US2096758A - Dynamic multiplier with automatic tone filter - Google Patents

Description

Oct. 26, 1937. E. s. PURINGTON DYNAMIC MULTIPLIER WITH AUTOMATIC TO NE FILTER Filed April 3, 1936 3 Sheets-Sheet 1 me .E.

JNVENTOR S PuringToh Ellison TTORIIVE Y Oct. 26, 1937. E, P NGT 2,096,758

DYNAMIC MULTIPLIER WITH AUTOMATIC TONE FILTEfi Filed April 5. 193a 3 Shee ts-Sheet 2 E m I Frequencies r 7* Low Noise and Signal 2E1 Grid 3o --4o -30 -zo v |o o Pdi'enfial of Third Grids INLENTOR Ellison S. Puringi'on.

Oct. 26,1937.

E. S. PURINGTON DYNAMIC MULTIPLIER WITH AUTOMATIC TONE FILTER Filed April :5, 1935 s Sheets-Sheet s vNN a INVEN'IOR H Elna s. Puring'l'on.

Patented Oct. 26, 1937 PATENT OFFICE.

DYNAMIC MULTIPLI TONE ER. wl'rn AUTOMATI FILTER Ellison S. Purington, Gloucester, Mass., asslgnor to John Hays Hammond Mass.

Application April 3,

16 Claims.

This invention relates to an improved system for reproducing sound from a sound record and provides an automatic tone filter in which the impedance of an inductance arm is changed in 6 accordance with the nature of the tone signals to be transmitted through that arm.

This invention further provides an improved method 'of expanding the volume output of a phonograph amplifier in accordance with the acoustic level of the tonal signals being transmitted through the amplifier.

This invention further provides a simplified control for systems of the above type.

In one embodiment an automatically variable filter device is provided which changes the relative transmission of impulses of different frequencies in accordance with the nature of those impulses.

In particular the present method accomplishes the control by including one winding of a transformer in the filter network and automatically variably loading another winding of the transformer by a balanced thermionic impedance, the value of which is determined by the recorded impulses. The balanced load permits the control to be effected without appreciable change of permeability of the core.

In dynamic multipliers using control upon electrodes of anamplifier device, it is desirable that the gain vary more rapidly with change of control potentials with weak signals, and less rapidly with strong signals. This is especially necessary because a greater change of decibel output is necessary to give a noticeable effect at Weak volume than at strong volume. Previously this has been accomplished largely by limiting devices which cut down the effectiveness of rectification with large signal inputs. The present invention provides a simplified method in which control is effected directly upon one electrode of an amplifier, and indirectly upon another electrode in such a manner as to give increased control eifect especially at low signal volumes.

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 tothe following description taken in connection with the accompanying drawings forming a part thereof, in which J r., Gloucester,

1936, Serial No. 72,518

Fig. 1 illustrates diagrammatically one embodiment of the invention as applied to the reproduction of sound from the sound record of a talking moving picture projector and phono-, r ph; Fig. 2 is a series of curves illustrating the relative frequency transmission characteristics of certain portions of the circuit shown in Fig. 1;

Fig. 3 is a series of curves showing the improvement in expansion performance in other portions of the circuit shown in Fig. 1; and

Fig. 4 shows a modified circuit arrangement according to the present invention.

Like reference characters denote like parts in the several figures of the drawings.

In thefollowing 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 accompanying drawings, and more particularly to Fig. 1, a phonograph pickup ill is shown which is connected to two con-' tacts of a double pole double throw switch II, the other contacts of which are connected to the circuit of a photo-electric cell l3 including battery l5. Light from a lamp I6 is focused by means of a lens I! through an aperture l8 in an aperture plate l9 upon the sound record of a moving photographic-film, 20. The light which passes through this film thenimpinges upon the photo-electric cell l3.

The'blades of the switch H are connected through a transformer 2i to the input circuit of a space discharge amplifier 22. The output circuit of this amplifier includes a filter network comprising a blocking condenser 23, an inductance 25, condensers 21, a double rheostat 28, and a manual volume control potentiometer 26. 'The double rheostat 28 constitutes a manual tone control for varying the characteristics of the filter network.

The adjustable contact of the potentiometer 26 is connected to the adjustable contact of a rheostat 30 in such a manner that they both move in unison. The rheostat 30 is connected through the primary 3| of a tone control transformer 32 to the primary of a coupling transformer 83. The secondary 34 of the tone control transformer 32 is connectedto the plates of two space controlled in the manner to be described. The

discharge tubes 35 and 36 by which the tone is cathodes of these tubes are grounded. The grids,

during operation, are variably negative with respect to ground as will be hereinafter set forth.

