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Retreiving The Best signal from The Groove


Page created February 2003

The Finishing Touch

A Word About Arm Mass and Compliance, MM - Capacitance,
MC - Impedance Loading, Replacement Styli, Break-in Time,
Climate and Seasonal Influences, and Playing Back Old Mono Records.

The Moving Magnet Cartridge
Phono Cartridge Moving Magnet - Original drawing by Rudolf A. Bruil


The signal in the record groove is followed by a diamond tip. This tip is glued to one end of a thin rod (cantilever) made of aluminum, boron, beryllium, or sapphire. At the other end of this rod a small magnet is attached. It moves close to a coil or in the space in between coils. The stronger the magnet and the larger the coil, the stronger the signal will be. The arrangement of magnet and coils (the topology) is of course of great importance. Manufacturers have often designed their own, special construction. Examples are Audio Technica and Satin both imitating the mechanical construction of the cutter head as much as possible.

Magnet and coils determine the quality of the signal being picked up: strength, frequency characteristic, harmonious build up, and the level of distortion.
The aim of course is to retrieve the maximum signal from the groove by optimizing the mechanical technique (the functioning) and by the application of specific materials for the various components: oxygen free coil wire, core material, magnetic material, rubber damper, cantilever, diamond tip, connecting wires, cartridge housing, and the distances between the individual parts and the arrangement (topology).
All these were as important in the early days as they are today. The final verdict is by listening to the cartridge in the appropriate arm, well set up and connected to the phono stage with the appropriate specifications (Ohm and picofarad).

A moving magnet cartridge should be connected to the phono stage of your (pre-)amplifier. This phono input should be marked MM. It should have the right capacitance to make the cartridge sound best. Consult the technical specifications of the preamplifier.

Test reports and specifications of cartridges should include the value of capacitance. These will tell if the cartridge can be optimally connected to the preamplifier you use.


The capacitance of the input of the amplifier should more or less match the value of the cartridge.
Of course a lot of scientific explanation can be given. In this respect mathematics and algebra are there for the technical buff. Just follow this rule:
The higher the capacitive value of the input of the phono stage, the duller the sound will be. The lower the capacitance of the phono input, the brighter the sound gets. A mismatch results either in dark dull sound or an overbright, distorted signal.

Naturally the capacitance of the phono cable should be taken into account when matching a moving magnet cartridge to the phono stage as the capacitance of the cable adds to the capacitance of the phono input.

The Moving Coil Principle

The drawing shows the principle of the cartridge:

Coils moving in a magnetic field. The movement generates a changing electric energy.

All parts and also the arrangement (the topology) are of course of great importance. They determine the quality of the signal being picked up: strength, frequency characteristic, harmonious build up, and the level of distortion.

The aim of course is to retrieve the maximum signal from the groove by optimizing the mechanical technique (the functioning) and by the application of specific materials for the various components: coil wire, core material, magnetic material, rubber damper, cantilever, diamond tip, connecting wires, cartridge housing, and the distances between the individual parts and the arrangement (topology).
All these were as important in the early days as they are today even if cartridge builders may choose different or new materials and maybe a slightly different construction, but the main principle developed by Ortofon is still the starting point of the designer.

Phono cartridge MC principle - Original diagram by Rudolf A. Bruil

Matching Transformers & Pre-Amps

 




Nakamichi produced the MB-150 preamp with an input impedance of 56 Ohm suitable for cartridges with 20 Ohm coils or thereabout. Kenwood manufactured the KHA-50, a phono preamp with an input impedance of 100 Ohm, exactly the value for the 40 Ohm Denon 103 and 300 series

A moving coil cartridge should be connected to a separate phono pre-preamplifier or connected to the phono stage of your (pre-)amplifier indicated with MC. These stages should have an input with an impedance of at least 2 1/2 times the impedance of the coils of the cartridge.

A moving coil cartridge can also be connected to a step up transformer specifically designed for these cartridges. Here the impedance value of the input of the transformer should be equal to the impedance of the coils.

That is why you should follow this rule: do not buy any moving coil step up transformer available, but one that matches the cartridge you use. The descriptions of the step-up transformers on this page tell exactly what it is all about.

