info about cartridge distortion (long)
Paul Guy
cartridges that I've had over the last 30 or 40 years.
Here's a history of my old cartridges:
early '60s: Astatic and Sonotone ceramic (Garard changer)
mid '60s: bottom of the line Shure magnetic (Dual 1019 changer)
1967: Empire series, I forget the model "" ""
late '60s: Shure V15 type II "" ""
early '70s: Micro Acoustics 2002 "" ""
mid '70s: later version of V15 "" ""
early '90s: Blue Point "" ""
1994: Blue Point Special Linn Valhalla
1998: Grado Platinum Reference "" ""
The sound definitely improved with the later choices. The first
few were really horrible, and the albums that were played with
them are completely chewed up. For a number of years I only
listened to CD's, but the cost of them versus used LP's pretty
well forced me back to vinyl.
As for the difference in sounds between the two, I'd lay blame
at the feet of the recording or mastering engineers. As far as
playback equipment goes, for the electromechanical parts like
turntables, cartridges, speakers, you get what you pay for - if
you buy from reputable sources. So as my system evolved, as I put
more cash into it, it started to sound better, and this especially
applied to phono pickups.
Among the many different tonal balances, they've all had one
drastic shortcoming - tracking ability (although I have been able
to get the Grado to behave fairly well if I track it at 2-2.25
grams). Tracking ability is a property of the cartridge to handle
high amplitude sounds, especially at the inner grooves. It usually
doesn't bother bass sounds. Loud piano, voice and other strong mid
to high frequency signals seems to set it off. It's a unique sound
that you won't hear on any other audio device. To me it sounds
like a crackling or scratchy sound that accompanies the loud
instrument or voice.
The effect is much more pronounced at the inner grooves of
the record. Most of the cartridges I've had do not badly at the
outside grooves, but sound irritating or disgustingly bad on loud
passages that are close to the centre of the LP.
Some manufacturers give you specs, some don't. Cartridges are
not the kind of equipment you can audition anywhere at your
average hifi store. You'd think that specs or features would be a
predominant part of the sales pitch, but they aren't. Some
companies like Grado have next to nothing describing the pickups.
Some like Van den Hul provide a wealth of technical and
subjective information. I suspect companies that know their
product is top of the line, have no fear about publishing full
technical details.
So, I did a fair bit of digging, and here's an excessive
amount of technical information relating to cartridge design,
sound quality and distortion. There's even some subjective
information, but not a lot! I refer to references by a number in
brackets that will correspond to a bibliography near the end of
this article.
After all the looking into technical information, not enough
is publicly available to make any decent decision. The result is
that just like everyone else, I must rely on audio reviews and
the opinions of those who write articles to the audio newsgroups.
However, if you're curious about WHY things affect the sound
quality, the following is a rather glossy introduction to some of
the technical reasons.
A few basic facts about recording levels and velocities:
linear groove velocity of a 33 1/3 12 inch LP:
inner outer grooves
8.5 20 in/sec (1)
21 51 cm/sec
standard recording level: 5.5 cm/sec @1000 Hz (1)
standard for cutting head 7 cm/sec @1000 Hz (1)
max. recording level (approx.) 21 cm/sec (1) as shown on
standard VU meter. Of course not everyone keeps it under control,
and also transients may go well over the needle response (which is
quite slow). I'd expect the maximum peak measured velocities to be
around 40 cm/sec, very dependent on the sound engineers doing the
recording and mastering.
The tip and the groove:
..................................
The groove is made with a cutter that determines the geometry
of the playback stylus. Here's some info:
groove: included angle 90 degrees, +- 2 deg.
top width .001 inches min., .003 in. nom.
bottom radius less than .0002
less than 225 grooves/inch
recommended tip radius: .0005 - .0007 in. (conical)
typical elliptical tip radii : .0007 front, .0002 side
pressure on vinyl using typical stylus: 4800-16000 psi (9)
surface temperature at tip: 1000 - 2000 deg F (1)
cutting tip radius: less than .00025 in.
cutting tip included angle: 88 deg.
