Why moving coil
 

Jim Lesurf wrote:

In article 441a6e41.0@entanet, Ian Bell wrote:

No I was thinking more of a small low mass pair of mirrors in place of
the MM for example with perhaps fibre optics to carry the data - no hum
issues for a start.


IIRC Denon did this about 25 years ago, but the result was not a
commercial success.


Optics were not common in consumer audio 25 years ago. A lot has changed
since then. Possibly time to look again.

Ian

Why moving coil
 

Trevor Wilson said

I took the question to be general. Do you mean that it is obvious that MC has
less inductance than MM? Why is that please?

**MUCH less wire in the coils (to keep moving mass within reasonable limits).
High output MC carts negate many of these advantages.

I see, although the restriction may have more to do with space than weight. But
it is possible to have low output MM with low inductance, or high output MC with
high inductance. Similarly, all magnets are not by definition heavier than all
coils. What is not simple is how the process of optimisation led MM and MC to be
so different in practice.

I know nothing of cartridges and the last one I heard was in a Dansette, but
there are other machines similarly categorised, such as ammeters and generators.
I remember the change from MC dynamos to MM alternators, which had much greater
output for a similar size and weight. Initially they were unreliable on
motorcycles because their extra rate of work had not been taken into account
properly, mechanically and thermally.

MM meters are generally not linear, if I remember, because the magnetic field is
not constant over the travel of the coil. With a moving coil, you can have a
magnet with a hole in it, or two magnets, or a ring magnet with a gap, with a
much more constant field in the space between, in which a small coil can move.
It would be impractical to have such a magnet assembly as the moving part.

A problem for MC dynamos is the need for commutator or slip-rings. A MC
cartridge with two coils needs at least three connections brought to a
stationary point, with attendant compromises in suspension.

I assume MM cartridges were initially high output, simple, cheap, not very
linear, and had a high mechanical impedance. MC were the opposite in all
respects. Advances in magnetic materials and their fabrication, and
smaller-scale boutique production, will have allowed carts to use more complex
and lighter magnets, and hence MM with more linearity, and MC with larger coils
perhaps? Or how else is linearity and high output achieved simultaneously?

Does one tend to have more crosstalk than the other?

BTW, has anyone mentioned the problem of noise in the early stages of
amplification from a low output source? Largely solved now, perhaps, but it was
a real problem during most of the development of cartridges.

cheers, Ian






"Trevor Wilson" wrote in message
...

"Ian Iveson" wrote in message
. uk...
Trevor Wilson ventured

**Er, not quite. The FR plots of MC carts, in some cases, CAN be shown to be
superior. The reason, of course, is blindingly simple. It's all about
inductance. The inductance of (LOW OUTPUT) MC carts is very low, indeed.
This enables them to produce a very flat, very wide (up to around 60kHz)
frequency response, with a correspondingly superior rise time. The real
benefit of such a system is that LC resonance effects are often well outside
the audible range. This means that a low output MC cart may exhibit a very
flat phase response within the audio band. This may not be the case with
high output MC carts nor with some MM carts.

Are you sure "blindingly simple" makes sense?


--

www.rageaudio.com.au

Why moving coil
 

AZ Nomad wrote:

On Fri, 17 Mar 2006 08:06:08 +0000, Ian Bell

No I was thinking more of a small low mass pair of mirrors in place of the
MM for example with perhaps fibre optics to carry the data - no hum issues
for a start.

No. You'd just have a 2lb brick with the electronics hanging off your
tonearm. :-)
Sure you could do better with today's microelectronics, but making custom
integrated circuits isn't a cheap endevor and I doubt any cartridge makers
forsee enough profits to cover development costs.


Since when has cost been an issue for audiophiles ;-)

Besides, would a mirror really be lighter than a coil? If you wanted to
improve the noise problem, move the preamp into the turntable. The
biggest noise problem has always been the cables.

Coils have problems with rising impedance, parallel capacitance and
susceptibility to external radiation. Optics has none of these.

Ian

Why moving coil
 

"Ian Bell" wrote in message
news:441ad477.0@entanet...
Jim Lesurf wrote:

In article 441a6e41.0@entanet, Ian Bell wrote:

No I was thinking more of a small low mass pair of mirrors in place of
the MM for example with perhaps fibre optics to carry the data - no hum
issues for a start.


