John Curl's Blowtorch preamplifier part II

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#80,141
Germany's "Auto, Motor and Sport" magazine, their annual edition, quotes Bentley's kurb weight as 2,295 kilos in its lightest shape and form (with the V8 engine). The Mulsanne sedan is the heaviest at 2,685 kg kerb weight, this model also being the only Bentley model with only rear wheel drive, all other models in all forms have full four drive. It takes a lot of power to just move that behemoth from standstill. In VW's language, the abbreviation "Ap" means permanent four wheel drive.
 
#80,142

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#80,143
dvv said:
Germany's "Auto, Motor and Sport" magazine, their annual edition, quotes Bentley's kurb weight as 2,295 kilos in its lightest shape and form (with the V8 engine). The Mulsanne sedan is the heaviest at 2,685 kg kerb weight, this model also being the only Bentley model with only rear wheel drive, all other models in all forms have full four drive. It takes a lot of power to just move that behemoth from standstill. In VW's language, the abbreviation "Ap" means permanent four wheel drive.
Click to expand...

...that's a lot of inertia!
"Ye cannae change the laws of physics, Jim!"
 
#80,144
dvv said:
Germany's "Auto, Motor and Sport" magazine, their annual edition, quotes Bentley's kurb weight as 2,295 kilos in its lightest shape and form (with the V8 engine). The Mulsanne sedan is the heaviest at 2,685 kg kerb weight, this model also being the only Bentley model with only rear wheel drive, all other models in all forms have full four drive. It takes a lot of power to just move that behemoth from standstill. In VW's language, the abbreviation "Ap" means permanent four wheel drive.
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The car IS heavy and it IS a pig on gasoline. 650#/ft at 1600 rpm takes care of the weight issue. It is heavenly to drive... even on mountain roads. Now the ZR1 and 427 Vettes I had modified (like audio... anything can be made better than stock)..... are also heavenly to drive. Extreme performance in different ways. It's all good, IMO. A passion for excellence is all that is required. Well, some money has to be spent to get the performance up - thats true of any thing (opamps, too). When I didnt have much money.... I did all the work on the car myself over time as funds allowed..... but eventually got there and enjoyed every minute and penny.


THx-Richard
 
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#80,148
bcarso said:
Yes carrier freezeout is real and already important at 77 Kelvin. Some experimental physics people used to shine light from a "grain-of-wheat" incandescent bulb on bipolar transistors to get them to work at 4.2 K. Another nuclear sci guy made a helium-cooled charge preamp using Germanium JFETs, although it still had so much excess noise it wasn't particularly high performance. That was a long time ago.
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It would be interesting to know if the semi's were in a helium bath, or in a vacuum at 4.2K. Given the complete lack of heat capacity at 4.2K for anything other than helium, a source of photons in proximity running over a thousand kelvin would raise the semis quite a lot. The amount of MLI we need to keep 4.2K objects at 4.2K is quite amazing, as even a square cm of line of sight to either 77K or 300K will prevent us getting to 4.2.

The problem is doubled at 1.88K.

John
 
#80,149
#80,150
You're right (of course your experience here is duly noted), John, vacuum makes an insanely good insulator, which case the self-heating of the device is often more than you want and need a conductive path out.

Certainly a Ge CCD sitting on a L-He cold head wouldn't have much noise. Nor signal.

MLI? Pardon my ignorance, but what does that stand for?
 
#80,151
jneutron said:
It would be interesting to know if the semi's were in a helium bath, or in a vacuum at 4.2K. Given the complete lack of heat capacity at 4.2K for anything other than helium, a source of photons in proximity running over a thousand kelvin would raise the semis quite a lot. The amount of MLI we need to keep 4.2K objects at 4.2K is quite amazing, as even a square cm of line of sight to either 77K or 300K will prevent us getting to 4.2.

The problem is doubled at 1.88K.

John
Click to expand...

Probably LN would be used.