The phono cartridge responds to velocity (at least magnetic
pickups). The record is recorded with constant amplitude below 500
Hz, and constant velocity above 500 Hz. In order to coordinate the
equalization set in the recording process and the pickup
characteristics, a playback equalization is needed. The advantages
are higher levels on the LP, i.e., better signal to noise ratio.
The types of distortion:
........................
displacement distortion:
This occurs when the groove has so much 'wiggle' that it
interferes with the next groove. This limits the displacement to a
certain max. value. One cure for this problem is to space the
grooves further apart for the loud passages. Knowing that the
sound level is dependent on velocity, the worst case velocity to
stay within the limits displacement distortion at reference
level, is about 6 cm/sec at around 1000 Hz (8). Above this value,
the stylus will most likely skip to another groove, so recording
engineers try never to let this happen.
Slope overload:
This happens when the slope of the groove being
cut is more than what the trailing cutter edge will allow. In
effect, what it says is that you can't cut a groove with an
angle more than that of the cutter. Cutters are claimed to
have about a 45 degree trailing angle, so NO angle can be any
greater than that. If you try to cut a 90 degree angle with the
front edge of the cutter, the back edge of the cutter will
flatten it to 45 degrees. That's the 'overload' limit. In terms
of velocities, that means about 70% of the groove linear velocity.
That works out to about 14 cm/sec at the inside grooves and about
35 cm/sec at the outer grooves on a 33 1/3 rpm 12 inch disk (8).
This is a physical limit - and it seems rather low, considering
all the market hype about cartridges that can track at 60 and 80
cm/sec. Perhaps the more recent cutters have a sharper cut than
those discussed in the 1963 paper. Or maybe I'm just a sucker for
all the hype.
Curvature overload:
This happens when the curve is so tight as a result of a high
frequency, high amplitude signal, that the stylus is to wide to
fit along it.
Here's a table showing the 'wavelength' (crest to crest) in
typical record:
groove wavelength @ 1 kHz 5 kHz 10 kHz
12 in. diam .020 in .004 .002
5 in. diam .008 .0016 .0008
As you can see, the needle tip diameter is not much smaller
then the wavelength on the inner grooves at 10 kHz. At low
amplitudes this isn't too much of a problem, but at large signals,
the tip can't fit inside the sharp turns of the groove 'valley'.
At the inside grooves, this places a maximum velocity of 10 cm/sec
at the reference level at about 4000 Hz. At the outside grooves
the limit is about 25 cm/sec at the reference level, at 10 kHz.
These are for a conical .0007 inch diameter stylus, an elliptical
one allows for considerably higher velocity limits. Severe
distortion and wear happen when these limits are exceeded.
The needle effectively tears or melts its way through the sharp
turns.
Since the above are for the reference level (0 db = 5.5 cm/sec
at
1000 Hz), it becomes very tricky setting the recording levels for
a reasonable volume, and keeping a minimum of overloads. Each
kind of music must be taken into account in keeping the number
of overloads to a minimum. The above forms of distortion depend
on the characteristics of the vinyl, groove geometry, cutting
stylus, and shape of the playback stylus.
Tracking Distortion:
..................................
This stems from the stylus having sufficient inertia, that it
can't stay down in the groove, and can actually lift away from
record on sharp turns. On turns going the other way, it digs into
the plastic. This gives a nasty 'crackly' kind of distortion
that shows up on voice, piano, or other loud relatively
high pitched sounds. Usually its much worse on the inner grooves,
and can make an operatic singer sound like a trumpet. Once it
happens, there is a 'noise' that is permanently embedded into
the album, even if you play it with a system that tracks
perfectly. The sound of this distortion is what has made me
appreciate CD's.
Since inertia is the culprit, one can mathematically
determine the things that affect this form of distortion.
The distortion varies as:
stylus mass to the 2/3 power
frequency to the 5/3 power
groove velocity INVERSELY to the 1/3 power
tip radius INVERSELY to the 1/3 power
That means higher frequencies make things much worse, the
stylus mass makes things somewhat worse, things get
somewhat better on the outside of the record, and smaller tips
make things a bit better. For that part of the distortion where
the tip actually leaves the surface of the vinyl, things improve
directly with the stylus pressure (the weight you set on the arm).
