

Or use a JJ array resistor, which operates very close to absolute zero but its very predictable.

(Talk about going off the rails. . . getting to -140 dB THD should be possible with none of this nonsense. May not be possible with tiny surface mount parts but its not difficult to work out solutions.)
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Old 18th May 2012, 12:07 AM   #23162
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Quote:
Originally Posted by simon7000 View Post
"If you build it they will come..."

But has SY compared a -120 db amp to a -140 db one?
And then they will love the -36dB one above all.
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Old 18th May 2012, 12:11 AM   #23163
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Quote:
Originally Posted by bcarso View Post
I think most of the time, unless the device temperature coefficient itself is a strong function of temperature, the notion that higher quiescent temperatures reduce the effects of signal-induced self-heating is a misconception, albeit intuitively plausible.

If one is working at very low temperatures where carrier freezeout starts to occur (say below 100K in conventionally-doped silicon) then it is a very different story. For example 77K (liquid nitrogen, hence very convenient as such things go) is a very poor temperature for operation of silicon JFETs.


Brad Wood
GASFET's do fine,IEEE Xplore - Abstract Page

20K noise temperature at 500MHz not bad.
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Old 18th May 2012, 12:39 AM   #23164
bcarso is offline bcarso  United States
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Quote:
Originally Posted by scott wurcer View Post
GASFET's do fine,IEEE Xplore - Abstract Page

20K noise temperature at 500MHz not bad.
Yes GaAs and even germanium do well, although their excess noise (as FETs) are not very good at low frequencies.

Nakamura (iirc) did a Ge JFET nuc sci preamp that worked pretty well at 4.2K, many many years ago.
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Old 18th May 2012, 07:36 AM   #23165
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Originally Posted by jacco vermeulen View Post
The $10K that Mr Sutherland charges for a twin case phono stage with low part count is more mind-boggling, imo.

+20 years ago, Avantgarde in Switzerland came out with an add-on battery PS for the MAS (Metaxas) preamp they distributed in Europe.
Bumped up the price of the pre considerably, and base retail wasn't exactly cheap to begin with. *

PS design has evolved in 20 years time, and batteries by no means deliver clean DC.
For a phono stage that has to stay under $1k retail, batteries may be a/the cheaper solution.

(* couple of years later, Mr Gassmann of Avantgarde, came out with an all-battery phono/pre/power amp combo under his own brand name. The powersupply of the power amp carried an automatic charging circuit, plus 8 Sonnenschein batteries of a size used for motorcycles. Total cost ~45K Swiss Francs in the early '90s. Afaig, Mr Gassmann still has the hots for batteries, his latest XA preamp, in a truly beautiful cast-alloy case, runs on can juice also)
Yes Fred did lot of work for this amps. I remember that every unit from Kostas Metaxas sounded slightly different and Fred Grassmann changed all the transistors before he delivered the amps.

The Avantgarde amps sounded quiet well but I never got involved into the music, the spark never jumped over. Anyway, Fred is a nice person to have good conversation. Is he still in business?
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Old 18th May 2012, 03:06 PM   #23166
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Demian,

I was looking for my results with DC bias to see if the 3rd harmonic changed, but did not find it. I do think I posted it way back on this thread.

But I did find one interesting test that I may not have shown before. 2 KHz & 50Hz IMD using the same everything except feedback resistor.

BTY if you know of any -140 db amplifiers I'd be curious. (-140 IM @ say 1KHz -30 and .03 Hz -3.)

ES
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File Type: jpg CC v MF 100 2K.JPG (384.2 KB, 179 views)

Last edited by simon7000; 18th May 2012 at 03:34 PM.
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Old 18th May 2012, 03:37 PM   #23167
jcx is online now jcx  United States
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pretty trivial at consumer line levels - 2 op amps in a multiloop

sub regulate the input op amp supplies, use modified integrator local feedback around the output (CFA) op amp and the input op amp only swings mV into 100s kOhm/pF driving the output op amp

it is easy to get to the point that common mode input device nonlinear currents are the biggest identifiable active device issue left - if you can use inverting mode or have low source Z you're done

and even Doug Self now shows how to treat the input current nonlinearity

have you seen this passive device distortion in large Bulk metal Foil R? - I didn't see any in common metal film with 10kHz+11kHz IMD test - maybe too high for the thermal time constants

Last edited by jcx; 18th May 2012 at 03:46 PM.
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Old 18th May 2012, 03:39 PM   #23168
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Are the yellow sidebands on 2kHz at +- 50Hz or +- 100Hz? If the latter, this would tie in with more yellow 150Hz but little 100Hz. CC gives more 3rd order.
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Old 18th May 2012, 03:55 PM   #23169
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Originally Posted by DF96 View Post
Are the yellow sidebands on 2kHz at +- 50Hz or +- 100Hz? If the latter, this would tie in with more yellow 150Hz but little 100Hz. CC gives more 3rd order.
If you expand the image fully you can see the 50, 100, 150 and 200 Hz sidebands!
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Old 18th May 2012, 03:57 PM   #23170
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As you can see from this 19 & 20 KHz IM plot, it is the low frequency modulation that makes the CC & MF perform significantly differently.
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