|Elliott Sound Products||Magnetic Phono Pickup Cartridges|
By Rod Elliott (ESP)
Page Created 28 December 2011, Published 19 Jan 2012
There is rather a lot of information on the Net regarding vinyl record pickup (phono) cartridges, and while some is very good, there's also a lot of nonsense. Even manufacturers seem to get things badly wrong, and this surprises me. Considering how long people have been making phono pickups, I fully expected that the information provided would be rather more useful than it often seems to be.
Note that this article concentrates on magnetic cartridges. Piezo ('crystal') pickups are not considered, simply because they do not fulfil the requirements of hi-fi. Most are guaranteed to cause irreparable damage to the vinyl disc in as little as one single playing. In addition, I focus on moving magnet/iron cartridges, as these seem to cause the most problems. Moving coil pickups are (generally) better behaved, but many of the issues are the same regardless of the type of cartridge - the impedance might be quite different, but the problems are simply scaled to suit.
The pickup cartridge is a relatively complex piece of electro-mechanical ingenuity, and requires high precision manufacturing techniques. Much of the internal structure is extremely small, and microscopes are needed to see the tiniest parts. However, no matter how one tries to get around it, the laws of physics still apply. The cartridge itself shows an electrical impedance that needs to be loaded properly if the full frequency response is to be obtained in practice.
Often the expected results are not achieved, and lovers of vinyl have vast numbers of websites and forum pages devoted to their cause. One of the issues faced is that there is a multiplicity of different issues - every part of the system causes something to happen at some frequency. There is the mechanical resonance of the pickup arm itself (with cartridge attached of course), and there is actually scope for several different resonant effects in this part of the system. Most effects will be noticed at the lowest frequencies, and it is desirable that the resonance be as low as possible (and below the lowest frequency to be reproduced).
Then we have to deal with the cantilever - the lightweight tube that carries the stylus and the moving magnet/ iron/ coil assembly. Being a mechanical device, it has a resonance too, and it will affect the high frequency end of the spectrum. It is preferable if the resonance is well above the audio range, but this cannot always be achieved in practice.
There is not a lot that we can do about the mechanical resonances in any turntable/arm/pickup assembly, other than a careful choice of the various components used. That this is not an exact science is to understate the matter - almost every manufacturer of these components thinks they have the answer, so it's no wonder that different combinations can sound very different from each other. Unlike with most other music sources (CD, SACD, FM, etc.), these differences are often not particularly subtle, and can be glaringly obvious in some cases.
The options for dealing with mechanical resonances are very limited - other than changing often very expensive equipment. Not so though with the electrical resonance(s), as they are much easier to model. Unfortunately, this doesn't mean that there is an easy fix. Some cartridges seem designed to thwart your every attempt to get a satisfactory result, often because of very high inductance. The basic electrical model of a cartridge is shown in Figure 1, and it is essentially a simple resistor, inductor, capacitor (RLC) filter. The inductance is split and one section is damped by a resistor - this simulates the semi-inductance of the cartridge (see below for more on this topic).
Figure 1 - Electrical Model Of A Pickup Cartridge (One Channel)
As should be obvious, there is an inevitable (and predictable) relationship between the inductive and capacitive elements, and this is moderated by the included resistances. As the value of inductance and capacitance increases, the resonant frequency falls. The ideal outcome is to ensure that there is no gradual rolloff, and no large peak at the high frequency end. Figure 2 shows the response of a (more or less) typical low inductance cartridge, having 230mH of inductance, and 1.2k winding resistance.
The 47k resistor is the terminating impedance of the phono preamp (this is standard), and the 100pF of capacitance is due to the cable between the cartridge and preamp. In some cases, manufacturers recommend (or at least mention) a 'suitable' range of capacitance, but in many cases the upper limit is much too high. Some suggested loadings are such that there can be a peak of 4dB or more at a frequency well below 20kHz. One I modelled peaked at 8kHz - this might be alright for a DJ playing scratch mixes at 110dB SPL in a nightclub, but is hardly hi-fi and is not likely to be well received at home.
Figure 2 - Electrical Frequency Response Of Cartridge In Figure 1
The red trace shows the response with 100pF of capacitance (typical of the average cable run), and the green trace shows what happens if the capacitance is increased to 500pF. It is fairly obvious that with this cartridge (and most others), the capacitance needs to be kept low. I checked the specifications of a large number of cartridges, and the majority of moving magnet and moving iron types have an inductance of 400mH or more. The highest I've seen in specifications is 930mH, although the test cartridge I used initially appeared to be even higher (based on (flawed inductance) measurements). Great care is needed to ensure that measurement results reflect reality.