But there is a down side, the extra weight causes more
pressure on the groove, and a resultant deformation. Trying to
reduce the tip diameter also causes more deformation since the
pressure increases - same weight, but a smaller surface area.
Not only do the accelerations from the signal waveform cause
problems, but also the curvature distortion mentioned above.
The two effects are opposite in character, and it happens that
distortion (2nd harmonics) can be canceled at certain levels.
One cartridge uses the 'complementary' distortion between
mis-tracking and curvature distortion (13) to its advantage.
This is the MIT-1 made by J-MAS, Inc. Its stylus mass is
adjusted to compensate for the distortion from curvature
distortion and vinyl 'give'. As far as I can tell, the
company no longer exists.
Here's some measured IM distortion data from paper #3:
pickup A elliptical stylus, 2 gm, 14 cm/sec signal (peak)
outside grooves inside grooves
IM
distortion
%
| |
1.6 | |
| o |
1.4 | o | o
| o o | o
1.2 | o | o
| | o
1.0 | o o | o
| | o
0.8 | | o
| o o | oooooo o
0.6 | |
| o | o
0.4 | |
| o | o
0.2 | oooooooo o |
| oo |
0.0 +....2....4....7....10...14. +....2....4....7....10...14..
Frequency (kHz) Frequency (kHz)
The behaviour near the outside of the record is very good until
we hit about 8 kHz, and then distortion quickly rises. For the
inner grooves, distortion is more than 3 times that at the outside
groove, and then rapidly increases at around 5 kHz. The reasons
for the drastic decrease in distortion above 10-12 kHz are not
understood. It is suspected that the characteristics of vinyl (as
it yields under the intense heat and pressure) play a large part
in the odd behaviour.
Since this is IM distortion, the audible effects can be at
much lower frequencies than the applied signals. The
resultant frequencies will be integer multiples of both the
sum and difference of the main signals. The sums will be at
very high frequencies, often beyond audibility, but the
differences can appear all across the audio band. It
would not be common to have high frequencies present at the
amplitudes shown in the previous two tables.
At lower amplitudes the data did not show the distortion 'peak'
at 10 kHz. For the same cartridge, at 3.5 cm/sec peak
velocities, the outside grooves gave almost a constant .07% IM
distortion, and on the inside grooves gave about 0.15% IM
distortion.
Here's the effect of vertical force (or weight):
IM distortion (%)
signal pickup A
weight amplitude left right
1.5 gm 3.5 cm/sec .05 - .1 .07 - .4
2.0 gm 3.5 .05 - .07 .07 - .1
2.5 gm 3.5 .05 .05 - .07
1.5 gm 5.6 cm/sec .06 - .5 .15 - .8
2.0 gm 5.6 .06 - .2 .15 - .4
2.5 gm 5.6 .05 - .08 .1 - .15
1.5 gm 8.9 cm/sec .1 -1.0 .2 -1.5
2.0 gm 8.9 .1 - .7 .15 -1.0
2.5 gm 8.9 .07 - .25 .1 - .7
1.5 gm 14 cm/sec .15 -2.0 .3 -3.0
2.0 gm 14 .15 -1.5 .25 -2.0
2.5 gm 14 .1 - .6 .2 -1.2
Most of the distortion occurred at frequencies around 10 Khz, a
rare kind of signal at this amplitude. You can see that
increasing the weight definitely decreases distortion. Being a
'scientific' article, the identity of this cartridge is kept
secret, although they corresponded to the best available at the
time (1967).
Also discussed in this paper (3) is the effect of anti-skating.
The testing included signals that would cause tracking problems,
so it would show if there was a relationship between the two. The
data showed small improvement going from no anti-skating to the
optimum amount. One channel (the one closest to the centre) got a
bit worse, the other one improved. The effect of the correct
antiskating is to correct for frictional forces pulling the arm
toward the centre because the groove is not parallel with a line
that goes between the tip and the arm pivot. When it is set up,
the forces on the tip are mostly vertical, and each side of groove
sees similiar forces. Otherwise, one side has less force to keep
the tip from flying off on a sharp curvature due to a loud musical
passage. That's the one that will distort more.
