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12th July 2019, 11:50 PM  #641  
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Quote:
Last edited by syn08; 12th July 2019 at 11:52 PM. 

13th July 2019, 12:02 AM  #642 
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I just remembered that when I first started working I looked at a Keithley electrometer manual and saw noise figure re: 10**12 Ohms and thought WTF.
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13th July 2019, 08:22 AM  #643 
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Hans graph is it showing S/N. It’s showing ‘relative improvement in noise performance’ so it’s quite useful in that regard.
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13th July 2019, 10:15 AM  #644 
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But I guess it was shunted buy the electrode capacitance of a few pF?
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13th July 2019, 10:16 AM  #645  
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Quote:
Now everything together in one image, showing the noise in nV/rtHz for all shown combinations in collector current and Rs, enabling the calculation of S/N for a given Cart. I have no prejudice against S/N for a phono amp, it's a good indicator how noisy the amp is. When it is in dBA after Riaa, that magnitude gives a clear sign of how silent a phono amp is, reasonably matching your hearing perception. 65dBA is the bottom line, 75dBA is very good, and above doesn't bring much benefit because being way below your hearing limit at normal listening levels with the PU arm in the air. For a S/N figure for a flat MC Head amp, 7.9 dB should be subtracted from the above figures, so resp 57.1dB as bottom line and 67.1dB as very good. So for a 0.1mV 3R AT36E cart, looking at the image, noise at Ic=8mA is 0.28nV/rtHz This gives a flat S/N of 20*log(1e4/(141*0.28e9)) = 68.1dB flat or 76.0 dBA after Riaa and Aweighting. More than excellent. For a Dynavector DV23R with 0.2mV and 35R, according to the image, noise at 4mA is 0.85nV/rtHz. 20*log(2e4/(141*0.85e9)) = 64.4 dB flat or 72.3 dBA, which is still very good for this Cart. It will be hard if not impossible to find anything bettering this figure. Hans 

13th July 2019, 07:05 PM  #646 
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To see what the Formula from #541 does for the above examples with the AT36E and the Dynavector, two Carts at both ends of the spectrum, here again a calculation along that way:
S/N = 20*log[17.7e3*(Vcart/(Sqrt(RTI^2+Rs/60)] Vcart the output in mV @5cm/sec/1KHz, RTI the Amps equivalent input noise in nV/rtHz without Cart Rs the Carts source resistance in Ohm. For the AT with 0.1mV and 3R I used 8mA collector current, were noise with 0.1 Ohm, thus without Cart, is 0.19nV/rtHz. Calculation then gives 20*log[17.7e3(0.1/(sqrt(0.19^2+3/60)] = 75.6 dBA For the Dynavector, 0.2mV and 35R, I used 4mA, were noise without cart is 0.21nV/rtHz. 20*log[17.7e3(0.2/sqrt(0.21^2+35/60)] = 73 dBA Both calculations are within +/ 0.7dB, also covering the span for all other combinations that I exercised. Quite accurate IMO because none of the ultra complex behaviour of the "Duraglit" topology was taken into account. I further investigated Scotts comment for high impedance MC amps, where a resistor is used to terminate the Cart. A termination resistor has two effects, it reduces the output voltage in a linear way but also the combined resistor noise with a sqrt. So for a termination 10 times Rcart, the effect is only a worsening in S/N of 0.4dB and even for a termination of 4 times Rcart, the effect is just 1dB loss in S/N, but this only for a completely unrealistic noiseless amp. In practice the effect will be much smaller, reason to not include this in the formula. Another comment was to include input noise current, contributing to additional voltage noise. This is easy to inplement with the addition of In. When unknown, or for a Fet inut, simply set In to zero. This alters the formula into: S/N = 20*log[17.7e3*(Vcart/(Sqrt(RTI^2+Rs/60 +In*Rs^2)] Vcart the output in mV @5cm/sec/1KHz, RTI the Amps equivalent input noise in nV/rtHz without Cart Rs the Carts source resistance in Ohm. In the input current noise in nA S/N outcome is after Riaa and after Aweighting. IMO it is safe to assume that the outcome of this formula is accurate to within +0dB/2dB. Hans 
14th July 2019, 02:54 PM  #647 
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Just FYI, it's easy to make noiseless passive filters in SPICE by substituting a gm with its input connected to output as a 1/gm value resistor.
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14th July 2019, 03:02 PM  #648 
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You can also write "noiseless" after the resistor part value.
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14th July 2019, 03:07 PM  #649 
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Location: Belmont MA

Not everyone's SPICE has that feature?
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14th July 2019, 03:51 PM  #650 
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Location: St. Wendel / Saar, SouthWest Germany

Spice 3 does not seem to have it:
< SPICE Circuit Components > LTspice has it. ngspice has it, chapter 3.2.1 Resistors, page 71/637 < http://ngspice.sourceforge.net/docs/ngspicemanual.pdf > ADS has noise=yesno
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