US2258990A - Electric piano - Google Patents

Description

' Oct. 14, 1941. E. s. LUNDIE ELECTRIC PIANO Filed March 30, 1940 I J Zinuentor Ernest 6'. Lundze Gttomeg Patented Oct. 14, 1941 snsc'rmc rmno Ernest S. Lundie, Haddon Heights, N. 1., asaignor to Radio Corporation of America, a corporation of Delaware Application March so, 1940, Serial No. 326,825

This invention relates to electric pianos of the 3 Claims.

type using strings in more or less the conventional manner but in which the strings are associated with electromagnetic pickups which convert the mechanical energy of the string into electrical energy which is thereafter amplified and reproduced through electrical reproducing means.

In the construction of such pianos in the past, there hasusually been considerable difliculty due to electrostatic and electromagnetic pickup of energy from extraneous sources, and this has been particularly noticeable when high amplifications were used. Various methods of preventing such pickup have been heretofore proposed which have not been altogether satisfactory for various reasons. It has been proposed, for example, to use single-ended pickups consisting of a bar magnet supporting a single winding and to reverse the connections of alternate coils, all the coils being connected in series. Such an arrangement is unsatisfactory because the windings and magnetic power required vary according to the location of the string in the piano and the windings therefore do not have uniform pickup of extraneous energy. Further, such a direct connective coupling of all the pickups permits any electrostatic impulses which may be applied to the circuit including magnets to be transmitted to the amplifier. A second form of avoiding pickup is described and claimed in Demuth application Serial No. 265,276, filed March 31, 1939, wherein equal numbers of similar pickup coils are connected to separate transformers hav- 1 ing their secondaries connected in opposition or are connected to independent primaries of a transformer having a single secondary. In such an arrangement, neither electrostatic nor electromagnetic extraneous pickup is transmitted to the amplifier provided the pickups are all identical. However, in some cases, it is desirable to use pickups of different magnetic properties or having different numbers of windings, and, in such an arrangement, the Demuth circuit is not satisfactory.

The magnetic pickups in themselves may be of the nature described and claimed in Demuth application Serial No. 232,526, filed September 30, 1938. In the said application, each pickup consists of two flat strips of material secured to a backing plate and cooperating with the strings corresponding to one note. One of the strips of material is necessarily of a-permanently mag-- netizable material, for which the alloy commerother strip may be of soft iron or steel for most of the notes. In such an arrangement, the pickup winding is preferably placed on the soft iron core member. For some notes, particularly those in the extreme treble, where the amplitude of vibration is small, it may be desirable to pick up additional energy and this may be accomplished by putting a second similar winding on the magnetic alloy pole pieces. Also, in the caseof the extreme bass strings, the amplitude of vibration is relatively large and the magnetic portion of the string forming the core thereof may be separated from the magnets by copper windings on the exterior of the string. In such a case, it is necessary that a correspondingly more intense magnetic field be used and this may be accomplished by making both of the core members of the magnetizable alloy and by placing pickup coils on both of those core members.

The foregoing modifications of the pickups render them nonuniform in their pickup of extraneous electrical energy. For example, a winding on a soft iron core will pick up extraneous energy with considerably greater intensity than will a similar winding on a magnetized alloy core and, if all the windings were connected as directed in the said Demuth application, the pickups of extraneous energy would not be balanced, and the apparatus would have a resultant A. C. hum and other induced interference'.

The apparatus of the present applicationis an improvement on that of the said Demuth application Serial NO. 265,276, filed March 31, 1939,

in that it uses the double transformer winding arrangement in the output circuit, but it differs therefrom in the manner of balancing the connections to the said output transformer. In the apparatus according to the present invention, some pickups having soft iron or steel cores are connected into each of the output transformer primary circuits, and some coils having magnetic alloy cores are also connected into each of the output transformer primary circuits, the num-,

ber of coils of each type connected to eachcircuit being so chosen as to give a balanced result.

One object of the invention is to provide an improved electric piano.

Another object of the invention is to provide an improved manner of connecting electromagnetic pickups to eliminate the pickup of undesired signals.

Another object of the invention is to provide an improved connection for electromagnetic cially known as alnico" may be used, and the pickups in an electric piano which will provide improved quality of. sound reproduction therepickups in an electric piano which will provide improved quality of sound reproduction therefrom with a minimum of reproduction of electrical interference.

Other and incidental objects of the invention will be apparent to those skilled in the art from a readmg of the following specification and an inspection of the accompanying drawing in which the single figure is a schematic diagram of the connection of pickups to the output circuit in my improved electric piano, the pickups themselves being indicated diagrammatically in their proper relative positions.

