US3342923A

US3342923A - Stereophonic electronic musical instrument

Info

Publication number: US3342923A
Application number: US394142A
Authority: US
Inventors: Edward J Henley
Original assignee: MP Moller Inc
Current assignee: MP Moller Inc
Priority date: 1964-09-03
Filing date: 1964-09-03
Publication date: 1967-09-19
Anticipated expiration: 1984-09-19

Description

T aci l Sept. 19, 1967 HENLEY 3,342,923

STEREOPHONIC ELECTRONIC MUSICAL INSTRUMENT Filed Sept. 5, 1964 2 Sheets-Sheet 1 $1 J M A g.

y SQ" W INVENTOR.

[OW/4P0 J Hex/15 BY IWQJ, when P 19, 1967 E. J. HENLEY 3,342,923

STEREOPHONIC ELECTRONIC MUSICAL INSTRUMENT Filed Sept. 3, 1964 2 Sheets-Sheet 2 s TAl/VG INVENTOR. fan/4,90 J. flEA/za United States Patent 3,342,923 STEREOPHONIC ELECTRONIC MUSICAL INSTRUMENT Edward J. Henley, Fanwood, N.J., assignor to M. P. Moller Incorporated, Hagerstown, Md., a corporation of Maryland Filed Sept. 3, 1964, Ser. No. 394,142 13 Claims. (Cl. 84-1.24)

ABSTRACT OF THE DISCLOSURE A stereophonic electronic musical instrument, such as an electronic organ, wherein an attenuative bus is employed to collect the signal outputs of the several oscillators employed in the instrument and to distribute these signals to a plurality of amplifier and speaker systems. The speakers are spatially arranged, and both the sources of oscillations and the inputs of the amplifiers are connected at spaced intervals along the attenuative bus, so that the accoustic output of the instrument has a spatial distribution similar to that of a pipe organ or other acoustic instrument.

My invention relates to electronic musical instruments and, more particularly, to elecrtonic organs.

Electronic musical instruments, such as electronic organs, have certain advantages over more traditional instruments, such as pipe organs. These advantages include low cost, great tonal variety, wide dynamic range, small size, and ease of construction. However, none of the electronic instruments produces the ensemble effect of a large pipe organ.

In a pipe organ, each type of tone is produced by a separate set of pipes, more commonly known as a rank of pipes. Even a moderately sized pipe organ has several ranks of pipes. With each rank having up to 97 pipes, the total number of pipes can readily exceed one thousand. These pipes are generally played from a console having two or more manual keyboards and one pedal keyboard. By means of coupler switches, it is possible to activate several pipes in each of several ranks by depressing a single key at the organ console.

It is evident that a plurality of organ pipes do not function as a point source of sound. Indeed, the charm of a pipe organ lies largely in the fact that the pipes are spread out over many square yards of area, each pipe sounding from its own particular location in space. The differences in time of arrival of the sounds from many pipes, as well as the many angles of arrival of direct and reflected sounds, help the listener to perceive that it is indeed a chorus of many independent sounds rather than a very limited number of tones radiated from a relatively few points in space. It is this same spatial distribution of independent tones which distinguishes a chorus from a quartet and a symphony orchestra from a small dance orchestra.

Many ingenious methods have been used in electronic organs to mask their deficiencies. These include several types of multiple speaker arrangements, such as a plurality of speakers radiating the same sounds, woofer-tweeter combinations wherein the sound is radiated from each speaker in accordance with the frequency response characteristic of that speaker and the associated dividing network, separate speakers for each tonal quality or group 3,342,923 Patented Sept. 19, 1967 of similar tonal qualities, and multiple channel arrangements in which, for example, odd-numbered notes are radiated from one speaker system and even-numbered notes are radiated from another. These may be classed as pseudo-stereophonic systems. Another such system is one in which both channels carry the same tones but one channel introduces a delay of many milliseconds. Still other systems employ speakers in motion or systems of sound dispersing bafiles to achieve similar effects. However, none of the systems employed to date has produced an ensemble effect comparable to that produced by a pipe organ with a similar tonal design.

Accordingly, it is an object of my invention to provide a multiplicity of independent sound sources in an electronic musical instrument.

It is also an object of my invention to provide a spatial distribution of tones comparable to that produced by a pipe organ.

It is a further object of my invention to reduce intermodulation distortion in an electronic musical instrument.

