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Old 30th October 2012, 02:33 PM   #531
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Have you tried to shunt R15 ?
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Old 30th October 2012, 03:01 PM   #532
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Quote:
Originally Posted by MiiB View Post
Thanks.... I think, a good solution... if you make a cascoded Jfet CCS. you have Tera-Ohm impedance for the first more than 100 KHz then dropping with the capacitance of the cascode Jfet. but never lower than a couple of hundred Kohms.
MiiB Jfet CCS are the simplest solution for making a CCS but the jfet parameters are not very accurate. but is a possibility.
im sorry but Tera-Ohm impedance is not possible. even 1pf of parasitic capacitance would bring it down even at 1Hz. The cascode serves to stabilize the voltage across the ccs, but in this case is not necessary as the input voltage is 9mv rms maximum.

Have you already read the paper about ccs that ken newton post ? if not read it , there is a lot of good information.
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Old 30th October 2012, 06:37 PM   #533
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You may be right..., however my LT simulator says different.... I wonder if the models aren't sufficient even to calculate CCS impedance.... hmmm So much for for believing that..
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Old 30th October 2012, 06:51 PM   #534
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Default CCS Performance

I did some sims of various CCSs a while ago.
Here are some of the results.
Schematic followed by graphs.

First, LED based:
Attached Images
File Type: jpg Circuit.JPG (11.1 KB, 224 views)
File Type: jpg Regulation.jpg (24.0 KB, 223 views)
File Type: jpg Rejection.jpg (54.6 KB, 217 views)
File Type: jpg Thermal.jpg (28.0 KB, 214 views)
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Old 30th October 2012, 06:52 PM   #535
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Next, Ring of two:
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File Type: jpg Circuit.JPG (10.7 KB, 212 views)
File Type: jpg Regulation.jpg (25.9 KB, 41 views)
File Type: jpg Rejection.jpg (57.0 KB, 55 views)
File Type: jpg Thermal.jpg (26.8 KB, 55 views)
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Old 30th October 2012, 06:53 PM   #536
MiiB is offline MiiB  Denmark
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pls try to do a BF862 casoded Jfet CCS
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Old 30th October 2012, 06:53 PM   #537
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And last, TL431 based:
Attached Images
File Type: jpg Circuit.JPG (11.0 KB, 56 views)
File Type: jpg regulation.jpg (29.2 KB, 47 views)
File Type: jpg rejection.jpg (57.6 KB, 48 views)
File Type: jpg Thermal.jpg (31.2 KB, 47 views)
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Old 30th October 2012, 06:54 PM   #538
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Just some food for thought...
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Old 30th October 2012, 07:06 PM   #539
MiiB is offline MiiB  Denmark
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the TL431 has high impedance but starts loosing it after only 1K, but is stiller better at 100KHz than the two other... Can you make one cascoded Jfet... ??
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Old 30th October 2012, 07:24 PM   #540
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Quote:
Originally Posted by georgehifi View Post
Thanks Ken, so this is not an ideal circuit for a PCM1704 then? as to get 2v out, the loading for the dac would be too high at 2.35kohms? When the ideal should be much lower, around 10-100ohms. Is this correct? still learning about I/V's.

Cheers George
George, whether a 2.35k output impedance is too high or not will mostly depend on the total amount of shunt capacitance presented by the interconnect cable (typically, about 50pF per foot) plus that of the input of the next gain stage. The output impedance forms a low-pass filter in conjunction with that shunt capacitance.

Output impedance in itself does not say anything about the sound quality of a line level gain stage. For example, the Hovland HP-100, for example, was a very highly regarded tube linestage having a 2.4kohm output impedance.
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Last edited by Ken Newton; 30th October 2012 at 07:27 PM.
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