US2220350A - Sound reproducing system for musical instruments - Google Patents

Description

NOV. 5, 1940. s PURINGTQN 2,229,350

SOUND REPRODUCING SYSTEM FOR MUSICAL INSTRUMENTS Filed March 4, 1959 2 Sheets-Sheet 1 POWER AMF'L! FIER POWER AMPLIFIER INVENTOR ELLISON S. PURlNGTON BY I I A ORNEY E. s. PURINGTON 2,220,350

SOUND REPRODUCING SYSTEM FOR MUSICAL INSTRUMENTS Filed March 4, 1939 2 Sheets-Sheet 2 POWER AMPLIFIER INVENTOR F S E ELLISON S. PURINGTO'N Patented Nov. 5, 1940 PATENT OFFICE SOUND REPRODUCING SYSTEM FOR MU- SICAL INSTRUMENTS Ellison S. Purington, Gloucester, Mass., assignor to John Hays Hammond, Jr., Gloucester, Mass.

Application March 4, 1939, Serial No. 259,731

5 Claims.

This invention relates to musical instruments having vibrating elements and more particularly to a piano having electrical sound propagating means associated therewith.

The invention provides for the use of an auxiliary oscillator, the operation of which is controlled by the variation of the capacity of the pick-up device,- which may be made to cause m amplitude, frequency or phase modulation of the auxiliary oscillator frequency. Audio frequency tonal currents may be produced by suitable demodulation of the oscillator output.

The invention also consists in certain new and [5 original features of construction and combinations of parts hereinafter set forth and claimed. Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the -invention itself, as to its objects and advantages, the mode of its operation and the manner of its organization may be better understood by referring to the following description, taken in connection with the accompanying drawings forming a part thereof, in which;

Fig. 1 isa circuit diagram illustrating the general principles used in the customary capacity pick-up arrangement;

Fig. 2 is a schematic diagram of the pick-up and amplifying system as applied to a piano and;

Fig. 3 depicts diagrammatically the circuits of a modified form of the invention Like reference characters denote like parts in the several figures of the drawings.

In the following description and in the claims parts will be identified by specific names for convenience, but they are intended to be as generic in their application to similar parts as the art will permit.

Referring to the drawings and more particu- 40 larly to Fig. 1 a plurality of grounded vibratory elements ID are provided such as the strings of a piano. Located adjacent to the strings in is a metallic plate ll forming with the strings l0 an electrostatic pick-up device. Connected across the strings l0 and the plate II is a battery l2 and a resistor l3. The plate II is connected through a condenser l5 to the grid of an amplifier tube IS. A resistor I1 is connected across the input circuit of the amplifier I6. 50 The output circuit of the amplifier I6 is connected through a second amplifier l8 and a transformer I9 to a power amplifier 20, the output circuit of which is connected to a loudspeaker 2|.

The capacity of the condenser formed between the strings l0 and the plate ll may be considered to vary in general in accordance with the law C=Co(1+Kf(t)) in which i (t) is a function of time which varies between the limits 60 --1 and 1, and Co and K are constants, which for example may be 00:50 micrornicrofarads and K='.005.

Assuming simple sine wave vibrations of frequency f cycles per second, f(t), for computational purposes, may simply be sin 21r ft. For a range of f in the audio scale, for example from 32 to 4096 cycles per second corresponding to the fundamental tones of the lowest and highest Cs on a piano, the voltage across the resistor ll may be computed for each value of f. The problem may be considered as divided into two parts; first the computation of the voltage e, which is the A. C. portion of the voltage across the condenser I0H assuming resistor ll to be disconnected and which, disregarding phase, is represented by the formula:

1 (Rim in which E is the voltage of the battery l2, assumed to be 200 volts, K is the capacity range factor .005, R is the resistance of the resistor l3, assumed to be 5 megohms is the frequency in cycles per second and C equals 50 micromicrofarads. e will be in R. M. S. volts; second the relation in which Rg is the resistance of the resistor l1 and Xe is the reactance of the condenser |0-l l.

The following table gives the computation of the various voltages corresponding to the different frequencies f;

These tables indicate the following points:

1. For the same values of K for each string the lower notes would be greatly discriminated against, so that the range of capacity variation for the lower strings should be much greater than for the highest strings.

2. The nature of the circuit is inherently such that the harmonic tones are more strongly represented in the electrical output than in the vibrational input.

The difiiculties encountered with the ordinary type of capacity pick-up as shown in Fig. l are as follows:

1. Because of the high impedance of the circuit, and the low pick-up level, it is susceptible to interference from stray electric fields, requiring especially good shielding.

