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Old 17th July 2003, 01:44 AM   #1
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Question Help with ccs

I started this thread in the chip amp section, asking for help designing a constant current source for Joe Rasmussen's tube gainclone project. Maybe there will be more interest here.

I read this article by Nelson Pass which describes some current source designs. I was thinking about using the mosfet version, with a 1N936 temperature compensated zener (since I happen to have some on hand).

Joe's circuit is attached for reference.

Any thoughts?
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Old 17th July 2003, 07:36 AM   #2
moamps is offline moamps  Croatia
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Hi,
why in solid state forum?
Isn't Tubes better ?

My suggestion
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Old 17th July 2003, 01:48 PM   #3
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Quote:
Originally posted by moamps
Hi,
why in solid state forum?
Isn't Tubes better ?
I put it in solid state because I was looking for a solid state current source. But I guess you're right; the tube forum probably would have been a better choice. Maybe I should have this thread moved.

A (probably stupid) question about your ccs suggestion: Does it depend on regulated supplies, since it has no voltage reference?
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Old 17th July 2003, 02:11 PM   #4
moamps is offline moamps  Croatia
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Hi,

Id is constant for Vds higher than ca 8V (I choose -Vcc=24V). Noise figure is here very important, be careful with zeners.

Regards
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Old 17th July 2003, 02:56 PM   #5
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Okay, it was a stupid question, since this is a JFET ccs, so it doesn't need a voltage reference. I guess this is the quietest way to go. I notice that you recommend a higher B+ voltage, too.
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Old 17th July 2003, 03:43 PM   #6
moamps is offline moamps  Croatia
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Quote:
Originally posted by Sparhawk
Okay, it was a stupid question, since this is a JFET ccs, so it doesn't need a voltage reference. I guess this is the quietest way to go. I notice that you recommend a higher B+ voltage, too.

Hi,

Only 35V for +B voltage is minimum, IMHO. For my taste, Va=ca +100V, and Ia=5-8mA are optimum values for this application.


Your Q are OK, don't worry.
(I have one stupid Q; mod's, anyone, help; how can I place more than one pic in one post??)

Regards
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Old 17th July 2003, 04:07 PM   #7
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Maybe I'll try a higher B+ voltage, then, assuming I can find an appropriate transformer.


Quote:
Originally posted by moamps
(I have one stupid Q; mod's, anyone, help; how can I place more than one pic in one post??)
You can use the IMG tag to post multiple pics, but in that case you're not actually uploading the pics with the post, so they must be accessible via a URL (ie, on a web server).
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Old 17th July 2003, 05:56 PM   #8
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Quote:
Originally posted by moamps
(I have one stupid Q; mod's, anyone, help; how can I place more than one pic in one post??)
not stupid... you can only attach a single picture, but if the picture is already out in webspace you can link to it and show multiple pics in a single post.

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Old 17th July 2003, 06:58 PM   #9
moamps is offline moamps  Croatia
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Hi,
thank you both for answers about posting pics. Here is one interesting PS schematic (voltage tripler) for getting high +B from 24Vac ct source. By adding -24V regulator (7924 or so), whole PS for HGC-CS can be implemented easily (just a thought).
You can find out more at: www.tubecad.com/march2001/page24.html

Regards
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Old 17th July 2003, 06:59 PM   #10
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Hi,

Moamps,

Would R2 be necessary? Or did you put it there to protect the Fet against turn-on transients?

From looking at the BF245 datasheet it seems to me the BF245B is the better candidate for the job or I'm I mistaken?

Another silly question showing my ignorance of all things solid state: are yuo regulating the -24VDC supply with something like an 7924 or similar?

EDIT: I just noticed you answered the last Q already, thks.

TIA,
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