If you apply double or treble the required anti-skating, or
apply it in the wrong direction (can't do that with most
turntables), the worst-case distortion can get very bad, typically
3 times that of an optimum setup. One channel gets really bad, the
other one actually gets better. It's a case of getting the best
compromise, and setup errors of less than about 30% really make
very little difference.
In the data from above we saw a problem with resonance
that caused a lot of distortion when high frequency signals
were played. Because of the nature of IM distortion, its
products will be heard at low frequencies.
The parameters we can 'play' with are the needle shape and
size, and the stylus mechanical parameters. Of course, at
the other end of the stylus is the mechanism that translates
mechanical motion into an electrical signal. Its mechanical
properties also come into play since it's part of the
mechanical system that couples vinyl 'shape' into the
electrical analog of sound.
One can also precompensate at the recording end for
tracking distortion, but that must make some assumptions
about the parameters of the cartridges being used to play
the records. RCA 'Dynagroove' records do some of this
pre-processing, but the audio community never fell in
love with its unique sound (a number of other forms of
processing were also applied).
Here's some trackability specs for different cartridges:
trackability 400 Hz 1 kHz 5 kHz 10 kHz
Shure V15vxmr 30 cm/s 46 cm/s 80 cm/s 60 cm/sec
Van den Hul Frog 70-80 uM
Van den Hul Grasshopper 70-80 uM
Van den Hul MC-10 70-80 uM
Ortofon MC15 60 uM
Ortofon X1MC 60 uM
Ortofon X3MC 70 uM
Grado Platinum ??
Benz Glider >80 uM
Most of the European cartridges use an amplitude spec at
315 Hz (I put it in the 400 Hz column). Since we know the
frequency and amplitude, by differentiating the signal with
respect to time we get the velocity, which would be omega
(2*pi*freq) times the amplitude, or about 0.2 cm/sec for
every uM @ 315 Hz. This is for RMS values, for peak you'd need
multiply by .28 (I think the industry uses peak values). After
you multiply all the uM numbers by .28 to convert the values to
cm/sec, you can see that the V15 has quite superior
trackability figures, and that is consistent with its much
lower stylus mass as shown in one of later tables.
If we assume there are about 25 uM to 0.001 inch, and the
maximum (roughly) width of a groove is 0.003 inches, then 75uM
to 80uM is the full width of a groove at approximately maximum
level. Thus the European test really tests more the linearity
of the system at full displacement with a moderate velocity.
The American (or Shure) method is to specify maximum velocities
at different frequencies, most of them well above the 'constant
amplitude' range. In effect, this is a more rigorous test (in
my opinion) to separate good and badly tracking cartridges.
The tip:
..............
As described in the section on curvature distortion
(tracing distortion), the shape of the tip determines how
small a corner that it can turn. It is quite possible (and
happens often) that signals are cut onto the vinyl that
make it very difficult for a tip to follow. As a result,
there is a demand to make the profile of the needle as small
as possible. The original shape was a cone, with a rounded end.
Here's the view looking down the tone-arm, or along the
record groove.
\ <-IA-> / IA=included angle
\ /
\ /
\ /
\ /
`` -tip radius (the circle formed by the tip)
The tip radius was standardised as 0.007 inch, although many in
the industry thought .005 in was a better compromise. The included
angle for the needle is 87 degrees, to fit comfortably in the
angle of the groove (90 degrees). The first improvement was to
make an elliptical cross section (when you look at the needle from
the top, along the vertical axis). This makes the tip wide enough
to negotiate the grooves properly, but narrow in the front-back
direction so it can fit into a sharp turn. Once you do that, of
course,the pressure goes up because there is such a small surface
area. In response to that, needle profiles are redesigned so as to
give a wider contact point in the vertical axis, i.e., the tip
radius as shown above is modified to make it less round. This is
the idea of the line contact tip. There is an artful compromise to
be made, it can vary from the original curvature which will work
easily, to an almost straight edge which requires very precise
adjustment to keep contact point the same size on either the left
or right side of the groove.
The vinyl:
..................