2. Because of the high impedance in combination with high voltage across the pick-up the circuit is susceptible to electrical noise produced by variable leakage across the condenser device.

In order to overcome these undesirable effects the novel types of capacit pick-up circuits shown in Figs. 2 and 3 have been developed. These circuits involve the use of an auxiliary oscillator, the operation of which is controlled by the capacity of the condenser, which may be made to cause amplitude, frequency or phase modulation of the auxiliary frequency, audio frequency tonal currents being produced by suitable demodulation of the oscillator output.

Fig. 2 illustrates the invention as applied to a piano 25, which is provided with the usual strings 26 mounted on a metallic frame 21. Located adjacent to the strings 26 are a plurality of metallic plates 28, which are insulated from the framework of the piano. The frame 21 is connected by a grounded conductor 29 to a fixed adjustable condenser 30 and a variable condenser 3|, the other sides of which are connected to one end of a resistor 32. The plates 28 are connected by a conductor 33 to one end of a resistor 35. The other ends of the resistors 32 and 35 are connected together and through a coil 36 to the conductor 29. The coil 36 is inductively coupled to a coil 31 which is the inductor of a tank circuit of an oscillator 38 and supplies A. C. voltage between the junction of the two resistors 32 and 35 and ground. The condenser formed between the strings 28 and the plates 28 and the two condensers 30 and 3| form two arms of a bridge arrangement 34 the other two arms being the resistors 32 and 35.

Connected across the two resistors 32 and 35 is the primary of a transformer 39 the secondary of which is connected in the input circuit of an amplifier 40. The output circuit of the amplifier 40 is connected through a transformer 4| to de modulator and audio amplifier 42. The output circuit of the tube 42 is connected through a transformer 43 to the input circuit of a power amplifier 45 the output circuit of which is connected to a loud-speaker 46. The amplifier 40 and following circuits may be standard circuits of a high grade radio receiver, such for example as the intermediate frequency amplifier and second detector circuits of a superheterodyne. Only so much thereof has been shown as is necessary to an understanding of the present invention.

For automatically controlling the bridge adjustment a D, C. relay 4'! may be connected to the variable condenser 3|. The relay 41 is operated by a D. C. relay tube 48 which is controlled from-the amount of D. C. rectified by the detector 42 as will be described.

In the operation of the form of the invention shown in Fig. 2. A. C. voltage is impressed from the oscillator 38 across the bridge arrangement 34 and the condenser 30 is adjusted so that a suitable unbalanced condition results, thus causing a proper amount of energy to pass through the transformer 39. When the degree of unbalance of the bridge 34 is varied, due to the variation in capacity of the pick-up device 2628 as the strings 26 vibrate, the energy delivered through the transformer 39 is modulated in accordance with the capacity fluctuations. This modulated energy is' amplified by the amplifier 40 and passes through the transformer 4| to the demodulator and audio amplifier 42, the audio frequency output of which corresponding to the string vibrations, passes through the transformer 43 to the power amplifier 45. Here the energy is further amplified and is fed to theloud-speaker 46 where it is reproduced as musical tones.

Some of the D. C. current from the detector device 42 will fiow through the resistor 49 in the direction of the arrow causing a potential difference to be built up across this resistor which is proportional to the signal strength. This potential will change the bias on the grid of the relay tube 48 thus controlling the amount of current flowing through the relay which in turn will change the setting of the variable condenser 3|. In this way the bridge arrangement 34 is adjusted so as to maintain correct off-balance conditions, as indicated by the detector energy, regardless of any variation in the average capacity of the pickup device. This control arrangement, however, is not essential to the operation of the system.

This system of amplitude modulation by capacity variations using the bridge method has been shown for purposes of illustration. Other systems of amplitude modulation by capacity variations may be used in its place without changing the scope of the invention.

This system has the advantage of freedom'from noise effects due to the intermediate frequency amplification and the low impedance of the capacity pick-up at its frequency of operation. Shielding against low frequency electromagnetic effects will not be so essential as in low frequency pick-up systems.

The form of the invention illustrated in Fig. 3 depicts the frequency variation method of pickup in which the conductor 33 from the capacity pick-up plates 28, shown in Fig. 2, is connected to one end of an inductance coil 50 and the conductor 29 from the strings 25 is grounded. The coil 50 forms part of an oscillatory circuit 5| which includes a tube 52 a coil 53, inductively coupled to the coil 50, and a battery 55. Connected between a tap on the coil 50 and ground is a potentiometer 58, the adjustable contact of which is connected to the first grids of two pentode tubes 5'! and 58 which are connected in push-pull arrangement.