Another object is to simplify the problem of voicing and regulating the various frequency ranges in an electronic musical instrument.

Still another object is to permit variation of the dynamic level in a manner that will compensate for characteristics of the human car.

A further object of my invention is to achieve high reliability by providing enough independent elements in an electronic musical instrument whereby a breakdown in any element will not seriously impair the over-all capacity of the instrument.

These and still further objects of my invention will become readily apparent from the following detailed description, accompanying drawings, and appended claims.

In the drawings:

FIGURE 1 is a simplified diagram showing the essential features of my invention.

FIGURE 2 is a diagram of a keyed oscillator which may be employed in an electronic musical instrument.

FIGURE 3 is a diagram of a continuously operative oscillator with provision for keying the audio output.

FIGURE 4 is a diagram of an oscillator showing an output path containing the combination of two signals taken from two different points in the oscillator circuit.

FIGURE 5 is a typical front view of four sets of three loudspeakers each, such as would be used in a small organ containing four combinations of the apparatus illustrated in FIG. 1.

For purposes of illustration, only the essential features of my invention are shown in FIG. 1 to facilitate an understanding of the operation and novel characteristics. Those skilled in the art recognize that a complete instrument may contain a large number of such assemblies under control of a single console. Various power and key switch arrangements are all well known and are not a part of this invention. Therefore, they have been shown only to the extent necessary for an understanding of this invention.

stereophonic reproduction of sound has been employed for many years, in the most popular form, uses -a two channel system. More sophisticated systems employ three channels in order to eliminate the hole in the middle which occurs when wide separations are used. However, certain technical problems have hindered development of a true stereophonic musical instrument which produces sounds having the desired spatial distribution. One of these problems is the precedence effect which causes the nearer of two sources of the same sound to appear as the true source even though the farther source generates more acoustical power. A power difference of twenty decibels or more is required to adequately minimize this limitation. My invention resides in a design employing three or more channels in a manner which successfully minimizes the precedence effect and has many other desirable features Which cannot be achieved in a two channel system. These results are not achieved simply because three channels are employed instead of two, but are due to the method disclosed in this invention. This is analogous to a comparison of a two-legged stool and a three-legged stool. A threelegged stool is not necessarily self supporting, but it can be if the three legs are arranged in other than a straight line. The system employing three or more channels, as disclosed in this invention, is the first ever proposed to my knowledge which provides true stereophonic generation and radiation of musical tones with results comparable to those produced by a pipe organ.

Referring now to FIG. 1 of the drawings, a series of seventy-three oscillators is used to generate tones corresponding to each key on one of the instruments keyboards plus an octave extension of twelve tones. This is in accordance with well-known practice and it is understood that a greater or lesser number of oscillators may be employed, depending upon the design requirements of the. particular instrument.

The output 11 of each of the seventy-three oscillators 10 is connected through an isolating resistor 12 to an attenuative collector bus 13 comprising a like number, less one, or seventy-two series-connected resistors 14. The output of the first oscillator 10, shown at the left of FIG. 1, is connected to one end of the attenuative collector bus 13, the output of the last oscillator 10, shown at the right of FIG. 1, is connected to the other end of the attenuative collector bus '13, and the output of each intermediate oscillator 10 is connected to a junction of two of the series-connected resistors 14, so that each oscillator output connection is separated from the next oscillator output connection by one of the series-connected resistors 14.

Since the series-connected resistors 14 do not, in themselves, provide the required attenuation in the collector bus 13, three terminating

resistors

15, 16 and 17 are provided. Resistor 15 terminates one end of the bus 13, resistor 17 terminates the other end of the bus and resistor 16 terminates the center of the bus \13. Termination of the bus 13 at an intermediate point to increase the total attenuation from end-to-end is very important in order to obtain an adequate stereophonic effect.

Bridged across each of the terminating

resistors

15, 16 and 17 is the input of an amplifier 18. A loudspeaker 19 is connected to the output of each amplifier 1,8. In accordance with common practice, volume changes of the amplifiers 18 are accomplished by a ganged control 20. The specific methods for doing this are well known in the art and may, for example, be mechanically ganged potentiometers or remotely controlled circuits employing lamps and light-sensitive resistors in attenuative circuits. However, the employment of three or more channels according to my invention permits new and novel effects to be achieved as I shall presently disclose.

