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Old 21st July 2011, 04:55 PM   #2401
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The Opamps should not affect noise much but the overload margin to drive the low impedance feedback network. One solution would be to give all Opamps a separate feedback resistor R26, R35. It could be anything from 100kOhm to 500kOhm i supose. Compensating caps C19, C20 could also be separated if that does not help.
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Old 22nd July 2011, 09:45 PM   #2402
hesener is offline hesener  Germany
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No progress today as I had some trouble with the external power supply, and tonight I renovated my LP12 that hadn't seen any service in 22 years apparently. Wanted to go full symmetric today with the FPS but, well, .......
More next week, thank you joachim for your help!
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Old 22nd July 2011, 11:55 PM   #2403
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We will make it work, thats for sure.
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Old 23rd July 2011, 01:02 PM   #2404
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Some time ago i presented a simple BJT/MOSFet stage that could be used for the second stage of a phono amp. MIIB and i where paying a bit around with it and the performance of that circuit was found to be very good considering the small component count. I build one circuit to test it in the lab. It worked just fine but i never listened to it because i did not build a stereo set. That has being done now and i am pleased with the result. The pre-pre section i use at the moment is based on an older design from 2 years ago. It is a parallel symmetric folded cascode. I had some difficulty at the beginning to DC couple that circuit. I managed now to do that, even without a servo.
How does it sound ? Silky sweet and liquid. Maybe not as squeaky clean and tight as my best OP stages but very easy on the ear. Sibilance are not a problem at all. I have not found a conclusive explanation but some stages have problems with vocal sibilance and some donīt. Many stages pronounce the sharp "SSS" too much and it disturbs me much during listening. Not this stage. I heard stages that are subjectively more extended in the stratospheric highs or it seems so at last. This stages highs are there too but very well integrated into a broad midrange so the attention shifts down into the vocal and lead instrument range. I think this can be a good thing for many systems that suffer from sharpness or anemia. I measured the speed of this stage and it is very fast so measurably there is no treble loss. The subjective impression of sweetness and low fatigue can even come from the high speed of that circuit. I can certainly understand listeners that prefer simple discrete circuits over certain elaborate OPamp stages.
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Old 23rd July 2011, 01:07 PM   #2405
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Here is a picture. I have of cause several ideas to improve the circuit so stay tuned.
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File Type: jpg Simple BJT-MOS Phono.jpg (84.9 KB, 359 views)
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Old 23rd July 2011, 02:18 PM   #2406
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When you watch the circuit a bit more closely it has some similarities with the symmetric Schlotzaur buffer so loading the input stage with a current mirror comes to mind. The mirror could also convenient set Vgs in the MOSFets. They differ quite a bit in this parameter.
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File Type: pdf Simple BJT-MosFet current mirror.TSC - TINA.pdf (10.7 KB, 94 views)
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Old 23rd July 2011, 02:47 PM   #2407
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Another possibility is to use a floating Fet differential input stage.
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File Type: pdf Simple JFet-MosFet current mirror.TSC - TINA.pdf (10.4 KB, 114 views)
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Old 24th July 2011, 08:52 PM   #2408
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I build the floating Fet version as the second stage of a phono stage. I have an offset problem i do not understand so i stop for today.
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Old 24th July 2011, 10:25 PM   #2409
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I felt a bit unsatisfied. I went back into the lab and there it was: the ground of the Pre-Pre was not connected to the second stage. Offset went down to less then 4V. The rest was possible to trim with the emitter resistors in the mirror. Ok, i floats around a bit. Therefore i use the output cap. A servo could be used of cause. Here are some pictures .... they do not upload i try again later
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Old 25th July 2011, 12:41 AM   #2410
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The circuit REALLY starts to fascinate me. Too bad. Where should i take all this time to fine tune it. What can also be done is a folded cascode as second stage. The circuit can then work non inverted. One other good thing with this solution is that the 2kOhm resistor that can be a source of noise is not used any more. The way i have drawn the circuit is suboptimal. The folded cascode can only swing so much voltage so there is some juggling to be done of how much dynamic range is available. I have of cause a better idea. I hope it does not get too much complicated.
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