The following was copied from ‘The Gramophone’ magazine while it was freely available on-line, some years ago. That archive is now only available to subscribers to The Gramophone; but we venture to include it here, as it was available to all, at the time when it was copied.
Gramophone, February 1935.
TUNING PICK-UPS by P. WILSON
BEFORE I embark on the principal subject of these notes, there is one item of general interest in connection with current broadcast transmissions to which I should refer. During the past few months a marked blasting has become evident on many transmissions of speech and some vocal items, but never so far as I recollect on instrumental music. The blasting seems to be somewhere in the 7,000-cycle region ; it can be eliminated entirely by cutting off the frequencies above about 5,000 cycles.
When I originally encountered it I first of all suspected my speakers ; but change of speaker made no difference. Then I suspected my receiver and even went to the trouble of remaking part of it with some improvements which I had thought of since the original design was worked out. This made the blasting rather worse. After a while I began to notice that only particular types of microphone were guilty those used by the announcers principally—and I remembered that modifications have been made in the Reisz microphones used for speech. I am pretty confident the trouble is there. I do not get the blasting on foreign transmissions and I have checked up both on the Hartley-Turner receiver whilst we had it for test and by inquiry from friends who possess other types of ” high fidelity ” receivers. All disclose the same defect in the transmission, but few ordinary commercial receivers show it, presumably because their response does not go high enough in the scale.
I have refrained from mentioning the matter until I became quite sure of my ground, but now I feel that many readers would like to have the information in case they strike the same trouble. Of course, the remedy is for the B.B.C. to bring their microphones up to date. Quis custodiet ipsos custodes?
While there are very few pick-ups on the market which lend themselves to precise adjustments, I have found that something may be done to improve most types. I have not yet attempted to do anything in the way of tuning a crystal pick-up, and I hesitate to begin since the cartridge is hermetically sealed. This type must therefore be excluded from the notes which follow. Fortunately, I have found that the samples which have passed through my hands have had almost the identical characteristics of their class, standard or de luxe, so that the need for tuning is not so obvious.
The chief points of attack in the electro-magnetic types are the pivot clamping and the fish-tail damping. At one time I tried to make a moving-iron pick-up with “free pivots,” but I found that, do as I would, high notes were severely attenuated. if a reasonable high note response is required some form of spring control at the armature pivots seems to be essential. Usually this spring control is secured by clamping the armature in some way between blocks of rubber. The main reason for this is to enable the needle screw to be situated in the rocking axis, thereby reducing inertia. Sometimes it is supposed that these blocks of rubber are for the purpose of damping. But this is a mistaken notion. So far as the leverage motion of the armature is concerned, their damping effort is negligibly small, whilst for transverse motion it is non-existent—or should be if the rocking axis of the armature is properly chosen. It is quite feasible to obtain the required degree of spring control by means of metallic springs. E.M.G. do it successfully and other folk have only failed because the springs have been too strong and. the magnetic gaps too small.
Now the interesting thing about this spring control is this : As it is increased beyond a certain point a high note peak begins to appear in the response curve. Usually this is in the 3,000-cycle region. The location of the peak in the scale, however, does not seem to depend upon the magnitude of the spring control, but its strength does. As the leverage motion of the armature is made stiffer so the strength of the peak goes up, its position in the scale remaining unaltered. Evidently the peak is a characteristic of the armature and not of the control. A little research shows that it is the second harmonic of transverse vibration of the armature considered as a free free bar. The fundamental mode, and in fact every odd mode, of transverse vibration is suppressed by virtue of the pivoting of the armature and no amount of damping at the pivot will affect the even harmonics. This will sound very abstruse, I dare say, to many readers, but so long as you take my word for it and for what it implies it doesn’t much matter whether you understand it or not !
Most pick-ups get their high note response through transverse vibration of the armature and by virtue of this second harmonic peak. If the peak is too strong its magnitude may be reduced by slackening off the spring control at the pivots. But here a word of warning is necessary. The spring control has another function in most pick-ups : that of counteracting the overbalancing pull of the magnetic field. If you slacken off too much you will in effect arrive at the position of having a negative spring and the last result will be worse than the first. The order of the day, therefore, is a little at a time.