IIRC Denon did this about 25 years ago, but the result was not a
commercial success.


Optics were not common in consumer audio 25 years ago. A lot has changed
since then. Possibly time to look again.

Ian

The Finial laser turntable exists and works, after a fashion. With a stylus,
small particles are pushed out of the way and don't affect the playback. The
Finial's main problem is that the laser and the playback electronics cannot
distinguish between a groove wiggle and a piece of dirt, and so both a
replayed faithfully. Chemically clean records apparently play well in the
Finial, but that's hardly a domestic proposition.

There was at least one photoelectric pickup and one electret capacitor
pickup, but both used a more-or-less conventional stylus for scanning. As
someone posted earlier, the high quality replay of vinyl (as opposed to home
DJs and scratching in clubs) is very much a minority interest, and there's
not sufficient mass-market volume to justify the development of new pickup
technologies.

S.

Why moving coil
 

Thus spake Adrian C:
Ian Bell wrote:

I am surprised there has been so little development of cartridges
beyond the two basic magnetic types and good old ceramic. What about
an optical cartridge for example?

Ian

I've wondered about that! If you take the electronics that is
currently used to read where a microsoft optical mouse is on its
surface, give its microcontroller a short sharp reprogramming
session, and hang the whole caboodle off a linear tracking arm, it
would work?
University final year project for some lucky student reading this!

Remember the Finnial deck? HFN used to mention it as vapourware from time to
time. I think it finally got made. Laser optics to allow LP archiving
non-destructively but prone to noise unless used in a cleanroom IIRC & with
a truly cosmic price. As for linear tracking, aren't records mastered on a
swinging cutter arm?

My brother had a Miniconic strain gauge cartridge 30yrs ago - had a supply
but can remember too much about it. I suspect cartridge development more or
less stopped shortly after March 1983. With the average manufacturing
quality of vinyl, was not a huge surprise to me at least. Records weren't
thin enough not to support a warp & not thick enough not have one either.

Why moving coil
 

"Paul B" wrote in message
...
Thus spake Adrian C:
Ian Bell wrote:

I am surprised there has been so little development of cartridges
beyond the two basic magnetic types and good old ceramic. What about
an optical cartridge for example?

Ian

I've wondered about that! If you take the electronics that is
currently used to read where a microsoft optical mouse is on its
surface, give its microcontroller a short sharp reprogramming
session, and hang the whole caboodle off a linear tracking arm, it
would work?
University final year project for some lucky student reading this!

Remember the Finnial deck? HFN used to mention it as vapourware from time
to time. I think it finally got made. Laser optics to allow LP archiving
non-destructively but prone to noise unless used in a cleanroom IIRC &
with a truly cosmic price. As for linear tracking, aren't records mastered
on a swinging cutter arm?

My brother had a Miniconic strain gauge cartridge 30yrs ago - had a supply
but can remember too much about it. I suspect cartridge development more
or less stopped shortly after March 1983. With the average manufacturing
quality of vinyl, was not a huge surprise to me at least. Records weren't
thin enough not to support a warp & not thick enough not have one either.



Don't forget the defunct Weathers FM cartridge as well. As Arnie mentioned
the Japanese ELP company also manufacture optical vinyl players. I
understand they've progressed a lot from that poor Finnial deck but they are
still very expensive. If you contact the company they'll send a CD so you
can hear their vinyl replay :- http://www.elpj.com/about/index.html

Mike

Why moving coil
 

In article , Mike Gilmour
wrote:


Don't forget the defunct Weathers FM cartridge as well.

Aha! I was just trying to recall the name and details of a cartridge that
IIRC used capacitance sensors and an AC signal to detect stylus movements.
Was that the "Weathers FM"? I can recall hearing mentions of this years
ago, but not any details.

Slainte,

Jim

--
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Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Why moving coil
 

In article , Ian Iveson
wrote:
Trevor Wilson said


MM meters are generally not linear, if I remember, because the magnetic
field is not constant over the travel of the coil. With a moving coil,
you can have a magnet with a hole in it, or two magnets, or a ring
magnet with a gap, with a much more constant field in the space
between, in which a small coil can move. It would be impractical to
have such a magnet assembly as the moving part.