In the drawing, the permanently magnetized pickup units which may be of Alnico are indicated by rectangles as shown at 22, while the soft core pickups are indicated at 23. me to the small scale of the drawing, the windings it, respectively. It will be noticed. however, that the t four notes in the unit it are PTO. vided with coils on the magnetized cores as well as on the agnetized cores. When this region of the piano is reached, not only are the strings wound with copper but the windings have become relatively heavy, thereby causing the pickups t bemparated a greater distance from the magnetic core of the string and requiring correspondingly more turns for a corresponding amount of pickup.

on the individual coils and the connections therebetween have not been shown but the connections to the end of each group of coils are shown. The pickups are preferably arranged on a series of soft. steel backing plates numbered it to it, inclusive, and are arranged in proper relation to the strings of the piano. The core members are pressed into apertures in these backing plates and the backing plates form a portion of the magnetic circuit of each pickup.

0n the plates i0 and II which carry the pickups for the notes in the higher portion oi! the treble section, the rectangular cores are arranged as shown substantially parallel wih the edges of the plates so that they will extend across all three strings corresponding to each oi these notes. On. these two plates, pickup coils are provided both on the magnetized and on the unmagnetized core units as these strings are very short and their amplitude of vibration is very small. It will be apparent from an inspection of the units it and ii that the coils on the magnetized cores 22 of the plate it are connected in series and these are then c nnected in series with the cells on the m u etized cores 23 of the plate ii. Similarly, the coils on the unmagnetized cores 23 of the plate 80 are connected in series with the coils on the magnetized cores 22 of the plate N. This arrangement provides an equal number of each type of coil in each circuit from this portion of the piano. In the unit 82 which cooperates with strings having a greater amplitude of vibration than those at it and ii, pickup coils are provided only on the unmagnetized cores at 23 and these coils are connected in series with the coils on the magnetized cores of the unit it and the coils on the unmagnetized cores of the unit It. Similarly, in the unit it 'coils are provided only on the unmagnetized cores and these are connected in series with the coils 'on the unmagnetized cores of the unit and those on the magnetized cores of the unit it. It will be noticed that the cores on the units 86 and it are turned at considerably more of an angle to the edges of the supporting plate. This is for the reason that these pickups cooperate with notes using only two strings to each note and this relative turning of the core members permits the entire magnetic field to be concentrated on those two strings. The coils on the magnetized cores of the units it and it are connected in a similar manner to those on the plates 52 and In the unit numbered 66, coils are provided on both the soft core and the magnetized core for all the strings in brder to provide suificient pickup. It will be noticed that the cores of this unit are set at apprommately the same angle as those of the unit numbered 95 to accommodate the double strings for each note.

In the unit numbered it, the coils are provided and connected as in the unit it but in this case the cores are turned at a still greater angle to the edge of the plate, since they cooperate only with tlie single bass strings. It will further be noticed that in the pickups corresponding to the lowest 6 of the base strings both of the cores are made of magnetic material so as to provide. a more intense magnetic field. This is for the reason that these strings have much heavier windings than the others, increasing the space from the magnetic material of the string to the pole pieces of the pickups to a considerably greater extent than in the case of the other strings and, due to their large amplitude of vibration, these strings must also be spaced a greaterdistance from the pole pieces to avoid mechanical contact.

It will be, noticed that the coils of the unit Iii having magnetic coijes are connected in series with the coils of the l'l ofthe upper tier including those which have unmagnetized cores as well as a few of those having magnetized cores and conversely th coils on the unmagnetized cores of the unit ii are connected to the coils on the magnetized core group of the unit I? so that these connections are in the same general relation as. those of the units i0 and ii. It will be noted that there are more magnetized cores than there are 'foi, the unmagnetized cores, but this does not interfere with the securing of the proper balance, since the four coils of the unit I5 are connected to the side of the circuit which will produce proper balance.

The arrangement of parts shown in the drawing is substantially that which appears in an electric piano viewed from the back and the output leads irom on end of the pickups pass downwardly from the high frequency end to the ampliher while the leads from the bass section pass a relatively short distance to the amplifier which is in approgziiately; the position indicated by the transformer. it. ;Due to the greater length of leads from the high end of the frequency pickups to the ampiifienthere is more tendency for electrostatie pickups in these leads and I accordingly preier to ground the high frequency end of the primary windings l9 and 20 to prevent any such electrostatic pickup from reaching the windings. One end of the secondary winding 2! oi reproduction ,over the entire range of the piano but also provides substantially no inductive pickup from evenvery intense electromagnetic I fields.

I claim as my invention:

1. In an electric piano, a plurality of vibratible elements, a plurality of electro-magnetic pickups in cooperative relation with said elements, means connecting said pick-ups in two matched groups to two output transformer primaries, and ground connections at the high frequency end of each of said primaries.

2. In an electric piano, a plurality of vibratible elements, a plurality of bi-polar electro-magnetic pickups including permanently magnetized members and soft core members, pickup coils on said soft core members and on some of said permanently magnetized members, an output circuit of coils in each group being determined by the total pickup of those coils.

ERNEST S. LUNDIE.

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