FIG. 2 shows a transistor oscillator circuit which is suitable for production of one type of tone found useful in an electronic organ. In this circuit, the oscillator 10 is normally inoperative and is made operative upon closure of a key switch 21 which applies operating potential from a battery 22. It is understood, of course, that in a complete electronic organ, the operating potential to operate all oscillators 10 is obtained from a common supply. The oscillator output connection 11 leading to the isolating resistor 12, shown in FIG. 1, is at point 23. The key switch 21 may be directly actuated by operation of the console keys or it may be a remotely controlled electro-mechanical or electronic circuit, such as a diode gating circuit.

FIG. 3 shows a transistor oscillator 10, similar to the one shown in FIG. 2, except that the oscillator is normally operating and its output connection 11 is established as required by an audio key switch 24. As in FIG. 2, this switch may be directly actuated or it may be one of the many well-known remotely controlled electronic switching circuits.

FIG. 4 shows a transistor oscillator circuit, similar to that shown in FIG. 2, except that the signal developed across a load resistor 25 at point 26 is combined in an out-of-phase relationship with the signal developed at point 27 because of the phase reversal occurring in the circuit. An additional isolating resistor 12 is provided to minimize loading effects and a capacitor 28 is provided to block the direct current in the transistor collector circuit from getting into the signal circuit. The output connection 11 at point 29 leads to the isolating resistor 12, shown in FIG. 1.

It is obvious from the above that a wide variety of oscillator circuits, including systems employing master oscillators and cascaded frequency dividers, may be used without departing from the scope of my invention.

FIG. 5 shows a four cabinet array of loudspeakers 19 in an arrangement such as might be used in a small organ. By employing modular techniques, with three loudspeakers 19 in each horizontal cabinet 30, it is possible to achieve great flexibility in the design of an organ. The array shown is part of an organ employing four combinations of loudspeaker equipment shown in FIG. 1. Each loudspeaker combination is designed to produce a different tone quality. By placing the loudspeaker combination carrying the strongest fundamental tones near the floor, it is possible to secure an enhancement because of the increased radiation impedance at the junction of the floor and the speaker cabinets 30. Each of the four cabinets 30 contains internal partitions (not shown) to separate it into three isolated speaker compartments. This is necessary to secure the acoustic isolation vital to a good stereophonic system.

Referring again to FIG. 1, it is apparent that the signal appearing at point 31 from the left oscillator 10, as viewed in FIG. 1, will be reproduced Without attenuation from having to pass along the collector bus 13. However, the signal appearing at point 32 will have been attenuated because of the voltage drop across resistors 14 between points 31 and 32. Resistor 16, paralleled by the seriesconnected resistors between point 32 and ground 33, including terminating resistor 17, presents a relatively low impedance at point 32 with a resulting substantial attenuation of the signal from the left oscillator 10, as viewed in FIG. 1. At point 34 the signal from the left oscillator 10 is still further attenuated because of the voltage drop across the series-connected resistors 14 between points 32 and 34, terminated by resistor 17.

In order to facilitate an understanding of further descriptions of the circuit, I list below a set of circuit values which have yielded excellent results.

Ohms Isolating resistors 12, each at 100,000 Series connected resistors 14, each at 47 Terminating resistors 15 and 17, each at 1,000 Terminating resistor 16 1,500

Using 10% tolerance resistors with the above nominal values, the performance is entirely satisfactory. It is to be noted that the impedance of the bus 13 is not uniform along its length. This is advantageous and helps to equalize the amplitudes of the radiated signals derived from the various oscillators 10. The attenuation ranges from 0.2 decibel to 0.5 decibel per section depending upon the points between which the measurements are made. If a signal from the left oscillator 10 is applied to the bus 13, it is radiated with reference amplitude by the left loud- Go speaker 19, as viewed in FIG. 1. The same signal is radiated by the middle loudspeaker 19 approximately 11.5 decibels below reference amplitude and by the right loudspeaker 19 approximately 22 decibels below reference amplitude. It is evident then that most of the sound is raddiated from the left loudspeaker 19 and the apparent source of the sound is in close approximation to the position of that loudspeaker. Those skilled in the art will, now appreciate that with the circuit shown, the distribution of energy, and therefore the apparent source of sound, will shift in accordance with which oscillator is furnishing the signal. For example, if an oscillator approximately midway between the left oscillator 10 and the middle oscillator 10 at point 32 of FIG. 1 is furnishing the signal, the left and middle loudspeakers 19 will each radiate sound approximately 4 decibels below the previously mentioned reference amplitude and the right loudspeaker 19 will radiate sound approximately 14.5 decibels below reference amplitude. In this case, the apparent source of sound is close to midway between the left and middle loudspeakers 19. If the oscillator 10 at point 32 is furnishing the signal, the middle loudspeaker 19 will radiate sound at reference amplitude and the two end loudspeakers 19 will each radiate sound approximately 10.5 decibels below reference amplitude. In this case the apparent source of sound will be the middle loudspeaker 19. In a similar manner, the apparent source of sound will continue to shift toward the right loudspeaker 19 as the oscillators 10 furnishing the signals approach to the right,