Coral T 100 Moving Coil Step Up Transformer - Impedance: 6 Ohm / 47 kOhm - Frequency Response: 5 - 50.000 Hz. Gain: 26 dB - Crosstalk: 34 dB - Weight: 130 gram.

Ortofon STM 72 Transformer for Ortofon SL-15 series, MC-10 and 20 - Frequency response 20 - 50.000 Hz., cartridge impedance 2 Ohms, and load impedance 10 - 15 kOhm.

As the Ortofon MC cartridges with their 3 Ohm impedance were very popular, manufacturers often designed a step-up transformer with an extra input for 3 Ohm cartridges. That is why Denon transformers do have the 40 Ohm load connection for Denon DL-300 series phono cartridges and a 3 Ohm load connection for Ortofon.

This is the simplest of Denon transformers: AU-310 with simply a 40 Ohm input.

Fidelity Research produced transformers specifically designed for their own cartridges. These transformers are not strictly suitable for other makes of cartridges, unless the impedance does match of course.

A transformer for Audio Technica cartridges should have an input with a 20 Ohm load. The Akai MT 200 can also be used for AT cartridges because of its input load of 20 Ohm.

If you use a preamplifier the technical specifications of both cartridge and preamplifier stage will tell you if the matching is correct.
It is of course possible to exceed the figure of 2.5 times. Say for a cartridge measuring 40 Ohms, the impedance of the MC section of your preamplifier should have a value of at least 100 Ohm. In certain preamplifiers the impedance of the Phono Input can be adjusted. Certain manufacturers of cartridges prefer/advise to use a higher figure than 2.5 times the impedance of the coils of their cartridges. For the same 40 Ohm cartridge they can advise 250, 500 or even 1000 Ohm. A higher value gives a more slender sound and gives less "massive sound". A higher value can be achieved by inserting a resistor in parallel, but that may also alter the sound quality and the punch (transient).Generally the impedance of the phono stage is an integral part of the circuit design as calculated by the designer.
You can alter the value if the impedance. If the value is rather high for the cartridge, it is possible to solder a resistor in parallel (inside the connector for instance). But then the sound may get slow, less defined and will have less slam (bad transient response).Sometimes it is logical to connect a separate transformer or phono preamplifier if the stage of the existing preamplifier results in a mismatch.

Check the specifications of your cartridge and of your phono preamp, preamplifier or step-up transformer and make corrections if there is a mismatch.

Compliance: Springs, Resonances and Mass


After you have found that downforce and loading are correct, it still can be that the low frequencies are not firm enough and that the mid and high frequencies have a fuzzy character and lack detail and definition.

Or it may be just the other way round: the sound is muddy, has a heavy quality and lacks speed.
Lightness, speed, firmness and correct tonal quality, they all depend on the right choice of arm for your cartridge. Or to be more precise: the right combination of the effective arm mass and the level of compliance of the phono cartridge, which actually is the elasticity, the easiness with which the diamond tip can be moved.

As the techniques of recording and record production and manufacturing were improving through the years and attained unquestionable heights, the manufacturers of turntables were conducting research and were designing new turntables which incorporated specific solutions in order to make the record player function in such a way that it does not interfere with the delicate signal contained in the groove.
It was more and more apparent that the isolation from the environment plays a key-role and had to be improved. Vibrations from the motor should be eliminated completely. Mechanical feedback (resonances) via floors, cabinets and racks also should be minimized or possibly eliminated completely. Hence the introduction of the floating chassis which was first incorporated in the famous Acoustic Research belt-drive turntable devised by Mitch Cotter. His construction deals with these two problems in a most effective way.

Suspension of floating chassis and the cantilver functioning as a spring to de-couple the cartridge - Original drawing by RABspring

The cantilever of the cartridge functions as a spring. The resonance of this spring should be in the 8 to 12 Hz. band. The suspension of the turntable should be much lower so it will not interfere with the functioning of the cartridge.

The use of springs is an effective solution. The (de-)coupling of platter and arm can also be achieved by means of rubber feet instead of supple springs. Another, very popular method applied to modern turntables is the use of cones or spikes. Because of the reduced contact surface of a spike, only very high frequencies can be transmitted and low frequencies are stopped completely. With large rubber feet the coupling of lower frequencies is taking place to a certain degree only, because of the large surface, and the midband and high frequencies may be damped somewhat.

