Technical: (coil characteristics)

[bauzace50]

Hi,

this is a technical query. An audiophile friend commented on the Non-Magnetic coil core (coil former?) on certain MC models. This might also pertain to the AT OC-9 And Empire MC-5 designs, which can be viewed as having an "air core". He says this will motivate adverse reaction with some preamps because of its effect on coil inductance.

This is the first time I've come accross this idea, and would like comments from the VE Brain Trust.

Although the following is probably UNrelated, I wonder if this has some relation with Dynavector Softened Magnetism with a coil of wire placed around the pole piece?

Any comments or elucidations?
THANKS ,
bauzace50

[phonomac]

Hello Bauzace,
There is a post on AC which quotes Jonathan Carr (Lyra designer) on this subject, looking in particular at matching Denon MCs to pre-amps.

It is titled 'Denon DL-S1, DL-304, and preamp considerations ' (not sure whether I'm allowed to post a link or not ) but you will find it makes an interesting read on this very subject.

regards

Angus

[dlaloum]

I have seen a couple of mentions of this... apparently permeable/magnetic cores make for a much higher efficiency MC - higher output (still LOMC) - but it trades off on some magnetic non-linearities.

The Air Core types, sacrifice some output (which makes them VERY low output MC's) in exchange for a purer sound with fewer non linearities.

And Johnathan Carr also mentioned this in the MM/MC thread on Audiogon.

The short list of cartridges that have been designed/built in this manner does not include a single clunker... all top notch - the DL304 is the most economical cartridge that has this feature/

bye for now
David

[phonomac]

Hello all,
The interesting aspect of this for me was the recommendation for different pre-amp loading between the two types, and the influence of the loading on the dynamics.

regards

Angus

[dlaloum]

I wonder whether the air core coils have lower inductance - this would shift the resonant frequency up, and cause them to behave a little differently to the "standard" MC's

The shift upwards in res f, would imply that it would interact differently than the standard core MC's - but this would vary depending on the phono stage design - and so would the loading required... (no hard and fast rules with MC's...)

[Ldg]

Hmmm. I'm not sure those Jonathan Carr extracts add much beyond a reasonable explanation of the issues. Nor tackle the crux of the nature of any real effects in permeable cores.

JC implies that the B field alternates, ie passes through zero. But i doubt that, because there is always a permanently applied field. I venture there is never a true flux reversal, just very small flux variations against a context of a non-zero operating point. Essentially a permanent bias.

I understand the description is better served by very small changes up/down the B-H curve, presumably at an operating point that is definitely non-zero and chosen to be linear. The devil is in the detail then. It depends on how well the B-H curve approximates a straight line in each direction over the small range of flux variations in play. I venture that the flux changes are so small that the relationship is very linear. As indeed seems to be the case.

The gradient of the B-H curve relates to permeability. Even if the curve is seriously curved, as it generally is, small variations always approximate straight lines.

There are downsides to air cored coils too. Not only the obvious serious lack of sensitivity/output, they generally need to be larger. That means that the field they sit in needs to be linear over a larger distance. Larger coils tend to be longer and require finer wire with loss implications. Also more masssive, depending on the construction and tradeoffs this is usually limiting. There are also non-ideals caused by imperfect flux linkage between turns on one end of the coil and turns at the other in air coils. Also self capacitance introduces more significant non-ideals, in conjunction with wire non-ideals and losses.

Lastly, I couldn't understand any link between loading and air-cored coils in those extracts. If it's real, it could be down to non-ideals. I wouldn't bet too much that the non-ideals aren't simply different, not necessarily better.

So I don't think it stands scrutiny, on the whole. Though much of the physics is reassuringly sound for a change, the devil is in the detail. Also, would be necessary to see some evidence of small signal non-ideals from permeable cores before getting too carried away. I've spent some reasonable time looking, and I'm still undecided, erring toward there is typically no notable effect.

[Ldg]

Just to illustrate, here's an estimate of likely flux density changes arising in MM/MI and MC cartridge cores which shows it in both cases to be very small in the context of being unaffected by the large scale non-linearities arising in B-H curves of highly permeable alloys.

Apols it's a bit mathy, but hopefully if the terms used in the JC extracts make sense to you, so should this !

The aim is to estimate the size of flux changes in typical MM/MI and MC cartridges, and relate this to published B-H curves for typical core materials and geometries:

Definitions :

Coil self inductance L [H]
Number of turns in coil N []
Current in coil I [A]
Induced voltage across coil V [V]
Induced voltage per turn in coil Vt [V] = V/N
Total flux linking coil Ø [Wb]
Flux Density in coil core B [Wb/m­­²]
Time t [s]
Angular frequency w [rads/s] = 2*pi*frequency
Coil core area A [m²]

Relations :

dØ/dt = dI/dt * (L/N)
as an inductor, rate of change of flux is proportional to rate of change of current, the constant is inductance per turn.

V = L * dI/dt
Vt = (L/N) * dI/dt
voltage per turn is proportional to rate of change of current, the constant is inductance per turn

Then Vt = dØ/dt ; ie coil volts per turn is exactly rate of change of core flux.

Calculations :

For an MM/MI N = 1000 , number of turns in the coil is about 1000 ref this thread : viewtopic.php?p=273610

For an MC, N = 50, number of turns in MC coil is about 50, same reference above.