that is, from point 32 towards point 34. This action takes place whether the oscillators are operated singly or in combination.

In the examples given, it has been assumed that the amplifiers 18 have equal gains. However, it is evident that an arrangement according to my invention lends itself to shaping of the volume characteristic according to the fundamental frequencies rather than according to the frequency components. For example, if the oscillators 10 are connected to the bus 13 in a chromatic progression with the lowest frequency oscillator 10 at one end and the highest frequency oscillator 10 at the other end, the volume of the tones from the higher frequency oscillators can be reduced by reducing the gain of the appropriate amplifier without affecting tone quality. If conventional tone control methods were applied, the higher string-type tones would begin to sound like flutes.

The three-channel arrangement which I prefer, having the oscillators 10 connected to the bus 13 in a chromatic progression, lends itself to another desirable variation. By making the controllable volume range of the amplifiers unequal and gauging the controls so that they are operated simultaneously by a foot-operated lever, it is possible to compensate for limitations of the human ear and to maintain a tonal balance over a wide dynamic range of volume levels. For example, the amplifier which favors the output of oscillators operating in the middle of the audio range can be made to have a thirty decibel range of gain adjustment while the other two may have only a twenty or twenty-five decibel range covered simultaneously by a ganged control.

In addition to permitting variable control of volumes over the frequency range, the three-channel arrangement is obviously adaptable to regulation of the tone color through use of conventional tone controls or formant filters in the individual amplifiers. Typical circuits are not shown because these are so well-known in the art. However, the stereophonic system according to my invention is the first which permits use of multiple formant filters which affect different frequency ranges without undesirable interaction. This is because the filters used in each amplifier can have characteristics differing from those of the filters used in the other amplifiers. The tone quality transitions are smooth and continuous in accordance with progression from one end of the range covered by the oscillators to the other end of the range.

My invention also results in a substantial reduction of intermodulation distortion. There are two types of distortion which commonly occur in electronic organs. One type occurs because of non-linearities in the oscillator output circuits. This is well understood in the art as being a source of intermodul-ation because of the paths which exist from each oscillator to all the other oscillat-ors. It is evident that the attenuative collector bus 13 provides more isolation between oscillators than is obtained by the usual type of collector bus. The other type of distortion is the Doppler modulation of high frequency tones which are radiated by loudspeakers that are simultaneously radiating high amplitude low frequency tones. It is evident from the previous discussion of my invention that the tones generated by the highest frequency oscillators are attenuated over 20 decibels by the collector bus, so that they are not radiated strongly by the loudspeaker carrying most of the power of tones generated by the low frequency oscillators. The converse is also true. However, it is an important feature of my invention that harmonics of the fundamental signal produced by any oscillator are radiated in. the correct proportion by the loudspeaker or loudspeakers carrying the bulk of the energy from that oscillator. This is because of the lack of any frequency discrimination in the attenuative collector bus. Conventional woofer-tweeter speaker arrangements result in a perceptible tonal discontinuity as a result of having the fundamental tones radiated from one loudspeaker and an apparently disconnected series of harmonics radiated from another.