The secondary of the coupling transformer 33v is connected tothe first grids of two pentode expander tubes 38 and 39 which are operated as a push-pull amplifier, the output circuit of which includes the primary of a transformer 48.

The secondary of the transformer 48 is connected in the input circuit of a push-pull amplifier 4| comprising two pentode tubes 42. The output circuit of this amplifier 4| includes a transformer 43 and the moving coil of a loudspeaker 44.

A high voltage power supply battery 48 or its equivalent such as a rectifier system, is provided for supplying current to determine the potentials of the electrodes of the various space discharge devices. The field coil 46 of the loudspeaker 44, a filter choke 41, and a resistor assembly 48 are connected across the battery 45.

The positive end of the resistor assembly 48 is connected, through a feed resistor 48 and a coupling resistor 48a to the anode; of the space discharge amplifier 22, through a feed resistor 58 and windings 34 of transformer 32 to the anodes oi. the tubes 35 and 36, and through the primary of the transformer 48 to the anodes of the pentodes 38 and 39. The positive end of filter choke 41 is connected through the primary of the transformer 43 to the anodes of the pentodes 42.

The cathode of the amplifier 22 is connected to ground. through a biasing resistor 52 and a by-pass condenser 53. A tone control resistor 54 and condenser 55 may be connected across the secondary of the transformer 2| to control the transmission characteristics'there'of. A by-pass condenser 56 is connected in theoutput circuit of the amplifier 22 between the junction of resistor 48 and resistor 48a and ground.

The center tap of the primary of transformer 40 is connected through a bleeder resistor 51 to the cathodes of the pentodes 38 and 38 thence through a biasing resistor 58 to ground, and also through a coupling and biasing resistor 68 to the second grids of the pentodes 38 and 33. The third grids of the pentodes 38 and 38 are variably negative in accordance with the control voltage to be described.

The signal voltage is applied to the first grids of the pentodes 42. The cathodes and third grids are connected to an intermediatepoint on the resistor assembly 48. The second grids are connected to the positive side of choke coil 41.

For controlling the operation of the tone control tubes 35 and 36, energy is delivered from the input side of the primary winding of transformer 32, through condenser to a,potentiometer 6|. For controlling the operation of the expander tubes 38 and 38, energy is delivered from the out-' put side of the primary winding of transformer 32, through condenser 15 to a potentiometer 16.

For tone control purposes, the adjustable contact of potentiometer 6| is connected to the grid of an amplifier tube 62. The output circuit of this amplifier is coupled through a high-pass filter and coupling network 63 to the input circuit of a second amplifier tube 65. The output circuit of the amplifier tube 65 is coupled to a rectifier circuit 66 comprising a rectifier tube 81 and a resistor 68. The positive side of the resistor 68 is connected through a second resistor 88 to the grids of the tone control tubes 35 and 36. A limiter tube 18 is connected between the resistor 68 and the adjustable contact 1| of a potentiometer 12. The negativeside of the resistor 88 is connected to a second adjustable contact 13 onmosaics the potentiometer 12 which is connected between the negative side of the battery 45 and ground.

For expansion control purposes, the adjustable contact of potentiometer 16 is connected to the grid of an amplifier tube 11. The output circuit of this amplifier is coupled by a filter and coupling circuit 18 to the input circuit of a'second amplifier tube 18. The output circuit of the amplifier 18 is coupled to a rectifier circuit which includes a rectifier tube 88 and a resistor 8|. The positive side of the resistor 8| is connected through a second resistor 82 to the third grids of the tubes 38 and 38. A limiter tube 83 is connected between the resistor 82 and the adjustable contact 84 of a potentiometer 85 which is connected between the negative side of the battery 45 and ground. The negative side of the resistor 8| is connected to a second adjustable contact 86 of the potentiometer 85.

Plate voltage is supplied to the tubes '62, 65, 11, and 18 from the positive end of the resistor assembly 48 through suitable feed resistors 81, 88,

-88,.and 88 respectively. The cathodes of these fpick up ill or photo-electric cell l3 passes to the amplifier 22, where it is amplified and fed through the filter network to the potentiometer 26. The double potentiometer 28 may be used as a manual tone control when desired.

Energy from the potentiometer 26 .is fed through the rheostat 30, primary winding 3|, and the transformer 33 to the push-pull amplifier 31. Here it is amplified and fed through the transformer '48 to the power amplifier 4|, where it is further amplified and fed through the transformer 43 to the moving coil of the loudspeaker 44, where it is reproduced as speech or music in a well known manner.