TBH neither MM nor MC carts are, electrically, inherently linear. Indeed, I
think that most 'MM' carts are actually induced field 'variable reluctance'
sensors which are nominally unstable without a mechanical restoring force.

However the 'geometric' and mechanical causes of nonlinearity tend to mean
that the above doesn't matter much as you'd end up with distortion
regardless of the magnetic design.

A problem for MC dynamos is the need for commutator or slip-rings. A MC
cartridge with two coils needs at least three connections brought to a
stationary point, with attendant compromises in suspension.

I assume MM cartridges were initially high output, simple, cheap, not
very linear, and had a high mechanical impedance. MC were the opposite
in all respects. Advances in magnetic materials and their fabrication,
and smaller-scale boutique production, will have allowed carts to use
more complex and lighter magnets, and hence MM with more linearity, and
MC with larger coils perhaps? Or how else is linearity and high output
achieved simultaneously?

The 'linearity' of MM and MC carts largely depends on the displacements
being 'small', and on "hiding un-noticed behind the mechanical and dynamic
nonlinearities"... :-)

Does one tend to have more crosstalk than the other?

BTW, has anyone mentioned the problem of noise in the early stages of
amplification from a low output source? Largely solved now, perhaps, but
it was a real problem during most of the development of cartridges.

This problem had two causes.

1) The engineers involved were used to designing low-noise amps with a
relatively high input impedance, so had becomed used to designs suitable
for that.

2) The devices widely available at the time tended to give optimum noise
performance with source impedance well above those of the MCs.

Again, though, I found that a common-base BP design worked just fine, even
20+ years ago.

The interesting thing was that some power transistors delivered very low
noise in such circumstances due to having multiple parallel emitter areas.
They also sometimes worked better if used 'the wrong way around' - i.e.
using the collector as the input and the emitter as the output! :-) This
was a trick I'd discovered some years before when making low noise amps for
low impedance sources in instrumentation. Careful selection could give
lower noise than using any of the devices sold as 'low noise' signal
transistors at the time. Not much use for production, though, because of
the need for selection.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Why moving coil
 

"Jim Lesurf" wrote in message
...
In article , Mike Gilmour
wrote:


Don't forget the defunct Weathers FM cartridge as well.

Aha! I was just trying to recall the name and details of a cartridge that
IIRC used capacitance sensors and an AC signal to detect stylus movements.
Was that the "Weathers FM"? I can recall hearing mentions of this years
ago, but not any details.

Slainte,

Jim

--
Electronics
http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html


It was a very good sounding cartridge for its time (or now even?) Seem to
remember it had an external oscillator unit fed to some sort of capacitance
sensor which modulated the frequency according to stylus movement then was
returned to demod in the external unit with some sort of detector circuit,
perhaps a Foster Seeley discriminator but I don't know. I'll Google it as
I've forgotten a lot,! It could be made even better nowadays, there was
very little for the stylus to move so it had an extended FR. I would really
like to hear one nowadays :-)

Why moving coil
 

On Fri, 17 Mar 2006 09:52:29 +0000 (GMT), Jim Lesurf
wrote:

In article , Bill Taylor
wrote:
On Thu, 16 Mar 2006 23:23:34 -0000, "Serge Auckland"
wrote:


"Bill Taylor" wrote in message
...

(Compliance only needs to be more than about 10-12c.u. to track all
records)


Interesting theories. Can you tell me a bit more as to why a compliance
of 10-12cu is sufficient for all records? Interesting that in vinyl's
heyday, some cartridges were providing 30-40 cu. It could be a
marketing exercise rather than having a sound engineering reason for
it, but it would be useful to know why such high compliances are not
necessary.


According to J Walton in "Pickups - The Key to Hi Fi" (published 1968,
but the physics haven't changed): the maximum excursion on an LP is
about .005cm, "if compliance were the only factor involved a compliance
of 2 c.u. is quite sufficient to track the largest stereo amplitude of
.005 cm at 3 gm tracking weight". It's possible that a small number of
modern LPs have slightly higher excursions, and tracking weights are a
bit lower, so a slightly higher compliance is needed, but not that much
higher.

Afraid I don't have a copy of Walton. However I suspect the above may
either be an over-simplification on his part, or is out of context.

