US1979036A - Monitoring system for push-pull amplifiers - Google Patents

Description

: Oct. 30, 1934- J. H. HAMMOND, JR

MONITORING SYSTEM FOR PUSH-PULL AMPLIFIERS Filed Nov. 23, 1929 3 Sheets-Sheet l PIC-5.1;

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J. H. HAMMOND, JR

MONITORING SYSTEM FOR PUSH-PULL AMPLIFIERS Filed Nov. 25, 1929 FIGA.

3 Sheets-Sheet 2 SIGNAL STRENGTH ouw we flwmx JOH N HAYS HAMMOND JR Oct. 30, 1934. v ,1. H. HAMMOND, JR 1,979,036-

MONITORINGSYSTEM FOR PUSH-PULL AMPLIFIERS Filed NOV. 23, 1929 3 Sheets-Sheet 5 FIG .5.

SIGNAL STRENGTH OLLVX! NW9 Fm/Mm I, JOHN HAYS HAMMOND JR.

.110 cum Patented Oct. 30, 1934 UNITED STATES PATENT OFFICE John Hays Hammond, Jr., Gloucester, Mass., as-

signor to Hammond Holding Corporation, Gloucester, Mass., a corporation of Delaware Application November 23, 1929, Serial No. 409,211

22 Claims.

This invention relates to an automatic control system, and more particularly to a system for automatically controlling the amplification of an electron discharge amplifier in accordance with the'amplitude of the signal being translated.

This invention is particularly applicable to pick-up devices, such as a phonograph pick-up or a photo-electric pick-up, and to an amplifying system for amplifying the signals picked up and applying them to a sound propagating device such as a loud speaker.

The volume ratio between the loud and soft tones of an orchestral selection is considerably greater than that which may be recorded on a phonograph record or photographic sound record. The present invention, therefore, provides for-expanding the volume ratio of the recorded sound and causing the same to more nearly approximate the originalselection. The invention also provides a limiting device for preventing the amplifier from becoming overloaded, with consequent sound distortion, and the amplifier itself is especially constructedto prevent distortion due to undesired modulation.

The invention particularly provides for the elimination of extraneous noises, such as needle scratch or film noises, and prevents the amplifier from operating in response to such noises when they are unaccompanied by desired musical tones.

The invention also consists in certain new and 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 ollowing description taken in connection with the accompanying drawings forming a part thereof, in which Fig. l is a schematic diagram of a monitoring system constructed in accordance with the present invention;

Fig. 2 is a schematic diagram having a modifled form of control;

Fig. 3 is a schematic diagram of a further modified form of control;

50 Fig. 4 is a diagram of the amplification characteristics of an amplifier 'having the control shown in Fig. 2; and

Fig. 5 is a diagram of the amplification characteristics of an amplifier having the control shown in Fig. 3.

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 ge- 50 neric in their application to similer parts as the art will permit.

Referring to the drawings more in detail, an amplifying system is shown in Fig. 1 as applied to phonograph pick-up 10 or a photo-electric pick-up comprising photo-electric cell '11 to which the amplifier may be selectively connected by double pole double throw switch 12.

The photo-electric pick-up includes a source of light 14, lens 15, aperture plate 16, having an aperture 17 therein, and photographic sound record or film 18, the above elements being arranged so that light rays from source 14 are focused by means of lens 15 through aperture 1'7 onto the photographic sound record and are then used to actuate photo-electric cell 11. Said cell is energized by a suitable source of potential 20 and is connected through transformer 21 with switch 12.

The amplifier is disclosed as comprising three stages of push-pull amplification including elec-- tron discharge tubes 25, 26, 27, 28, 29 and 30 and coupling transformers 31, 32, 33 and 34 to which said tubes are connected in the manner well known in the art. The secondary of transformer 34 is connected to the armature coil 35 of a loud speaking device 36 which may have a stationa y field coil 37 energized by battery 38. Tubes 25 to 30 have the usual sources of potential for their filament, grid and plate circuits, which are well known in the art and will not be described in detail.

A variable resistance 40 is connected across the input terminals of transformer 31 to permit the voltage applied to the system to be manually adjusted as desired. It is obvious, however, that the energy derived from the various pick-up devices may be amplified as desired before being applied to the amplifying system disclosed herein and the manual voltage control may be associated with any convenient part of the apparatus.