The vinyl plastic has behaviour that greatly affects playback,
and it also has properties that aren't well understood.
Vinyl subsurface 'yielding' begins at weights of about 0.15 grams,
and plastic deformation starts at 1 - 1.5 grams. It also appears
that below a few grams, the material appears to be harder than it
should be. Vinyl shows signs of work hardening (material gets
harder after playing), and excessive wear if played again without
giving it time to 'recover' (9). Information that's in the public
domain seems pretty sketchy about some of the properties of vinyl.
There are sets of equations by Hertz that describe what happens
in the case of a needle in a plastic groove, but taking into all
the other effects, like curvature and tracking distortion is quite
a task. The paper by Shiga (13) apparently is one of the better
analyses, but it's very complicated. What is also a problem that
you'd understand, is that although many of the forms of distortion
can be calculated, there is very little ability to determine how
unpleasant those shortcomings will sound. In other words, despite
all the math, there is still an art to getting the best
compromise.
The shank (cantilever):
..................
The shank is the tiny little shaft that has the needle
mounted on it, it has a pivot that usually consists of a rubber
like substance , and on the other side of the pivot is the
mechanism that converts vibration to electrical signal.
Sometimes the transducer mechanism will be on the same side
of the pivot as the needle.
The shank determines a lot of how the system performs. Its
mass (weight) affects high frequency response, resonance,
tracking distortion, and phase response. Weight will
affect distortion (tracking), while the combination
of weight and stiffness determine resonances and frequency
response.
Ideally, it should be extremely stiff and light. Resonance
modes should be out of the audio bands. Technology has made it
possible to use some rather outlandish material in order to
achieve this. Here's a table of stylus masses that have been
at the edge of technology over the years (12):
1959 1.5 mg - milligrams
1968 0.6 mg
1971 0.40 mg
1973 0.35 mg
1976 0.25 mg
1978 0.23 mg
1979 0.15 mg
1980 0.098 mg (Matsushita)
And some typical values for todays cartridges:
tip shape stylus mass shank
Shure V15vxmr 4X75 uM .17 mg(?) beryllium tube
Van den Hul Frog 2X85 uM .32 mg boron solid
Van den Hul Grasshopper 2X85 uM .32 mg boron solid
Van den Hul MC-10 3X85 uM .35 mg boron solid
Ortofon MC15 8X8 uM .50 mg
Ortofon X1MC elliptical .75 mg
Ortofon X3MC fineline .75 mg
Grado Platinum elliptical ?? ??
Benz Glider 4X80 uM .25 mg(?) boron solid
A 'factor' that combines the stiffness and
lightness shows how the following materials fare (12):
Titanium 2400
Aluminum 2750
Carbon Fibre 6200
Titanium Nitride 6240
sapphire, ruby 8000
Beryllium 14670
Titanium Boride 14710
Boron 19560
Diamond 28500
Some of the substances like diamond are tricky, since they
are crystalline, and show quite different strengths depending
on the axis or alignment.
As with a bicycle frame, you can get greater strength for the
same weight by making the shaft hollow, or tubular. And, with
the same principle as a fishing rod, quite often the thickness
is tapered, with the thick end closest to the pivot. Cheaper
cartridges will use aluminum tubes, the better ones will use
beryllium tubes (Shure V15), or solid boron shafts (most
of the high-end cartridges). At high frequencies, the stylus
-cantilever-transducer system behaves like a fishing rod
with a small weight on the end. At low frequencies, the
whole thing moves together, but at high frequencies you can
flex it fast enough that the middle of the rod moves, but there
is almost no motion at either end. At this frequency the
response is almost zero. Prior to getting to this frequency,
the phase response is pretty poor. Of all the measures of a
cartridge (including type like moving iron, moving coil,
moving magnet, etc), this measure of phase deviation at higher
frequencies, is the best measure of the sound quality (11). So
cantilever design is an important part of the sonic design
of a phono pickup.
Transducer:
.......................
This is the part that converts the stylus motion into
a voltage. Most of the reasonable cartridges used in hi-fi
stuff use some form of changing a magnetic field to induce
voltages in a coil of wire. Cheapy cartridges will use the
flexing of a crystal or ceramic element to generate a voltage.