For controlling the operation of the automatic variable filter some of the energy from the rheostat passes through the stoppage condenser 66 to the potentiometer 6|, from which any de sired proportion is fed to the amplifier tube 62. Here the energy is amplified and passes through the high-pass filter 63, the high frequency component passing to the second amplifier tube 65. Here the high frequency energy is further amplified and is fed to the rectifier 61 where it is rectified and causes a current to flow through the resistor 68 in the direction of the arrow. This causes a potential difference to be built up across this resistor which is proportional to the intensity of the high frequency energy in the input signal. The positive potential produced at the upper end of the resistor 68 is applied through the resistor 68 to the grids of the filter ccntrol tubes and 36.

In the absence of control signal, the tubes 35 and 36 are biased sufficiently to reduce the space currents substantially to zero, so that the secondary winding 34 is not loaded and the efi'ect of the presence of the transformer 32 is principally the choking eifect due to the actual inductance of the primary winding 3| which discrininates against the passage to transformer 33 of the higher frequency signals, such as surface noise, for example in the range of 1000 to 4000 cycles. 1

This discrlmination'against the higher tones is shown by curve I00 in Fig. 2'which shows the transmission characteristics when only weak high frequency signals are present. The curve shows the lowered transmission of the higher frequencies in the background noise range. The curve I00 represents the transmission character! istics when the needle. is in the record groove in the absence of any appreciable recorded signal.

When there are recorded signals, suficient to mask the presence of the background noises, the rectifier circuit is operated to lower the bias of the tubes 35 and 36 as already described. This decrease in bias is in accordance with the characteristics of the control channel. This in turn lowers the thermionic resistance shunted across the secondary winding 3t thus increasing the secondary loading which produces the general effect of lowering the primary equivalent impedance especially for the higher frequencies. The relative transmission of currents of different frequencies throughthe network in which the primary winding 3! is connected may therefore be modified by secondary thermionic impedance control.

The effect just described is depicted by the eurvesin Fig. 2, which show transmission characteristics relative to frequency. It has been found that the surface noises of the higher frequency range may be masked by a sufficient amount of signal in that range or by a much greater signal of lower frequency. The curve IIiI shows the relative ability of currents of different frequencies to produce rectification by tube 61. The elements are suitably proportioned so that whenever a combination of lows and highs is present in suflicient amount to operate the rectifier 61, they will also be sufilciently strong to mask the surface or ground noise effects which the control channel then permits the primary- Winding 3i to pass.

When the combination of lows and highs is sufficiently strong so that the grid potential of the tubes 35 and 36 is equal to their cathode potential, the relative transmission of signals through the primary of transformer 32 for different frequencies is as shown by curve I02 in Fig. 2. This shows improved transmission for high frequencies over the corresponding curve I 00 corresponding to weak control signals.

By this process the transmission .is automatically controlled so that surface noise effects will not be transmitted to the loudspeakers unless the volume level and volume distribution is at all times such as to mask the noise effects. The constants of the circuits are preferaby so chosen that the control action will be rapid. There is no apparent loss of hightones except under conditions which would otherwise allow the passage of high tones to be accompanied by serious background disturbances. In this manner it is practicable to use a system that will transmit higher frequencies than if surface noises were suppressed by manually controlled or fixed scratch elimination.

The primary winding 3| may be shunted by a condenser, or magnetically coupled to a tuned circuit network 99 for the purposes of tuning the effective impedance of the primary winding 3i, and modifying the transmission characteristics for weak signals.

It will be noted that a balanced load is placed on the secondary 34 of the transformer 32. In this waythe effect on the D. C. "core flux is balanced out and the load may be varied without altering the saturation of the transformer. The control is thus made rapid and distortionless.

The details of this tone control system are further explained and claimed in my co-pending application Serial No. 72,520, Patent No. 2,006,- 760.

With reference to the expansion operations, the first grids of the pentodes 38 and 39 are at ground potential, the cathodes-are biased positively with respect to ground by the potential drop in resistor 58 through which bleeder current fiows from the bleeder resistor 51 and also the space currents to the cathodes. The second grids are maintained at a positive potential by the plate battery 45 acting through the coupling resistor 59. The third grids are normally maintained at a negative potential determined by the setting of the adjustable contact 86 of the potentiometer 85.

Some of the energy which passes through the transformer 32 passes through the condenser and thence through the potentiometer I6 to the amplifier tube 'IT.- The output of this amplifier tube is further amplified by the second amplifier tube 19 and then passes to the rectifier tube 80. The rectified output of this tube passes through the resistor 8| in the direction of the arrow and thus causes a potentialdifference to be built up across this resistor which is proportional to the current passing through it andtherefore to the signal strength.

