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Richard Lee's Ultra low Noise MC Head Amp
Richard Lee's Ultra low Noise MC Head Amp
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Old 12th July 2019, 11:50 PM   #641
syn08 is offline syn08  Canada
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Quote:
Originally Posted by Hans Polak View Post
I think I may not have explained it well enough.
Output signal or gain depends on Ic, that's correct.
But in the noise calculation I have divided the output noise by the gain, to get the circuits input noise independent of gain.
I could have adjusted gain in all situations to the same amount by changing the output resistor, but for the few situations I tried to do this, it had marginally effect on the input noise, so I left the output resistor the same for all cases.

If you are convinced that this could introduce a significant error, I'm willing to redo the graphs.

But now coming back to the question concerning relation between Y axis and S/N.
For a given Cart with a certain certain Rs, the graph shows a relative noise change of X dB when changing Ic.
The S/N for this given Cart will then change by -X dB for this change in Ic.


As mentioned as an example, when going with a 40 Ohm Cart from 5 mA to 20 mA, S/N will become 0.25 dB worse independent of Vout at 5cm/sec@1KHz.
When for a 1 Ohm Cart going from 2.77mA to 12 mA, S/N will improve by 3.2dB again inpendent of Vout at 5cm/sec@1KHz.
I'll think about, not sure why you made it so complicated when the textbook S/N ratio definition is rather straightforward. But then something that strikes me is that the S/N ratio fundamentally depends on the signal level. "S/N ratio of X dB" makes sense only if you add "ref. 0.4mV input". This is one reason why IMO S/N ratio is not a good metric for audio, and I just shared a few posts above my opinion that for the case of a MC head amp this S/N metric is straight useless and misleading.

Last edited by syn08; 12th July 2019 at 11:52 PM.
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Old 13th July 2019, 12:02 AM   #642
scott wurcer is online now scott wurcer  United States
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Quote:
Originally Posted by syn08 View Post
I just shared a few posts above my opinion that for the case of a MC head amp this S/N metric is straight useless and misleading.
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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Old 13th July 2019, 08:22 AM   #643
Bonsai is offline Bonsai  Europe
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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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Old 13th July 2019, 10:15 AM   #644
Bonsai is offline Bonsai  Europe
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Quote:
Originally Posted by scott wurcer View Post
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.
But I guess it was shunted buy the electrode capacitance of a few pF?
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Old 13th July 2019, 10:16 AM   #645
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by syn08 View Post
I'll think about, not sure why you made it so complicated when the textbook S/N ratio definition is rather straightforward. But then something that strikes me is that the S/N ratio fundamentally depends on the signal level. "S/N ratio of X dB" makes sense only if you add "ref. 0.4mV input". This is one reason why IMO S/N ratio is not a good metric for audio, and I just shared a few posts above my opinion that for the case of a MC head amp this S/N metric is straight useless and misleading.
My previous images were meant to show the relative change in dB when changing the collector current for a given Rs.
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 dB-A after Riaa, that magnitude gives a clear sign of how silent a phono amp is, reasonably matching your hearing perception.
65dB-A is the bottom line, 75dB-A 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(1e-4/(141*0.28e-9)) = 68.1dB flat or 76.0 dB-A after Riaa and A-weighting. 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(2e-4/(141*0.85e-9)) = 64.4 dB flat or 72.3 dB-A, which is still very good for this Cart.
It will be hard if not impossible to find anything bettering this figure.


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Old 13th July 2019, 07:05 PM   #646
Hans Polak is offline Hans Polak  Netherlands
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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 dB-A

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 dB-A

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 A-weighting.

IMO it is safe to assume that the outcome of this formula is accurate to within +0dB/-2dB.


Hans
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Old 14th July 2019, 02:54 PM   #647
scott wurcer is online now scott wurcer  United States
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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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Old 14th July 2019, 03:02 PM   #648
gerhard is offline gerhard  Germany
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You can also write "noiseless" after the resistor part value.
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Old 14th July 2019, 03:07 PM   #649
scott wurcer is online now scott wurcer  United States
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Quote:
Originally Posted by gerhard View Post
You can also write "noiseless" after the resistor part value.
Not everyone's SPICE has that feature?
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Old 14th July 2019, 03:51 PM   #650
gerhard is offline gerhard  Germany
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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/ngspice-manual.pdf >

ADS has noise=yes|no
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