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Old 15th July 2010, 11:17 PM   #1471
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Sorry, there is a mistake. R14 ( 10kOhm) and the RIAA should go to PIN3 ( +In) of the output Opamp.
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Old 15th July 2010, 11:26 PM   #1472
jason12 is offline jason12  United States
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Joachim,

I was more hopeful to see a symmetrical NPN/PNP BJT preamp with single ended (unbalanced) input like in post #814. My guess the one in post 814 will not work as it is.

I understand the main issue is the biasing of NPN/PNP pair which should be done in a way similar to Essex preamp.

Anyway, thanks.
Jason
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Old 16th July 2010, 12:02 AM   #1473
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You, are right, the circuits in post 814 where experimental and do not work the way i have drawn them. i do that sometimes to provoke reactions but i should not do that any more. Some people here are really serious about mistakes i make and i can understand that.
Here is a circuit you could like. It is a transimpedance circuit and has DC offset cancelation. It is extremely simple but performs very well. I was at a very influential German magazine and it compared well in sound to stages that cost 10.000,-€.
If you do not beleave me read Holger Barskes mini review on his website.
Attached Files
File Type: pdf Nobrainer 3.TSC - TINA.pdf (65.7 KB, 235 views)
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Old 17th July 2010, 06:11 PM   #1474
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I have not listened to a good MM cartridge in ages. The last time was one of the big Grados at Wittmann in Stuttgart in an unfamiliar system. Because a friend of mine undusted his Thorenz TD160 i had the chance to mount the top Clearaudio MM, Maestro Wood. It was not easy to adjust correctly because the Maestro has tapped holes and the Thorenz arm has another mounting scheme. I used small diameter screws to circumvent the problem. The sound i got with the Inductive RIAA was a surprise. A lot has happened in MM design it seems. It had more dynamics and resolution then i hoped for but still there was this imposible to describe homogenious sound that a good MM makes. This Clearaudio is certainly the best of it´s type i heard. One problem remains.
The motor of the Thorenz made a clicking noise that was also audible thruw the loadspeakers at low level. It is the typical cogging problem that older Thorenz have so i have to repair that. Otherwise the sound was very musical and entertaining. As linestage i used my Velleman 8020 that i had rebuild to an Aikido Buffer with very good success.
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File Type: jpg Thorenz TD160 Maestro Wood.jpg (59.6 KB, 744 views)
File Type: jpg Thorenz TD160.jpg (84.4 KB, 733 views)
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Old 18th July 2010, 02:40 AM   #1475
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During the course of this thread i reported about the difficulty to get the input DC offset to zero in a common base transimpedance ( or transresistance, i think this is the correct term ) circuit. Interestingly the first circuit i posted did not have that problem because it is ballanced. The problem comes because Ube is different in NPN and PNP bipolars at the same idle current and because the cartridge is connected to the emitters is seas the difference voltage to ground. Ube is a very weak function of idle current so even biasing the input transistors grossly different does not help much. Later i found several solutions, one is a floating supply like in the Rosi and one is a voltage divider from the bases to the emitters like in the Nobrainer stage. It ocured to me some time ago that there is another more elegant option if emitter resistors are present. I developed several versions and i show you now one of them. Emitter resistors have the disadvantage of some voltage noise added but you can make them quite small or take several transistors in parallel. The advantage of emitter resistors is that they provide some linearising current feedback that also lowers tolerances in the transistors so paralleling is easy without matching. You may now ask why i always come back to transresistance circuits when the problem can be solved with Fets to do a transconductance circuit. Well, it is that dramatic and dynamic sound that circuits like that can give provided the cartridge fits. The other advantage is that they can be used as I/U converters in DA converters. Yes, most of the circuits here i presented can be used in the analog stage of DA converters. The problems are nearly the same.
So here is the circuit. The potmeter can adjust the DC input offset to zero and can be substituted with a servo. The cascoded current mirrors give enormous gain so an All In One Go Phonostage is posible too. Second i show a circuit elaboration that already works on my bench. It is a BJT-FET cascode. I am shooting thruw the eye so to say. The task now is to find N and P channel Fets that have a high enough Gate-Source voltage at the desired current and do not add noise.
Attached Files
File Type: pdf Transconductance Inputstage 2.TSC - TINA.pdf (43.8 KB, 182 views)
File Type: pdf Common Base Fet Cascode.TSC - TINA.pdf (24.6 KB, 156 views)
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Old 18th July 2010, 06:21 AM   #1476
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Hi Joachim,
and the saga goes on!
Cascoding FET: provided, the current through your input Q's is under 2mA, the 2sk246/2sj103 would be a match. The datasheet doesn't contain a noise claim, but borbely uses those as cascoding FETs as well in input stages of phono amps.

Rüdiger
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Old 18th July 2010, 09:37 AM   #1477
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Thanks, Onvinyl for this tip. I need around 3.5mA, so i could maybe use two in parallel.
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Old 18th July 2010, 09:47 AM   #1478
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Studying the datasheet, forward transfer admittance is given as 4mS so noise can be calculated indirectly. I asume that they have a low 1/F frequency because they have been
designed for audio. Two of them seems to be just right.
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Old 21st July 2010, 10:57 AM   #1479
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Default JG-Self Linestage

A fellow DIYer has put the JG Self linestage on a PCB so this is one more circuit i posted here that can now be build easily. Actually it is a modular system with a mother board, two line boards, the stereo-mono switch and the potmeter.
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File Type: jpg JG Self Linestage Front.jpg (87.5 KB, 671 views)
File Type: jpg JG Self Linestage back.jpg (105.3 KB, 662 views)
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Old 21st July 2010, 02:02 PM   #1480
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Quote:
Originally Posted by Joachim Gerhard View Post
A fellow DIYer has put the JG Self linestage on a PCB so this is one more circuit i posted here that can now be build easily. Actually it is a modular system with a mother board, two line boards, the stereo-mono switch and the potmeter.
I like those 3D pcb's. Short connections, compact build. Nice!

jd
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