The sharpness of the high note peak depends on the damping in the pick-up. This damping must have resistance or energy-absorption characteristics and not those of a spring. The most effective positions for it are the two ends of the armature, that is, the needle point and the fish-tail. The former indicates one of the virtues of non-metallic needles they are all to some extent self-damping, though the triangular fibre is the most effective in this respect. The high note peaks are always flatter when non-metallic needles are used, so that the general characteristic is smoother and less keen, and since the production of surface noise depends to a large extent on the sharpness of a high note peak, the surface noise with a non-metallic needle is usually less pronounced. There are other, and not all of them happy, effects that arise from the use of non-metallic needles, but this is not the place to discuss them.
Most pick-ups have some form of damping at the fish-tail nowadays, but only too often this is not very effective in its absorption of energy at the particular frequency which is obstreperous. There is, unfortunately, very little space in which the damping arrangements can operate and in small quantities materials like rubber do not absorb high-frequency vibrations very readily. They function better if they are compressed, but in a pick-up the very compression has untoward effects on the leverage motion of the armature and therefore on the impedance to motion for low notes. This leads to excessive record wear.
Until recently the best material I had come across for damping purposes was that commercially known as Art Gum, but it is very fragile. Messrs. Rothermel have now sent me a sheet of soft Viscaloid which seems to me to be about as near perfection as ever we are likely to see and it is unaffected by water, air or light. Its damping efficiency is almost as great as that of the human thumb, than which there is no better !
I will wager that most pick-ups will be improved by the substitution of Viscaloid for the usual rubber damping. it is obtainable only in sheet form, either tic in. or in. thick.
Well, I must halt here for the moment. In a further article, I propose to set at rest the minds of a number of readers who have written to me asking for detailed instructions for tuning a Meltrope pick-up. The special points about that particular pick-up for this purpose are, firstly, that I happen to know all that is known about it *, and secondly that it is constructed so as to be easily adjusted. So, as Henry Hall would say, Here’s to the next time !
* Wilson is very modest here, for as we now know, it was he who designed the Meltrope!
Gramophone, March 1935.
TUNING PICK-UPS II by P. WILSON.
ALTHOUGH from this point on these notes are mainly concerned with the Meltrope pick-up, the principles involved, with two or three exceptions which I shall specify, apply equally well to most other types of moving-iron pick-up.
Look first of all at Fig. 1, which shows the pole-pieces and armature, the polarity being distinguished by the letters N, NI, S, Si. For clearness at this point the pivoting and damping arrangements and the pick-up coil, which fills the space between the pole-pieces, have been omitted.
As the armature is moved by the stylus with the fish-tail going over to the left, the magnetic flux from N through the armature to Si is greater than that from Ni up the armature to S, and so there is a net downward flux through the armature. The opposite result occurs when the fish-tail is moved towards S. In both cases, however, the upper pole tends to pull the armature farther over, and in the absence of any restoring force the armature would freeze to one or other of the upper poles.
Usually the restoring force is provided by the spring control at the pivots and in the fish-tail damping. In the Meltrope two additional magnetic restoring forces are provided. First of all note the shape of the magnetic gaps at the lower poles. They are such that when the armature is displaced these gaps both open. Since magnetic force always acts in such a way as to make the air gaps a minimum, the displacement of the armature sets up a restoring force at the lower poles.
The second magnetic restoring force comes from the use of magnetic balls in the damping arrangements at the fish-tail. I will deal with this later.
The points to notice at the moment are that to get the best results (1) The air gaps on each side of the fish-tail should be exactly equal when the armature is in its mid-position ; (2) The air gaps at the lower poles should likewise be exactly equal ; and (3) (in the Meltrope) the wedge-shaped projections on the armature should be mounted exactly in line with the corresponding wedges on the two pole-pieces.
Equality in the sizes of the gaps can be accurately judged by eye. If necessary a fine needle file should be used on the armature and/or the pole-pieces to obtain equality. But remember to remove the magnet (and keeper it with a piece of soft iron) before wielding the file. Otherwise, to remove the filings may be troublesome later on.
One additional precaution in the case of the Meltrope should be mentioned. The lower air gaps should be not less than -inch across, to the eye. The temptation is to have very small gaps in order to increase efficiency. There are two dangers in this. In the first place, owing to the wedge-shaped formation, a gap which seems to give a clearance before the armature is pressed down when the pivoting device is screwed up may in fact disappear altogether as soon as the wedges get into line. In the second place, if the gaps are very small it only requires a slight give in the rubber sleeving of the pivots to cause one of them to close up and thereafter the armature will rub against the bottom pole-pieces, causing a most objectionable distortion in reproduction. The expansion of the rubber when the pick-up is warmed may restore the status quo for a while, but the trouble will recur.