-RNM
 
#80,152
jneutron said:
It would be interesting to know if the semi's were in a helium bath, or in a vacuum at 4.2K. Given the complete lack of heat capacity at 4.2K for anything other than helium, a source of photons in proximity running over a thousand kelvin would raise the semis quite a lot. The amount of MLI we need to keep 4.2K objects at 4.2K is quite amazing, as even a square cm of line of sight to either 77K or 300K will prevent us getting to 4.2.

The problem is doubled at 1.88K.

John
Click to expand...

There have been some good results lately with GAS and other exotic stuff at cryo temperatures.

https://science.nrao.edu/facilities/cdl/low-noise-amplifiers
 
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#80,153
Derfnofred said:
You're right (of course your experience here is duly noted), John, vacuum makes an insanely good insulator, which case the self-heating of the device is often more than you want and need a conductive path out.

Certainly a Ge CCD sitting on a L-He cold head wouldn't have much noise. Nor signal.

MLI? Pardon my ignorance, but what does that stand for?
Click to expand...

We use a three prong insulation approach. Vacuum prevents convection, we have interesting methods of conduction prevention, and radiation warming from objects that are not at 4.2K.. Infrared radiation of the cryostat walls will warm up the guts so much that we can't afford even a square cm of the cold part seeing the 50K shield or the 300K cryostat. We block the infrared by using MultiLayer Insulation, a blanket made by alternating a half mil(IIRC) mylar sheet with aluminum vapor deposited reflection surface with a very thin gauzy sheet to break conduction. Typically we'll use a hundred layers or so from room to helium. During the space race, NASA developed MLI, but I recall they used gold vapor deposition. I'll always remember that commercial with the boiling beaker and the box floating inside, they boiled all the water off, then opened the box, there was cute little chick inside that was still alive. Right up there with the V-8 engine hovering over the sand running down the beach, or "whiplash driving school"...

John
 
#80,154
RNMarsh said:
Probably LN would be used.

-RNM
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For what? I don't think we can get LN2 to remain liquid at 4.2K.

scott wurcer said:
There have been some good results lately with GAS and other exotic stuff at cryo temperatures.

https://science.nrao.edu/facilities/cdl/low-noise-amplifiers
Click to expand...
Cool.
Right now, the only silicon I work with at 4.2K is a 3 inch dia bypass diode, and it's supposed to block current at the 4 to 6 volt level in the forward direction. It switches to it's on state quickly when it is required to carry a few kiloamps.

Unless it get's ""blowed up....sir""...
https://www.google.com/imgres?imgur...ZjMAhUFTCYKHXqBB4UQMwh4KDwwPA&iact=mrc&uact=8

John
 
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#80,155
jneutron said:
We use a three prong insulation approach. Vacuum prevents convection, we have interesting methods of conduction prevention, and radiation warming from objects that are not at 4.2K.. Infrared radiation of the cryostat walls will warm up the guts so much that we can't afford even a square cm of the cold part seeing the 50K shield or the 300K cryostat. We block the infrared by using MultiLayer Insulation, a blanket made by alternating a half mil(IIRC) mylar sheet with aluminum vapor deposited reflection surface with a very thin gauzy sheet to break conduction. Typically we'll use a hundred layers or so from room to helium. During the space race, NASA developed MLI, but I recall they used gold vapor deposition. I'll always remember that commercial with the boiling beaker and the box floating inside, they boiled all the water off, then opened the box, there was cute little chick inside that was still alive. Right up there with the V-8 engine hovering over the sand running down the beach, or "whiplash driving school"...

John
Click to expand...

Gotcha. Thanks!
 
#80,157
Yup, Samuel was complimentary in terms of overall noise, but not so much on distortion. At least compared to its price peers.

The ada4898 is the one that has my attention. Probably the best transimpedance (I/V) bipolar opamp right now. Opa161x OS right there too. Both sige. Both (probably) wildly overkill for most any other use, not that buying overkill is too expensive here compared to all the other bits of a preamp/DAC. $3.50ish/channel in a dual is hard to get upset with.

A paralleled opa1644 is probably the best gig for an opamp based riaa right now, but there could be some opamps in other folks lines that I missed (Scott, any recs from AD for a modern jfet opamp?).
 
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