The automatic control device is shown as applied to the grid circuit of tubes 2'7 and 28 which comprise the second stage of amplification. The impedance of tubes 27 and 28 is controlled by the normal grid potential thereof. This is deter- Y mined by the potential of battery 45, the setting manually adjusted to cause tubes 27 and. 28 to pass the desired amount of plate current as indicated by meter 48.

The current through resistance 47 which determines the potential drop of said resistance is controlled by the volume of the input signal applied to tubes 27 and 28. For this purpose an impedance chain, comprising condensers 50 and 51 and resistances 52 and 53, is connected across the two grids of tubes 27 and 28.

The constants of this impedance chain may be chosen by making condensers 50 and 51 sufficiently small to substantially block low frequency tones.

Resistance 52 is connected in the input circuit of space discharge amplifier 55, the output circuit of which is connected through transformer 56 to a pair of rectifiers 57 which are connected across resistance 47. A condenser 58 may be connected across the output circuit of an amplifier to bypass the high frequency tones and prevent the rectifiers from being responsive thereto.

In order to prevent the current in resistance 47 from varying with the instantaneous value of the signal current a low-pass filter net-work 59 may be connected in the output circuit of rectifiers 57 and a condenser 60 may be connected across resistance 47.

In the operation of the above described control system rectifiers 57 are operatedin accordance with the value of the intermediate frequency signals present in the amplifier circuit. The rectified current is passed through resistance 47 and causes a potential drop in that resistance which is proportional to the current flowing therein. This potential drop is in such direction that an increased signal in the amplifier circuit decreases the normal potential or bias of the grids of tubes 2'7 and 28, thereby lowering the impedance of said tubes and causing increased current to flow in the output circuit thereof. Condenser 60 serves to iron out variations in voltage across resistance 47 causing the control system to vary the gain ratio according to the average value of the signal.

Condensers 50 and 51 prevent the rectifiers from being operated by the low frequency signals and thereby prevent the control system from operating in response to sharp percussive sounds, such as a kettle drum, The higher frequency signals, such as those produced by needle scratch or film noises, are by-passed by condenser 58 and thereby prevented from operating the control system.

i Tubes 27 and 28 may be normally adjusted to pass a minimum amount of current when no signalis being applied. The amplification characteristics will then be determined by the change in grid bias due to current flowing in resistance 47 when a signal is applied to the amplifier. It is to be noted, therefore, that high frequency noises, such as needle scratch and film noises, are not amplified except when accompanied by other signals since rectifiers 57 are not responsive to those frequencies.

For preventing a loud signal from overloading the amplifier and therebycausing distortion, an

automatic limiting device may be employed which is operated by the output signal. This limiting device comprises a pair of rectifiers 61- which are connected through transformer 62, filter 63 and resistances 64 with the output circuit of transformer 34. Rectifiers 61 are connected through a filter network 65 with resistance 66 battery 67 and by the potential drop in resistance rectifiers cause acurrent to pass through resistance 66 in such a direction that the grid bias of tubes 25 and 26 is increased, whereby their amplification characteristics are lowered.

The overload control may be applied to the input circuit of other stages of amplification if desired. It is preferably applied to the input circuit of the first stage, as shown, since the signal amplitude therein is smaller and a smaller potential drop may be used to effect the desired control. Furthermore, the effect of the limiter is increased due to the action of I the succeeding stages of amplification and the above described control system.

Distortion in the output circuit due to rectifiers 61 is reduced to a minimum by the use of balanced rectifiers, as shown. The desired frequency characteristics may be impartedv to the -in Fig. 2 in which amplifying tube 55 is controlled by separate secondary winding 75 of transformer 32. The frequency characteristics of the control circuit are determined by band-pass filter 76 which is preferably designed to pass frequencies in the'middle tone range so that control will not be unduly affected by low or high tones.

The output circuit of transformer 56 in the system of Fig. 2 is connected through filter 76 to rectifiers 77 which supply direct current through resistance 47 in the manner previously pointed out.

The elements of the circuit identical with those shown in Fig. 1 are given similar reference numerals and will not be again described.