It is probably true to say that careful adjustment of pivoting arrangements will effect a greater improvement in all movingiron pick-ups than any other single thing.
The tightness of the spring control is important here, too. In the Meffrope, owing to the magnetic restoring forces, the mechanical spring can be much less stiff than in other types. Remember, too, that if the magnetic gaps are very small a stronger spring is needed. It is well worth while to sacrifice the larger output obtained by the use of smaller gaps to gain a more satisfactory pivot control.
Now a word or two about the real damping arrangements. Usually these take the form of a pad of rubber operating on a slight extension of the fish-tail above the upper pole-pieces. If the spring control at the pivots is adequate improvement in the way of smoothness of response can nearly always be effected by substituting for the rubber a material of greater energy-absorbing property such as Art Gum or, best of all, soft Viscaloid.
In the Meltrope, the Collar°, and one or two others the damping material is disposed in recesses in the upper poles. Usually these recesses are much too small to accommodate enough damping material. In the Meltrope a tubular cavity is cut right through each of the upper pole-pieces, closed at the ends remote from the fish-tail by screws (which, by the way, are locked by small grub screws on the faces of the pole-pieces), which can be used to adjust the damping pressure.
This damping arrangement has three different functions. First, as the armature is moved towards one pole-piece the iron balls on that side are attracted towards it more than those in the other pole-piece and thus exert a restoring force on the armature. This is the second magnetic restoring force referred to above. Secondly, part of the flux between the pole-piece and the armature is passed on through the iron ball, and since the washer next to the fish-tail is thin and has a small central hole, the gap between the ball and the fish-tail is quite tiny. If the washer is made of the right sort of springy material the response due to the passage of this part of the magnetic flux may be made to have a characteristic rising with frequency. This is especially useful since the response due to the passage of flux direct from the pole-piece to the fish-tail has a characteristic falling as the frequency increases. Theoretically, I suppose, it should be possible to obtain any desired balance between the two, but in practice I have not yet found any material for the front washer stable enough to give the required rise, without risk. IL can get a combined response ‘rising from 1,000 to 5,000 cycles by making the hole in the washer rather larger, but after a short time the ball begins to penetrate through the hole and to give a magnetic short circuit. Normally a slightly falling response is aimed at.
The third feature about the damping is that the mass reactance of the balls is opposite in sign to the spring reactance in the Art Gum and the two thus tend to cancel out, leaving a substantially resistive characteristic over a wide range of frequency, which, of course, is what is really wanted.
It will be appreciated, therefore, that the sizes of the various elements have been chosen after a lot of laborious experiment and should not be modified without caution.
The details of adjustment I must reserve for a further article.
Gramophone, April 1935.
THE TUNING OF PICK-UPS–Ill by P. WILSON
IF the features which I have described in previous notes are borne in *mind it is possible to retune a pick-up and make really startling improvements in its quality in quite a short space of time. I should say not more than half an hour on the average.
The principal points to remember are these:
(1) No adjustment will have a chance of being stable unless the two air-gaps at the upper poles are equal and those at the lower poles are likewise equal. In most pick-ups it is an advantage, from the point of view of efficiency of output, to have the lower gaps smaller than the upper gaps. In the Meltrope, however, this is not the case: and there is danger in making the lower gaps too small.
(2) The balance between high and low notes is mainly determined by the stiffness of the spring control at the armature pivots. As this stiffness is increased a high-note peak becomes more prominent. In the Meltrope, the magnitude of the highnote response is also affected by the characteristics of the upper-pole damping. Owing to this the stiffness at the armature pivots can be much less than in other types; in this way the mechanical impedance of the pick-up at the needlepoint is reduced and record wear is saved.
(3) The general smoothness of the response is determined by the fish-tail damping at the upper poles. This should be virtually resistive in character and should extend well past the frequency region where the high-note peak is situated. Unless this damping is sufficient the response is bound to be peaky and the surface noise will be pronounced and not distributed in character.
Now it will probably be convenient if I indicate briefly and in order the operations to be performed in retuning a Meltrope pick-up.
