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Simplistic NJFET RIAA
Simplistic NJFET RIAA
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Old 27th August 2012, 10:08 AM   #7901
merlin el mago is offline merlin el mago
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Its an experiment I don't recommend it before I test it. Certainly fits best to low MC carts. There can be Jxxx subs for the European BF although noisier, but BF are everywhere. K369 give benefits in the 1/F region and for wide-band 2 are like using 4 K170s. But it can be tried with same IDSS K170s too. Q5 can be used or not to taste. Without Q5, C2 should be 15nF due to more Miller.
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Ok, here it is for classic 4XK170BL. You may relax the matching for the quads since they are degenerated enough to share current at ~0.75*IDSS and the total transconductance (~130mS) will be steady for 7.5-9mA samples at shared ID. But make sure that the sum total of the quads IDSS between channels is nearly the same. So the folded cascodes will be sparing the same drive current to R16 and same gain. No loop feedback here to match the gain if not thorough with JFETs picking in both stages. It should present ~15 Ohm input equivalent noise resistance, not bad. And some better input headroom, adequate for normal MC also. Pick a ~5mA BF245A for Q8 and circa 15mA BF245C for Q5.
Latest design share the possibility to use or note of Q5 & C2 15nF?
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Old 27th August 2012, 10:59 AM   #7902
RCruz is offline RCruz  Switzerland
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Originally Posted by merlin el mago View Post
Latest design share the possibility to use or note of Q5 & C2 15nF?
Hi Merlin

Using Q5, you reduce miller on second stage so you might need to add some capacitance to C2 (use 16n instead of 15n).

If you do not cascode second stage, use "normal" C2 values.
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Old 27th August 2012, 12:30 PM   #7903
lohk is offline lohk  Europe
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This is very long thread now, indeed, and very interesting, but not easy to read it all again.
Besides of the missing symmetrical input your preamp should be useful as a high gain/low impedance microphone preamp, isn't it?
If I let out the RIAA network, do I have to leave a resistor between the stages?
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Old 27th August 2012, 01:38 PM   #7904
merlin el mago is offline merlin el mago
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Hi Merlin

Using Q5, you reduce miller on second stage so you might need to add some capacitance to C2 (use 16n instead of 15n).

If you do not cascode second stage, use "normal" C2 values.
Thanks Ricardo, so it's the same like with 369s
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Old 27th August 2012, 02:33 PM   #7905
RCruz is offline RCruz  Switzerland
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Yes
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Old 27th August 2012, 05:45 PM   #7906
Salas is offline Salas  Greece
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Quote:
Originally Posted by lohk View Post
This is very long thread now, indeed, and very interesting, but not easy to read it all again.
Besides of the missing symmetrical input your preamp should be useful as a high gain/low impedance microphone preamp, isn't it?
If I let out the RIAA network, do I have to leave a resistor between the stages?
It has been used for NAB curve open reel head amplification once but not for microphone. Provide the input level, source impedance and gain needs so we can evaluate if and how it can fit that purpose.
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Old 27th August 2012, 05:47 PM   #7907
Salas is offline Salas  Greece
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As load resistor is refered to GND, this means current flows inversely in Q3.

Output impedance of first stage is 2k2r right ?
Yes.
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Old 27th August 2012, 09:18 PM   #7908
RCruz is offline RCruz  Switzerland
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I will start to work on it
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Old 27th August 2012, 10:14 PM   #7909
lohk is offline lohk  Europe
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Originally Posted by Salas View Post
It has been used for NAB curve open reel head amplification once but not for microphone. Provide the input level, source impedance and gain needs so we can evaluate if and how it can fit that purpose.
Microphones are very different in out level and impedance, but generally have about 100Ohm source impedance upwards, usually around 200 Ohms. Because level and sound pressure changes the gain needed varies from 50-60db to -say- 20-30db. Real life dynamics exceed the dynamics we know from music productions by far, so the headroom should be very high or the distortion characteristics smooth.

Usually mic preamps have either differential amps as inputs or transformers, also because they are usually balanced.
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Old 28th August 2012, 12:10 AM   #7910
Bksabath is offline Bksabath  United Kingdom
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Have a look at FDW Balanced Phono stage
I got that on the bench at present
Or you can try INA stile made up with LT1038
30dB gain is easy and not much problem with Jhonson noise
http://www.diyaudio.com/forums/analo...aa-preamp.html
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