In some cases, it may be desirable to connect two sets of oscillators to the same attenuative collector bus. For example, it may be considered advantageous to have both sets of oscillators which constitute a mixture or a celeste to be connected to the same bus. The second set of oscillators 10 would 'be connected to the bus 13 through isolating resistors 12 at the points 31, 35, 36, etc. in FIG. 1. Alternatively, it may be desirable to provide each set of oscillators 10 with its own attenuative collect-or bus and to use only the amplifiers and loudspeakers in common with both combinations of oscillators and collector buses. In this case, the

points

36, 37, and 38 in FIG. 1 would be appropriate to establish the common connections. Sharing of the amplifiers and loudspeakers could also be on an either-or basis with the desired circuit transfers being made at

points

36', 37, and 38 on FIG. 1. 7

All of the preceding discussions have been centered around the chromatic arrangement which I prefer, that is, the oscillators 10 are connected to the attenuative collector bus 13 in the order of their operating frequencies starting with the lowest frequency oscillator being connected to one end of the bus and ending with the highest frequency oscillator being connected to the other end of the bus. The invention is also applicable to other arrangements such as the one in which the seventy three oscillators are numbered in series and starting with number one, so that all odd-numbered oscillators are connected in order of frequency to one half of the bus and all evennumbered oscillators are connected in the reverse order of progression, according to frequency, proceeding toward the other end of the bus. For the purpose of explaining the circuit action, therefore, the oscillator tone sources shown in FIG. 1 may be arbitrarily numbered from left to right and it is understood that any desired sequence can be used.

One reason the chromatic arrangement is preferred is that it is the most flexible for a given number of amplifiers and loudspeakers. To secure the same capability for adjustment of the low, middle, and high frequency ranges in the divided arrangement, having odd-numbered sources on one end and even-numbered sources on the other, it would be necessary to have a five-channel system instead of the three channels required for the chromatic arrangement. However, it is within the scope of my invention to employ this arrangement by connecting another amplifier and loudspeaker channel to an intermediate point on eachhalf of the attenuative collector bus.

My invention is readily adaptable to many well known systems of tone generation, such as the one disclosed in my United States Patent No. 2,941,435. The sine wave and harmonic containing buses disclosed in that patent can be made attenuative according to the present invention and the number of amplifiers and loudspeakers can be increased as required to achieve a spatial distribution of sound according to the invention disclosed herein.

Others may readily adapt my invention to other conditions of use, such as an electronic piano, or the like. As various changes may be made in the form, construction, and arrangement of the parts herein, without departing from the spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all matters are to be interpreted as illustrative and not in any limiting sense.

I claim:

1. In an electronic musical instrument, means for producing a spatial distribution of tones in accordance with their fundamental frequencies comprising, in combination, at leastthree sources of oscillations of various frequencies having desired tonal characteristics, an attenuative collector bus, an isolating resistor for each of said oscillation sources having one end connected to its associated oscillation source and its other end connected to said collector bus, a plurality of resistors connected in series in said collector bus, each of said series-connected resistors being disposed between collector bus connections of each pair of said isolating resistors, a plurality of terminating resistors each having one end connected to said collector bus and its other end connected to a point of common potential, at least three amplifiers each having its input connected to said collector bus at juncture of one of said terminating resistors and said collector bus, a loudspeaker connected to output of each of said amplifiers, and control means for said amplifiers.

2. In an electronic musical instrument according to claim 1, wherein said sources of oscillations are oscillators.

3. In an electronic musical instrument according to claim 1, wherein the input of one of said amplifiers is connected to one end of said collector bus, the input of a second one of said amplifiers is connected to the other end of said collector bus, and the input of a third one of said amplifiers is connected intermediate the two ends of said collector bus.

4. In an electronic musical instrument according to claim 1, wherein said plurality of terminating resistances constitute at least three terminating resistors.

5. In an electronic musical instrument according claim 1, wherein one of said terminating resistors connected to one end of said collector bus, another said terminating resistors is connected to the other end of said collector bus, and still another one of said terminating resistors is connected intermediate the ends of said collector bus.

6. In an electronic musical instrument according to claim 1, wherein said sources of electrical oscillations are connected to points along said attenuative collector bus in a sequential progression starting with the source of electrical oscilltaions having the lowest fundamental frequency being connected to one end of said attenuative collector bus and ending with the source of electrical oscillations having the highest fundamental frequency being connected to the other end of said attenuative collector bus.

7. In an electronic musical instrument according to claim 1, wherein odd-numbered sources of said electrical oscillations are connected to said attenuative collector bus in a sequential progression starting with the odd-numbered source having the highest fundamental frequency being connected to an approximate mid-point of said attenuative collector bus and ending with the odd-numi is bered source having the lowest fundamental frequency being connected to one end of said attenuative collector bus, and even-numbered sources of said electrical oscillations are connected to said attenuative collector bus starting with the even-numbered source having the highest fundamental frequency being connected to an approximate mid-point of said attenuative collector bus and ending with the even-numbered source having the lowest fundamental frequency being connected to the other end of said attenuative collector bus.