The blue trace in Figure 2 shows the response of the cartridge when the standard 47k resistor is increased to 100k, and capacitance maintained at 100pF.
If you do want to measure the cartridge inductance, use the setup shown in Figure 3. You need to measure at a low frequency to gain a reference. 10Hz is a good place to start, but it must be resistive at the reference frequency - at least 2 octaves below the frequency where the voltage starts to rise. This is used to find the +3dB frequency. When the signal level has increased by 3dB (1.414 times the voltage measured at 10Hz), the inductive reactance (XL) is equal to the DC resistance of the cartridge. Now you can calculate the inductance ...
L = XL / ( 2 × π × f )
From the test I performed, the following values were obtained ...Voltage at 10Hz = 26.4mV
+3dB Voltage = 37.5mV ( 26.3 × 1.414 )
+3dB Frequency = 840Hz
Inductance = 232mH
To verify that this works in practice, I also modelled the response in a simulator, using the measured and calculated values. The result was close to being an exact match (these data were used for Figures 1 & 2) - the process works!
In the two drawings, the 232mH is made up by 77mH as 'pure' inductance, with 155mH (paralleled by 68k) as the semi-inductance. When modelled in the simulator, this combination matched the voltages measured on the physical cartridge to a degree that one can be reasonably sure that the equivalent circuit is correct.
Figure 3 - Setup For Inductance Measurement
While it may seem a bit drastic to subject a pickup cartridge to such high voltages (compared to the 5mV or so you get from them), there is no reason to expect that any damage will occur. Even if the stylus and cantilever is deflected (I couldn't detect any movement), it will be far less than that caused by using a stylus brush.
Measurement of the cartridge parameters is not an especially easy undertaking, and determining the model from the measured electrical parameters is also somewhat irksome. One thing that is clear (but mentioned in only one reference I could find [ 1 ]), is that the 'inductance' of the cartridge is actually a 'semi-inductance'. It is imperfect, because of eddy current losses within the magnet/coil assemblies. When inductance figures are provided by the maker, they sometimes (but not often enough) specify the frequency. Knowing this is important to be able to characterise the electrical parameters properly, however the best you can expect is a figure for inductance at an unspecified frequency, and DC resistance. This is not enough to allow you to work out the real effects of loading on the cartridge's frequency response.
Using an inductance meter to measure the cartridge's inductance won't work! The DC resistance is high compared to the inductive reactance, so the meter will lie, and indicate that inductance is much higher than it really is. In addition, the test frequency is determined by the meter, and is unlikely to be appropriate for the task. Most meters don't even tell you what frequency is in use, so you don't get the opportunity to decide if it's appropriate or not (most will satisfy the 'not' criterion). A pickup I measured showed 1.55H (1550mH), which is silly - no cartridge will have that much inductance, however, the actual inductance calculated to be 1.15H, which is still silly and makes the cartridge pretty much unusable for anything other than very casual listening.
When the cartridge is measured, the amplitude rise with increasing frequency does not follow the ~6dB/octave one would expect from a 'perfect' inductor. This is partly because of the finite source impedance (I tested using 47k and 100k), but also because of the losses within the cartridge assembly itself that result in the 'semi-inductance' behaviour.
While I'd love to be able to tell you that I devised a simple formula to allow you to separate the inductance and semi-inductance to obtain an reasonably accurate model, I cannot. I figured out the circuit shown in Figure 1 by trial and error using a simulator - a tedious exercise to put it mildly. Another (completely different) cartridge I measured gave me the following data points ...
The error column referred to in the table is based on the figure that should be obtained if the inductor were a 'true' inductance, supplied from an infinite voltage via an infinite impedance (so don't fret too much that it can't be achieved). The low frequency end is of little consequence - the LF models perfectly due to the series resistance. At the higher frequencies, it is obvious that the effective inductance falls with increasing frequency. For this particular pickup, the inductance is fine up to somewhere between 2kHz and 5kHz, with the losses becoming more pronounced as the frequency increases further. The measured impedance response of the test cartridge is shown below.
Figure 4 - Ideal Vs. Measured Response With Cartridge As Load
The second test unit was subjected to an input signal so I could determine its parameters. The measurement data are shown in Table 1 (above). The red trace shows the simulated response (based on an ideal inductance), and green is the plot based on the measured values (I only tested this between 100Hz and 20kHz). To take these measurements, a signal generator puts a signal into the cartridge, via the normal 47k resistor. The voltages shown were measured across the cartridge (see the methodology shown in Figure 3). You can see that the green trace starts out with a little more level than the simulated (ideal) response, but is equal at ~1.5kHz, and falls below the ideal response at higher frequencies. This is the direct result of eddy current losses, which show that the 'semi-inductance' is a real phenomenon. As noted earlier, this is not the same cartridge shown in Figures 1 to 3 - it's a completely different unit.