There were a few 'high-end' cartridges that used ceramic
mechanism. I had an old 'Micro Acoustics 2002E' high-end
ceramic pickup which competed with the magnetic pickups of
that time (middle 70's?), but they don't seem to have a
market presence any more. If I remember correctly, replacing
it with a V15 improved things quite a bit, but it could be
that the needle was badly worn.
It is possible to have distortion from the magnetic
transducer, primarily because of non-linear magnetic field.
This happens as a result of the design of the pole
pieces, gap length and other parameters related to
magnetic circuits. Fortunately, the movements are so tiny
compared to the pole gaps that non-linearity is not a
problem. These designs must also cope with the different
positions of the magnet, coils, iron, etc., that occur
as a result of different weights that the user sets, and also
the variation in compliance of the pivot or 'damper'
material. The transducer is not designed for efficiency. As
a result, circuit loading does not have a large effect on
the stylus motion.
The choice of magnet material, core or yoke material, wire
gauge and conductor type play important roles. I'd be
reluctant to admit that any of the tweako stuff is important
(like copper crystal, silver, etc.) except for their
different mechanical properties, which are measurable, and
quite different. I know Van den Hul uses copper crystal
wires, I'd assume he's using them for their greater strength.
There has been a lot of concern whether the different
transducer types (moving iron, moving coil, moving magnet),
have inherently unique sound and fidelity. Tests amongst
trained, untrained listeners, audio reviewers have shown (11)
that when the tests are sufficiently unbiased, the preferred
cartridges do not correlate well with any particular
technology. What does correlate well is the quality of the
cartridge design (whatever technology) and especially the
phase variation at high frequencies (11). That highly depends
on the design of the entire stylus subsystem.
Pivot (damper):
.......................
The pivot material provides positioning or centering
of the shank. It also determines the 'compliance' or the
degree of displacement of the cartridge toward the record
when the user changes the weight or stylus force. This
parameter determines at what low frequency the arm will
resonate at, given its total effective mass. The ideal is
around 10-12 Hz. In the 70's they got overly obsessed with
high compliance cartridges. That meant it didn't take very
much weight before the cartridge would drag on the record,
and you needed a low mass tone-arm to bring the low-freq.
resonance up to 10-12 Hz.
Here's some figures for compliance and frequency response
in a sampling of cartridges:
compliance max freq. resp.
Shure V15vxmr n/a 25,000Hz
Van den Hul Frog 35 uM/mN 55,000Hz
Van den Hul Grasshopper 35 uM/mN 65,000Hz
Van den Hul MC-10 28 uM/mN 50,000Hz
Ortofon MC15 15 uM/mN 25,000Hz
Ortofon X1MC 13 uM/mN 30,000Hz
Ortofon X3MC 13 uM/mN 40,000Hz
Grado Platinum ?? 60,000Hz
Benz Glider 15 uM/mN 50,000Hz
These numbers don't mean a whole lot in determining sound
quality. The only useful data suggests that the ones with
compliance much higher than 15 would need a relatively lower
mass tonearm to keep the tonearm resonance around 10-12 Hz.
Since that resonance varies with the square root of the
compliance, the values have to be way out before you need to
worry. In other words, a doubling of the compliance only
causes a 40 percent change in resonant frequency, all other
things being equal.
The frequency response figures mean even less, since they don't
specify the conditions or what the level is relative to the
nominal output.
The pivot also dampens out resonances in the stylus shank.
Without that, certain frequencies and their harmonics would
cause extreme shank movement, mistracking, wild frequency
responses and crummy sound. The whole system behaves
very much like a car suspension, and a poor damper would
give mechanical performance similiar to that when your
car shock absorbers have died.
Since the materials are visco-elastic, they have a
reaction force that varies with frequency. Poor choice
of material, or the wrong geometry can cause loss of high
frequencies. I sometimes suspect that this may be used to
suppress wild stylus motion at high signal amplitudes
(velocity) and tame some of the nasty sound of mistracking.
