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Old 14th August 2011, 01:47 AM   #2531
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Stage A and B can also be build with negative feedback. As drawn they have feedback too but local current feedback that some people call zero feedback. That is not true. A working stage with stable parameters can not be build without feedback. Feedforward or better error correction is an option but mathematically it is possible to express feedforward as a special case of feedback too. As i said many times, it is not so much the topology, it is the execution. That makes it so difficult to me to recommend "the best" stage. For beginners the Nobrainer is a very cool solution and when you want what is technically possible build the FPS from Pilgham. If that does not suit you follow the process and learn. All the others have already found nirvana.
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Old 14th August 2011, 11:48 AM   #2532
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After my little tutorial on parallel symmetric design i went back to the MPP Summer 2011 1.
I think this circuit deserves a PCB and also to be build and listened too. Maybe i can persuade Jan Didden to do the layout or somebody else from here can do it. I put now most of the component values and types into the circuit. The Fets that provide constant current to the Leds can also be substituted with resistors as long as 1 - 4mA flow in the Leds without much disadvantage. I have drawn only one Fet and Led per type but actually 4 of each are needed. I have not dimensioned the Gyrators. That can be a simple BJT cap multiplier like the one used by B.Vogel ( it works well, i tried it ), a Fet multiplier JC style, an Opamp multiplier, a mixture or simply an RC combination that can sound good too. The circuit could even be supplied from the DC direct because the mirrors have excellent PSU rejection.
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File Type: pdf MPP Summer 2011 FET AIOG 1.1.TSC - TINA.pdf (15.9 KB, 135 views)
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Old 14th August 2011, 12:28 PM   #2533
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The chance to get a fuctional example of your circuit makes me nervous.
Let's do it.
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Old 14th August 2011, 12:53 PM   #2534
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Yes, that would be good. It is very close to what i do commercially.
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Old 14th August 2011, 02:33 PM   #2535
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I have finished now building the JLH JG in the elcap version and it works just fine. I also found out why my DC coupled experiments failed. I had connected the collector of one output transistor in a wrong way so i was running that thing on single ended. It did amplify though and that confused me. Feel free to try the DC coupled versions if you wish. I will stop here for the time being. I got it to lower noise and i am happy with that.
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Old 14th August 2011, 03:15 PM   #2536
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I made some measurements on the JLH JG. Idle in the input stage is 6mA and 17mA in the output stage. Amplification factor is x30 with a 5.1 Ohm feedback resistor. The output transistors can drive 17mA x 300 Ohm = 5.1 V in class a so this circuit will not leave class a any time soon. Bandwidth is 500kHz with a 150pF compensation cap. 10kHz square wave
looks good. I use 2 x 9V blocks for supply. Under this conditions the voltage is plus-minus 6V after the 80 ohm resistors. DC offset at the input is 70uV measured with a precision meter, that is under my 100uV criterion. Here the rather high Hfe of the Hitachi transistors helps although i run the stage on 4 times the current of the original JLH. See some pictures. I could keep the size compact by using 3900uF/9V Pannasonic SMD elcaps. The caps measure 30mOhm impedance at 100kHz. When i started building at the age of 11 a 10.000uF cap with 35V had the size of a cola can. We have come quite far in the meantime.
Attached Images
File Type: jpg JLH JG 10kHz square.jpg (92.0 KB, 275 views)
File Type: jpg JLH JG up 2.jpg (89.1 KB, 264 views)
File Type: jpg JLH JG down 2.jpg (91.4 KB, 261 views)
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Old 14th August 2011, 06:33 PM   #2537
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I posted more then i wanted the last days. I am alone at home. The family is in vacation.
When i get lonesome i land on the web usually. Maybe too many posts for some that have lost the guiding line again. As i said many posts ago my intention is not so much to bring up a "best" recipe but show the process of how things come together and happen in my place. Including mistakes and dead ends. Anyway, i have to keep a promise. Show you the zero impedance balanced transimpedance stage. Actually i would call it a balanced complimentary compound stage. It was designed by Smedegaard when he was at Becker Automotive as an I/V converter in a DA converter. I have not seen it used though so it is somewhat obscure. Many I/V stages can be used as MC input stages. Malcolm Hawksford told me that 20 years ago and also the engineering challenge is not that different. Low noise, high speed, low distortion etc.
So please look at the concept. I will elaborate later.
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Old 14th August 2011, 08:23 PM   #2538
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J.G. Home Alone
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Old 14th August 2011, 09:04 PM   #2539
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The circuit post 2537 may not work the way i have drawn it. Take away the plus and minus signs ( thanks AS ) . How the input could be configured is open to discussion.
Maybe it is totally useless for our purpose but who knows. Does anybody have a working idea ? For your degustation, here is the original article.
http://www.linearaudio.nl/Miscellane...20aes%20pp.PDF
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Old 14th August 2011, 09:20 PM   #2540
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Thank you for this PDF file.

The circuit, Fig. 34 (p. 25) in this text, is a little bit different from what
was shown in post 2537, it has a summing function for the output of
two DA-converters. The outputs are offset by a single Ube drop each.

Fig. 24 (p. 20) is also interesting and has no input offset.

Both circuits are non inverting, the inverting inputs being grounded.

Last edited by as_audio; 14th August 2011 at 09:21 PM. Reason: .. retype
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