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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 27th October 2009, 12:54 AM   #1901
john curl is offline john curl  United States
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Ok, you cannot directly put a cap across a Zener reference and get much noise improvement. You have to add an additional R in series between the reference and the cap to get the cap to lower the self noise significantly. Start with 80 nV/rt Hz or so for 3 series regulatiors adding as to the square root of 3 multiplier and 50 nV/rt Hz, the specified noise of the Zener reference. At Zero Hz the output impedance of the filter will be 3-5K. Want to bet on it?
For everyone else, IF you add an RC filter then you screw up the DC regulation. No big deal, but it should be understood that you are not looking directly at the Zener reference once you add the filter, and its low Z is next to useless.
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Last edited by john curl; 27th October 2009 at 12:57 AM.
 
Old 27th October 2009, 01:22 AM   #1902
syn08 is offline syn08  Canada
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Originally Posted by john curl View Post
Ok, you cannot directly put a cap across a Zener reference and get much noise improvement. You have to add an additional R in series between the reference and the cap to get the cap to lower the self noise significantly. Start with 80 nV/rt Hz or so for 3 series regulatiors adding as to the square root of 3 multiplier and 50 nV/rt Hz, the specified noise of the Zener reference. At Zero Hz the output impedance of the filter will be 3-5K. Want to bet on it?
For everyone else, IF you add an RC filter then you screw up the DC regulation. No big deal, but it should be understood that you are not looking directly at the Zener reference once you add the filter, and its low Z is next to useless.
I'm up to the bet: how much is the filter output impedance to match the JFET Norton equivalent noise at whatever frequency you choose. You are wrong by an order of magnitude, at least. The total output impedance of the LM329 + filter will still be another order of magnitude lower than the JFET Norton.

But then, I have already mentioned that LM329 is not for you, the idea to put 3 x LM329 in series is indeed stupid. I understand that all you want is to reproduce the 30 years old Vendetta performance. You have zero interest in building a modern, better version, gain stage and power supply, which is again understandable. However, teaching and preaching that design in 2009 is like teaching students about the Archeropterix bone structure.

Last edited by syn08; 27th October 2009 at 01:46 AM.
 
Old 27th October 2009, 12:56 PM   #1903
john curl is offline john curl  United States
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I would like to point out that most electronic concepts go back a long way, often many, many decades. The Norton equivalent concept is one such 'old' concept.
With more modern IC's, some improvements have been noted. It is thus true that the LM329 voltage reference is about 10 times lower noise than a 'low noise' Zener available in the past.
However, I would maintain that using my 'Norton' approach, the typical noise of the Norton equivalent, consisting of a 'low noise' low Gm fet of 4 ma and a 5K resistor would be less and certainly, equal, worst case to 50nV/ rt Hz that the LM329 is rated at. With the 100uf filter cap added across the resistor, the noise performance improves considerably. Those who doubt me, check it out for yourself. Of course, I have the ADVANTAGE of merely changing the value of the resistor to get the required output voltage. This is not possible with the LM329, and more complex circuitry must be added to get variable voltage, and I DOUBT that the approach in the app note to get higher voltage is very quiet, as well. Can't everyone see that a reduction in the self noise of the Norton equivalent at 35 Hz is going to be 100 times lower than 50nV/rt Hz or .5nV/rt Hz? This is due to the filtering action of the cap and the resistor. If I am wrong, show me where my calculations are mistaken.
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Old 27th October 2009, 01:15 PM   #1904
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John Curl's Blowtorch preamplifier part II
Someone should be able to measure the noise on both. I'll do it when I come across an lm329.
 
