There's no need for this roundabout way of looking at things.
Check out any elementary text on transformers.
At any given frequency, there will be a maximum AC voltage you can apply to a winding and this is governed by the onset of saturation flux levels in the core.
If you can somehow keep the (AC) voltage across this winding constant (eg by wiring to the mains) while drawing loadsa current you'll find the flux doesn't change .. because the voltage hasn't changed.
If you draw current by shorting out the winding however, the flux will drop but it drops not because loads current is flowing but cos the voltage across the windings has dropped.
The 'opposing current' is a sorta explaination but the EXACT relationship is between flux & voltage.
This holds for transformers and also for MM & MI cartridges. Don't forget the frequency dependence of flux/voltage too. 🙂
For a transformer, core flux is a function of primary current, not voltage. How much voltage it takes for a particular current depends on the secondary load. If the secondary is open, the transformer primary looks like an inductor, so e= -L(di/dt). If the transformer secondary is shorted, then (for an idea transformer) the voltage needed approaches zero since the short on the secondary is reflected to the primary.
In a phono cartridge, there is only one winding, which moves relative to a magnetic field. As flux changes in the coil, a current will be induced to flow in the wire, and enough voltage should be generated to make that happen (as a first approximation).
If some external circuit forces current back into the coil, then a magnetic field will be generated that will oppose the motion of the stylus and/or distort the internal magnetic field inside the cartridge. In that case, neglecting nonlinearities in the permanent magnet core, the resulting field should be the superposition of the original field and the externally induced field, to the extent the stylus assembly resists compensatory motion.
That's my, perhaps distorted, old memory of how it should work. Apologies, if I got it wrong, or if I stated it in a confusing way.
In a phono cartridge, there is only one winding, which moves relative to a magnetic field. As flux changes in the coil, a current will be induced to flow in the wire, and enough voltage should be generated to make that happen (as a first approximation).
If some external circuit forces current back into the coil, then a magnetic field will be generated that will oppose the motion of the stylus and/or distort the internal magnetic field inside the cartridge. In that case, neglecting nonlinearities in the permanent magnet core, the resulting field should be the superposition of the original field and the externally induced field, to the extent the stylus assembly resists compensatory motion.
That's my, perhaps distorted, old memory of how it should work. Apologies, if I got it wrong, or if I stated it in a confusing way.
well we showed earlier in this thread that the generator is at best 1% efficient so good luck with resisting movement by sending current back through it 🙂
Right, I included whatever little effect there may be to try to help make sense out of the physics.
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If it helps any, the voltage viewpoint can be seen from integrating both sides of e= L(di/dt), which is hard to write in text format, but maybe we could say i= (1/L)(int v(dt)), where int is an integral symbol. When people want to talk in terms of volt-seconds, that's where this form of the relationship may be useful to keep in mind.
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I remember very well that it came across my path and that I mentioned this test by name in a forum, just because DBLT's can sometimes result in confusing outcomes.
But since it is not exactly on the top of the list of things that have my attention, I kept no copy of this article.
But I will give it a try to do a search and let you know when I found it.
Hans
You're best disputing that with Ampere and Maxwell, Richard 🙄
A cartridge differs from a transformer in that the 'primary' is replaced by a mechanical source of flux changes. Increasing load current from the coil increases mechanical impedance, albeit by a trivial amount. If armature motion remains unaltered, so does the source of associated flux changes. But the load current also invokes flux change in a direction to reduce net flux in the coil. Whereas this also happens in a transformer, electrical impedance of the primary consequently falls and primary current increases to compensate so that net flux remains constant. It's different.
LD
Yes, that's it. And the sense is such that the load current itself produces a field which opposes motion.
LD
Except, I would say generating whatever voltage is necessary to force the induced current to flow though the load is what creates the opposition to stylus motion.
As a thought experiment, as load resistance is increased, and more power or energy dumped into the load for a given induced current due to stylus motion, then the stylus has to do more work to generate the necessary voltage to make that happen, since the magnetic field doesn't do any work. That is to say, the stylus pushes back harder against the record groove walls (neglecting elasticity for the moment) as load resistance is increased (although the effect is small in this case).
The EXACT relationship is of course
emf = dΦ/dt hence the frequency dependence.
according to Maxwell & Ampere 🙂
The relationship between current & flux is much more ambiguous. Current generates a Magneto Motive Force MMF .. often in units of AmpereTurns. How much flux results depends on loadsa other factors.
But for the purpose of this exercise, lucky is right that feeding your MM/MI cartridge into a virtual earth will reduce the peak flux in the core and hence the magnetic losses. See your transformer textbook for details.