8. In an electronic musical instrument according to claim 1, wherein odd-numbered sources of said electrical oscillations are connected to staid attenuative collector bus in a sequential progression starting with the oddnumbered source having the lowest fundamental frequency being connected to an approximate mid-point of said attenuative collector bus and ending with the oddnumbered source having the highest fundamental frequency being connected to one end of said attenuative collector bus, and even-numbered sources of said electrical oscillations are connected to said attenuative collector bus starting with the even-numbered source having the lowest fundamental frequency being connected to an approximate mid-point of said attenuative collector bus and ending with the even-numbered source having the highest fundamental frequency being connected to the other end of said attenuative collector bus.

9. In an electronic musical instrument, means for producing a spatial distribution of tones in accordance with their fundamental frequencies comprising, in combination, a plurality of sources of electrical oscillations of various frequencies having desired tonal characteristics, an attenuative collector bus, an isloating resistor for each of said oscillation sources having one end connected to its associated oscillation source and its other end connected to said collector bus, a plurality of resistors connected in series in said collector bus, each of said seriesconnected resistors being disposed between collector bus connections of each pair of said isloating resistors, at least three terminating resistors each having one end connected to said collector bus and its other end to a point of common potential, at least three amplifiers each having its input connected to said collector bus at juncture of one of said terminating resistors and said collector bus, and at least three loudspeakers, at least one of said loudspeakers being connected to the output of each of said amplifiers, each of said sources of electrical oscillations being connected respectively to a different point on said attenuative collector bus through one of said isolating resistors, one of said terminating resistors being connected to one end of said attenuative collector bus, another of said terminating resistors being connected to the other end of said attenuative collector bus, and at least one other of said terminating resistors being connected intermediate the two ends of said attenuative collector bus, the input of one of said amplifiers being connected to one end of said attenuative collector bus, the input of another of said amplifiers being connected to the other end of said attenuative collector bus, and the input of at least one other of said amplifiers being connected intermediate the two ends of said attenuative collector bus.

10. In an electronic musical instrument according to,

claim 9, wherein said sources of electrical oscillations are normally inoperative but are made operative as required for the performance of a musical composition by application of operating potentials by means of circuits including manually operated key contacts.

11. In an electronic musical instrument according to claim 9, wherein said sources of electrical oscillations are normally operative and the connections to said attenuative collector bus are established by means of circuits including manually operated key contacts.

12. In an electronic musical instrument according to claim 9, wherein the gain of said amplifiers is varied simultaneously by common control means to a predetermined degree for each of said amplifiers whereby compensation is introduced for the variation in tonal sensitivity of the human ear with changes in dynamic level.

13. In an electronic musical instrument according to claim 9, wherein said three loudspeakers are physically disposed in a row with the loudspeaker at one end of the row being connected to the amplifier source having the lowest frequency, and the loudspeaker at the other end of the row being connected to the amplifier source having the highest frequency.

References Cited UNITED STATES PATENTS ARTHUR GAUSS, Primary Examiner.

10 D. D. FORRER, Assistant Examiner.

Claims

1. IN AN ELECTRONIC MUSICAL INSTRUMENT, MEANS FOR PRODUCING A SPATIAL DISTRIBUTION OF TONES IN ACCORDANCE WITH THEIR FUNDAMENTAL FREQUENCIES COMPRISING, IN COMBINATION, AT LEAST THREE SOURCES OF OSCILLATIONS OF VARIOUS FREQUENCIES HAVING DESIRED TONAL CHARACTERISTICS, AN ATTENUATIVE COLLECTOR BUS, AN ISOLATING RESISTOR FOR EACH OF SAID OSCILLATION SOURCES HAVING ONE END CONNECTED TO IS ASSOCIATED OSCILLATION SOURCE AND ITS OTHER END CONNECTED TO SAID COLLECTOR BUS, A PLURALITY OF RESISTORS CONNECTED IN SERIES IN SAID COLLECTOR BUS, EACH OF SAID SERIES-CONNECTED RESISTORS BEING DISPOSED BETWEEN COLLECTOR BUS CONNECTIONS OF SAID PAIR OF SAID ISOLATING RESISTORS, A PLURALITY OF