The complex recorded signal is picked up by the cartridge which should always be fitted firmly at the end of the arm. Never loosen the cartridge screws or place a thin plate of rubber, metal or other material in between the cartridge and the headshell. If you do, the result will be an incorrect attack and no real transient at all. Tonearm and cartridge are to be considered as an entity.

The suspension of the turntable should have a very low frequency so it does not interfere with the music.

The cantilever of the cartridge also functions like a spring and (de-) couples the arm at the cartridge's end from the record, platter and base, while at the other end the arm is fixed at the pivot (arm base).

A heavy object needs a strong spring. Likewise heavy arms are only suitable for cartridges with a sturdy cantilever. In other words: a cartridge with a low compliance figure needs a heavy arm in order to obtain a fundamental resonance in the region of 8 to 12 Hz.
A lightweight arm needs to be matched with a supple spring, in other words a very compliant spring which is the cantilever of a high-compliance cartridge.

The mass of the tonearm can vary from 4,5 grs. of an SME 3009 III to even 38 grams of the Fidelity Research FR-66S. And the Ortofon SMG 212 for instance has such a heavy counterweight that the use with moving magnet cartridges is not advised. That arm was meant for heavy Ortofon MC cartridges for which the step-up transformer was incorporated in the headshell. These Ortofons had a low compliance.

The very heavy Fidelity Research FR-66S has an effective mass of 38 grs.
With the Fidelity Research moving coil cartridge FR1-Mk3F the resonance is
very low: 6 Hz. Also the FR-64(S) arm has a mass of 30 grs.
which still is too heavy for a Denon DL-103

In all cases the resonance of the arm-cartridge-system should be kept outside the audioband and should not be disturbed by the fundamental resonance of the suspension of the turntable. There should be a clear division between turntable, arm and audio signal.


Turntable suspension: 2 to 4 Hz.

Arm-cartridge resonance: 8 to 12 Hz.

Audio band: 20 to 20.000 Hz.

Naturally you will consult the literature of the cartridge manufacturer and check reviews and technical tests in order to find out what arm mass is advised. Likewise you will try to find the best cartridge for a given arm. And: you can measure the cartridge-arm resonance by using a test record (if you have one) and see if it is in the right region.

The question remains: What is the frequency of a specific arm-cartridge-combination that makes the system sound best? Is it 8 Hz.? 12 Hz.? Or somewhere in between, say 9.4 Hz.?

This you can find out by carefully listening to the tests of a special technical record, or just the music you are used to. So if the sound is muddy, has a heavy quality and lacks speed, than the mass of the arm is too high. You can try to find a lightweight headshell and see if that helps. But you probably will come to the conclusion that the only thing you have to do is to buy a cartridge with a lower compliance which is suitable for the heavy arm, or buy a lighter arm suitable for the cartridge you already have.

If the low frequencies are not firm enough and they miss power, and the high frequencies sound fuzzy not well defined, than the arm mass is too light for the cartridge you are using. Try a heavier headshell or even a heavier arm.

In that case the situation is better because you just have to add a bit of weight to the arm. Sometimes a little piece of metal of just one gram glued on top of the headshell can already mean a significant improvement. After adding the weight you have to adjust the downforce and the antiskating force anew. Here also listening carefully will tell you if you have to add more or less mass.

Another important factor is the distribution of the mass of the arm.

If you find that the arm mass should be greater you should nevertheless take care that the added weight is distributed more or less evenly over the length of the arm.

If the arm is too light to match the correct compliance of the cartridge, adding a little piece of metal to the headshell will lower the resonance and can improve the tracking, and the signal will show less distortion.

In this way you can optimize the quality the functioning of the cartridge and the retrieval of the signal out of the groove so that the most natural sound will be heard through your speaker systems.
Do not think it is done in a minute or two. Take your time. Because after optimization you might think you did the job, until you listen the next day and find that it needs readjustment.
Good luck and good listening.

Mounting the Cartridge and Arm Base
Ever since the long playing record exists one standard question has been "How tight should a cartridge be screwed to the headshell?" Another important question was "How rigidly should the tonearm base be mounted to the plinth?"
Yes, turntable, arm and cartridge should be considered as one entity. Yet the answers to these questions are important.

