Even more universal as the graph in #502, Richard Lee's Ultra low Noise MC Head Amp
Here a general formula to calculate the S/N after Riaa and after A-weighting for a MC Head Amp.
This is without taking the mostly very low contribution of current noise into account.
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.
As an example with ABOOS cart, 35R /0.2mV with Richards 0.28nV/rtHz amp, gives 20*log(17.7e3*(0.2/(sqrt(0.28^2+35/60)] = 72.77 dB-A.
Hans
Here a general formula to calculate the S/N after Riaa and after A-weighting for a MC Head Amp.
This is without taking the mostly very low contribution of current noise into account.
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.
As an example with ABOOS cart, 35R /0.2mV with Richards 0.28nV/rtHz amp, gives 20*log(17.7e3*(0.2/(sqrt(0.28^2+35/60)] = 72.77 dB-A.
Hans
Interesting formula, any idea about how it is derived? The term Rs/60 looks rather strange.
It needs units of V squared.
Hans there's nothing wrong with a figure of merit normalized this way but what about the folks that want a pre-amp with control on cartridge loading, there are LOTS of them. Then you need to include the circuits where the Ibias is not negligible or FET circuits.
He talks about differences between sine sweeps and pink noise as if the test LP's are perfect.
So far all evidence is that it behaves as a voltage source in series with it's internal Z. MMs behave the same way into virtual gnd, which was a shock to me at first but has useful consequences as discussed elsewhere.
If we may believe Stereophiles test results, they behave very well with virtual gnd MC Amps.
Hans
As Bill said check the other threads, I rely on folks with far better LP setups than me for the data. I don't find the tinkering with gear much fun.
Unfortunately several folks with vast knowledge of LP history no longer participate here. The number of LP's cut with pre-distortion for spherical stylii is unknown but it is far more than most think.
Adding noise is by taking the sqrt of the sum of the squares.
Since a resistor produces sqrt(R/60)nV/rtHz, the square of it is R/60.
Hans
Hi Bill,
Kindly learn to pick your own battles.
Anything that brings some practicality for the diy community such as Han's graph is useful. That is why I bought up the test record. Having read section 3.8 of the Sounds of Silence pp 125 there is no specific mention of test records but a very detailed analysis of vinyl noise.
I had some tests done by a friend some time ago with is system similar if not this product.
I make no endorsements or affiliations but they seem to have recognition from Stereo Sound Magazine which is the most influential Hifi publication on the planet.
AnalogMagik All in One Cartridge Alignment Software
I could talk to a mastering engineer with 10,000 hours on the job and get his opinion but this entertainment.
I meant no offence but in my diligent re-reading of the entire thread am hardly the only one who finds Scott's response's difficult to comprehend. Scott's ability is not question. It's the way he responds that matters. He need to understand we are not time wasters. I opt for a growth mind set and that means there is always more than one way of looking at a problem or an issue.
As me me I come hear like most to relax. I get things done through guys like Scott in business so expect less attitude and more fun here.
So lighten up.
Lets leave it there.
https://www.diyaudio.com/forums/ana...tra-low-noise-mc-head-amp-30.html#post5837744
https://www.diyaudio.com/forums/ana...tra-low-noise-mc-head-amp-51.html#post5846625
https://www.diyaudio.com/forums/ana...tra-low-noise-mc-head-amp-51.html#post5846826
Quote:Hans this way of looking at it leaves out too many things like MM carts and their loading/ pre-amp topology.
For one thing the curves are asymptotic on the right to simply the fact that the self noise of the cart is already at -75dB losing the point that any pre-amp at 10dB below that adds less than a dB more. Taking away the message that the pre-amp has to be noiseless in order to not increase the net noise one bit is rather pointless.:Quote
https://www.diyaudio.com/forums/ana...tra-low-noise-mc-head-amp-52.html#post5846999
Quote: In this thread we are discussing ultra low noise MC head amps.
This has nothing to do with MM Carts. That's why I restricted the graph to MC Carts from 0.1mV to 0.5mV to make it more clear in what situation you really need an ultra low noise Head Amp. Where is the Cart with -75dB and where is the message that the preamp should be noiseless ? The lowest noise for a preamp in the graph without Cart is 200pV/rtHz and nowhere noiseless. Could it be that you did not fully get the meaning of the graph and that it concerns S/N after Riaa and after A-Weighting ? Or maybe you can give an example illustrating what you mean, because I don't get it.
Hans :Quote
https://www.diyaudio.com/forums/ana...tra-low-noise-mc-head-amp-52.html#post5847038
Quote:
Originally Posted by aboos
I have another candidate for a lowest noise MC amp: Dynavector DV23R. Despite it has a nominal sensitivity of 0.2mV it has an unusual high impedance of 35 Ohms for such an output. Extrapolating your graph up to 35 Ohms will end up below 0.2nV/rtHz RTI to reach the practical 75 dB-A S/N level.
