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.
Hans
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.
I'm kind of on the fence here but I think the graph is a good visualisation of where common sense ends and the madness of TM* begins. It also shows what is sensible and what isn't for cartridges. Which may not affect anyone's buying decisions but fun nonetheless.
And not sure I've seen this sort of presentation before so it's good and people can understand it. And for me working backwards from the equalised output to give a measure of the flat gain stage noise requirements seems sensible. It's just a FOM after all, unless I've completely missed the point again?
*I assume these initials are known as trying to be polite here!
Hi Scott,
In practice reducing the resistive loading of LMCC over a range of Cart Rs x 10 has a non linear effect versus frequency. This is also dependent of the Cart impedance. With suitable test record this can be verified. The take away is that the resistance loading should be adjustable for a flat response.
In practice reducing the resistive loading of LMCC over a range of Cart Rs x 10 has a non linear effect versus frequency. This is also dependent of the Cart impedance. With suitable test record this can be verified. The take away is that the resistance loading should be adjustable for a flat response.
I've never heard of that or seen measurements of it either. If you load too high you can start to see an effect on a square wave response but staying around 10x cart resistance you only normally change level (for obvious reasons).
We're talking virtual ground pre-amps here or at least that the loading is not a variable considered in the analysis. The cart response is assumed flat irrespective of loading.
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
Here is something for you to use. For the complementary Duraglit, the equivalent input current noise is
Ini=630/[Rc*SQRT(Ic)]
where Rc[ohm] is the cartridge resistance, Ic[mA] is the collector current. This is valid if gm*Rc is much larger than 1, so for large collector currents and/or large cartridge impedances.
For Scott's extreme example in (Rc=14ohm, Ic=12mA, making the above approximation valid) this formula gives Ini=13pA/rtHz. Compare this with the equivalent input noise provided by your formula (62pA/rtHz), you'll see the complementary Duraglit has a much lower equivalent input current noise.
By simulation, Scott got in #490 6.5pA/rtHz for each transistor, or a total of 9.1pA/rtHz. Not too bad compared to the above 13pA/rtHz, given the error sources (in principal, gm*Rc=7 so not really >> 1, an exact calculation gives 11pA/rtHz).
I have no trouble at all, just that the voltage sensitivity and series R for a MC cart is all you need for a figure of merit.
It’s a case of actual experimentation. The point being your LTspice discussion on lowest noise by loading of the Cart by the input stage may not prove in practice to be the optimal subjective performance.
A model is only as good as the assumptions behind the model. You might put all noise sources into a lumped Sum and work out which noise components have dependence’s and which are constant or non linear. The document l referred to early spells this out. I don’t see any of this particularly difficult once some real data is applied.
As l said get hold of a suitable test record and build buy an suitable analyser and do done practical experiments on different classes of MC and MM carts.
Some conclusions might then be useful to the wider diy community.
Are you familiar with actual LOMCC total THD? Something else to investigate with noise.
I had my Cart measured recently. I will see if l can locate the results.
Low output moving coil cartridge. The distortion of vinyl is in the crapper but that is not the issue. IME all the fancy stylus geometries make little difference with any real test record, in fact folks can't even determine definitively if the LP's were cut with pre-distortion.
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As l recall it was around 0.25% (-54db)
The point looking at whole topic to define the context of noise of the input stage and other sources of noise in a graphical red presentation
With all due respect your wrong. They can if they have the means. Don’t assume. Be curious.
In practice the best result is actual about careful arm set up. If you actually measure it with the right test equipment is obvious.
These systems are available off the shelf.
In regards to distortion it might be assumed not the issue in an academic discussion on input stage noise. But in context of the process from pick up to phono stage output input stage noise cannot to looked at in isolation. Compromises need to be considered in over-load margin, the driving current requirements of of low impedance feedback networks ect before settling on any one aspect of the design.
I recall Doug Self bring this up in his original Precision Preamplifier article about in the late 70’s WW article.
Without wishing to be rude there are a number of other threads discussing test records and what is wrong with a lot of them. It is best to discuss that there to keep SNR good here.
I guess you have not been around much recently. Scott has considerably advanced the DIY state of phono testing with some of the work he has done. He is more curious that most of us. Again all covered in other threads.