The potential drop across the resistor 8i will decrease the negative bias on the third grids of the tubes 38 and 39 making them more positive in proportion to the increase of input signal strength. This acts directly upon the plate-cathcreases the current drawn by the second grids,

thereby decreasing the potential drop through the biasing resistor 59 which increases the positive potential on the second grids. This in turn lowers the internal impedance and increases the mutual conductance of the first grids upon the anode-cathode circuit.

These two effects are in a like sense and. are additive for extending the range of expansion, and increasing the expansion at low acoustical volume.

The third grid potentials are limited by the current flow through the resistor 82 so as to prevent these grids from becoming positive with respect to the cathodes.

The magnitude of the effects which are produced by this portion of the circuit are indicated potential of the second grid by the indirect method just described. This curve shows. a greater sharpness of cut-oil! as well as a greater range of gainfresulting in improved expansion control and performance at weak signal values with small rectified potential.

The limiter tubes 10 and 83 function when their anodes become positive with respect to their cathodes to prevent further build up of potential on the respective control grids.

The details oi operation of the pentode expander tubes are further explained and claimed in my copending application Serial No. 72,519, Patent No. 2,096,759.

In Fig. 4 is shown a system for practicing the invention, suitable for nontechnical operation, in, which the controls available for adjustment are minimized by use of a gang switch which presets the controls in a best average manner for different kinds of phonograph signals.

Pick-up device I2I delivers electrical energy to operate the amplifier tube I22, which is pro-' vided with an input coupling resistor I23 connected from grid to ground. This energy is suitably treated by attenuators I24, choke I25, transi'ormer I26, resistor I21, and condenser I28 to cznnpensate for recording deficiencies. The attenuator I24 is preset in accordance with the design characteristics of pick-up I2I, and in a coordinated design, may be omitted. Choke I25 is'so proportioned as to out down higher frequency tones which have no musical value, as for example above the order of 4000 to 5000 cycles. Resistor I21 and condenser I28 serve to relatively strengthen the lower tones to compensate for undercutting of has notes. This is accomplished by proportioning resistor I21 and condenser I28 to load the secondary of transformer I26 for frequencies of the order of 400 cycles and over, and to produce practically no loading for frequencies of the order of 40 cycles.

These fixed adjustments are made to balance up the system tonally and as to volume, and are not an essential part "of the invention. 7

The signal energy amplifier tube I22 is arranged as a resistance fed amplifier, with plate current supplied from a plate supply source I29,

ilirough feed resistor I30 and coupling resistor I3I, with condenser I32 serving as a by-pass to ground. The space current passes from the cathode of tube I22 to ground through a resistor I33, by-passed by a condenser I34 which serves to bias the cathode positive with respect to ground. Energy amplified by tube I22 is passed through blocking condenser I35 and through the primary winding of a transformer I36, to 9. volume control potentiometer I31 to ground. The passage of currents from the plate of tube I22 to the volume control potentiometer J31 is influenced by. the loading of the transformer primary by two secondary windings, one of which is loaded by variable resistor I30, manually controlled, and the other of which is loaded by the automatically controlled tube impedances from plate to plate of tone control tubes I39, in a manner to be described. The plate of tube I22 is by-passed to ground for high frequencies by condenser I40, and the volume control potentiometer I31 is similarly by-passed to ground by a condenser I4I These two condensers, in combination with the effective primary impedance of transformer I36, form a filter section of the lowpass type, with cut-of! automatically and manually adjustable.

Energy from the potentiometer I31 is fed sped-ins through a transformer I to the first grids-of push-pull connected expander pentodes I46. Between the potentiometer I31 and the transformer I45 may be included suitable circuits for varying the fldelityot transmission in accordance with the manual adjustment of the volume control. In particular, resistor I41, choke I48, condenser I49 and potentiometer I50 may serve to discriminate somewhat against medium valued tonal frequencies when the potentiometers I31 and I50 are set for lower signal volumes.

The pentodes I45 serve to produce volume expansion in accordance with the acoustical volume output of the tube I22. The anodes of the pentodes I46 are connected to the primarywindings of a transformer I5I which corresponds to the transformer 40 of Fig. 1, the secondary of which is connected to the subsequent transmission line, or amplifier stages.

The transformer I5I may be ballasted with primary resistors I52 to the mid point of which the plate supply battery I26 is connected. Potential for the second grids of pentodes I46 is supplied from the source I29 through coupling resistor I53. Bleeder current from the source I29 passes through bleeder resistor I54 to the cathodes, and thence through biasing resistor I56, which determines the bias of the first grids with respect to cathode, and through potentiometers I51 and I58 to ground. Condensers I59 may be used, if desired, to by-pass the biasing resistor I56 to ground.