But his assertion that 'slew rate distortions' are improved is MUCH more dodgy. As these will be TOTALLY swamped by the slew rate distortions of the cartridge's mechanical system we can't really check this ... so dis pseudo Magnetic Guru will stop wanking on the subject. 😀
BTW, a moving coil transducer obeys different rules so the THD mechanisms are different. The end results can be similar but most moving coils are more efficient than most MM/MI. Realising the extra efficiency for better S/N may not be easy though 😉
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Faraday, actually.
And emf = dΦ/dt = -L di/dt
So dΦ = -Ldi . No current change means no flux change, and no emf, in an inductor. In a cartridge dΦ has mechanical origin.
And, strictly, the dependence is on instantaneous slew rate.........
So we're back to cantilever flex, which I agree has the same form as eddy current losses in principle. But as to relative magnitudes, or significance in the scheme of things, what makes you think the mechanical cause is the larger ?
Attached is another one from the archive: a method for measuring cartridge slew rate and stylus acceleration, based on a method by Baxandall.
Surely not, quite the opposite. Show us the evidence ?! We've all heard there are special laws of physics for MC carts.......🙄
LD
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These are the relationships of the magnetic, mechanical and electrical derivatives:
Φ = flux = stylus displacement
dΦ/dt = rate of change of flux = stylus velocity = cartridge output emf
d2Φ/dt2 = stylus acceleration = slew rate of cartridge output emf
d3Φ/dt2 = stylus jerk = rate of change of cartridge output slew rate emf
LD
Φ = flux = stylus displacement
dΦ/dt = rate of change of flux = stylus velocity = cartridge output emf
d2Φ/dt2 = stylus acceleration = slew rate of cartridge output emf
d3Φ/dt2 = stylus jerk = rate of change of cartridge output slew rate emf
LD
Thinking about it some more, yes, and that field distorts the primary field, as you say. The field distortion effect seems to be sometimes referred to as Armature Reaction, at least in motors and generators. And, I suppose a phono cartridge could be considered a type of generator...
This just can't be true..........
Into a transimpedance preamp, a MM/MI cartridge perhaps typically converts more than a decade more power than MC, and there's never a decade difference in mechanical power via suspension compliance............
LD
Mark, I had the same picture in my mind.
http://www.diyaudio.com/forums/analogue-source/100408-results-m97xe-mod-7.html#post1208582
But after so many tests in which I see there is no indication of reciprocity, I have strong doubts.
Of course I still don’t know if with the cartridge the reciprocity brakes at the mechanical element-magnetic element interface or it breaks at the magnetic element-electric element interface
And I see there is some hope left as we are returning to the murder scene again and again in search for some unnoticed elements of proof
George
George, After thinking about it some more, I think I had some of that wrong. LD seems to have a pretty good handle on it. Regarding some expected effects, say reciprocity as you mention, it's probably just very small. Mechanical resistance to motion of the cantilever, etc., is much greater than electrical loading, so maybe not a whole lot to measure.
One thing I'm still wondering about is the Armature Reaction effect when using a virtual earth preamp. The effect is function of coil current, which seems like it ought to be maximum when coil load resistance is minimum. But, it may be a very small effect too.
In addition, there are computer codes to model magnetics these days, but I don't know if it really would be worth the effort.
Probably easier and more directly to the point to build one or two or a few test preamps and go with the one you like best. And, if you trust test records, make the best use of them to verify the designs perform as expected. What else to do?
One thing I'm still wondering about is the Armature Reaction effect when using a virtual earth preamp. The effect is function of coil current, which seems like it ought to be maximum when coil load resistance is minimum. But, it may be a very small effect too.
In addition, there are computer codes to model magnetics these days, but I don't know if it really would be worth the effort.
Probably easier and more directly to the point to build one or two or a few test preamps and go with the one you like best. And, if you trust test records, make the best use of them to verify the designs perform as expected. What else to do?
A funny slip in spelling here creates some real confusion. I think LD has computed the ratio of mechanical work vs electrical power several times here.
BTW I opened the other thread.
I was just reading about new rubidium based fluids with negative mass. Could you fill a tubular cantilever with M- fluid and achieve net 0 effective tip mass? Could you tweak arm/cartridge resonance by adding some M- and thereby reducing tonearm effective mass? Or maybe just package it in tiny vials which audiophiles could purchase and scatter around their rooms. Ok
🙂🙂
If only the cartridge generator was anything like efficient as a motor, a servo might be made to achieve the same result......?!?
LD
A negative mass means with F=M*A that when a force is applied, the negative mass will push back, thereby preventing motion.
Is that what you want for the cantilever ?
Hans