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Old 25th July 2011, 01:37 AM   #2411
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I try again to upload the pictures....
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File Type: jpg Floating Fet second stage 1.jpg (95.9 KB, 404 views)
File Type: jpg Floating Fet Second Stage 2.jpg (86.0 KB, 403 views)
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Old 26th July 2011, 09:05 AM   #2412
hesener is offline hesener  Germany
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After re-reading the article once more, I found a serious stuffing error I made - well.... Will try to rework everything tonight and see what happens. Now your suggestions should make sense! Thanks
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Old 26th July 2011, 11:30 AM   #2413
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I hope you find the bug. Good luck.
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Old 26th July 2011, 10:06 PM   #2414
tvi is offline tvi  Australia
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Joachim, I was wondering if you set your pdf printer to landscape it might just spit out 1 page?
Don't know if you care?

Regards
James
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Old 27th July 2011, 02:11 AM   #2415
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Sometimes the circuit diagram appears at page two. The reason is that i remove component names and they appear on page one. Sorry for that inconvenience. I try to be more careful.
By the way, i build the floating Fet version and it has the magic. I do not know if it is the current mirror or the Fets or both. I also bypassed the NOS trustworthy Wima 10uF MKT with a 19nF KP so any of this changes may have brought the benefit.
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File Type: jpg Floating Fet Second Stage.jpg (72.1 KB, 342 views)
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Old 28th July 2011, 07:46 AM   #2416
hesener is offline hesener  Germany
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OK, found two more (ridiculous so I won't mention) errors and now the FPS is up and running. Adjusting the currents in the input stage took some time due to thermal settling, but it works!

The high power dissipation in the opamps is still there, however. Internet research and the other usual suspects did not reveal a good idea. The problem seems to be caused by the fact that the DC offset that any given opamp sees is unrelated to its own offset or corrective action, so it will "fight" it with all the DC gain it has available. That also explains why adjusting the offset is hopeless, since the trimpot position where it is correct is extremely small, and also drifting with time and temperature.

(Of course, capacitor coupling at the output would solve this, but this idea is so ridiculous for this circuit that I won't even mention it ;-)))

So, at this point I guess I will just live with this situation. As it stands, the opamps sit at about + or -2V or thereabouts, meaning the swing is reduced by 4Vpp, so the overload margin at this point is reduced to 16Vpp, a reduction of 2dB. Not that terrible.

Increasing the output resistors might help a little bit with reducing the power consumption, but the higher DC output voltage the opamps would settle to would further reduce the overload margin, I guess.

Any further ideas highly welcome!
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Old 28th July 2011, 07:53 AM   #2417
hesener is offline hesener  Germany
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Hi Joachim, looking at the picture of your lab table, I was wondering what you need the grey water tube for - cooling your opamps? ;-)))))
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Old 28th July 2011, 09:47 AM   #2418
FdW is offline FdW  Netherlands
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Rolling dough for baking cookies on the opams.
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Old 28th July 2011, 01:13 PM   #2419
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The tube is actually needed as a reflex tube in a desktop monitor i am developing at the moment.
Good that the FPS works. We experienced the same. The Opamps get quite warm but the stage works fine nevertheless. Yours is working now too so am quite eager to know how it sounds.
My solution would be to mix the opamps in such a way so that both channels are the same.
Actually the offset they are showing is not a bad thing in many ways. It drives the output stages into class a. Another solution would be to use opamps with less input offset but the same GBW ( gain - bandwidth ) like the OPA277 or OPA1461. I mentioned that option in my original article. We then decided on the NE55534A out of the main reason that the kit should not be too expensive. Besides getting warm it works very well in this application because it is inverting ( no common mode distortion ) and it has low current noise for a bipolar. For example the LME49710 has 4 times more current noise but a bit less voltage noise. P.S. : I also tried the OP27 successfully. This is also a precision op with low offset.
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Old 28th July 2011, 10:43 PM   #2420
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It must be over two years ago now. I fantasized with Salas about a really decent pre regulator for a phono stage. I am now in the good position that i am building a non compromise phono stage for a generous friend of mine. It will be called The Magnum Opus. Today i drilled the bottom board to fix the bigger components. The design is double mono. There are even 4 24VA spilt bobbin transformers in it. One for each voltage. The caps are Slit Foils, the diode bridges are Ixys Silicon Carbite. I use 4 in an arrangement that avoids direct current flow in the transformers. One of that bridges cost nearly 100,-$ so this is a real luxury. Common mode ckokes are fitted. This Lundahl LL2733 are not cheep ether. Snubber caps are Relcap Teflon. This guy is serious about sound. I used brass screws to fix the parts and i think the whole schebang came out nice, even a bit spectacular.
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File Type: jpg Phono Preregulator MO 1.jpg (76.9 KB, 281 views)
File Type: jpg Phono Preregulator MO 2.jpg (80.1 KB, 237 views)
File Type: jpg Phono Preregulator MO 3.jpg (83.6 KB, 161 views)
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