First of all, the nickelled cover-plate is removed and a keeper of iron, about 2 in. by ij probably 0.25” – NF) in., is placed across the upper poles. The grub screws on the front of these poles are then released and the end adjusting screws in the hollow poles are withdrawn. Most of the damping elements should then shake out of the hollow poles. The front balls, however, will probably stick and can be left for the moment.
Next unscrew the bottom square pivoting plate and pull out the armature. After this is done a piece of rubber tubing, or a pipe cleaner, can be pushed through the upper poles, thus removing the remaining damping elements. Be careful at this stage not to use such force as to break up the little rods of art gum, and not to lose any of the elements ; the tiny front washers can be very elusive.
Now look at the armature. It should be symmetrically formed without any burrs or projections left by the tools. Use a rat-tail file if necessary to ensure that this is so. The rubber sleeves which engage in the recesses in the pivoting plates will probably need renewing. I have not yet found a completely satisfactory material for this position ; red rubber tubing is the best I have come across and the size actually used is a standard article at many rubber stores. Note that the recesses in the pivoting plates are serrated. The object of this is to make the bearing surfaces so irregular that inequalities in the material of the sleeving become negligible. If the serrations have sharp edges, however, these can well be smoothed down a little with the point of a round rat-tail file so as to obviate the risk of cutting through the rubber sleeving.
Between the armature and the bottom pivoting plate is a little pad of red rubber. Preserve this : it is of special quality. Its main function is to seal the bottom of the pick-up and prevent magnetic dust finding its way into the lower magnetic gaps. It also exerts a certain restoring force on the armature, but this has been allowed for. Remove the pad and work it between the finger and thumb so as to restore its flexibility.
Now, having renewed the pivot sleeving, reinsert the armature and examine the gaps to see that they are equal and free from dirt. Try the armature both ways* to see which gives the better spacing, and remember that each gap should appear to be about in. across or rather more. If necessary, file the wedge on one side or other of the armature rocking plate so as to make the gaps equal.
Inspect both upper and lower poles again and clear away any metallic dust, reinsert the armature, put on the rubber pad symmetrically, and replace the pivoting plate. When screwing up the four screws in the latter, make sure that the armature fish-tail rests evenly in the gap between the upper poles.
Having screwed up as far as possible, insert a needle collet and move the armature from side to side, making sure that the motion is free and spongy, but resisted equally on both sides of the mid-position ; any feeling of rubbing or jerkiness at this stage is fatal to good results. Remove the magnet-keeper temporarily : the armature should just click over and adhere to the upper pole on the side to which it is displaced. Anything more than a bare click will mean a poor response to high notes.
Having satisfied yourself on these points, replace the keeper and begin to insert the damping elements in this order for each upper pole (1) The thin washer. Ram it down with a piece of rubber tubing (I use white sound-box gasket since it is more easily seen) so that it bears centrally on the fish-tail (2) A ball; (3) A cylinder of art gum; (4) A second ball; (5) Two cylinders of art gum; (6) The adjusting screw with rubber tip.
In the first pole tackled, only half screw up the adjusting screw until the elements have been introduced into the other pole. Eventually the ends of the adjusting screws should be flush with the pole faces.
Having tightened up the grub screws and removed the keeper once more, you are now ready for a trial. If the surface noise is coarse and low-pitched, the screws for the pivoting plate are not screwed up enough, and possibly the adjusting screws in the upper poles are too tightly screwed up. Aim at getting the reproduction rather on the keen, almost nasal, side to start with. Within 24 hours, when the elements have settled down, the keenness will go. If it is found necessary to adjust, tightening the pivoting will bring up high notes and adjusting the damping will smooth them out. If the damping is too fiercely screwed up, however, the quality will become tight ” and restricted and the voltage output will fall substantially.
Do not adjust the damping screws, however, when the coverplate is on or without releasing the grub screws, and do not forget to tighten the grub screws after each adjustment. You must make sure at all stages of adjustment that the fish-tail lies centrally between the upper poles and the resistance to motion on each side should be equal. This should be the case whether the keeper is across the poles or not : there should be both mechanical and magnetic balance.
Finally, remember that, as in other matters, expertness in tuning only comes with practice.
* This only applies in cases of armatures fitted with needle collets.
Unfortunately, the diagrams to which Wilson refers, were not available to us on these old downloads.