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Simplistic NJFET RIAA
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Old 28th August 2012, 06:38 PM   #7921
vdi_nenna is offline vdi_nenna  United States
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Is there a C3 in the 1.0.5 shunt power supply?
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Old 29th August 2012, 08:51 AM   #7922
RCruz is offline RCruz  Switzerland
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Please specify what is 1.0.5 shunt ?
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Old 29th August 2012, 08:53 AM   #7923
Salas is offline Salas  Greece
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The pdf one.
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Old 29th August 2012, 09:03 AM   #7924
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Quote:
Originally Posted by vdi_nenna View Post
Is there a C3 in the 1.0.5 shunt power supply?
No there is no C3, it was an optional designation for bypassing C2 that was left out for clarity.
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Old 29th August 2012, 09:50 AM   #7925
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Old 29th August 2012, 10:38 AM   #7926
Salas is offline Salas  Greece
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@ lohk

Here is what I came up with, regarding your request. The challenge is the second stage JFET to have some Vgs that can afford a bit of swing. I actually breadboarded a BF245C Fairchild and reconstructed its sim model to be sure. Since higher Vgs JFETs are short for gain most work is done by first stage and that dominates the noise spec to a good measure. With 47R degeneration there is enough Vgs to take 100mVpk-pk input before asymmetrical compression and clip eventually. Soft knee. With that degeneration being 3 times lower than a 150 Ohm capsule, the mic still sets the noise spec, so no real addition there beyond the ~1nVsqrt of K170's. It could marginally reach 40dB gain in that config which is what I saw in Jensen mic trafo specs for the active part along with 15K secondary termination in FET interface trafos. Some BF245s may show a bit more Yfs even but it won't change the gain drastically. Watch the channels gain to match if using X-Y mics anyway.
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Old 29th August 2012, 11:41 AM   #7927
lohk is offline lohk  Europe
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Ok, I get it. Thank you very much.
Can I reduce R2 even more, say, to 25Ohms? Or do I drastically reduce the headroom?
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Old 29th August 2012, 11:45 AM   #7928
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You end up with 42dB gain and 60mVpk-pk max input.
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Old 29th August 2012, 12:12 PM   #7929
lohk is offline lohk  Europe
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Ok, no, that wont work. With a transformer it is even worse.
The crucial thing with mic-pres is the undistorted headroom, this is my experience. Some of the better mics can put out up to 1V peak, so I have been told, but I did not measure that yet.
Well, I do have the parts (maybe not with the very sophisticated psu) in my box, so I should just try it. Thanks for the effort.
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Old 29th August 2012, 12:17 PM   #7930
Salas is offline Salas  Greece
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With a proper output spec capsule and gain structure it should be interesting. 1:1 trafo looks safer. Low noise JFETs don't give us much margin in this config unfortunately. If you will have some success let us know.
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