The plates of the tone control tubes I39 are supplied with operating potential from source I29 through feed resistors I63 and I64. The source is also connected through bleeder resistor I65,to

the cathodes, thence through biasing resistor I66 to ground. Suitable by-pass condensers I61 to I69 may be connected across the various resistors and between the grids oi the tubes I39 and ground.

The gain of the amplifier stage including pentodes I46 is controlled in accordance with the voltage between the third grids and cathodes, and the transmission through primary of transformer I36 is controlled in accordance with the grid to cathode voltage of tubes I39. These voltages are automatically controlled in accordance with the signals transmitted to an-amplifierrectifier system from. the plate of tube I22, through a coupling condenser I12, which impresses control signal voltage across a coupling resistor I13 connected in the input circuit of control preamplifier tube I14. The values of condenser I12, and resistor I13 are suitably proportioned with respect to each other, and with respect to the other combinations of resistors and condensers so that the expansion produced by the control channel will be in accordance with the acoustical volume instead or the electrical volume level at the output of tube I22.

The operating potential is supplied to the anode of control preamplifier tube I 14 from the source I29 through feed resistor I15, by-passed by a condenser I16. The cathode is connected to ground through a biasing resistor I11 and a by-pass .condenser I18.

The output of this tube I14 is fed through a blocking condenser I 80 to a resistance coupling network to actuate the grids of expander-control amplifier tube I84 and tone-control amplifier resistance coupling network, comprising a tapped resistor I82 and a fixed resistor I83 connected in parallel across the grid-cathode circuit of expander-control amplifier tube I84. The switch I8I successively engages a contact I85 connected to ground and contacts I88 to I88, connected to taps on the resistor I82. The arrangement is such that the energy supplied to the grid-cathode circuit of the tube I84 is increased as the switch I8I engages the successive contacts I88 to I88, but is zero when it engages contact I85.

The cathode of the tube I84 is connected to ground through a biasing resistor I89 and a bypass condenser I90. The output is applied through a blocking condenser I9I to a rectifier output resistor I92, connected across one anodecathode path of a double rectifier I93. Plate voltage is supplied to the tube I84 from the source I29 through feed resistor I94 and coupling resistor I94a. By-pass condensers I85 and 'I95a serve as a by-pass on feed resistor I94 and coupling resistor I 94a for high frequencies. The

, positive end of the resistor I92 is connected through a resistor I96 to the third grids of the pentodes I46.

The potential thus applied to the third grids of the pentodes I46 is limited by the second anode-cathode path of the double rectifier tube I93. For this purpose the second anode is connected through a resistor I91 to a tap on the resistor I96 and the second cathode is connected, by a switch I98 having contacts I89 'to 282 to taps on a resistor 203 which is connected between the positive end and adjustable contact of the potentiometer I58. The second cathode is thus adjustably positive with respect to the first anode which is connected to the ad- Justable contact of the potentiometer I58. A filter and timing condenser 284 may be connected across the resistors I92 and I96 to by-pass'the A. C. component of the rectified energy. 1

The output from the amplifier tube I14 is also.

impressed through a resistance capacity network, comprising resistors 2H), 2H, and 2I2 and condensers 2I3, 2I4, and 2I5 on the input circuit of a tonecontrol amplifier tube 2I6. The cathode of this tube is connected to ground through a biasing resistor 2H and a by-pass condenser 2I8. The output of the amplifier tube M6 is applied through a coupling condenser 2I9, to a rectifier output resistor 220 connected across one anodecathode path of a double rectifier 22I-. Plate voltage is supplied to the tube 2I6 from the source I29 through a feed resistor 222 and coupling resistor 222a, the former having a by-pass condenser 223.

The positive end of the resistor 229 is connected, through a resistor 224 to the grids of the tone control tubes I39. The potential thus applied to the grids is limitedby the second anodecathode path of the double rectifier tube 22I. For this purpose the second anode is connected, through a resistor 225, to a tap on the resistor 224 and the second cathode is connected to a tap on the resistor I66 which is negative with respect to the cathodes of the tubes I39. A filter condenser 226 may be connected across the resistors 220 and 224 to by-pass the A. C. component of the rectified energy.

The timing of the expander control exercised by the control channel including the expandercontrol amplifier tube I84 and the rectifier I93 may be varied by a switch 230 having contacts 23Ito 234. The switch blade is connected to the cathode of the expander pentodes I46. The

contact 23I is connected directly to the third to the third grids through condensers 235 and 238 respectively. When the contact 23I is closed the third grids are maintainedat the cathode potential and the pentodes I46 operate as standard amplifiers. When the contacts 233 or 234 are closed the timing otthe control potential on the third grids is governed by the capacities of the condensers 235 or 238.

