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Old 5th July 2011, 03:24 PM   #21
sonnya is offline sonnya  Denmark
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Here are the boards. I dont think i will be able to assemble them before next week :/

You can see an DIP8 and a SO-8 on one of the pictures for comparison.
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File Type: jpg DSC01822-2.JPG (161.1 KB, 481 views)

Last edited by sonnya; 5th July 2011 at 03:27 PM.
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Old 2nd August 2011, 12:59 PM   #22
sonnya is offline sonnya  Denmark
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Talking First samples made...



Now the first samples are made...

I am going to test them shortly...
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Old 2nd August 2011, 10:47 PM   #23
sonnya is offline sonnya  Denmark
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In the first test i discovered that the PMBFJ620 jfet has a vgs difference of ~440mV...

I need to find a fix for it...
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Old 2nd August 2011, 11:08 PM   #24
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Ok, anything else is like it should be.....

Sound is very very detailed even at low level...

Despite of the offset of 440mV is does not drift more than a mV...
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Old 3rd August 2011, 10:11 AM   #25
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that's impressive.
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Old 3rd August 2011, 10:19 AM   #26
sonnya is offline sonnya  Denmark
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I did try mount them in an Quad 34 running at +/- 7.5V. Works also...
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Old 3rd August 2011, 09:11 PM   #27
sonnya is offline sonnya  Denmark
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Okay, the problem with the offset is due to a poor vgs matching in the PMBFJ620 . At id=~3mA the is a difference in the package of 430mV..

Sadly enough there is no matching jfet with the same layout... Also no dual npn bjt with the same layout.

So they are for scrap... They work, but the have the hugh DC offset...

The fix for this module is a trimmer between the rails with a series resistor to either in- or in+.

Or add an external servo...
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Old 5th August 2011, 03:27 PM   #28
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How much is much DC? Can't you adjust it with R2/R3 (or maybe R5/R6)?

If you make a version with and one without the VAS cascode, it would be interesting to hear your oppinion on the sound of folded cascode vs cascoded folded cascode. I've tried it. It reduces distortion both in sim and in real life, but I find that it softens the sound for good and for bad.

Click the image to open in full size.
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Old 5th August 2011, 04:18 PM   #29
sonnya is offline sonnya  Denmark
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Hi Nelson, Thanks for showing interrest...

I would Maybee have expected less than 100mV vgs difference but it shows ~2V and 1.6V..

I am lucky that i did not buy a full reel of the pmbfj620 from NXP ...

I will try make a version with ssm2212 from analog instead of jfet's .. It has high gain of ~600 so offset current Will be small compared to bc8xx or that3xx

I Will certainly make it as a module but not that small..

They Will be mounted on a motherboard.. I Will try compare different version of the folded cascode.. And i Will let you know on the outcome
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Old 27th April 2013, 07:58 PM   #30
sonnya is offline sonnya  Denmark
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After a long period have had sparetime to test the modules.

The attached measurement show a well behaved THD.

No load, the 2. harmonic is the only present distortion. 0.003% or as low as my equipment goes.

into 220R load 3. is present at 1KHz and 3. and 4. harmonic is present at 10KHz.

The same circuit will be no different with an bipolar input (SSM2212 from analog devices).

More to follow.
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File Type: pdf fft1k noload jfetopampv10.pdf (36.7 KB, 25 views)
File Type: pdf fft1k 220r jfetopampv10.pdf (36.7 KB, 17 views)
File Type: pdf fft10k noload jfetopampv10.pdf (36.8 KB, 11 views)
File Type: pdf fft10k 220r jfetopampv10.pdf (36.8 KB, 20 views)
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