Assume an output of 5mV (MM), or 0.5mV (MC), which is near full scale output ref 0dB@1kHz sine wave. We wish to find Ø, the peak amplitude of total flux in the core, and H the peak flux density in the core.

dØ/dt = Vt = 5mV/1000 = 5E-6 V

Because the stimulus is a sinewave, the peak value of Ø = Vt / w

Then for MM peak Ø = c 8E-10 [Wb]
and for MC peak Ø = c 1.6E-9 [Wb]

For MM core section area A might be 5mm², for MC A might be 3mm²

Then peak core flux density B for MM/MI is estimated to be Ø/A = 0.00016 Wb/m²
And peak core flux density B for MC is estimated to be 0.00053 Wb/m²

And the amplitude of flux density changes in coil cores associated with full scale output of the cartridge at 5cm/s@1kHz is estimated to be circa

0.00016Wb/m² for MM, and 0.00053 Wb/m²

Discussion

A datasheet for what seems a fairly ubiquitous permalloy core material is listed here : http://cartech.ides.com/datasheet.aspx? ... &c=TechArt

B-H curves are listed, which show the onset of saturation being at flux densities c 1 Wb/m².

Then the working range of flux density changes within coil cores for MM and MC cartridges is estimated to be 0.016% and 0.053% of the working range of that permalloy material. Associated with cartridge outputs representative of 5cm/s@1kHz, 5mV and 0.5mV outputs respectively.

Therefore, it appears that flux density changes represent a very small part of the working range of the core material, and therefore approximate a linear B-H transfer function. Even allowing for normal peak programme level overloads, there appears to be several orders of magnitude margin. Sufficient to support the notion that non-linearities in the B-H transfer curve aren't significant at any level or frequency.

Interesting ? I'd welcome this being checked over, but i think it's right. Comments and questions welcome, as always.

[bauzace50]

ld,
Your analysis is beyond my present capabilities without a tutor at hand. And, thankyou for your dedicated reply!

Would you say performance differences between air core coils versus iron core coils are marginal or meaningless?

Thankyou, again,
bauzace50

[Ldg]

You're welcome b_50. It's never my intention to post stuff which can't be readily understood. In this case I thought it best to set out the basis of why I disagree with the venerable Mr Carr, rather than expect anyone to just take my word for it.

What I disagree with is the proposed explanation, as to whether non-linearities in the B-H curve and remanance are significant in high permeability cartridge coil cores. Otherwise it would be profound for more or less all magnetic carts, but i seriously doubt it is, for reasons set out above.

One further thought as to an upside of air cores in MCs. There would be no saturation limit to flux density associated with the permanent field, whereas with permeable cores there is. This means that air cored equipped MC cartridges could potentially have far higher permanent field strength, narrower gaps, and smaller diameters. Especially these days with seriously decent rare earth magnets. This might mitigate some of the natural lack of sensitivity and need for more turns etc etc. Whether it is used in practice, I don't know. Nor how that might convert to advantage.

So yes, b_50 I'm skeptical of any benefit to air core over permeable cores in MCs, and would want to see some evidence and/or some explanation that stands scrutiny of any difference. At best, I venture the non-ideals might be different, and even that would be very design specific as I see it. Unfortunately, it's not as though one can just swap the armatures and leave everything else unaltered, it's so wrapped up in the whole cartridge as to be inseperable. As is often the case with such things, it's impossible to associate cause and effect when so much changes between 'comparable' MC designs.

BTW, as to the OP, is the OC9 air cored, I can't see any AT reference to it ?

[bauzace50]

ld,
thanks!
As to the OC9 cartridge: visual observation (without opening the cartridge body) with a 10x lens reveals two air-core coils inside this cartridge, which are cradled in a plastic carrier in a "V" shaped arrangement.

Thanks, again,
b50

[Hanuman]

In this case I thought it best to set out the basis of why I disagree with the venerable Mr Carr, rather than expect anyone to just take my word for it.

I'm not completely sure that you are disagreeing with him because his designs certainly don't incorporate air cores as far as I know.

What I disagree with is the proposed explanation, as to whether non-linearities in the B-H curve and remanance are significant in high permeability cartridge coil cores.

As above, I think John Carr seems to share your judgement on this if the choices he's made in the Lyra range of cartridges are any guide.

From the Kleos Specs:

"Coil Former: 5N iron core & 6N copper coils"

[bauzace50]

Non-engineers, or non-physicists certainly need to learn some of the terms. One new one for me is "remanance". Got to look that up, as well as other terms. It's re-education all over again.
Regards,
b50

[Ldg]

Non-engineers, or non-physicists certainly need to learn some of the terms. One new one for me is "remanance". Got to look that up, as well as other terms. It's re-education all over again.

There's a good resource describing the terms here: http://info.ee.surrey.ac.uk/Workshop/ad ... terms.html which manages to be both plain and correct. Unfortunately it's tough to present without at least some maths. Mostly, the maths is elegantly simple. Mr Maxwell's stuff even more so, but that's generally avoided here. BTW, I reckon it's always a bad sign when little diagrams appear with lines going round in circles, and this seems to mostly avoid that

[Ldg]

As above, I think John Carr seems to share your judgement on this if the choices he's made in the Lyra range of cartridges are any guide.

From the Kleos Specs:

"Coil Former: 5N iron core & 6N copper coils"

That's reassuring, Hanuman. The extracts which led me to believe otherwise were from part of a dialog and might have been out of context, from the AC thread referred to by phonomac, which were in turn only partial quotes.