Cartridge loading

I for one would love to see documentation of the ability of a cartridge to generate a 1MHz signal to excite this resonance.

Quite simply it does not need to! Audio energy can cause the excitation. A resonant circuit can be driven into excitation with a single pulse; it should be no surprise that on-going audio signals can do this as well.

Dear @atmasphere and friends: "" and as I mentioned earlier, when you load the cartridge it stiffens the cantilever. ..""

"" It will be stiffer, less compliant. """

both statement from you failed for something very simple: no explanation about, no explanation why that: less/limited ability to trace high frequencies by the cartridge.

I would have thought that the reason for the reduced compliance (stiffer cantilever) would have been obvious! A cartridge is a simple magnetic motor/generator, just like a dynamic microphone or loudspeaker, in that a coil has an audio signal transduced into it by a magnetic means- either by moving the magnet with relation to the coil (MM) or moving the coil in relation to the magnet (LOMC). It is easy to demonstrate this principle with a woofer of a loudspeaker with the grill removed (dynamic speakers operate on the moving coil principle of course). With nothing connected to the loudspeaker, simply depress the woofer cone and see how easy it is to move. Now short out the speaker terminals and do it again. You’ll find that the woofer has become much stiffer! This is exactly what happens with a cartridge as the modus operandi is identical.


As I mentioned earlier, if this were not to happen, a new branch of physics would thus come into existence :) because it would violate Kirchhoff’s Laws. The operating principle is similar to how motors and generators work so you can study them as well. In short, its impossible for a cartridge to drive a lower resistance load and *not* have a stiffer cantilever!

This is just a well known electromagnetic breaking force, proportional to the velocity of the movement (in turn proportional to the frequency) and inversely proportional to the R. It is just plainly ~f/R, like any other linear damping force. It adds to the total damping force, acting on the cantilever (the rest comes e.g. for the mechanical damping in the suspension). Lowering the R, just lowers the output across the entire spectrum but the nature of the output (its functional dependence on f) does not change at all. No additional damping of higher frequencies beyond the normal behavior of a damped oscillator. Just the damping coefficient increases.

I think you might be over-thinking this.

You got most of this right, right up until your conclusion. Think about a generator, one with no load and one with a load, which will be harder to turn? By your logic above (if I’m reading it right) somehow the loaded generator is easier to turn, which certainly isn’t going to happen. I think where you’re getting into trouble is the idea that the output goes down with reduced R load, which it does. The problem is: a certain amount of energy is used to make the stylus move. Where does that energy go? It is of course applied to the input load of the preamp in the form of a voltage. Now if you decrease the voltage by reducing the R load value, where is that same energy going? The Law of energy conservation says it has to go somewhere! It does not just ’vanish’. It is dissipated in the load and also by the cartridge coils themselves, both in the form of heat. But I think you will find if you do some measurements that the output does not go down as fast as it appears you are thinking. This is because the stock 47K load is easy to drive and the output of the cartridge will stay pretty constant until the load is decreased to some point below 10x the impedance of the cartridge; IOW probably less than 100 ohms.

Hey Ralph,

this is where we got last time we had this discussion.

Quite simply it does not need to! Audio energy can cause the excitation. A resonant circuit can be driven into excitation with a single pulse; it should be no surprise that on-going audio signals can do this as well.

I would like to see some documented proof of this or point me to a way to measure it. I have no doubt that an unstable phono will have problems with spurious HF info but I do not see the cartridge ever generating anything in the megahertz realm to excite this.

I do not see anyone debating that a loaded MC cartridge will stiffen its suspension, what I think is up for debate here is that this stiffening of the suspension will lower the cartridges ability to accurately trace high frequency info.

The 800 pound primate hiding in the corner here is the trend for some to insist that a cartridge is inherently a current generator and should feed a current amplifier for optimal performance.  This necessitates the cartridge driving a near dead short which by your reasoning would have a sever impact on the HF tracking ability.

dave

I have no doubt that an unstable phono will have problems with spurious HF info but I do not see the cartridge ever generating anything in the megahertz realm to excite this.

I do not see anyone debating that a loaded MC cartridge will stiffen its suspension,

The cartridge will not generate MHz output. But think about it this way- if that resonance is out there and it never goes into excitation, this conversation would be moot. But obviously it does and here we are.
Raul has been challenging me on the stiffer cantilever thing. So I think we have to get past that first.
You can certainly run a cartridge into a near dead short. Its output will lower of course. But now that you point this out, the reaction by the cantilever would seem to be a downside. What is needed right about now is some sort of measurement, perhaps a sweep tone from 20 to 35KHz so we can play a cartridge back and see how the loading affects it. 35KHz is probably overkill but should be well within any modern LOMC cartridge and phono section. Its been on the record side for decades if my Westerex 3D is any indication. But to my knowledge other than conversation with others in the industry and my own research on the matter (I attempted to design a loading box that would sort out the correct loading for any cartridge about 20 years ago) I've not seen any actual measurements. Its for dead sure that stiffening the cantilever will have adverse effects in some situations, but in some cases it could help. One example of that is a Grado cartridge on a Graham 2.0 unipiviot. Normally the mismatch between the two results in something called the 'Grado dance'. But I've seen that with loading this dance is eliminated.

I think you might be over-thinking this.

@atmasphere  I doubt it, I'm quite ok with physics and I apply it to the situation.

You got most of this right, right up until your conclusion. Think about a generator, one with no load and one with a load, which will be harder to turn? By your logic above (if I’m reading it right) somehow the loaded generator is easier to turn, which certainly isn’t going to happen.