The quote does start with "if compliance were the only factor...."
which I thought did imply other considerations as well. I was just
about to leave for work when I posted that so I didn't have time for a
full precis, and I type slowly!. It was meant for illustration rather
than comprehensive analysis!

I may need to re-read some of the articles by Stan Kelley, but the above
looks to me as if is simply dealing with static compliance at LF. Thus the
physics may not have "changed", but may also not be as simple as the above
implies...

I also note that it quotes a playing 'weight' of 3g, which is high. A
small-radius tip could be expected to risk vinyl deformation even with an
unmodulated groove at such weights. Was he assuming a mono stylus which
would probably have been larger than a stereo one, and spherical?


Bear in mind that this was published in 1968 when 3 g was a typical
downforce for run of the mill cartridges. He was talking about stereo
cartridges, in this case of .5 thousands of an inch radius, and he
does say 3g is the maximum downforce before permanent groove damage
occurs.

If I look at Goddard's article in the 1963 "hi fi yearbook" he gives
a graph of the minimum acceptable compliance as a function of weight.

Simply to ensure contact this rises from 8 cu at 3g up to 20 cu at
1g playing weight. This ignores tip mass which will also contribute,
s you'd want a lower value to ensure avoiding mistracking. Also,
due to the finite compliance of the vinyl - which is more significant
with small contact profiles - you'd want a low compliance to minimise
vinyl deformation - and hence reduce distortion and wear.


Tip mass adds to the downforce neede to keep the stylus in contact
with the groove but I don't think you can trade off tip mass by
increasing compliance. According to Walton the demands of the tip mass
and mechanical resistance add to the downforce required by compliance,
so in this example he adds 2 gm to accomodate a 2mgm tip mass and 3 gm
to accomodate the mechanical damping appropriate to the tip mass and
compliance of the cartridge, giving a downforce requirement of 7-8 gm.
For a cartridge of 0.5 thou tip radius designed to operate with a 3gm
downforce he suggests a compliance of 5c.u. and a tip mass of 1 mgm.

The very high compliances of the 70s were very much a marketing
excercise.

They became so, but my recollection was that they grew for very good
reasons. Namely that unless the compliance was high and the mass was low,
the result was mistracking, high distortion, etc. (Also increased record
wear.) Personally, I'd regard a complaince as low as 10 cu would be too
low for comfort. I'd much prefer well over 20 cu.

A value as low as 2 cu would strike me as being unusuable for a modern
stylus profile - although the magazines rarely give any useful data
on this any more, so for all I know the profiles may be poor to
lower the pressure on the groove walls...

An advantage of high compliance and low tip mass is that you can have
a smaller contact area to improve the response and lower the distortion
level as well as keep down the wear on the LP.


Walton makes a very convincing case that the most significant cause of
groove damage is high stylus tip mass. He reckoned that a tip mass of
..6mgm or less would cause relatively low rates of wear. In 1968 this
was something of a stringent rquirement, I doubt that there were more
than one or two cartridges that could meet that requirement. (There
probably aren't that many around today.)

FWIW the best cartridge that I have owned was a Technics P205CMk4.
This tracked better than the Shures v15s that I've owned and has a
flatter FR. According to the makers spec it has a compliance of only
12c.u. but an effective tip mass of 0.11 mg, which agrees with Waltons
argument about the importance of low tip mass. I think that it is a
better cartridge than the V15-V, but my sample has got a collapsing
suspension and replacement styluses seem to be unavailable.

I can't comment on modern MCs. But my experience with some of the early
ones that were enthusiastically welcomed by 'reviewers' was that they
mistracked to an audible extent on many LPs, and this was the main
difference I noted when comparing them with something like a Shure V15.

That's my experience as well, but I haven't used any really pricy MCs.

I suspect the reviewers liked the way the mistracking and groove
deformation alterted transient peaks. It was noticable at the time
that the reviewers who liked the early MCs (e.g the Asak) also liked
'pop and rock' music, not classical music, and may have perhaps
liked the 'enhancement' on the transients of electric guitars and
drums... :-)

Also, recordings of pop music seem to be cut with lower peak
velocities than a classical releases, which makes cartridge tracking
ability less important.

Bill