Cartridge Loading.....Part II

The virtual ground doesn't care what the origin is of the impedance in series with the voltage source. An ideal virtual ground simply means that all of the current that enters a node is conveyed somewhere else with a zero change in the voltage at that node.

An ideal opamp with infinite gain and bandwidth will be a perfect virtual ground, just as a perfect ground will have zero impedance to "real ground".

From Wikipedia.

"Lenz's law, named after the physicist Emil Lenz (pronounced /ˈlɛnts/) who formulated it in 1834,^[1] says that the direction of the electric current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes changes in the initial magnetic field.

It is a qualitative law that specifies the direction of induced current, but states nothing about its magnitude. Lenz's law predicts the direction of many effects in electromagnetism, such as the direction of voltage induced in an inductor or wire loop by a changing current, or the drag force of eddy currents exerted on moving objects in a magnetic field."

To paraphrase, a changing magnetic field is created that opposes the changing magnetic field that creates it.  That in turn will create a "restoring force" to oppose the motion that created it in the case of a cartridge.

Lenz's law says nothing about the magnitude of that field and hence that force.

That's a much more complicated exercise as there is no simple, closed form, equation for it. It can be estimated/approximated very roughly assuming some level of reciprocity, but I have no intention of going into the arcana of this.

The point is that IT IS small compared to the mechanical forces being applied to the cartridge due to the reaction of the walls, and that it does increase as a function of increasing frequency.

Incidentally, I don't intend to further participate in this exchange.

Dear @wynpalmer4 : I understand you but due that some gentlemans as lewm just did not reads the whole cartridge loading threads then are reduntdant on the issue one and again and other gentlemans what want is to " win " the discussion or " hit " to some one else and of course always exist the stupid and the stupidity.

. Sorry that disturb you and sorry for what I will posts next.

 

R.

@lewm : this is what Wyn posted before your post:

""

I was the one who wrote previously about Faraday’s Law and Lenz’s law in support of Carr’s assertion.

For your information, I have designed DIY phono stages that embody the characteristics that he espouses- very high supersonic overload characteristics for example- and are extremely compliant to the RIAA characteristic, very low noise, and essentially unmeasurably low distortion. There are several hundred of them out there...

In any case, there are no conservation of energy issues here. The mechanical energy of the groove wall reaction to the gravity induced downforce (i.e. the forced motion of the stylus) "uses" Faraday’s law to produce an output EMF (voltage). That voltage produces a current that complies with Lenz’s law- which essentially defines the inductance of the coil and occurs as an energy conservation consequence- and that current is defined by the total impedance of the coil- the inductance, the capacitance and resistance- the equivalent load impedance in fact.

The back emf is just due to the inductance, and is proportional to the frequency.

For a 10uH inductance at 20kHz, the impedance is about 1.6ohms, so relative to a 100 ohm R the back emf generated that opposes the input voltage is about 1.6/100 of the input voltage, and 90degrees out of phase, so it’s about 0.1dB of the signal amplitude.

Yes, the back emf opposes the motion of the cartridge, but it’s very small compared to the generated voltage- which is due to the conversion of mechanical energy to electrical energy as described above- and essentially can be ignored in calculating the dynamics of the cartridge arm system. """

 

 

and years before in the same issue he posted:

 

 

""" effects of heavy resistive loading you state could be definitively true- certainly not on tracking which is demonstrably false based on IM tests on tracking performance that I have incidentally performed as a function of load. While mechanical impact does occur as a result of electrical load- there is some back emf necessarily generated by the signal current that affects the mechanical motion, but a quick back of the envelope calculation using Lenz’s law and the 10uH cartridge suggests a 2 orders of magnitude difference between the generated signal and the back EMF for a 100 ohm load at 20kHz- certainly not enough to cause tracking issues """

 

and this post too years ago by Wyn:

 

"""" By the way, I constructed a model for the cartridge back EMF using Lenz's law and incorporated it into my simulations.
For those who are interested, the simplest version of the law is V(t)= -LdI/dt.
In this case the parameters can be measured (the LC100A meter from Ebay is a great way to do it) and the back EMF acts to oppose the voltage developed in the coil. The fractional change (attenuation) in the signal voltage is easy to calculate as it approx. equal to -L*2*pi*frequency of interest/Rload. So, it's inversely proportional to the load R and proportional to the frequency. """"

 

After all those years why you did not learn or at least try to understand about that issue? sorry.

 

R.

 

 

Incidentally, I don't intend to further participate in this exchange.

@wynpalmer4 

That's unfortunate.

I was/am familiar with Lenz's Law. I was simply hoping you could elucidate on how the input of the phono preamp (where the load is) provides a back EMF, or it I simply was misunderstanding what you wrote. I'm perfectly open to learning something new.

Dear Raul, I recently posted a URL from Wiki that contains a figure which I thought is instructional in understanding Lenz' Law.  Some describe Lenz' Law as the electromagnetic equivalent of Newton's Third Law of Motion. My opinion-less post with the URL preceded the one from Wyn that explains Lenz' Law in words rather than pictures. Other than that, I have been sitting on the side lines. I did not and do not take any position, because I am not qualified at the level of the best contributors, but I am here to learn. So I wonder why I am now the target of your invective. What makes you think I have not been reading the posts by Wyn, Ralph, and Dave, the only 3 people on this thread who are qualified to discuss the subject at hand? 

Cut back on the coffee, take a tranquilizer, get some therapy.  Do whatever the f*** you want but leave me alone, please.