Seems you did not understand what I wrote: lower R presents obviously more breaking force, opposing the stylus movement. This is the electromagnetic induction law in action: the current (flowing through R) creates the magnetic field that opposes the stylus movement. This force behaves like ~f/R.

I think where you’re getting into trouble is the idea that the output goes down with reduced R load, which it does. The problem is: a certain amount of energy is used to make the stylus move. Where does that energy go? It is of course applied to the input load of the preamp in the form of a voltage. Now if you decrease the voltage by reducing the R load value, where is that same energy going? The Law of energy conservation says it has to go somewhere! It does not just ’vanish’. It is dissipated in the load and also by the cartridge coils themselves, both in the form of heat.

Just to be precise, the energy is not presented in a form of a voltage because voltage alone cannot perform work. The energy is presented in a form of a heat, dissipated in the combined resistance of the circuit (R_load, the coil DCR, the cables etc), caused by the induced voltage applied to the resistance. This is ok. The question is so what? To speak of energy conservation, you have to look at all the forces acting on the stylus:
- the driving force, coming from the diamond tracking a rotating, modulated groove, say at freq. f; this force is the source of all the energy flows- the restoring force of the suspension- various damping forces, including the electromagnetic one ~f/R

In a simple case of a linear suspension, you can solve it (for the speed of the stylus as the signal is proportional to it) and you will see that the movement has two components:
1) the transient, exponentially decaying self oscillations of the stylus; the frequency is the usual cart-tonearm combo but decreased due to the electromagnetic damping; the decay time is inv. proportional to the damping so ~R in the EMF part
2) the steady state, the forced movement with the freq. f, dictated by the tracked groove;this is the signal we want; the amplitude of this movement will have an R dependence too

The only *qualitative* change in R can happen is in the 1st, spurious part. Lowering  the R changes the suspension character from underdamped to critically damped to overdamped. But this is not the signal we are trying to get! This is an artefact added to the real signal of freq. f.
Of course if the motor is so weak that the stylus tracing a HF track into low R will make it slow, we are in trouble but let's assume a healthy TT design.

But I think you will find if you do some measurements that the output does not go down as fast as it appears you are thinking. This is because the stock 47K load is easy to drive and the output of the cartridge will stay pretty constant until the load is decreased to some point below 10x the impedance of the cartridge; IOW probably less than 100 ohms.

I suspect you are trying to describe a behavior of the 1/R function. Yes, far from zero it flattens out so changes say 1k to 47k can be negligible, but the closer to zero the steeper the changes.

Summarizing, you seem to selectively use the bits of the whole picture of the stylus motion and draw conclusions from them, like the fact that lower R changes the compliance, but you completely neglect that there is a very strong driving force here coming from the rotating platter, and this is the force setting the stylus into the motion.

lower R presents obviously more breaking force, opposing the stylus movement. This is the electromagnetic induction law in action: the current (flowing through R) creates the magnetic field that opposes the stylus movement. This force behaves like ~f/R.

This is what I've been maintaining all along.

Just to be precise, the energy is not presented in a form of a voltage because voltage alone cannot perform work. The energy is presented in a form of a heat, dissipated in the combined resistance of the circuit (R_load, the coil DCR, the cables etc), caused by the induced voltage applied to the resistance. This is ok. The question is so what? To speak of energy conservation, you have to look at all the forces acting on the stylus:
- the driving force, coming from the diamond tracking a rotating, modulated groove, say at freq. f; this force is the source of all the energy flows- the restoring force of the suspension- various damping forces, including the electromagnetic one ~f/R

In a word, yup.

The only *qualitative* change in R can happen is in the 1st, spurious part. Lowering the R changes the suspension character from underdamped to critically damped to overdamped. But this is not the signal we are trying to get! This is an artefact added to the real signal of freq. f.
Of course if the motor is so weak that the stylus tracing a HF track into low R will make it slow, we are in trouble but let's assume a healthy TT design.

Since we have been in agreement all along on the first two bits, maybe its this last bit that is the stumbling block. I used to load MM cartridges to critical damping by simply ringing the cartridge/cable combination with a square wave and observing the resultant output and taming it with a loading resistor. MM cartridges have a lot more inductance so its easy for that inductance to ring. But attempts to do this with LOMC failed, simply because with any loading I could not detect anything other than a nice looking square output since the inductance is so low. So I am challenging the idea of critical damping of the mechanical aspect of the suspension, not because I don't think it can happen but more because I'd like to see the evidence. Its an interesting idea- I am assuming that the electrical damping used to do this is similar to a shock absorber in a car; with the right amount the stylus in better contact with the groove, just like a shock absorber keeps a wheel on the road.


My exposure to all this is through phono preamplifier design; about 35 years ago I discovered that the phono section itself can contribute to ticks and pops. I discovered this serendipitously but once I understood it was real it was then a matter of sorting out why.  And the answer (as I have mentioned earlier on this thread) has a lot to do with this ultrasonic/RF resonant tank circuit that I've been talking about. I've also noticed that while I can cut a 35KHz groove on my Scully lathe, depending on loading you can't always play it back, depending also on the cartridge.


So now I am curious- at what frequencies did you make your measurements?  At this point it appears that the taming of the resonant peak requires a different value as opposed to that which might tame the cantilever; the two aspects are caused by entirely different mechanisms. However, **any** resistance in parallel with a tank circuit will detune it; for most phono sections to be happy the detuning must be enough to kill the tank circuit altogether.