A switch 238 is provided to vary the initial setting of the biasing potential of the cathodes oi the pentodes I48 with respect to their signal grids as determined by the potentiometers I51 and I58. This switch 238 is provided with contacts 238-242 certain of which are connected toresistors 243 and 244 which are respectively connected, by the switch 238, across the potentiometers I51 and I58 for varying the effective resistance thereof.

In the operation of this embodiment, the signal energy derived from the pick-up I2I is amplified by the amplifier tube I22, passes through the filter network including the primary oi the transformer I36 and is further amplified by the push-pull amplifier stage comprising the pentodes I46 after which it is applied through the transformer IBI to'a utilization circuit comprising a power amplifier and a loudspeaker driven thereby.

A portion ,of the energy passes from the output of the amplifier tube I22 to the control preamplifier tube I14, thence through the resistance coupling network above described to the input circuits oi the expander-control amplifier tube I84 and the tone-control amplifier tube 2I8. The energy is amplified by these tubes and is rectified by the double rectifiers I93 and 22I to cause a current to flow through the resistors I32 and 228 in the direction oi the arrows to build up a positive potential proportional to the input energy which is applied to the third grids of the. pentodes and to the grids of the tone control tubes I39, to control the expansion and tone as described in connection with Fig. 1.

In addition to control of the expansion properties in accordance with the signal characteris-' tics at the plate of tube I22, thereis an additional control by the four switches I8I, 230,238, and I98 for controlling the strength of signal to the volume control rectifier, the timing, the expander gain for weak signals, and the expander gain for strong signals. In addition there may he controls, as for example of the gain for weak signals, which vary with the setting of the manual volume control. It is understood that the four switches are mechanically connected with respective switch points as indicated by the numberings.

In particular, when switch I8I is connected to contact I85, no signal is impressed upon the volume control circuit, while atthe same time switch 230 is connected to contact 23 I for connecting the third grids of expander tubes I46 tothe cathodes, producing maximum gain irrespective of signal volume. For the next position, with contacts I86 and 232 closed, the switches may be set for average dance records; with contacts I81 and 233 closed, for average operatic records; and with contacts I88 and 234 closed, for average. symphonic records. It will be understood, of course, that this classification is for illustration only, and that any desired number of switch connections may be used. I

The switch I 8| is connected to tapped resistor I82 and grid-input resistor I83 to vary the signal III , path. This secondary fiow decreases the efiiciency.

when higher frequency tones are sufficiently' present to mask the scratch frequency tones.

The second cathode-anode branches of the tubes I93 and 22I are not used for rectification purposes, but rather to decrease the efiiciency of rectification when strong signals are impressed. This action will beillustrated with reference to tube 22I. The second cathode is connected to a point on the resistor I66 which is positive with respect to ground and with respect to the negative and grounded end of resistor 220. In the absence'of signals, no current passes through the second anode to cathode path, because the anode is negative with respect to the cathode. When sufficiently strong signals are impressed, peaks of surges will be suificiently great in the system to cause the second plate to become positive with respect to its cathode whereby rectified current will flow through the second plate to cathode of rectification of tube HI and prevents further substantial increase in potential across resistance 220, thereby limiting the final potential of the grids of tubes I39 to only slightly more positive than the potential of the point on the resistance I89 to which the second cathode is'connected. The sharpness and definiteness of cut-oi! may be increased by making the tap on resistor 224 closer to the end of the resistor which is connected to condenser 226 and by making the resistor 225 smaller.

Similar control is exercised by the second anode and cathode of tube I93, to decrease the eillciency of rectification of that tube at increased signals and to limit the potential applied to the third grids of the pentodes I48.

It will be noted that abrupt termination of rectification efllciency results when the grid potentials of tubes I39 are raised to cathode potential, due to conduction within the tubes I39 themselves, and similar results occur in tubes I48 to limit the control potential.

, Tubes I93 and 2,2I are commercially available, type 6H6, in which two, independent diodes are included in the same structure. One of the diodes is used for rectification purposes, the other is used for controlling the eiflciency of rectification, giving increased efllciency of rectification for weaker signals.

It will be further observed that upon decrease in signals the condensers 224 and I49 first discharge through the grid-to-cathode path of tubes I39, and through the second anode-to-cathode path in tube 22I. Thereafter the discharge path available is through resistors 224 and 220 in series. The time constant determined by the condensers 223 and I39, and resistances 224 and 229 in series may be of the order of .02 ohmfarad or less, but the effective time constant when the condensers are being charged will be very much less. This provides very little delay in control of the impedance of tubes I39, so that i the change of transmission is very quickly accomplished.