The output circuits of the two tubes 51 and 58 include two tuned circuits 59 and 50, the former being tuned higher and the latter lower than the mean frequency of the oscillator 5|. The tuned circuits 59 and 60 are connected through two condensers 6| and 62 to two rectifiers 65'and 66, the output circuits of which are connected through two filter networks 61 and 68 to two audio amplifiers 68 and 10. The output circuits of the amplifiers 69 and 10 are connected through a transformer H to a power amplifier 12 the output circuit of which is connected to a loud-speaker 13.

In the operation of the form of the invention shown in Fig. 3 the oscillatory circuit 5| generates a high frequency voltage across the output potentiometer 56, the frequency of which varies in accordance with the variations in capacity of the plck-up device 25-28, shown in Fig. 2. due

to the vibrations of the strings 26 when the piano is played. This frequency modulated signal is impressed upon the tubes 51 and 58 where it is amplified and is fed to the two tuned circuits 59 and 60 which are respectively tuned above and below the mean frequency of the oscillator 5|.

The tuned circuits 59 and 60 convert the frequency modulated signal to amplitude modulated signals with the modulation for the circuit 59 out of phase with the modulation for the circuit 60. These modulated signals are then detected by the rectifiers and 66 to produce audio frequency currents out of phase. These audio signals are then filtered by the filters 61 and 68 and are impressed upon the input circuits of the audio amplifiers 69 and 10, where they are amplified and pass through the transformer H to the power amplifier 12. Here the signals are further amplified and are impressed upon the loud-speaker 13 where they are reproduced as musical tones in the usual manner.

In this system the push-pull nature of the demodulators and the audio amplifier minimizes the effects of any amplitude variations of the oscillator which might be caused by stray disturbances.

It is to be understood that the system includes the usual sources of potential and control circuits and that the types of tubes may be varied as desired. Only the essential features have been described herein.

Although only a few of the various forms in which this invention may be embodied have been shown herein, it is to be understood that the invention is not limited to any specific construction but may be-embodied in various forms without departing from the spirit of the invention or the scope of the appended claims.

What is claimed is:

1. In a musical instrument, a vibratory element, and means for'amplifying the sound produced therefrom, comprising a source of superaudible oscillations, a variable reactor, means varying the impedance of said reactor in accordance with said mechanical vibrations, said reactor being connected to modulate the frequency of said oscillations, a transmission channel in cluding means to demodulate said frequency modulated oscillations to derive audio frequency components therefrom, and means to amplify and convert said components into sound waves.

2. In a musical instrument, a vibrating string, a capacity reactance including said string as an element, a source of superaudible oscillations, means modulating the frequency of said oscillations in accordance with variations in capacity of said reactance, a loud speaker adapted to propagate sound waves, a transmission channel including means to demodulate said frequency modulated oscillations to derive audio frequency components therefrom, and means to actuate said loud speaker in accordance with said components.

3. The invention set forth in claim 1 in which the transmission channel includes an amplifier for the modulated oscillations adapted to amplify said oscillations prior to demodulation thereof.

4. In a piano, a plurality of vibrating strings, a capacity element mounted adjacent said strings and forming therewith a variable capacity device, a source of super-audible oscillations, a demodulator therefor, means including a bridge circuit for impressing said oscillations upon said demodulator, means including said variable capacity device for varying the transmission characteristics of said bridge circuit so as to modulate said oscillations, said demodulator being adapted to derive audio frequency components from said modulated oscillations and a loud speaker actuated by said components.

5. The invention according to claim 4, in which the bridge circuit is varied in accordance with the energy delivered from the oscillator to the demodulator so as to maintain the necessary un-' balance therein for transmission and modulation purposes.

' ELLISON S. PURINGTON.

DISCLAIMER 2,220,350.-Ellz's0n S. Purington, Gloucester, Mass. SOUND REPRODUCING SYSTEM FOR MUSICAL INSTRUMENTS. Patent dated November 5, 1940. Disclaimer filed August 12, 1941, by the inventor; the assignee, John Hays Hammond, Jr., consenting. Hereby enters this disclaimer to claims 1, 2, and 3 of said Letters Patent.

[Oflicz'al Gazette September 2,1941;]

DISCLAIMER 2,220,350.Ellz's0n S. Purington, Gloucester, Mass. SOUND REPRODUCING SYSTEM FOR MUSICAL INSTRUMENTS. Patent dated November 5, 1940. Disclaimer filed August 12, 1941, by the inventor; the assignee, John Hays Hammond, Jr., consenting. Hereby enters this disclaimer to claims 1, 2, and 3 of said Letters Patent.

[Ofiicz'al Gazette September 2, 1941;]

Images