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Old 11th August 2011, 11:33 PM   #2491
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Man,
u're really unique
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Old 12th August 2011, 07:14 AM   #2492
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Thanks, yes, i am in the flow. Before i publish the balanced transimpedance stage i will show a diagram how i would build the JLH stage today.
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Old 12th August 2011, 08:36 AM   #2493
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This is the circuit i would build. I call it the JLH Homage Pre-Pre. I let it run on plus-minus 7V but that is not critical. I upped the idle currents quite a bit. 11mA in the input stage and 35mA in the output stage. I lowered the impedance of the feedback network. JLH uses 12 to 20 Ohm where i use 3.3 Ohm so i loose at least the noise of a 8.7 Ohm resistor. I inserted transdiodes instead of Elcaps. That makes the circuit DC coupled but in theory it is 3dB more noisy. I solve that problem by using ultra low noise transistors on high current. The specimen i chose are from Renesas ( Hitachi ). I think they are not made any more but production stopped only a short time ago so i see them here and there in quantity for around 0.5€ a piece. If you have them you can use similar Ultra Low Rbb´transistors from Toshiba or ROHM but they are virtually extinct or very expensive. As an alternative you can use medium power transistors from Sanyo, Fairchild or Zetex. Some of them are exceptional. I assume the noise performance of this circuit as 0.5nV/qHz so it is around 4dB more quiet. Shunting the transdiodes with 10000uF caps will lower the noise by an additional 3dB. In fact you could then take out the transdiodes again.
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File Type: pdf JLH Homage Pre-Pre.TSC - TINA.pdf (9.2 KB, 115 views)
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Old 12th August 2011, 08:37 AM   #2494
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I made a small mistake. R11 should be lower. Something between 50 and 330 Ohm. It works well without too.
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Old 12th August 2011, 10:42 AM   #2495
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Here is the correct version.
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File Type: pdf JLH Homage Pre-Pre.TSC - TINA.pdf (9.4 KB, 65 views)
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Old 12th August 2011, 12:23 PM   #2496
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What is bias current flowing in MC cartridge?
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Old 12th August 2011, 01:28 PM   #2497
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I will measure that today. My precision meter had empty batteries but i got some now. With my normal meter it measures under 0.1mV. Theoretical it is 2,2mA / 150 = 15uA x 6 Ohm ( my Titan ) = 90uV in the original JLH design. In my design it is 22mA / 400 = 55uA x 6 = 330uV.
I know, that is a lot. You can look at it in two ways : the DC pushes the cartridge somewhat in one direction. That can be partly compensated by changing the tracking force. The other is, just relax and enjoy. What i know with many experiments i did with much more offset that the cartridge will not be harmed. If you can not life with that you can put in an offset cancelation. How that can be done i have shown before here. I will draw something for you that should work.
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Old 12th August 2011, 01:35 PM   #2498
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Here we go .....
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Old 12th August 2011, 01:41 PM   #2499
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The other solution is a cap, see post 2482. JLH uses a 100uF elcap with a 1uF film bypass.
To prevent distortion in the bass i would use 1000uF. I tried Pannasonic FM and FC with good results. Niichicon Muse KZ is even better and the Elna Silmic 2 i can not differentiate from a film cap. Even a Pannasonic FM is good because the levels are so low. Now you have some options.
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Old 12th August 2011, 01:51 PM   #2500
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I noticed the problem with the elcaps too, using the circuit as headphone amp the distortion increased quite considerably at low frequencies with the recommended value of 100uf as given by JLH. The cure was 5600uf elcaps instead.

Nice results from just 4 transistors.
Semelab in the UK produced a second source equivalent of the 1085 and compl, RS electronics still had some the last time I checked, hopefully semelab they will produce more.
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