Similar control operation is exercised by the rectifier limiter I93 and associated circuits The timing of this circuit is controllable by switch 230 with time constant for example of the order of .2 ohm-farad to .02 ohm-fared for symphonic and speech purposes respectively.

In addition to timing control of the expander,

the initial setting control-switch 239 provides for 'a wider range of expansion on one type of recordthan on another type. Resistor units 243 and 244 serve to lower the effective resistance of the potentiometers I51 and I50 which may be varied to adjustfor characteristics of diiferent tubes I46. For example, with the switch 238 engaging contact 24I the amount of possible change of gain by tubes I46 may be 30 db, while with the switch 238 engaging contact 242 it may be 20 db, assuming volume-co trol set at full volume. The linking of manua volume-control setting of potentiometers I31 and I50 to the potentiometer I50 serves to reduce the initial negative bias on the third grids of the tubes I46 when the volume control is set for lower volume. This increases the amplification characteristics of the tubes for weak signals and thereby cuts down the amount of possible expansion as the general volume levelis reduced :0 asto maintain the stray noise level in the absence of signals substantially independ ent of the manual control setting. In this way no signals will be lost when the reproduction is made at reduced volume.

The remaining control switch I98 is for limiting cpcration control, the second cathode of tube I93 being connected to the center point of the switch. The tappcdresistor 203 is of relative high resistance compared with resistance of potentiometer I58. As a result, the contact points 200 to 202, connected to the tapped resistor 203, are at a potential positive with respect to the first anode of tube I93, irrespective of the setting of potentiometer I58. I

It will be seen, therefore, that there is provided a phonograph circuit with a minimum of controls, namely manual volume and manual tone controls, such as would be required for a conventional type amplifier. In addition there is a signal switch which should be preset only in accordance with thetype of record which is to be reproduced. Furthermore, there are suitable linkages between the manual volume-control and expander adjustments so that undesirable results are not produced when the reproduction is set for reduced volume. circuits, the choices of adjustments to be made by the signal switch. and the linkages between the manual volume and manual tone-control cir- 1. In a system for reproducing sound from a sound record, a transmission channel including. a

filter network and an amplifier, a transformer having a primary connected in said filter network to control the transmission characteristics It will be understood that the type of' thereof, means applying a variable balanced load to the secondary of said transformer for altering the effective primary impedance, means controlling the amplification characteristics of said amplifier and a control channel energized in accordance with the applied signal, said control channel having means controlling said load and controlling the amplification characteristics of said amplifier in accordance with the impressed signal for simultaneously varying the dynamic range and the frequency distribution of the reproduced signal.

2. In a system for reproducing sound from a sound record, a signal channel comprising a filter network and an amplifier, a transformer having a primary connected in said filter network to control the transmission characteristics thereof, a pair of space discharge devices connected in balanced relation across the secondary of said transformer to variably load the same and to thereby control the effective primary impedance of the transformer, said amplifier comprising a third space discharge device and means varying the impedance of all of said space discharge devices in accordance with a characteristic of the applied signal so as to simultaneously vary the dynamic range and the frequency distribution of the reproduced signal.

3. In a system for reproducing sound from a scund record, a signal channel comprising a filter network and an amplifier, a transformer having a primary connected in said filter network to control the transmission characteristics thereof, a pair of space discharge devices corinected in balanced relation across the secondary of said transformer to variably load the same and to thereby control the effective primary impedance of the transformer, said amplifier comprising a third space discharge device means deriving a control voltage from the applied signal, and means applying the control voltage to vary thegrid bias on all of said space discharge devices for changing the load on said secondary and the gain ratio of said amplifier in accordance with a characteristic of the applied signal.

4. A system for reproducing sound from a sound record comprising a transmission channel having a filter network and an amplifier, a varia ble impedance device included in said filter network and adapted to change the transmission characteristics thereof, said amplifier comprising a multi-grid space discharge device having a signal grid and a plurality of control grids at least one of which is positively biased, means applying the signal voltage from the output circuit of said filter to said signal grid, means applying a control voltage to both of said control grids so as to control the impedance of said tube thereby in the same sense, and means responsive to the applied signal for varying the impedance of said variable impedance device and for producing said control voltage.

5. In a system for reproducing sound from a sound record, a signal channel including a nonlinear amplifier adapted to expand the dynamic range of the signal, a manual volume control for adjusting the volume of thereproduced signal and means for changirig the amount of expansion produced by said amplifier, said last means being mechanically linked to said manual volume control so as to reduce the expansion when the volume control is set for a lower volume.