Since this damping extracts energy out of the stylus system,
it will put an additional burden on the vinyl, with
possibly higher record wear. On the other hand, it may
reduce impact and shock as the needle leaves or touches
back down on the groove.
The conventional material for pivots is iso-butylene/
isoprene rubber. This is very good material for dampening
(its also used in car suspensions in the movement limiting
bumpers), but it suffers from variation in its parameters with
temperature and age. Silicone rubber has been used successfully,
it lasts longer, and is less affected by environmental factors
unless they vary quite a bit from room temperature.
The coil and output circuit:
............................
For a magnetic pickup, the signal is generated in the coils
of wire that are in the magnetic field. The characteristics
of the cartridge are such that there are a wide variety of
optimum loads. Most manufacturers try to make sure that
a load of 47,000 ohms in parallel with a few hundred picofarads
of capacitance will work well. The capacitance is due to the
length of shielded wire between the pickup and the preamp
input. Since the pickup coil is inductive, there is a good
possibility of it resonating with the external cable capacitance.
As a result, it is necessary to keep the inductance below a
range of values that might cause audible or ultrasonic
resonance. Inductances higher than 100 millihenries are going
to be a problem.
And some typical values for todays cartridges:
coil coil nominal load
cartridge inductance(mh) resistance resistance
Shure V15vxmr 425 1000 47 kilohm
Shure V15 type II (old) 720 625 47k
Van den Hul Frog 47k (>1k)
Van den Hul Grasshopper 47k (>10k)
Van den Hul MC-10 200 ohm
Ortofon MC15 7 20 ohms or greater
Ortofon X1MC 80 47k
Ortofon X3MC 80 47k
Grado Platinum 45 475 47k(I like 8-10k)
Benz Glider 38 1k-47k
My very old V15 type II had an inductance of 720 mh. With a
capacitive load of 300 pf (quite typical), there would be a
resonant circuit at 10 kHz that would give rise to a really crummy
transient response, and possible 'zingy' sound. For a cartridge
like that, you'd need to get the total cable capacitance less than
40-50 pf to get resonance above 25 kHz. That would be very
difficult. For the new V15vxmr, you'd need to achieve 100 pf,
still not easy. The resonance of the V15 with typical cable
capacitances might explain why it is not a well liked cartridge.
Remember, that excessive phase shifts at higher frequencies
correlates with poorer subjective assessment of a cartridge.
Resonance would be one cause of such phase shifts.
High values of inductance not only cause resonances with cable
capacitance, but even in the absence of capacitance, will cause a
drop of high frequency output. Into a 47K load, the V15 will have
a 3db loss at just over 17 kHz because of the inductance. The old
V15 type II would have a -3db point at 10 kHz (neglecting possible
resonance effects).
Values of load resistance below the nominal 47,000 ohms should
only be used when specified by the manufacturer, unless you are
deliberately trying to tailor the response characteristic to your
own taste, as I did for the Grado Platinum. This is the only area
where the average user can fiddle with a cartridge, without
destroying it or completely messing up the sound. Other than
changing the overall frequency response there's not much effect,
unless you go way off in terms of load value.
There are cartridges that have very low output levels
(typically moving coil cartridges), and usually work best into
quite low valued loads, i.e.., 100 ohms. Since the voltage output
is also very low, you must either use specially designed preamps,
or step up the signal with a special transformer. These are not
your usual audio or interstage coupling transformer, since stray
power line frequency magnetic fields will induce significant hum
voltages relative to the signal level. The expense of these
transformers is due to special hum balancing cores, winding
techniques and magnetic shielding, and of course a very small
market.
There are some sorry devices sold to 'demagnetize' cartridges,
and 'enhance' or condition the wires. Unless you're really
gullible, it's best to stay away from them. Some can damage
preamps or cartridge wiring insulation. There's really no need for
them. The rationale for them is similiar to that of needing to
mark the edges of CD's with a green felt pen.
the end...(phew!)
Please tell me if there are any mistakes or inaccurate
information in the above, or even better, if you know of
interesting gems of information that I can add to this, should I
repost it. If there is info you'd like me to expand upon, please
tell me so, since I have copies of most of the papers referenced.