Don't expect the slight loss of inductance at high frequencies to cause reduced attenuation at high frequencies - the signal amplitude will also fall as the losses increase. This too can be modelled, but to do so requires a great deal more complexity in the model, and it can't be verified by any sensible (i.e. non-destructive) test methodology that I can think of. Cartridge manufacturers often use cantilever resonance to attempt to get a flat response up to the highest frequencies, but this can add further complications. For example, a cantilever carrying a dirty stylus will be heavier than one where everything is nice and clean, and will have a slightly lower resonant frequency (and perhaps some additional damping as well). A change in HF response is likely, but it will probably be inaudible amongst the greatly increased distortion caused by the dirty stylus (plus, the vinyl will be damaged as well).
On top of everything else, there can be some interesting (but usually not good) phase anomalies created when any form of EQ is applied, and this is especially true of a mechanical resonance. Whether it actually causes audible problems is unknown (to me at least), but some  claim that the results are very poor. I can't confirm this, but I expect that the audibility effects may be overstated - at least to a degree.
The cable is another issue that must be considered. There is the cable that runs from the headshell to the sockets on the back of the turntable, and also the capacitance of the cable between the TT and phono preamp. I measured a fairly typical cable, and got a figure of 326pF for 1.2 metres of cable - this is not good, and IMO is generally far too high. By comparison, a 1.5m length of miniature RF coax (RG174/U) was only 155pF (close enough to 100pF/metre). Without extensive further research into cable types and their capacitance (outside the scope of this article), I am unable to make any useful comments on this. The issue with cable capacitance is that if it's too high, the only way to reduce it is to change cables - hit and miss at best. This also applies to phono preamps that have a shunt capacitor built in to the preamp - again, if it's too high, there may be no way to disable it - especially for commercial products that are still under warranty, and lack a switched capacitance option.
As always, beware of the snake oil! There are some utterly outrageous claims made for all cables, and tone arm/phono leads are no different. It matters not a jot if the cable is 6N pure (6 nines, or 99.9999% purity), and anyone who claims otherwise is lying. The use of Litz wire is common and fairly normal for the tone arm cable, because it needs to be very flexible to cope with swinging back and forth and up and down movements for years on end. Low capacitance is also highly desirable - remember, you can always add capacitance, but you can't take it away. The use of precious metals is a benefit for the contact areas, particularly gold because it doesn't tarnish. Silver cables are just a way to separate you from your money - they don't (and can't) sound 'better'. No double blind test has ever shown that anyone can hear the difference between any two cables with similar inductance and capacitance, regardless of price. Nor can any (other) differences be measured, even with the most sophisticated equipment. Cable distortion? Complete nonsense, provided that all connections are well soldered and wiping surfaces are free of oxides! You don't have to spend $1k/metre to get that.
Ultimately, there is only one way you can accurately characterise the response of any cartridge and cable arrangement, and that is to use a reference disc with recorded tones at different frequencies. This takes everything into account ... electrical characteristics, mechanical resonances, and anything else that may influence the cartridge's response. This includes the RIAA equalised preamp.
While this may ultimately allow you to get perfectly flat frequency response, this still does not guarantee that it will sound any good. It's also unfortunate (but true) that vinyl records don't like to be played over and over again, and display their displeasure by losing the high frequencies first. Test discs aren't cheap nor very easy to find any more, so the final adjustment may well end up being purely subjective.
It's little wonder that there is a vast discrepancy between the maker's specifications and (amateur) listener reviews with many phono pickups. With few exceptions, I consider commercial (magazine or Web) reviews to be useless, because it's very rare that any product gets the thumbs down, regardless of how badly it may perform. Unfortunately, any subjective assessment is also likely to be flawed unless it has been conducted using double-blind techniques - a very difficult proposition with phono cartridges.
Resistive and capacitive loading can alter the performance of a phono cartridge rather dramatically at high frequencies, and tone arm resonance can have a significant effect at low frequencies (although hopefully not with quality units). Since this is obviously the case, it's very hard to argue that the accuracy of a phono preamp's RIAA equalisation is especially significant. Certainly, it should be as close as possible, but any deviation of a dB or so either way is of little consequence. This is particularly true since no-one knows (and/or those who do won't say) what other equalisation was applied to the master recording or the disc-cutting lathe. What is important is that the two channels should track each other very well to preserve the stereo image.