There are a few spots were a mechanical connection infuences the reproduction:

 


1. armbase-plinth
2. ball bearing / knife bearing (armbase-arm tube)
3. headshell-arm
4. cartridge-headshell


In case you buy a readily assembled turntable like a Technics SL 1200, you only have to worry about the connection of headshell and arm and of cartridge and headshell. And even then you can omit the mounting of the cartridge if you by a ready combination like the Ortofon Concorde.

Take care that the headshell is firmly secured by turning the nut to the maximum. There should be no play. Also be careful when mounting the cartridge in the headshell. Check the result o a tight and less tight connection. The best fit is the one that provides detailed bass, a good transient, an extended sound characteristic, and no coloration.

If you are a music listener who wants the ultimate in refinement, most likely you will have a turntable on which you mount the arm and cartridge of your own choice. In that case you may experience that the quality of the signal changes with the way the arm and cartridge are mounted. If the armbase is too tightly bolted to the plinth, the sound will loose openness and warmth. The same goes for the cartridge and the headshell. The possible influence of the bearing should not be a concern as the bearing should be well adjusted by the manufacturer who has the least friction ion mind and the less play. Only in case of an ill treated second hand arm the bearing needs a closer look and maybe adjusting.

In general this is how you can go about 'optimizing' the mechanical performance of the arm/cartridge/turntable. Take a record which sould be reproduced with a full blooded sound, yet with detail, and no distortion. Play the choosen fragment and listen carefully. Assess the quality. If the sound lacks subtlety, losen the screws of the arm base by half a turn and play the music again. If there is improvement, leave it for the moment. Now check the connection of the cartridge in the headshell. Adjust the screws in such a way that the screws are indeed not too tight, and listen again. If the connections are too tight, the reproduction will show distortion. This means that you will have to adjust and readjust until you will find the best sound, that s the sound without stress, the sound that is clean and at the same time is as natural in character as can be.

Replacement Styli for Moving Magnet Cartridges

& Replacing an MC Cartridge


Many music lovers go for a moving coil cartridge when it comes to a precise and natural reproduction. After all the record was cut by a diamond moved by coils. Others prefer the sound of a moving magnet cartridge when they talk about high fidelity. They find that a moving magnet cartridge has more speed than a moving coil cartridge and therefor gives a better transient and is more pleasant to the ear. If you have more than one turntable, you can of course use both techniques.

Each design has its specific positive features.

Yet the moving coil adepts have it easier when the time comes to replace a worn needle tip by a new one. For them the road to constant happiness is a straight one. They just exchange their used cartridges for a new of the same brand, same type. And they will seldom be disappointed.

The performance of the moving magnet cartridge is not always guaranteed.

Especially when you are using an older cartridge for which original styli are no longer available, you depend on the needle of a different manufacturer. Generally these needles are not of the same quality as the original ones. This is because the materials used are not the same as used by the original manufacturer:

1) rubber damper (suspension block),
2) magnet (how large is it and of what type, material),
3) the cantilever (how strong is it, of what material is it made, what is the shape and mass),
4) and the parameters of the diamond tip. Is it a nude diamond, what is its shape, what is the mass, and is it well polished?

If you order a needle for a discontinued cartridge, be sure it is an original specimen. If that is not available, check the functioning of the replacement stylus and compare it to the original one, both by eye and by ear.

It is generally impossible that the replacement made by a different factory will meet the standards of the original stylus.

But the difference in sound should not be too great. If a replacement stylus does not perform well enough, you should contact the seller. Because an impaired stereo image, a limited frequency band, and the lack of openness, will only irritate the listener. Of course, the suspension of the stylus needs a break in time. You need more than one week. This is certainly true. The replacement stylus does not show the qualities right from the moment you have put it into the cartridge.
In order to avoid these problems it is advised to buy a modern quality cartridge of which original needles will be available for at least another five years. Another option is to send the MM cartridge to a service for a new tip placed at the end of the original cantilever. That will guaranty another 2000 hours of enjoyable music listening. Ask The Cartridgeman or Alt Jouk van den Hul for possibilities and prices.