-Paul
.................................................................
papers and books referenced:
(1) Tremaine H.M., "Audio Cyclopedia", Howard Sams & Co., 2nd
edition - 1973
-monstrous book with a lot of good but dated info.
(2) Hunt F.V., "The Rational Design of Phonograph Pickups",JAES,
Vol.10, #4,Oct. 1962
- Author with Acoustics Research Laboratory, Harvard University
- broad overview of design requirements and limitations of
cartridge design
(3) Woodward H.V. and Werner R.E., "High Frequency Intermodulation
Testing of Stereo Phonograph Pickups", JAES, Vol.15 #2, Apr. 1967
- Authors from RCA Laboratories
- effects of various parameters on IM distortion in cartridge
design
- watch out for typos that confuse 'A' and 'B' cartridges
(4) Kantrowitz P., "High Frequency Stylus-Groove Relationships
In Phonograph Cartridge Transducers", JAES, Vol.11 #3, July 1963
- Author with Sonotone Corporation, elmsford, New York
- detailed mathematical analysis of distortion and losses in
stylus and vinyl groove.
(5) Walton J., "Stylus Mass and Reproduction Distortion", JAES,
Vol.11 #2, April 1963
- Author with Decca Record Co., London, England
- analyzes distortion and wear with respect to stylus mass
(6) Walton J., "Stylus Mass and Elliptical Points", JAES,
Vol.14 #2, April 1966
- Author with Decca Record Co., London, England
- determines factors that cause distortion, and noise, and
compares conical to elliptical styli.
(7) Kogen J.H., "Tracking Ability Specifications for Phonograph
Cartridges", JAES, Vol.16 #2, April 1968
(8) Woodward J.G. and Fox E.C., "A Study of Program-Level
Overloading in Phonograph Recording", Vol.11 #?, Jan. 1963
- Authors from RCA Laboratories, Princeton, New Jersey
- defines the sources of distortion in record playback.
Unfortunately uses simulations to generate statistics.
(9) Bastiaans C.R., "Factors Affecting the Stylus Groove
Relationship in Phonograph Playback Systems", JAES, Vol.15 #4,
Oct. 1967
- author with Westinghouse Research Laboratories, Pittsburgh, PA
- The effect of the vinyl plastic on the sound and stylus
performance (resonance frequency).
(10) Bastiaans C.R., "Further Thoughts on Geometric Conditions
in the Cutting and Playing of Stereo Disks", JAES, Vol.11 #?,
Jan. 1963
- Author with Philips Phonograph Industries, Baarn, Netherlands
- Mostly about the effects (distortion, separation, response)
of vertical tracking angle, cutter orientation.
(11) Hansen., V., "Development of a Pickup Cartridge", JAES,
Vol.32 #5, May 1984
- Author with Bang & Olufsen, Denmark
- results of a lot of subjective tests
- waterfall plots of cartridge response
(12) S. Obata, M. Itoh, K. Azuma, " A Hi-Fi Moving Magnet
Using Recent Technology", JAES, March 1984
- Author with Matsushita Electric Industrial Company
- a lot of info about cantilever design
(13) N.O.G. Shiga, "Regenerative Distortion in Stereophonic
Disc Recording", Japan Acoustics Society Gazette 18,
#1, 1962
(14) L.R. Happ, "Dynamic Modeling and Analysis of a
Phonograph Stylus", JAES, Vol.27 #1/2, Jan./Feb. 1979
- Author with Shure Brothers Inc., Evanson, Illinois
- Discussion of stylus shank design and limitations
Some papers I never got my hands on, but might have some good
info:
Pierce, J.A., and F.V. Hunt, "Theory of Tracking Distortion
in Sound Reproduction from Phonograph Records" J.A.S.A., Jan.
1941
Cooper, D.H., "Compensation for Tracing and Tracking Error",
JAES, Oct. 1963
Cooper, D.H., "Integrated Treatment of Tracking and Tracking
Error", JAES, Jan. 1964
Cooper, D.H., "On Tracking Error Measurements", JAES, Oct.
1964
Kogen, J., and R. Samson, "The Elliptical Stylus", Audio,
May 1964
............................................................
-Paul