A further modified form of control circuit is shown in Fig.- 3- in which the amplifier 55 is coupled by transformer 56 to a filter net-work 80 which determines the frequency characteristics of the circuit. The output of filter 80 is applied through amplifier 81 and transformer 82 to rectifiers 77 which are similar to those illus- 1 5 trated in Fig. 2. A limiting device comprising rectifier 83 and biasing battery 84 is connected across the input of rectifier 77 for limiting the current in resistance 47 so that on high signal strengths the bias on the input circuit of tubes 27 and 28 will not become sufliciently small to cause said tubes to operate in an abnormal manner.

It is to be noted that by placing the filter system in the control circuit instead of in the input to the amplifier, the system is capable of distinguishing between currents caused by needle scratch and desired currents caused by overtones or high notes w ich are present in the amplifier. These high frequencies are not discriminated thereof prevents distortion due to undesired modulation which might otherwise occur in the amplifier tubes and insures a practically undistorted signal in the output circuit.

While certain novel features of the invention have been shown and described and are pointed out in the annexed claims, it will be understood that various omissions, substitutes and changes in the forms and details of the device illustrated and in its'operation may be made by those skilled in the art without departing from the spirit of the invention.

What is claimed is:

1. In a sound reproducing system, a sound record, a pick-up device therefor, a plurality of stages of amplification fed by sa'id pick-up device, and means fed by said pick-up device controlling the gain ratios of said stages in accordance with the strength of the signal derived from said record.

2. In a sound reproducing system, a sound record, a pick-up device therefor, a plurality of stages of amplification fed by said pick-up device, separate control circuits fed by said pickup device controlling the gain ratios of said stages in accordance with the strength of the signal.

3. In a sound reproducing system, a sound record, a pick-up device therefor, at least three stages of amplification fed by said pick-up device, each stage comprising a push-pull amplifier, a normal dynamic control circuit fed by the input of an intermediate of said stages, said circuit having means for varying the gain ratio of the push-pull amplifier of that stage in accordance with the strength of the signal applied to the input thereof, a limiting dynamic control circuit fed by the output of the last stage and having means for varying the gain ratio of the first stage inversely as said normal control circuit, said normal control circuit operating to increase the gain ratio with increase in signal strength, said limiting control circiut operating to decrease the gain ratio with increase in signal strength.

4. In a sound reproducing system, a' sound record, a pick-up device therefor, at least three stages'of amplification fed by said pick-up device, a normal dynamic control circuit fed by the input of 'an intermediate of said stages, said circuit having means for varying the gain ratio of the amplifier of that stage in accordance with the strength ofthe signal applied to the input thereof, a limiting dynamic control circuit fed by the output of the last stage and having means for varying thegainratio of the first stage inversely as said normal control circuit.

5. In a sound reproducing system, a sound record, a pick-up device therefor, an amplifier fed by said pick-up device, a control circuit fed by said pick-up-device having means for rectifying v and averaging the signal impressed thereon, said control circuit including a full wave rectifier and afilter to eliminate the eifect of residual tones on said amplifier, said control circuit also having means for preventing undesired tones from being rectified and averaged.

6. In a sound reproducing system, a sound record, a pick-up device therefor, a push-pull amplifier fed by said pick-up device, a control circuit fed by said pick-up device controlling the gain ratio of said amplifier in accordance with the strength of the signal impressed thereon, whereby the effect of noise in said control circuit is alleviated.

7. In a sound reproducing system, a sound record, a pick-up device therefor, an amplifier fed by said pick-up device, a control circuit also fed by said pick-up device for varying the gain ratio of said amplifier in accordance with the strength of the signal impressed upon said control circuit, said control circuit having a band filter for preventing the passage therethrough of low frequency tones and high frequency tones and consequent control of gain ratio thereby.

8. In a sound reproducing system, a sound record, a pick-up device therefor, an amplifer fed by said pickup device, a control circuit also fed by said pick-up device for varying the gain ratio of said amplifier in accordance with the strength 100 of the signal impressed upon said control circuit, said control circuit having means for preventing control by undesired tone frequencies while permitting control by desired tone frequencies, whereby the amplification of said undesired tone frequencies is not affected unless accompanied by desired tone frequencies.