That would simply mean that you don’t get 75dB-A S/N even with a 0.2 nV/rtHz MC Head Amp. But the 75dB-A was just meant as a sensible upper limit.
With the Richard Lee Duraglit Special, producing 0.28nV/rtHz without Cart, you will still get an excellent 72.7dB-A, meaning complete silence with the arm in the air and still quite a bit above the surface noise from the LP.
Hans:Quote
https://www.diyaudio.com/forums/ana...tra-low-noise-mc-head-amp-52.html#post5847160
Quote:
Originally Posted by scott wurcer
Where did you say a minimum pre-amp noise was factored in? It makes sense to plot a line with two asymptotes on the left cart with 0 R and on the right where the cart R equals your reference of choice. For MC only which are flat with frequency you could pick spot noise at 1kHz and it would make no difference.
We are still not synchronised along the same line.
A MC Headamp has a flat noise spectrum, but to listen to LP’s, somewhere in the line a Riaa correction will be applied changing the overall noise spectrum.
But a noise spectrum has to be weighted to correspond to what we actually hear.
In this case I have used an A-weighting.
In the given graph you can enter Rcart on the horizontal axis. Go upwards from there to the curve that represents the output voltage of the Cart.
Go from here horizontal to the Y axis and you will find what equivalent input noise without Cart the MC Headamp should produce to get a S/N of 75dB-A after Riaa and after A-weighting. This figure differs significantly from the S/N without Riaa and without A-weighting by almost a factor 2.5 !
Hans:Quote
https://www.diyaudio.com/forums/ana...tra-low-noise-mc-head-amp-53.html#post5847247
Let's just drop it, for flat carts the same eq applied to all is just a constant factor. There is no difference in weighting from cart to cart.
https://www.diyaudio.com/forums/ana...tra-low-noise-mc-head-amp-53.html#post5847282
Quote:
Originally Posted by scott wurcer
Let's just drop it, for flat carts the same eq applied to all is just a constant factor. There is no difference in weighting from cart to cart.
You are right about the constant factor, but first you have to find out what this factor is, which I did here Richard Lee's Ultra low Noise MC Head Amp.
Without knowing this factor, you can't calculate the S/N after Riaa and A-Weighting out of the flat noise at the input. From there you can simply use the flat noise and divide it by 2.5 without having to weight from Cart to Cart.
I don't know what gave you that idea.
The end result is that you have a simple graph to find out what noise the head amp should have for your specific Cart to achieve 75dB-A after Riaa.
I still don't understand why you have so much trouble with this graph.
Hans: Quote
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With all due respect YOU picked the battle. Scott has actually analysed test records and posted the results on DIYaudio. He has contributed a huge amount to the DIY community so telling him what he should be doing is laughable.
Let's move on, I apologize Hans there is nothing wrong with your graph I just prefer the simpler presentation of the formula. Now it's clear that for some combinations you can't reach -75dB, and you can plot SNR vs Ein which I think let's those that want to see how far they can go (either way) experiment for themselves.
Ok, so no frequency response issues, correct?
This has an interesting consequence... If you increase the collector current (into the mA range) in a common base stage (to lower the noise, RTI goes with SQRT(1/2gm)), the gain also decreases (since the input impedance goes with 1/gm). So the S/N decreases ~1/SQRT(gm) or ~1/SQRT(Ic).
So increasing the collector current (in the mA range) actually lowers the overall S/N...
At very low collector current (gm is small) increasing the collector current (in the uA range) actually increases the S/N since the gain is not affected, but the noise decreases.
Between these two extremes, there could be an optimum Ic for each cartridge Rs, to maximize the S/N. Perhaps this is what Richard was thinking when talking about "noise matching", and we assumed he was talking about maximum power transfer (that doesn't apply to audio).
I need to do more algebra...
I mentioned that for my 14 Ohm example 12mA was a minimum at least in LTSpice. More current and SNR degraded.
None so far. Hans has found some interesting anomalies with virtual ground MM stages (which are bonkers and only eejits like me would build) but I don't think the MC equivalent model is anywhere near as complex. Hans does have a rig and may have measured his Benz to confirm.
I'm not seeing anything interesting. A couple of things that can confuse the issue, the self biasing resistors are in parallel with the load (and also beta dependent) so when you increase the bias current eventually the gain will suffer by simple attenuation at the output. The SNR seems monotonic with Ic for different Rs.
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SNR does not depend on the gain, since both the noise (Rs +Rb + 1/2gm equivalent noise) and the signal (past the Rs-Zin divider) are equally amplified. The bias resistor is much larger than the load, anyway. Once again, I am looking at the basic common base, no complementary stuff.