John Curl's Blowtorch preamplifier part II

Status
Not open for further replies.
Jump to Latest
#76,062
gerhard said:
cascoded source follower? 😕

regards, Gerhard
Click to expand...
Yes, bootstrapped cascode, BF862 and ~PN4391. Mostly eliminates the variable voltage across the BF862, and with it various distortions, while also lowering Vds to about 4V and reducing dissipation and signal-induced dissipation shifts. The current source loading of the source and light loading by the op amp input means 862 Cgs is also rendered of little effect.

It eats up common-mode range, but for phono is not a concern.

Borbely has done this sort of cascoding with 2SK170s and 2SK246 parts.
 
#76,063
PMA said:
Please see the attached file, OPA1641 was measured.
Click to expand...

With a 1k loading, how much of what you're seeing is OPS and how much of that is common mode error? It's also one of the lower bandwidth parts on that list (not by such a huge margin as to explain the 20x difference, however).

Edit to add: John Caldwell (BB/TI App engineer) has said that the 1642 moved to a different process to help with common mode issues. Forgot the thread. Not sure if your part is before/after that change (not sure if that'd affect this load condition, however).

(And thanks for that data!)
 
Last edited:
#76,064
scott wurcer said:
gerhard,

Here's a nice paper on thermal drift issues and 1/f noise with some measured data. MAT03's at .5 Hz or so corner. This is the first time I saw a theory vs. data on the 1/f^4 spectra due to thermals even though I have observed similar things over the years.

http://arxiv.org/pdf/physics/0503012.pdf
Click to expand...
That's interesting. They didn't look very hard for lower-noise JFETs, and their Wilsonian mirror could be quieter (although its noise is largely nulled). But I like the discussion about the LF noise.
 
#76,067
bcarso said:
I was looking at the part for a budget MM phono pre application, and noted the 5.1nV/sq rt Hz noise density, the preceding part having been the OPA2134.

But I think I will allow a trifle higher parts count and use an AD797 or LME49990 preceded by a BF862 cascoded source follower and servo. The overall input voltage noise will still be acceptably small, although could be reduced by a more elaborate series feedback arrangement for the JFET. Things rapidly spiral into complexity, and I'd like frequency compensation to be simple for a change.
Click to expand...

Keep in mind the intrinsic noise of the cartridge and input network. How much below that do you need to be for an uncorrelated noise source?
 
#76,068
scott wurcer said:
gerhard,

Here's a nice paper on thermal drift issues and 1/f noise with some measured data. MAT03's at .5 Hz or so corner. This is the first time I saw a theory vs. data on the 1/f^4 spectra due to thermals even though I have observed similar things over the years.

http://arxiv.org/pdf/physics/0503012.pdf
Click to expand...

Yes, I have a similar design. The principle is around since Precision Monolitics times.

< https://picasaweb.google.com/lh/photo/AWKl7tzKFwX5wMVbdLFvrNMTjNZETYmyPJy0liipFm0?feat=directlink >

And somewhat cheaper:
< https://picasaweb.google.com/lh/photo/Bz6NkIibR7_yQyuKdwpPzNMTjNZETYmyPJy0liipFm0?feat=directlink >

Rubiola is a big shot in the time/frequency scene. His website is Enrico Rubiola home page.
Lots of interesting stuff. His book is excellent, also.

regards, Gerhard

ps: does anybody still make these nice plated-through boards with
ground, formerly Vero Powerplane boards?
Those from Roth are not plated-tru and are un-tinned.
 
Last edited:
#76,071
SY said:
Keep in mind the intrinsic noise of the cartridge and input network. How much below that do you need to be for an uncorrelated noise source?
Click to expand...
Essentially no input termination noise using the cooled termination approach, so the overall noise will be dominated by thermal cartridge noise, which will be nice. I'd share the schematic, but this is for a client (and indirectly two of them in fact) and they would be upset.
 
#76,073
Derfnofred said:
Edit to add: John Caldwell (BB/TI App engineer) has said that the 1642 moved to a different process to help with common mode issues. Forgot the thread. Not sure if your part is before/after that change (not sure if that'd affect this load condition, however).
Click to expand...
If the JFETs had a lower output conductance it would help, so perhaps that is part of the process change.

PS: Mouser shows the LME49990 as at an imminent end of life. But TI doesn't mention this on the datasheet. Anyone know what is up?
 
#76,074
bcarso said:
That's interesting. They didn't look very hard for lower-noise JFETs, and their Wilsonian mirror could be quieter (although its noise is largely nulled). But I like the discussion about the LF noise.
Click to expand...

From the data sheet, I suppose the 1/f frequency is 99.5 Hz or so 🙂
Not really interesting when you are into 1/f.

JFET manufacturers seem to abhor data sheets. Yes, that's all.

regards, Gerhard
 

Attachments

  • IF3902.png
    IF3902.png
    239.9 KB · Views: 260
#76,075
gerhard said:
From the data sheet, I suppose the 1/f frequency is 99.5 Hz or so 🙂
Not really interesting when you are into 1/f.

JFET manufacturers seem to abhor data sheets. Yes, that's all.

regards, Gerhard
Click to expand...
I seem to recall that Danyuk measured a rather lower corner for the BF862, but I would have to dig out the article he did to verify this. But you are right, specs on FETs are meager.
 
Status
Not open for further replies.
Top Bottom