Now that we can determine the cartridge parameters, we can go about determining the optimal loading for the cartridge. What we don't know is the effect of any HF boost caused by cantilever resonance, so we can only model based on the electrical parameters. In almost all cases, it is reasonably safe to assume that the lowest possible capacitance will give the flattest response, but I wouldn't want to bet on that. What we do know with certainty is that if the capacitance is higher than desirable, the response will peak at some frequency that's within the audio band (see Figure 2). Add cantilever resonance and any other effects that no-one will tell us about, and the results become highly unpredictable.
In some cases, the cartridge might benefit by using a higher than normal load impedance. See Figure 2 again, and note the blue trace. In this case, a higher load resistance means that even less capacitance is tolerable. The blue trace was done using the model in Figure 1, but with 100k resistance and 100pF. The capacitance has to be reduced to 30pF to get flat response!
It is probable that you will not be able to get capacitance much lower than ~100pF, although mounting the phono preamp within the turntable eliminates the output cable's contribution altogether. Other than using unshielded cables, this is the best way to minimise the capacitance. Unshielded leads of any kind are generally a really bad idea for phono pickups, because such leads will pick up any interference that is present. Hearing a random radio station or other noises rarely adds anything useful to the recorded material (although there may be exceptions with some 'music' genres).
Given that there is a practical lower limit for the capacitance, cartridges with comparatively low inductance are easier to work with, but they will generally have lower output. In general, we expect these cartridges to have an output level of around 2.5mV to 4mV at 1000 Hz, with a 5cm/sec recording velocity. Inductance of around 400mH or lower seems to be the most desirable, but this limits the range of available cartridges quite dramatically, and may still not give you optimum results. In very generalised terms, I suggest that anything over 550mH may cause problems with high frequency response, which must be augmented by cantilever resonance with any realistically achievable cable capacitance.
Phono pickup cartridges exhibit many different effects, many of which we are completely unable to model because the information is simply not available. It should be clear that most cartridges are likely to perform at their best with no more than 100pF ('typical' RCA lead capacitance) of shunt capacitance, although there will be exceptions. In some cases, personal preference will guide the decision that either a higher capacitance or higher load resistance gives a subjectively better result. It's also probable that some cartridges never manage to sound quite right in some systems.
At least one thing should be very clear - the common 'wisdom' that higher capacitance makes cartridges sound dull is obviously wrong. If the capacitance is too high, you will get a resonant peak at some frequency within the audio band, and this will often give the illusion of 'brightness', but the highest frequencies are lost. Figure 2 shows this very well - the peak shown with 500pF is +4dB at about 15kHz, and this will sound very bright indeed. If the cartridge has more inductance, the peak frequency is lowered, and can easily fall below 10kHz.
For reasons that I can't quite fathom, I was unable to find inductance data for any moving coil pickup. Not just the low output varieties, but the high output (around 2mV at 5cm/sec) types as well. It's obvious that the inductance will be much lower than a high impedance moving magnet/iron cartridge, but so too is the recommended load (or terminating) impedance for low impedance MC pickups. At 100 ohms, far less inductance will cause HF rolloff than with a higher impedance, but capacitance becomes irrelevant. No sensible cable will ever have enough capacitance to cause a problem.
This is probably one reason that the (low output) moving coil construction is thought by many to be 'superior' to moving magnet/iron types. Even high output moving coil pickups are likely to be looked down upon by many an audiophile (for example, those who also consider $10k speaker cables to be a bargain are likely candidates). There seems little doubt that many moving coil cartridges are extremely good - but one is still limited by the available source material, as well as the need for a very low noise 'head' amplifier or an expensive transformer.
The final result can really only be measured using a test disc and the cartridge of choice. Subjective 'listening test' evaluations may well give you a result that you like and can live with, but there is no guarantee that this will be accurate or result in an overall flat response. In the end, it doesn't really matter a that much - you listen to your system, and if you like the performance then you have achieved your objective.
Regardless of what you do, there will be discs that sound superb, and others that are rubbish. This probably has nothing to do with your system, but can be the result of over-enthusiastic equalisation or compression during mastering or cutting. It is unrealistic to expect that everything will sound good. This doesn't happen with CDs, SACDs, FM, DAB, Blu-Ray or any other medium, and to expect it from vinyl (with all its additional complexities) is ... well, ... unrealistic.
|Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott (Elliott Sound Products), and is Copyright © 2011 - all rights reserved. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.|