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Simplistic NJFET RIAA
Simplistic NJFET RIAA
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Old 5th June 2014, 01:17 PM   #12811
Salas is offline Salas  Greece
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Simplistic NJFET RIAA
What is the lowest DC rail your assortment allows via VR2x with R3x at 9.1k?
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Old 5th June 2014, 06:21 PM   #12812
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I will check in the morning but the voltages I listed were with VR2 at the end click.
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Old 5th June 2014, 06:40 PM   #12813
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If you got the rail range for proper TP readings now then you need change nothing. MM arrangement won't drift as much as the MC anyway, so you aren't after much trimming.
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Old 6th June 2014, 10:28 AM   #12814
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Quote:
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What is the lowest DC rail your assortment allows via VR2x with R3x at 9.1k?
Lowest that I can go is 34.8v on one channel and 35.1v on the other.TP1/2 readings are 3.25v and 3.124v.
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Old 6th June 2014, 11:34 AM   #12815
Salas is offline Salas  Greece
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Obviously the IDSS of your Q3x Vref biasing JFETs falls somewhere between usual R3x value matches for providing balanced enough target trimming range.
Engage VR1 from this point on and try for 3.6V TP1/2. If it again won't reach you may use 8.2K for R3x pursuing extra DC Rail bottom range but you are not in unacceptable readings area already.
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Old 6th June 2014, 01:08 PM   #12816
Marra is offline Marra  United Kingdom
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I forgot to mention that altering VR1 makes very little differance to the TP1/2 reading.At least not enough to hit 3.6V.Will have a listen to it as I don't think I have any 8k2 resistors.
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Old 6th June 2014, 04:45 PM   #12817
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In MC VR1 would make more difference, in MM can be used for better matching mainly. By lifting one leg of R12 you can much extend its influence.

You made the 40dB with 91R R2 version I presume. That one uses the least bias current. For what cart model, and is it drifting comfortably little on TP1/2 readings after warm up?
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Old 6th June 2014, 07:32 PM   #12818
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Yes the MM with 91R R2 for use with a Decca Microscanner Gold.TP1/2 fluctuate very little unlike the 63db version I built previously.
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Old 7th June 2014, 01:48 PM   #12819
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Quote:
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Yes the MM with 91R R2 for use with a Decca Microscanner Gold.TP1/2 fluctuate very little unlike the 63db version I built previously.
Quintessentially British cart maker. Everything in it designed different than the norm.
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Old 7th June 2014, 02:02 PM   #12820
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So true.
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