6. A system for reproducing sound from a sound record comprising a variable filter and an amplifier adapted to expand the dynamic range of the reproduced sound, a control channel connected to control the amount of expansion produced by said amplifier, means energizing said control channel in accordance .with the signal energy, adjusting means for variably adjusting the proportion of the -signal energy applied to the control channel, a second adjusting means for adjusting the timing of the control action, a third adjusting means for adjusting the amplifier gain for ,weak signals and a fourth adjusting means for adjusting the amplifier gain for strong signals and means operating all of said adjusting means in unison for presetting the amplifier for a plurality of different types of records.

7. A system for reproducing sound from a sound record comprising a variable filter and an amplifier adapted to expand the dynamic range cf the reproduced sound, a controlchannei connected to control the amount of expansion produced by said amplifier, means energizing said control channel in accordance with the signal energy, adjusting means for variably adjusting the proportion of the'signal energy applied to the control channel, other adjusting means for adjusting the amplifier characteristics, and means operating all of said adjusting means in unison for presetting the amplifier for a plurality of different types of records.

8. A system for reproducing sound from a sound record comprising a variable filter and an amplifier adapted to expand the dynamic range of the reproduced sound, a control channel connected to control the amount of expansion produced by said amplifier, means energizing said control channel in accordance with the signal energy, means for adjusting the control action, means for adjusting the expansion, means for adjusting the amplification, and means operating all of said adjusting means in unison.

9. A system for reproducing sound from a sound record comprising a variable filter and an amplifier adapted to expand the dynamic range of the reproduced sound, a control channel connected to control the amount of expansion produced by said amplifier, means energizing said control channel in accordance with the signal energy, adjusting means for variably adjusting the proportion of the signal energy applied to the control channel, a second adjusting means for amplifier adapted to expand the dynamic range of the reproduced sound, a control channel connected tocontrol the amount of expansion produced by said amplifier, means energizing said control channel in accordance with the signal energy, adjusting means for variably adjusting the proportion of the signal energy applied to the control channel and other adjusting means for adjusting the amplifier characteristics.

11. In a system for recording or reproducing sound, a signal channel including an amplifier adapted to expand the dynamic range of the re-v produced signal, a control channel therefor, means supplying said control channel with a portion of the sound energy and a rectifier in said control channel, means utilizing said rectifier for producing a control voltage varying in accordance with the applied signal and means for decreasing the emciency of rectification in said rectifier when the control voltage produced thereby exceeds a predetermined value, and means utilizing said control voltage to vary the amplification characteristics of said amplifier.

12. In a system for recording or reproducing sound, a signal channel including an amplifier adapted to expand the dynamic range of the reproduced signals, a control channel therefor, means supplying said control channel with a portion of the sound energy and a double rectifier in said control channel having two anode-cathode paths, means utilizing one of said anode-cathode paths for producing a control voltage varying in accordance with the applied signal and means utilizing the other of said anode-cathode paths for decreasing the'eillciency of rectification in said first path when the control voltage produced thereby exceeds a predetermined value and means utilizing said control voltage to vary the amplification characteristics of said amplifier.

13. In a system for recording or reproducing sound, a signal channel including a variable impedance device, a control channel having a rectifier connected to produce a control voltage, and means decreasing the efllciency of rectification when the control voltage exceeds a predetermined value, and means utilizing the control voltage to a predetermined value, and means utilizing said control voltage to vary the impedance of said device.

15. In a system for reproducing sound from a sound record, asignal channel including an amplificr tube having a negatively biased control grid, means adjusting said bias to vary the amplification characteristics of the tube, a manual volume control for adjusting the volume of the reproduced signal, and means interconnecting the bias adjusting means and the manual volume control so as to decrease said bias when the volume control is set for lower volume whereby the stray noise level is maintained substantially independent of the volume control setting.

' l6- In a system for reproducing sound from a sound record, a transmission channel including a filter network and an amplifier, a transformer having a primary connected in said filter network to control the transmission characteristics thereof, means applying a variable balanced load to the secondary of said transformer for altering the effective primary impedance thereof, said amplifier comprising a multi-grid space discharge de-' vice having a signal grid and a plurality of control grids, at least one of which is positively biased, means applying the signal voltage from the output circuit of said filter to said signal grid, means applying a control voltage to both of said control grids so as to control the impedance of said tube in the same sense and means responsive to the applied signal for controlling said load and for producing said control voltage for simultaneously varying the dynamic range and the frequency distribution of the reproduced signal.

ELLISON S. PURINGTON.

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