Burning in and seasonal influences.

Each new cartridge needs many hours of playing time to make the suspension of the cantilever supple and bring the compliance of the cartridge to the original values as specified by the manufacturer. In the beginning the downforce will need a bit extra weight. After a week or so of playing, the downforce can already be adjusted. If the down force is too low or too high can easily be heard. And do not forget that the climate plays its role. During a hot summer the temperature can improve the tackability of the cartridge and you may choose a slightly lighter down force. But in autumn - and certainly in winter - the downforce may need a little adjustment. It all depends on where you live and the way you heat your listening environment.

RIAA Curves, Characteristics, Turnover, Rolloff

Gradual Attenuation of Low Frequencies

The long playing record can not contain a normal dynamic level of frequencies in the lower register. Therefore the low frequencies have to be attenuated when cutting a record. If the bass level would not be attenuated, any cartridge would jump out of the groove right after the moment the needle touches the groove. Sounds in a linear curve cannot be engraved.

Boosting the High Frequency Content

On the other hand, loud high frequency sounds can be engraved far more easily. They have minute amplitudes, the dynamics can easily be increased when cutting the lacquer disc.

It is therefor practical while attenuating the low frequencies, to boost the high frequency content at the same time. Hence the signal is corrected when cutting a record.

Improved Signal to Noise

When playing back a record it is necessary to correct this odd frequency characteristic again by boosting the bass and attenuating the high frequencies.
Attenuating the high frequencies to their normal level has the advantage that the signal to noise ratio is improved because the surface noise of the vinyl is attenuated at the same time. (This phenomenon inspired Ray Dolby to devise his Dolby Noise Reduction System for tape and cassette recorders.)

Rumble

The disadvantage of boosting the lower frequencies is that the rumble caused by the cutting lathe and also the rumble of the motor and transmission in the record player/turntable will be amplified too. Hence the importance of a rumble filter and the necessity of practically non existent rumble values in a turntable.

What Curve?

In the early years record record companies used different values for attenuating the lower frequencies and amplifying the high frequencies when cutting a lacquer. There was no standard adopted by all companies. In those days gramophones and amplifiers had a knob for switching on various circuits. They were marked Columbia, 78 rpm, Decca, etc. Each circuit gave the appropriate turnover frequency, the boost of the low frequencies and the attenuation of the highs.

But what to do if you were playing back a record of another manufacturer. Which was the best selection?

 

DIALING YOUR DISCS

In the early mono days when not everything concerning records and recordings was standardized, High Fidelity, the American magazine for music listeners, regularly published tables with data for adjusting the characteristic of the frequency curve when playing records of various manufacture.

Not all records were cut with the same characteristic. Important are the turnover frequency and the roll-off. The table at left gives data for most record labels. The rolloff for the RIAA curve was 13.7 dB. Several record labels did not comply with this standard. That is why many old recordings do sound differently. It is important to take the number of decibels into account. Strange to see that the value for His Master's Voice, Westminster and Remington was 16 dB instead of the prescribed 13.7 dB. Concert Hall only needed 10.5 dB attenuation, the lowest figure in the list.

For the die hards who can and will afford it, there are preamplifiers and special units available today that allow the listener to choose the appropriate turnover frequency and the amount of bass boost and treble rolloff. The table at left can be of help, although these units do allow you to adjust the playback characteristic to your own taste as well in relation to the rest of the chain.

This table was copied from the October 1955 issue of High Fidelity Magazine.

McIntosh Laboratory, Inc. designed a Professional Audio Compensator which made it possible to make adjustments for characteristics of all available record labels to ensure correct playback curves.

(Advertisement from High Fidelity Magazine of July, 1954.)

Want to know the technical specifications of cartridges?

Go check this CARTRIDGE DATABASE.

Not for all cartridges in this database the pF (capacitance) values of the MM cartridges are given. The best way is to determine the best match with a specific (pre-)amplifier of which you have the specifications of the Phono Input and add it to the capacitance of the phono cable you use. If necessary choose a cable with less capacitance or a cable with a higher pF.

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Audio&Music Bulletin - Rudolf A. Bruil, Editor - Copyright 1998-2012 by Rudolf A. Bruil and co-authors