Old 27th October 2009, 02:25 PM   #1905
dimitri is offline dimitri  United States
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Quote:
However, I would maintain that using my 'Norton' approach, the typical noise of the Norton equivalent, consisting of a 'low noise' low Gm fet of 4 ma and a 5K resistor would be less and certainly, equal, worst case to 50nV/ rt Hz that the LM329 is rated at.
Assume 2SK246 is a 'low noise' low Gm fet.
It has 3.3mA/V transconductance at 4mA, or 5k*3.3mA/V = 16 gain
3nV/sqrt of 2SK246 will give you 50nV/sqrt on the output plus the noise of 5k itself 9nV/sqrt. Total noise is 51nV/sqrt or 5uV in 10kHz BW
 
Old 27th October 2009, 02:49 PM   #1906
syn08 is offline syn08  Canada
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Originally Posted by john curl View Post
I would like to point out that most electronic concepts go back a long way, often many, many decades. The Norton equivalent concept is one such 'old' concept.
What is infuriating is that (a) you reject the concept of (super) regulator to feed a low noise amp and (b) you reject the idea to add some decent PSRR to the Vendetta gain stage (at least a cascode, as mr. Pass does in his XONO design).

Again, I understand your plan to change as little as possible in the Vendetta Research old design, but telling this is an optimal solution (and extending this particular solution as ideal, to other audio applications) is totally incorrect. If I have your permission I can post the entire Vendetta Research schematic, including those ugly paralleled JFETs in the power supply, for everybody to make an educated opinion. BTW, the second stage in your Vendetta Research has feedback, so it's only the input stage that is open loop. Otherwise, the idea to feed the RIAA network from a high impedance source is correct.

True, a decent regulator has feedback, but then again, only a poor designed regulator has overshoot, and getting 1milliohm output impedance up to 10MHz from a small parallel regulator is not a problem using modern components. Similarly, the noise can be made arbitrary low.

P.S. Dmitri's calculations are correct. J201 has though 6nV/rtHz at best, so you may double those numbers. And K246 is not at all cheap. Here you go.

Last edited by syn08; 27th October 2009 at 02:54 PM. Reason: P.S. added
 
Old 27th October 2009, 02:56 PM   #1907
simon7000 is offline simon7000  United States
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The noise from a Panasonic M series 1000 uf 16 V electrolytic capacitor biased to 10 volts is greater than 20 nv/sqrt(hz) from normal vibration. (.005 Gs)

It is among the quietest capacitors I have measured.

So using a capacitor to lower noise has it's limits.
 
Old 27th October 2009, 03:05 PM   #1908
syn08 is offline syn08  Canada
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The noise from a Panasonic M series 1000 uf 16 V electrolytic capacitor biased to 10 volts is greater than 20 nv/sqrt(hz) from normal vibration. (.005 Gs)

It is among the quietest capacitors I have measured.

So using a capacitor to lower noise has it's limits.
Agreed. That's another good reason to provide a decent PSRR to the low noise gain stages, then the pressure to build ultra low noise power supplies would be significantly reduced. In my 0.32nV/rtHz preamp, I live happily with around 100nV/rtHz power supply noise.
 
Old 27th October 2009, 03:16 PM   #1909
dimitri is offline dimitri  United States
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Quote:
getting 1milliohm output impedance up to 10MHz from a small parallel regulator is not a problem
DIP package has a 20 nH loop between corner pin 7 and pin 14, or 1.2 Ohm@10MHz
0805 cap package inductance 0.5 nH or 30 mOhm @10MHz

Yes, syn08, I omit 10^-3

Last edited by dimitri; 27th October 2009 at 03:38 PM.
 
Old 27th October 2009, 03:22 PM   #1910
syn08 is offline syn08  Canada
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Originally Posted by dimitri View Post
DIP package has a 20 nH loop between corner pin 7 and pin 14, or 1.2kOhm@10MHz
0805 cap package inductance 0.5 nH or 30 Ohm @10MHz
Force and sense wiring and proper screening. And SMD parts, of course. And that 0805 inductive reactance @10MHz would be of course 30 MILLI ohm. And 20nH @10MHz would be 1.2 ohm.

Last edited by syn08; 27th October 2009 at 03:28 PM.
 

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