9. In a sound reproducing system, a sound record, a pick-up device therefor, an amplifier fed by said pick-up device, a control circuit fed by said 110 pick-up device having means for rectifying and averaging the signal impressed thereon, said control circuit including a full wave rectifier and a filter to eliminate the effect of residual tones on said amplifier.

10. In a sound reproducing system, a sound record, a pick-up device therefor, an amplifier fed by said pick-up device, a control circuit fed by said pick-up device having means for rectifying and averaging the signal impressed thereon, said 120 control circuit including a full wave rectifier to alleviate the effect of residual tones on said 11. In a sound reproducing system, a sound record, a pick-up device therefor, an amplifier fed by said pickup device, a control circuit also fed by said pick-up device for varying the gain ratio of said amplifier in accordance with the strength of signal impressed upon said control circuit, said control circuit having means for discriminating against low frequency signal tones so as to prevent these tones from controlling the amplification to an undesired extent.

1 In a sound reproducing system, a sound record, a pick-up device therefor, an amplifier fed by said pick-up device, a control circuit also fed by said pick-up device for varying the gain ratio of said amplifier in accordance with the strength of signal impressed upon said control element, said control element having means for discriminating against high frequency signal tones so as to prevent thesetones from controlling the amplification to an undesired extent.

13. In a sound reproducing system, a sound record, a pick-up device therefor, a dynamic expander fed by said pick-up device, a sound propagating device fed by said dynamic expander, an output limiting arrangement fed by the output of said dynamic expander for determining the maximum amount of dynamic expansion, said limiting 150 dill arrangement having a critical point corresponding to an output below which limiting control is not exercised.

14. In a sound reproducing system, a sound record, a pick-up device therefor, a dynamic amplifier fed by said pick-up device, a sound propagating device fed by said dynamic amplifier, a limiting circuit fed by the output of said dynamic amplifier for controlling the expansion caused by said dynamic amplifier, said limiting circuit having a critical point below which limiting control is not exercised.

15. In a sound reproducing system, a sound record, a pick-up device therefor, a dynamic expander fed by said pick-up device, a sound propagating device fed by said dynamic expander, a control arrangement fed by said pick-up device for controlling the passage of energy through said dynamic expander, said control arrangement having a limiting device for limiting the amount of dynamic expansion.

' 16. In a sound reproducing system, a sound record, a pick-up device therefor, a dynamic amplifier fed by said pick-up device, a sound propagating device fed by said dynamic amplifier, a control circuit fed by said pick-up device for controlling the gain ratio of said amplifier in accordance with the average intensity of signal, said control circuit having a limiting device for limiting the maximum control energy.

-17. In a sound reproducing system, a sound record, a pick-up device therefor, a push-pull amplifier fed by said pick-up device, a sound propagating device fed by said amplifier and a control circuit for controlling the gain ratio of said amplifier according to a moving average of the signal intensity.

18. In a sound reproducing system, a sound record, a pick-up device therefor, a plurality of stages of push-pull amplification fed by said pickup device, the gain ratios of said stages being controlled in accordance with the strength of the signal derived from said record.

19. In a sound reproducing system, a sound record, a pickup device therefor, an amplifier fed by said pick-up device, a control circuit also fed by said pick-up device, said control circuit having a rectifier supplying a sub-audio current for controlling the gain ratio of said amplifier in accordance with the strength of signal impressed on said control circuit, and means for preventing high frequency tones from reaching said rectifier, whereby dynamic control of said amplifier by said high frequency tones is prevented.

20. In a sound reproducing system, a sound record, a pick-up device therefor, a sound propagating device fed by said pick-up device, said record having audio tones of 1ow,intermediate and high frequencies, and means controlled exclusively by the presence of the intermediate frequencies in the signal'picked up from the record for determining the dynamic expansion of tones of all frequencies.

21. In a sound reproducing system, a sound record, a pick-up device therefor, a sound propagating device fed by said pick-up device, said record having audio tones of low, intermediate and high frequencies, and means controlled exclusively by the presence of the intermediate frequencies in the signal picked up from the record for determining the intensity of tones of all frequencies.

22. In a sound reproducing system, a sound record, a pick-up device therefor, a sound propagating device, a plurality of stages of push-pull amplifiers between said devices and a dynamic volume control associated with an intermediate one of said amplifier stages.

JOHN HAYS HAMMOND, JR.

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